New alloy design approach to inhibiting hot cracking in laser additive manufactured nickel-based superalloys

doi:10.1016/j.actamat.2023.118736

Acta Materialia 材料学报

Volume 247, 1 April 2023, 118736
第 247 卷，2023 年 4 月 1 日，118736

https://doi.org/10.1016/j.actamat.2023.118736 Get rights and content 获取权限和内容

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Abstract 摘要

Avoiding the formation of cracks to ensure a reliable printability and a good stability is crucial in the laser additive manufacturing of alloys. Contrary to previous studies that have generally tried to decrease the liquid film and solidification range, in this work, we innovatively utilized segregation engineering and abundant cell boundaries to introduce liquid backfilling as well as a network of segregation phases to alleviate thermal stress, consequently eliminating hot cracking. More specifically, zirconium was introduced into a nickel-based superalloy to form a continuous interdendritic liquid film during the laser additive manufacturing process. It was found that the continuous intermetallic Ni₁₁Zr₉ segregation phase decorated the cell boundaries, and cracks were completely eliminated in the as-printed Haynes 230 alloys when their Zr content reached 1 wt.%. Moreover, this continuous Ni₁₁Zr₉ network layer was able to act as a “skeleton” to significantly improve the yield strength of the as-printed samples. Following appropriate heat treatment, these Zr-modified Haynes 230 alloys exhibited an extraordinary combination of strength and plasticity, which were superior to those of the previously reported Haynes 230 alloy. These findings provide a new alloy design route for the laser additive manufacturing of crack-free alloys with excellent mechanical properties.
在激光增材制造合金过程中，避免裂纹的形成以确保可靠的可打印性和良好的稳定性至关重要。与以往通常试图减少液态薄膜和凝固范围的研究不同，本研究创新性地利用偏析工程和丰富的晶胞边界，引入液态回填以及偏析相网络来缓解热应力，从而消除热裂纹。更具体地说，在镍基高温合金中引入锆元素，在激光增材制造过程中形成连续的枝晶间液态薄膜。研究发现，连续的金属间化合物 Ni ₁₁ Zr ₉ 偏析相装饰了晶胞边界，当锆含量达到 1 wt.%时，打印出的 Haynes 230 合金中裂纹完全消失。此外，这一连续的 Ni ₁₁ Zr ₉ 网络层能够作为“骨架”，显著提高打印样品的屈服强度。经过适当的热处理，这些掺锆改性的 Haynes 230 合金表现出卓越的强度与塑性组合，优于先前报道的 Haynes 230 合金。这些发现为激光增材制造无裂纹且具有优异机械性能的合金提供了一条新的合金设计途径。

Graphical abstract 图示摘要

Keywords 关键词

Additive manufacturing

Hot cracking

Cell boundary

Heat treatment

增材制造热裂纹晶界热处理

1. Introduction 1. 引言

Metal additive manufacturing, also known as metal 3D printing, offers tremendous advantages over traditional subtractive manufacturing techniques in terms of the intricate integral component fabrication, lightweight engineering and material utilization [1], [2], [3]. However, only a limited number of commercial alloys, such as 316L, IN718, Ti6Al4V, and AlSi10Mg, have been widely applied to this new technology [4,5]. The overarching challenge is the inability to produce defect-free components. The majority of alloys cannot accommodate the rapid cooling rate and spatially variable temperature gradients during the laser additive manufacturing process, and eventually exhibit severe cracking under thermal stress [6], [7], [8]. Indeed, even some alloys with good weldabilities, such as Hastelloy X and Haynes 230, also exhibit obvious cracking during laser additive manufacturing. Such cracking is mainly represented by hot cracking, also known as solidification cracking in casting, which usually occurs in the terminal stage of molten pool solidification. Hot cracking is generally regarded to be caused by elemental segregation, and as a result, the solidification range of the alloy is expanded to form a liquid film that impedes bonding of the dendrites. The synergistic effect between the liquid film and the thermal stress arising from cooling shrinkage then leads to the development of hot cracking [9], [10], [11]. Michael et al. [12] used atom probe tomography (APT) to show that Zr segregates at the grain boundaries in the IN738LC alloy fabricated by laser powder bed fusion (LPBF), resulting in an enlarged solidification range and promoting the formation of solidification cracking. Similarly, elements such as Si, Mn, and B, which exhibit low partition coefficients in nickel-based superalloys, have also been reported to lead to intergranular low melting γ + γ' eutectics and borides [13], [14], [15].
金属增材制造，也称为金属 3D 打印，在复杂整体部件制造、轻量化工程和材料利用方面，相较于传统的减材制造技术具有巨大优势[1]，[2]，[3]。然而，目前只有少数商业合金，如 316L、IN718、Ti6Al4V 和 AlSi10Mg，被广泛应用于这项新技术[4,5]。主要挑战在于无法生产无缺陷的零件。大多数合金无法适应激光增材制造过程中快速冷却速率和空间上变化的温度梯度，最终在热应力作用下表现出严重的裂纹[6]，[7]，[8]。事实上，即使是一些焊接性能良好的合金，如 Hastelloy X 和 Haynes 230，在激光增材制造过程中也会出现明显的裂纹。这种裂纹主要表现为热裂纹，也称为铸造中的凝固裂纹，通常发生在熔池凝固的末端阶段。热裂纹通常被认为是由元素偏析引起的，结果导致合金的凝固范围扩大，形成阻碍枝晶结合的液态薄膜。液态薄膜与冷却收缩产生的热应力之间的协同作用，进而导致热裂纹的形成[9]，[10]，[11]。Michael 等人[12]利用原子探针断层扫描（APT）显示，在激光粉末床熔融（LPBF）制造的 IN738LC 合金中，Zr 在晶界处偏析，导致凝固范围扩大，促进了凝固裂纹的形成。同样，Si、Mn 和 B 等在镍基高温合金中表现出低分配系数的元素，也被报道会导致晶间低熔点γ+γ'共晶体和硼化物的形成[13]，[14]，[15]。

One of the general considerations in the fabrication of nickel-based superalloys by laser additive manufacturing is a reduction in the content of the interdendritic segregation components, such as Zr, Hf, Si, Mn, and B, which have been identified to be critical in determining the crack sensitivity of superalloys [16]. Until now, only limited types of nickel-based superalloys, such as Hastelloy-X, CM247LC(Hf-free), and IN738LC, have achieved hot cracking alleviation by decreasing the contents of such segregation components [6,7,12,16]. However, it is worth noting that reducing the levels of these minor alloying elements can adversely affect the mechanical properties of the printed alloys, such as sacrificing their strength and creep life [6,17,18]. For example, Zr atoms can fill vacancies and reduce the diffusion rate of elements at grain boundaries, thereby slowing dislocation climbing and strengthening the bonding of grain boundaries [19,20]. Neglecting these elements completely in nickel-based superalloys is therefore not optimal for crack suppression. Considering that the stress concentration and the presence of vulnerable regions (high angle grain boundaries, liquid film regions, etc.) are two necessary factors for crack generation, we assume that if the residual stress concentrated in the vulnerable region can be effectively alleviated, hot cracking could be prevented in laser additive manufacturing. Recently, considerable studies have focused on the use of inoculation treatment by adding Ti, Sc, and Zr solutes into Al alloys to obtain fine equiaxed microstructures that accommodate stress/strain and effectively suppress hot cracking [5,21,22]. However, inoculation treatment is not common practice in nickel-based or cobalt-based superalloys manufactured by the laser additive method, owing to the lack of suitable inoculants.
激光增材制造镍基高温合金时的一般考虑之一是减少枝晶间偏析组分的含量，如 Zr、Hf、Si、Mn 和 B，这些组分被认为是决定高温合金裂纹敏感性的关键因素[16]。迄今为止，只有有限类型的镍基高温合金，如 Hastelloy-X、CM247LC（无 Hf）和 IN738LC，通过降低这些偏析组分的含量实现了热裂纹的缓解[6,7,12,16]。然而，值得注意的是，减少这些微量合金元素的含量可能会对打印合金的机械性能产生不利影响，例如牺牲其强度和蠕变寿命[6,17,18]。例如，Zr 原子可以填补空位，降低晶界处元素的扩散速率，从而减缓位错攀爬并增强晶界的结合力[19,20]。因此，在镍基高温合金中完全忽略这些元素并非抑制裂纹的最佳方案。考虑到应力集中和脆弱区域（高角度晶界、液膜区域等）的存在是裂纹产生的两个必要因素，我们假设如果能够有效缓解集中在脆弱区域的残余应力，就可以防止激光增材制造中的热裂纹。近年来，许多研究集中于通过向铝合金中添加 Ti、Sc 和 Zr 溶质进行接种处理，以获得细小的等轴晶微观结构，从而缓解应力/应变并有效抑制热裂纹[5,21,22]。然而，由于缺乏合适的接种剂，接种处理在激光增材制造的镍基或钴基高温合金中并不常见。

During the laser additive manufacturing process, the molten pool is significantly smaller than the casting and welding pools; hence, the solidification associated with an ultra-fast cooling rate distinctly restrains the growth of dendrites and increases the solute solubility [2,23]. The interface (cell/grain boundary) in the laser additive manufactured microstructure is 2–3 orders of magnitude higher than that achieved by conventional casting and welding, which effectively disperses the liquid film caused by elemental segregation. In addition, the segregated elements present in conventional alloy compositions are strictly controlled at low levels [6,24], and so it is difficult to further decrease the volume of the liquid film and control the alloy solidification range for laser additive manufacturing. In contrast, certain segregated liquids can take part in backfilling to relieve the thermal residual strain, thereby reducing hot cracking during the casting process [10,25]; examples of such systems include Al-Si and Al-Cu alloys [26]. Thus, to prevent hot cracking in laser additive manufactured alloys, potential approaches could take advantage of the abundant cell boundaries, adjust the segregated components to uniformly introduce a liquid film among the dendrites, and alleviate the stress concentration. However, detailed studies into the mechanism of liquid backfilling during the hot cracking of various alloys remain scarce, and only limited insights have been gained into the importance of cell and grain boundary segregation [27,28].
在激光增材制造过程中，熔池明显小于铸造和焊接熔池；因此，与超快冷却速率相关的凝固显著抑制了枝晶的生长并提高了溶质的溶解度[2,23]。激光增材制造微观结构中的界面（细胞/晶界）比传统铸造和焊接高出 2–3 个数量级，有效分散了由元素偏析引起的液膜。此外，传统合金成分中存在的偏析元素被严格控制在低水平[6,24]，因此难以进一步减少液膜体积并控制激光增材制造的合金凝固范围。相比之下，某些偏析液体可以参与回填以缓解热残余应变，从而减少铸造过程中的热裂纹[10,25]；此类体系的例子包括 Al-Si 和 Al-Cu 合金[26]。因此，为了防止激光增材制造合金中的热裂纹，潜在的方法可以利用丰富的细胞边界，调整偏析组分，在树枝晶之间均匀引入液态薄膜，并缓解应力集中。然而，关于各种合金热裂纹过程中液体回填机制的详细研究仍然较少，对于细胞和晶界偏析重要性的认识也仅限于有限的见解[27,28]。

In this work, segregation engineering is innovatively employed to introduce a continuous uniform interdendritic liquid film in the terminal stage of solidification, and to eliminate hot cracking during the laser additive manufacturing process. Essentially, this strategy employs the characteristics of the low partition coefficients of Zr in nickel-based superalloys to form a continuous and stable liquid film at the cell and grain boundaries, which then achieves liquid backfilling to relieve the stress concentration. The ability of this process to eliminate hot cracking is evaluated, and the formation of network intermetallic compounds at the cell and grain boundaries of the as-printed sample is investigated. Subsequently, the dissolution of intermetallic compounds, the refinement of intergranular carbides, and the transformation of M₆C-to-MC during the heat treatment process are systematically characterized. Finally, the mechanical properties of the final samples are examined and compared with those of previously reported samples.
本研究创新性地采用偏析工程，在凝固终末阶段引入连续均匀的枝晶间液膜，以消除激光增材制造过程中的热裂纹。本质上，该策略利用锆在镍基高温合金中低分配系数的特性，在细胞和晶界处形成连续且稳定的液膜，从而实现液态回填以缓解应力集中。评估了该工艺消除热裂纹的能力，并研究了打印样品细胞和晶界处网络状金属间化合物的形成。随后，系统表征了热处理过程中金属间化合物的溶解、晶间碳化物的细化以及 M ₆ C 向 MC 的转变。最后，检测并比较了最终样品的力学性能与先前报道样品的性能。

2. Methods 2. 方法

2.1. Materials and printing
2.1. 材料与打印

Four types of gas-atomized Haynes 230 powder were used for the LPBF process with compositions (wt.%) of Zr-20.08Cr-12.19W-1.92Mo-1.46Fe-1.1Co-0.33Mn-0.36Al-0.38Si-0.1C-0.05B-bal.Ni. Therein, the Zr contents of the four powders were 0, 0.5, 1.0, and 1.5 wt.%, respectively, and the oxygen contents were <150 ppm in all cases. The prepared powders exhibited spherical morphologies with a size range of 15–53 μm. These powders were subjected to LPBF using a Concept Laser M2 printer equipped with a 400 W fiber laser source. The optimized LPBF printing parameters were a laser power of 300 W, a scanning speed of 1100 mm/s, a hatch spacing of 100 μm, and a layer thickness of 50 μm. A strip scanning strategy was employed along with a 67° rotation in the laser path between subsequent layers. All fabrication processes were carried out under a protective argon atmosphere and the oxygen content in the forming chamber was controlled below 100 ppm. Samples with Zr contents of 0 and 1 wt.% were used for detailed microstructure observations and analysis of the mechanical properties. Samples with Zr contents of 0.5 wt.% and 1.5 wt.% were also fabricated to determine the effect of the Zr content on the microstructural evolution.
采用四种气体雾化的 Haynes 230 粉末进行 LPBF 工艺，其成分（质量百分比）为 Zr-20.08Cr-12.19W-1.92Mo-1.46Fe-1.1Co-0.33Mn-0.36Al-0.38Si-0.1C-0.05B-余量 Ni。其中，四种粉末的 Zr 含量分别为 0、0.5、1.0 和 1.5 wt.%，且所有粉末的氧含量均低于 150 ppm。所制备的粉末呈球形，粒径范围为 15–53 μm。使用配备 400 W 光纤激光器的 Concept Laser M2 打印机对这些粉末进行了 LPBF 处理。优化的 LPBF 打印参数为激光功率 300 W，扫描速度 1100 mm/s，扫描间距 100 μm，层厚 50 μm。采用条带扫描策略，并在相邻层之间激光路径旋转 67°。所有制造过程均在保护性氩气氛围下进行，成型室内氧含量控制在 100 ppm 以下。选取 Zr 含量为 0 和 1 wt.%的样品进行详细的显微组织观察及力学性能分析。还制造了含锆量为 0.5 wt.%和 1.5 wt.%的样品，以确定锆含量对显微组织演变的影响。

2.2. Thermodynamic calculations and differential scanning calorimetry (DSC) analysis
2.2. 热力学计算与差示扫描量热法（DSC）分析

Thermal-calc software based on a database dedicated to nickel-based superalloys (TCNI9) was used to simulate the sequential steps from the liquidus temperature to the solidus temperature using the Scheil-Gulliver solidification model (no diffusion in the solid state and complete mixing in the liquid) [29]. DSC experiments (STA 449 F3 Jupiter) were carried out to measure the dissolution temperatures of the segregated phases and the liquidus temperatures of the alloys. The differential heat flux was recorded at a heating rate of 10 °C/min in the temperature range of 30 °C–1450 °C.
基于专用于镍基高温合金数据库（TCNI9）的 Thermal-calc 软件，采用 Scheil-Gulliver 凝固模型（固态无扩散，液态完全混合）模拟了从液相线温度到固相线温度的连续步骤[29]。采用差示扫描量热法（DSC，STA 449 F3 Jupiter）实验测量了偏析相的溶解温度和合金的液相线温度。差热流在 30 °C 至 1450 °C 的温度范围内以 10 °C/min 的加热速率记录。

2.3. Heat treatment 2.3. 热处理

To study the effect of heat treatment on the microstructures and mechanical properties of the LPBF fabricated original and Zr-modified Haynes 230 alloys, the as-printed samples were heat treated at 1200 °C for 1 h under an argon atmosphere, and then air-cooled to room temperature.
为研究热处理对激光粉末床熔化（LPBF）制造的原始及 Zr 改性 Haynes 230 合金显微组织和力学性能的影响，将打印后的样品在氩气氛围下于 1200 °C 加热 1 小时，随后空冷至室温。

2.4. Microstructural analysis
2.4. 显微组织分析

Crack analysis was performed on the images captured using an optical microscope (OM). Microstructural investigations were performed using scanning electron microscopy (SEM) combined with energy dispersive spectroscopy (EDS) and electron back scatter diffraction (EBSD) detection. The EBSD mapping area was 400 μm × 400 μm, and the step size was 1.2 μm. The SEM samples were electrolytically etched in a solution consisting of 20 vol.% sulfuric acid and 80 vol.% ethanol at 5 V for 10 s. Transmission electron microscopy (TEM) analyses were conducted using a Tecnai G2 F30 S-TWIN microscope operated at 300 kV to determine the phases and characterize the microstructures. The TEM samples were prepared by standard ion-milling using a Gatan PIPS II 695 instrument. X-ray diffraction (XRD) analyses were conducted on the scanning plane (XY plane) using Bruker D8 Advance diffractometer with Cu-Kα radiation and a step size of 0.2° The residual stresses of the samples with different Zr contents were evaluated using the sin²ψ method by calculating the spacing of the (311) peak at different ψ angles. The XRD scans were collected in the 2θ range of 89° – 92.5° at various ψ angles (i.e., 0, 15, 20, 25, 35, and 45°).
裂纹分析通过使用光学显微镜（OM）拍摄的图像进行。微观结构研究采用扫描电子显微镜（SEM）结合能谱分析（EDS）和电子背散射衍射（EBSD）检测进行。EBSD 扫描区域为 400 μm × 400 μm，步长为 1.2 μm。SEM 样品在由 20 体积％硫酸和 80 体积％乙醇组成的溶液中以 5 V 电解蚀刻 10 秒。透射电子显微镜（TEM）分析使用 Tecnai G2 F30 S-TWIN 显微镜，工作电压为 300 kV，以确定相组成并表征微观结构。TEM 样品通过使用 Gatan PIPS II 695 仪器的标准离子研磨法制备。X 射线衍射（XRD）分析在扫描平面（XY 平面）上使用 Bruker D8 Advance 衍射仪，采用 Cu-Kα射线，步长为 0.2°。不同 Zr 含量样品的残余应力通过 sin ² ψ方法评估，计算不同ψ角度下（311）峰的间距。 XRD 扫描在 2θ范围为 89°–92.5°的不同ψ角度（即 0、15、20、25、35 和 45°）下进行采集。

2.5. Tensile tests 2.5. 拉伸测试

All tensile samples (sliced from the as-printed block samples, gage length: 35 mm; gage width: 3 mm; gage thickness: 2 mm) were built with the longitudinal axes perpendicular to the building direction. The tensile tests were performed at room temperature (RT) using a CMT5105 testing apparatus at a strain rate of 10³ s ⁻ ¹. The tensile strain was measured using an Epsilon axial extensometer with a gage length of 25 mm.
所有拉伸样品（从打印块样品切割而成，有效长度：35 毫米；有效宽度：3 毫米；有效厚度：2 毫米）均沿纵轴垂直于构建方向制造。拉伸试验在室温（RT）下使用 CMT5105 测试仪以 10⁻³ s⁻¹的应变速率进行。拉伸应变通过有效长度为 25 毫米的 Epsilon 轴向引伸计测量。

3. Results and discussion
3. 结果与讨论

3.1. Crack elimination in the as-printed material
3.1. 打印材料中的裂纹消除

Fig. 1a shows OM images captured along the building direction of the LPBF printed samples. Although extensive cracking is observed in the original sample (0 wt.% Zr), it can be seen that cracking is effectively inhibited in the Haynes 230 alloy after Zr modification. Similarly, no microcracks are present in the high-magnification SEM images of the Zr-modified Haynes 230 sample (Suppl. Fig. 1). In addition, the open fracture presented in Fig. 1b shows a smooth cell surface without obvious plastic deformation in the original sample, which is typical of the hot cracking that occurs in the terminal stage of solidification [27]. Cracks are prone to propagate at high angle grain boundaries because their lower coalescence temperatures favor the accumulation of liquid, thereby resulting in localized strain concentrations during solidification [15]. The EBSD map shown in Fig. 1c reveals the epitaxial growth characteristics of the grains along the building direction, wherein the cracks propagate along the columnar grain boundaries in the original sample, as indicated by the arrows. Although the cracks were completely suppressed in Haynes 230 after Zr modification, the microstructure was still composed of columnar grains (Fig. 1f). This indicates that the role of Zr in nickel-based superalloys is different from that in previously reported aluminum alloys, wherein Zr can promote the columnar-to-equiaxed grain transition, ultimately enhancing the resistance of the aluminum alloys to hot cracking [5,30]. Furthermore, the SEM image presented in Fig. 1d indicates that only a few precipitated particles are decorated at the cell boundaries in the original Haynes 230 sample. The EDS line scan results show that no significant segregated elements are present at the cell boundaries of the original sample (see Fig. 1g and Suppl. Fig. 2a). In contrast, the cell boundaries of the Zr-modified Haynes 230 sample shown in Fig. 1e are clearly decorated with a continuous network of segregated phases. The elemental distribution maps confirm the obvious Zr enrichment, as well as the depletion of Ni, Cr, W, and Co along the cell boundaries (see Suppl. Fig. 2b). Following further characterization of the precipitates by TEM, it was apparent that the precipitated particles in the original sample were mainly Ni₂W₄C (M₆C) with a diameter of 60–90 nm (Fig. 1g and h). A certain amount of M₂₃C₆ was also found to be present in the microstructure, as reported in a previous study [31]. The network precipitates in the Zr-modified sample were identified as intermetallic Ni₁₁Zr₉ with a thickness of 30–40 nm (Fig. 1i and j). The above microstructural analyses indicate that the elimination of hot cracking may be closely associated with Zr solute segregation.
图 1a 显示了沿 LPBF 打印样品构建方向拍摄的光学显微镜（OM）图像。尽管在原始样品（0 wt.% Zr）中观察到了大量裂纹，但可以看出，在经过 Zr 改性的 Haynes 230 合金中，裂纹得到了有效抑制。同样，在 Zr 改性的 Haynes 230 样品的高倍扫描电子显微镜（SEM）图像中（补充图 1），也未发现微裂纹。此外，图 1b 中展示的开口断口显示原始样品的细胞表面光滑，且无明显塑性变形，这典型地反映了固化终末阶段发生的热裂纹[27]。裂纹易沿高角度晶界扩展，因为其较低的共晶温度有利于液相的积聚，从而导致固化过程中局部应变集中[15]。图 1c 所示的电子背散射衍射（EBSD）图揭示了晶粒沿构建方向的外延生长特征，其中裂纹沿原始样品中的柱状晶界扩展，如箭头所示。尽管在添加了锆（Zr）后，Haynes 230 中的裂纹被完全抑制，但其显微组织仍由柱状晶粒组成（图 1f）。这表明锆在镍基高温合金中的作用不同于先前报道的铝合金，在铝合金中锆能够促进柱状晶向等轴晶的转变，最终增强铝合金对热裂纹的抵抗能力[5,30]。此外，图 1d 所示的扫描电子显微镜（SEM）图像显示，原始 Haynes 230 样品中仅有少量析出颗粒分布在晶胞边界。能谱线扫描（EDS）结果表明，原始样品的晶胞边界处没有显著的元素偏析（见图 1g 及补充图 2a）。相比之下，图 1e 中锆改性后的 Haynes 230 样品的晶胞边界明显被连续的偏析相网络所装饰。元素分布图确认了晶胞边界处锆的明显富集，同时镍（Ni）、铬（Cr）、钨（W）和钴（Co）元素的含量有所减少（见补充图 2b）。通过透射电子显微镜（TEM）对析出物进行进一步表征后，显然原始样品中的析出颗粒主要是直径为 60–90 纳米的 Ni₃W₃C（M₂C）（图 1g 和 h）。微观结构中还发现了一定量的 M₆C₄，正如之前的研究所报道的那样[31]。Zr 改性样品中的网络析出物被鉴定为厚度为 30–40 纳米的金属间化合物 Ni₅Zr₆（图 1i 和 j）。上述微观结构分析表明，热裂纹的消除可能与 Zr 溶质偏析密切相关。

To gain a deeper understanding of the solidification process, the solidification pathway was calculated under the Scheil-Gulliver non-equilibrium condition (Fig. 2a). Alloying with Zr shifted the solidification curve of the Haynes 230 alloy to a lower temperature, wherein the solidification range (F_S=0 − F_S=1) sharply increased from 134 °C to 477 °C. Generally, alloys are considered to have the highest hot cracking sensitivity (HCS) in the terminal stage of solidification. The critical temperature range (CTR) is generally adopted to predict the HCS of an alloy, and a higher value of ΔT_CTR (ΔT_CTR = T_Fs=0.95 − T_Fs=1) indicates a higher HCS due to that the incomplete bridging grains have not yet developed significant ductility to accommodate the solidification shrinkage strain [27,28]. The ΔT_CTR values of 49 °C and 210 °C were obtained for the original and Zr-modified samples, indicating that the Zr-modified Haynes 230 alloy should have a higher HCS and a poor printability. However, the calculation results for the solidification path were opposite to those obtained experimentally. It also did not work well in predicting the HCS of the LPBF fabricated Al_0.5CoCrFeNi high-entropy alloy [27]. Similarly, the low melting B2 phase was found to be distributed in the cell and grain boundaries after the introduction of 0.5 wt.% Al into the CoCrFeNi alloy. Although a low solidification range and ΔT_CTR have previously been pursued to obtain an excellent crack resistance and a good printability in laser additive manufacturing alloys [32,33], such systems have so far shown limited success. This prediction discrepancy can be attributed to the fact that the model does not reflect the effect of the hierarchical microstructure on the elemental distribution caused by the rapid cooling conditions during the additive manufacturing process [14]. In the case of conventional casting, the segregation phase with a high volume fraction is mainly decorated at the grain boundaries [34]. Thus, benefiting from the numerous cell boundaries generated by ultrafast cooling during the LPBF process, the segregation phase was effectively dispersed.
为了更深入地理解凝固过程，在 Scheil-Gulliver 非平衡条件下计算了凝固路径（图 2a）。掺杂 Zr 使 Haynes 230 合金的凝固曲线向较低温度移动，其中凝固温度范围（F _S=0 − F _S=1 ）从 134°C 急剧增加到 477°C。通常，合金在凝固的末端阶段被认为具有最高的热裂敏感性（HCS）。临界温度范围（CTR）通常用于预测合金的 HCS，较高的ΔT _CTR （ΔT _CTR = T _Fs=0.95 − T _Fs=1 ）值表明更高的 HCS，因为未完全桥接的晶粒尚未形成足够的延展性以适应凝固收缩应变[27,28]。原始样品和掺杂 Zr 样品的ΔT _CTR 值分别为 49°C 和 210°C，表明掺杂 Zr 的 Haynes 230 合金应具有更高的 HCS 和较差的可打印性。然而，凝固路径的计算结果与实验获得的结果相反。它在预测激光粉末床熔化（LPBF）制造的 Al _0.5 CoCrFeNi 高熵合金的热裂纹敏感性（HCS）方面也表现不佳[27]。同样，在向 CoCrFeNi 合金中加入 0.5 wt.% Al 后，发现低熔点的 B2 相分布在细胞和晶界处。尽管之前为了获得优异的抗裂性能和良好的激光增材制造合金打印性能，曾追求低凝固范围和ΔT _CTR [32,33]，但此类体系迄今为止显示的成功有限。这种预测差异可归因于模型未能反映增材制造过程中快速冷却条件导致的分层微观结构对元素分布的影响[14]。在传统铸造的情况下，具有高体积分数的偏析相主要装饰在晶界处[34]。因此，得益于 LPBF 过程中超快速冷却产生的大量细胞边界，偏析相得到了有效分散。

The solidification path predicted the formation of Cr₂B and Ni₅Zr during the terminal stage solidification of Zr-modified Haynes 230, as indicated in Fig. 2a, which dramatically increased the ΔT_CTR. However, the Cr₂B and Ni₅Zr phases were not detected in the microstructure, and the existence of Ni₁₁Zr₉ was not predicted during solidification. This may be due to the low volume fractions of the Cr₂B and Ni₅Zr phases, or the ultrafast cooling rate of the LPBF fabrication process causing a deviation between the actual solidification microstructure and the predicted result [27]. Considering that it is difficult to accurately reflect the solidification process of the Zr-modified Haynes 230 alloy using only the solidification curve, DSC was used to measure the dissolution temperature of the segregated phase in the as-printed samples. The DSC heating curves of the original sample and Zr-modified Haynes 230 sample are compared in Fig. 2b, where liquidus temperatures of 1403 °C and 1384 °C were recorded, respectively; these values are relatively similar to the calculated results obtained using Thermal-calc. Notably, the Zr-modified sample exhibited an obvious endothermic peak at 1077 °C, which can be attributed to the dissolution of intermetallic Ni₁₁Zr₉ according to the microstructural characteristics. However, the solidification temperature range of the Zr-modified Haynes 230 alloy from liquidus to the formation of Ni₁₁Zr₉ was determined by DSC to be 307 °C, which is lower than the calculated result of 477 °C. This can be attributed to the fact that the Scheil-Gulliver model does not consider solute back-diffusion, and thus, it overestimates the solidification range of the alloy. Overall, the obtained results are consistent with previous reports indicating that Zr enlarges the solidification temperature range of nickel-based superalloys [12,35]. Furthermore, as shown in Fig. 1e, the cell boundaries are decorated with a continuous network of intermetallic Ni₁₁Zr₉, suggesting that intercellular regions are still filled with abundant liquid during the terminal stage of solidification. The enlarged solidification range prolongs the liquid duration; therefore, the solidification shrinkage strain can be effectively accommodated by liquid backfilling [10], as indicated in Suppl. Fig. 3a.
如图 2a 所示，凝固路径预测在 Zr 改性 Haynes 230 的终端凝固阶段形成 CrB 和 NiZr 相，这显著增加了ΔT。然而，在显微组织中未检测到 CrB 和 NiZr 相，且凝固过程中未预测到 NiZr 的存在。这可能是由于 CrB 和 NiZr 相的体积分数较低，或 LPBF 制造工艺的超快冷却速率导致实际凝固显微组织与预测结果存在偏差[27]。考虑到仅用凝固曲线难以准确反映 Zr 改性 Haynes 230 合金的凝固过程，采用 DSC 测量了打印样品中偏析相的溶解温度。图 2b 比较了原始样品和 Zr 改性 Haynes 230 样品的 DSC 加热曲线，分别记录到液相线温度为 1403°C 和 1384°C；这些值与使用 Thermal-calc 计算得到的结果相对接近。值得注意的是，Zr 改性样品在 1077°C 处表现出明显的吸热峰，根据微观结构特征可归因于金属间化合物 Ni ₁₁ Zr ₉ 的溶解。然而，通过 DSC 测定，Zr 改性 Haynes 230 合金从液相线到 Ni ₁₁ Zr ₉ 形成的凝固温度范围为 307°C，低于计算结果的 477°C。这可归因于 Scheil-Gulliver 模型未考虑溶质反扩散，因此高估了合金的凝固范围。总体而言，所得结果与先前报道一致，表明 Zr 扩大了镍基高温合金的凝固温度范围[12,35]。此外，如图 1e 所示，晶胞边界被连续的金属间化合物 Ni ₁₁ Zr ₉ 网络装饰，表明在凝固终末阶段，晶胞间区域仍充满丰富的液相。固化范围的扩大延长了液态持续时间；因此，固化收缩应变可以通过液态回填得到有效缓解[10]，如补充图 3a 所示。

While solutes, such as Zr, Hf, and B, are regarded as being detrimental to the printability and hot cracking resistance of nickel-based superalloys [6,12,32,34], these elements are mainly present in the form of segregated atoms or isolated precipitates in laser additive manufactured alloys because of their low contents. Such limited segregation in the microstructure implies that a lesser quantity of liquid takes part in backfilling during the terminal stage of solidification. To understand the relationship between the liquid content and hot cracking resistance, the effect of the Zr content on the hot cracking of the printed Haynes 230 alloys (0, 0.5, 1, and 1.5 wt.% Zr addition) was further examined. It is clear that the crack density decreases with increasing Ni₁₁Zr₉ content at the cell and grain boundaries (Fig. 3), and so it can be inferred that the volume fraction of liquid at the cell and grain boundaries increases with increasing Zr content. As the Zr content was increased to 0.5 wt.%, although the proportion of precipitates increased compared with the original (0 wt.% Zr) sample (Fig. 3e), the precipitates were still mainly distributed at grain boundaries in the form of a lamellar morphology (Fig. 3f). In particular, the isolated precipitates at the grain boundaries correspond to the localized liquid film during solidification, as indicated by the arrows shown in Fig. 3f, which forms a region with low shear strength and possesses a higher cracking tendency under thermal stress [32]. When the Zr content reaches 1 wt.% (Fig. 3c and g), the cracks are completely suppressed, and continuous network precipitates are formed at the cell and grain boundaries. It has been reported that grain boundary cracking is the result of strain localization. The strain can be transmitted by the grains and concentrated in the intergranular liquid film, while the cell boundaries can share the strain when the intercellular liquid is sufficient [10]. Although the addition of the Zr solute increases the volume fraction of the intergranular liquid, which is equivalent to increasing the number of vulnerable regions in the microstructure, hot cracking can also be suppressed when the stress in these regions is relieved. Therefore, it is reasonable to assume that the residual stress can be shared among the intercellular liquids/phases, thereby alleviating the residual stress concentration at the grain boundaries.
虽然像 Zr、Hf 和 B 这样的溶质被认为对镍基高温合金的可打印性和抗热裂性能有害[6,12,32,34]，但由于其含量较低，这些元素主要以偏析原子或孤立析出物的形式存在于激光增材制造的合金中。微观结构中这种有限的偏析意味着在凝固终末阶段参与回填的液相量较少。为了理解液相含量与抗热裂性能之间的关系，进一步研究了 Zr 含量对打印 Haynes 230 合金（添加 0、0.5、1 和 1.5 wt.% Zr）热裂纹的影响。显然，随着细胞和晶界处 Ni ₁₁ Zr ₉ 含量的增加，裂纹密度降低（图 3），因此可以推断细胞和晶界处液相的体积分数随着 Zr 含量的增加而增加。当 Zr 含量增加到 0.5 wt.%时，尽管与原始样品（0 wt.% Zr）相比，析出物的比例有所增加（图） 3e)，析出物仍主要以层状形态分布在晶界处（图 3f）。特别是晶界处的孤立析出物对应于固化过程中局部液态薄膜，如图 3f 中箭头所示，该区域具有较低的剪切强度，在热应力作用下更易产生裂纹[32]。当 Zr 含量达到 1 wt.%（图 3c 和 g）时，裂纹被完全抑制，且在细胞和晶界处形成连续的网络析出物。据报道，晶界裂纹是应变局部化的结果。应变可以通过晶粒传递并集中在晶间液膜中，而当细胞间液体充足时，细胞边界可以分担应变[10]。尽管添加 Zr 溶质增加了晶间液体的体积分数，相当于增加了微观结构中易受损区域的数量，但当这些区域的应力得到缓解时，热裂纹仍然可以被抑制。因此，可以合理地假设残余应力可以在细胞间液体/相之间分担，从而缓解晶界处的残余应力集中。

It is difficult to directly measure the stress during solidification of the molten pool. However, the residual stress is known to accumulate in the final microstructure [36], and therefore, the residual stresses of the samples with different Zr contents were characterized using the sin²ψ method based on the XRD results [31]. The detailed measurement results are presented in the supplementary materials. When the Zr content was increased from 0 to 1.5 wt.%, the residual tensile stress decreased from 201 MPa to 41 MPa, as shown in Fig. 2d. Remarkably, the volume fractions of the precipitated phases were determined to be 0.2, 2, 8, and 13% for the four samples with Zr contents of 0, 0.5, 1, and 1.5 wt.%, respectively. It should be noted that Sun et al. [27] reported that the residual stress can be effectively relieved under the effect of the molar volume expansion and plastic deformation of the intercell precipitated phase during intercell liquid film solidification. They also found that residual tensile stress can be transformed into compressive stress when the content of the intercell precipitated phase is sufficient in the LPBF fabricated CoCrFeNi alloy. Combined with the above experimental results, the suppression of hot cracking in the Haynes 230 alloy was attributed to alleviation of the residual stress with an increase in the intercell liquid/phase.
在熔池凝固过程中直接测量应力是困难的。然而，已知残余应力会积累在最终的微观结构中[36]，因此，基于 XRD 结果，采用 sin ² ψ方法对不同 Zr 含量样品的残余应力进行了表征[31]。详细的测量结果见补充材料。如图 2d 所示，当 Zr 含量从 0 增加到 1.5 wt.%时，残余拉应力从 201 MPa 降至 41 MPa。值得注意的是，四个样品中析出相的体积分数分别为 0、0.5、1 和 1.5 wt.% Zr 含量时为 0.2%、2%、8%和 13%。需要指出的是，Sun 等人[27]报道，在胞间液膜凝固过程中，胞间析出相的摩尔体积膨胀和塑性变形作用下，残余应力可以得到有效缓解。他们还发现，当胞间析出相含量足够时，LPBF 制造的 CoCrFeNi 合金中的残余拉应力可以转变为压应力。结合上述实验结果，Haynes 230 合金中热裂纹的抑制归因于随着胞间液相/相的增加，残余应力得到了缓解。

The corresponding XRD patterns (Fig. 2c) confirm that all samples are typical of the γ phase. Interestingly, the dominant diffraction peak can be seen to gradually transform from the (002) plane to the (220) plane, and the value of the corresponding intensity ratio I₍₂₂₀₎/I₍₂₀₀₎ increases with increasing Zr content, as shown in Fig. 2d. In addition, the inverse pole figure (IPF) and pole figure (PF) maps confirm that the cubic texture ({001}<001>) is gradually transformed into a Goss texture ({101}<001>) with the addition of Zr, and this is accompanied by a decrease in the texture intensity value (Suppl. Fig. 4). The evolution of the crystallographic texture in the Zr-modified Haynes 230 samples was supposed to stem from changes in the constitutional supercooling (ΔT_CS) during solidification. Because the majority of the Zr solute was rejected at the solid/liquid (S/L) interface during molten pool solidification owing to the low solid solubility of Zr in the nickel matrix, and the solidification temperature ahead of the S/L interface changes, resulting in the generation of a CS Zone [24]. The enlarged ΔT_CS provided an additional driving force to activate grain nucleation [37], and ΔT_CS was positively correlated with the Zr solute concentration. As presented in Suppl. Fig. 5, the average grain size can be seen to decrease from 42 μm to 28 μm, as the Zr content in the Haynes 230 samples is increased from 0 to 1.5 wt.%. This demonstrates that the increased ΔT_CS promotes nucleation events during solidification. Owing to the effect of the heat flux direction and preferential crystal orientation (<001> for cubic crystals), the original Haynes 230 sample shows an obvious epitaxial growth of columnar grains and a strong cubic texture, which are typical microstructure characteristics of laser additive manufactured nickel-based superalloys [38,39]. With the promotion of nucleation events, the growth probability of grains exhibiting other crystal orientations increases during competitive growth, thereby limiting the epitaxial growth of grains and reducing the intensity of the cubic texture [40,41]. Therefore, the texture evolution and grain refinement can be attributed to the change in ΔT_CS in the Haynes 230 alloy after the introduction of Zr. This reduction in the texture intensity and the observed refinement of the grain size are also conducive to improving the hot cracking resistance of LPBF manufactured Haynes 230 alloys, due to their enhanced ability to alleviate the residual stress through coordinated deformation of the grains [33,42].
相应的 XRD 图谱（图 2c）确认所有样品均为典型的γ相。有趣的是，主衍射峰可见逐渐从(002)晶面转变为(220)晶面，且对应的强度比值 I ₍₂₂₀₎ /I ₍₂₀₀₎ 随 Zr 含量的增加而增加，如图 2d 所示。此外，逆极图（IPF）和极图（PF）图谱确认立方织构（ ₍₂₀₀₎ <001>）随着 Zr 的加入逐渐转变为 Goss 织构（{101}<001>），且织构强度值降低（补充图 4）。Zr 改性 Haynes 230 样品中晶体织构的演变被认为源于凝固过程中成分过冷（ΔT _CS ）的变化。由于 Zr 在镍基体中的固溶度低，大部分 Zr 溶质在熔池凝固时被排斥至固/液（S/L）界面，导致 S/L 界面前方的凝固温度发生变化，从而产生了成分过冷区（CS Zone）[24]。增大的ΔT _CS 提供了额外的驱动力以激活晶粒成核[37]，且ΔT _CS 与 Zr 溶质浓度呈正相关。如补充图 5 所示，随着 Haynes 230 样品中 Zr 含量从 0 增加到 1.5 wt.% ，平均晶粒尺寸从 42 μm 减小到 28 μm。这表明增大的ΔT _CS 促进了凝固过程中的成核事件。由于热流方向和优先晶体取向（立方晶体的<001>）的影响，原始 Haynes 230 样品表现出明显的柱状晶粒外延生长和强烈的立方织构，这些是激光增材制造镍基高温合金的典型显微组织特征[38,39]。随着成核事件的促进，在竞争生长过程中表现出其他晶体取向的晶粒生长概率增加，从而限制了晶粒的外延生长并降低了立方织构的强度[40,41]。因此，织构演变和晶粒细化可归因于引入 Zr 后 Haynes 230 合金中ΔT _CS 的变化。这种织构强度的降低和观察到的晶粒细化也有助于提高 LPBF 制造的 Haynes 230 合金的热裂纹抗性，因为它们通过晶粒的协调变形增强了缓解残余应力的能力[33,42]。

Fig. 4a and b show the bright-field TEM images along the 〈011〉 zone axis of the transverse and longitudinal cell microstructures in the original Haynes 230 sample, respectively, in which the high-density dislocations packed at the cell boundaries are indicated by arrows. This result indicates that significant residual stress exists in the as-printed sample, which stems from the localized thermal inhomogeneity of the repeated heating/cooling cycles during the laser additive manufacturing process, ultimately resulting in the concentration of thermal residual strain at the cell and grain boundaries [43,44]. In comparison, although a continuous Ni₁₁Zr₉ phase can be seen at the cell boundaries, the dislocation density is significantly decreased in the matrix of the Zr-modified Haynes 230 sample, as shown in Fig. 4d and e. A large number of stacking faults can also be observed in the Ni₁₁Zr₉ phase (inset of Fig. 4e). Furthermore, the high-resolution transmission electron microscopy (HRTEM) image further confirms that the Ni₁₁Zr₉ phase has an orientation relationship with the γ matrix of (-101) [010]Ni₁₁Zr₉//(1-11)[-112]γ and (201)[010]Ni₁₁Zr₉//(220)[-112]γ (Fig. 4c), indicating the presence of a highly coherent interface. Moreover, high-density dislocations can be seen in the corresponding Inverse Fourier Transformation (IFF) image of these Ni₁₁Zr₉ phases (Fig. 4f). In the terminal stage of solidification, thermal strain is transmitted to the cell and grain boundaries and is relieved by coordinated deformation between grains; meanwhile, high-density dislocations and microcracks nucleate at the boundaries to accommodate internal stress [10,45]. The segregation of Zr at the cell and grain boundaries can then form a continuous liquid film that exists stably over a wide temperature range and effectively backfills solidification shrinkage. The decreased dislocation density in the matrix of the Zr-modified Haynes 230 sample demonstrates that the continuous liquid film can act as a strain absorber to avoid the detrimental effect of stress/strain concentration. Moreover, the high-density dislocation storage and severe lattice distortion observed in the intercellular Ni₁₁Zr₉ phase indicate that this phase was formed under severe thermal stress. The coherent Ni₁₁Zr₉/matrix interface and the lower elastic modulus of Ni₁₁Zr₉ (Ni₁₁Zr₉: 122 GP, γ matrix: 214 GPa [31,46]) favor the preferential deformation of Ni₁₁Zr₉ to relieve the stress concentration of the matrix during repeated heating/cooling cycles in the laser additive manufacturing process. The variation in the dislocation density in the microstructures of the Haynes 230 samples is consistent with the evolution trend of the residual stress, as determined by XRD.
图 4a 和 4b 分别显示了原始 Haynes 230 样品中沿〈011〉区轴的横向和纵向细胞微观结构的明场透射电子显微镜（TEM）图像，其中箭头所示为聚集在细胞边界处的高密度位错。该结果表明，打印后的样品中存在显著的残余应力，这源于激光增材制造过程中反复加热/冷却循环导致的局部热不均匀性，最终导致热残余应变在细胞和晶界处的集中[43,44]。相比之下，尽管在细胞边界处可以看到连续的 Ni ₁₁ Zr ₉ 相，但如图 4d 和 4e 所示，Zr 改性 Haynes 230 样品基体中的位错密度显著降低。在 Ni ₁₁ Zr ₉ 相中（图 4e 插图）还可以观察到大量堆垛层错。此外，高分辨透射电子显微镜（HRTEM）图像进一步确认了 Ni ₁₁ Zr ₉ 相与γ基体之间存在取向关系，具体为(-101)[010]Ni ₁₁ Zr ₉ //(1-11)[-112]γ和(201)[010]Ni ₁₁ Zr ₉ //(220)[-112]γ（图 4c），表明存在高度相干的界面。此外，在这些 Ni ₁₁ Zr ₉ 相的对应逆傅里叶变换（IFF）图像中可以看到高密度位错（图 4f）。在凝固的末端阶段，热应变传递到细胞和晶界，并通过晶粒间的协调变形得到缓解；同时，高密度位错和微裂纹在晶界处成核以适应内部应力[10,45]。Zr 在细胞和晶界的偏析随后形成一层连续的液态薄膜，该薄膜在较宽的温度范围内稳定存在，有效填补凝固收缩。Zr 改性 Haynes 230 样品基体中位错密度的降低表明，连续液态薄膜可以作为应变吸收体，避免应力/应变集中带来的不利影响。此外，在细胞间 Ni ₁₁ Zr ₉ 相中观察到的高密度位错储存和严重晶格畸变表明该相是在剧烈热应力下形成的。相干的 Ni ₁₁ Zr ₉ /基体界面以及 Ni ₁₁ Zr ₉ 较低的弹性模量（Ni ₁₁ Zr ₉ ：122 GPa，γ基体：214 GPa [31,46]）有利于 Ni ₁₁ Zr ₉ 的优先变形，从而在激光增材制造过程中反复加热/冷却循环时缓解基体的应力集中。Haynes 230 样品微观结构中位错密度的变化与 XRD 测定的残余应力演变趋势一致。

3.2. Evolution of the microstructure and the mechanical properties after heat treatment
3.2. 热处理后微观结构和机械性能的演变

Heat treatment is a generally accepted method for controlling the microstructures and optimizing the mechanical properties of laser additive manufactured alloys [31,47,48]. Thus, for the purpose of this study, solution heat treatment at 1200 °C was employed to relieve the residual stress and facilitate microstructural homogenization of the as-printed samples. As shown in Fig. 5a, in the heat-treated original sample, coarsened carbides precipitated at the grain boundaries with an average size of 1.2 ± 0.2 μm, and the carbides precipitated within grains exhibiting an average size of 440 ± 30 nm. In addition, the bright-field TEM image presented in Fig. 5d reveals that intragranular carbides are located at the cell boundaries, and the inset SAED pattern indicates that the carbides at the grain and cell boundaries are both Ni₂W₄C (M₆C). Owing to the ultrafast cooling rates during the LPBF process, the insufficiently diffused W atoms segregated in the γ matrix to form a supersaturated solid solution. When subjected to heat treatment, these W atoms tend to migrate from the interior of the grains to the cell/grain boundaries [49]. Coarsening of the M₆C carbide during heat treatment is facilitated by the micro-segregation of solute elements and the rapid diffusion channel of dislocations along the cell boundaries in the as-printed sample [31,50]. Meanwhile, the tangled dislocations at the cell boundaries are essentially annihilated with stress relief, compared with the case of the as-printed original Haynes 230 sample (Fig. 1g).
热处理是一种公认的控制激光增材制造合金显微组织和优化机械性能的方法[31,47,48]。因此，本研究采用了 1200°C 的固溶热处理，以缓解残余应力并促进打印样品的显微组织均匀化。如图 5a 所示，在热处理后的原始样品中，晶界处析出了长大的碳化物，平均尺寸为 1.2 ± 0.2 μm，晶粒内析出的碳化物平均尺寸为 440 ± 30 nm。此外，图 5d 中呈现的明场透射电子显微镜（TEM）图像显示，晶粒内的碳化物位于细胞边界处，插图中的选区电子衍射（SAED）图案表明晶界和细胞边界的碳化物均为 Ni ₂ W ₄ C (M ₆ C)。由于激光粉末床熔化（LPBF）过程中超快的冷却速率，未充分扩散的钨（W）原子在γ基体中发生偏析，形成了过饱和固溶体。经过热处理后，这些钨原子倾向于从晶粒内部迁移到细胞/晶界处[49]。在热处理过程中，M₃C 碳化物的粗化是由溶质元素的微观偏析以及沿细胞边界的位错快速扩散通道促进的[31,50]。同时，与原始打印的 Haynes 230 样品相比，细胞边界处缠结的位错在应力释放后基本被消除（图 1g）。

Based on the above points, it should be considered that coarsening of the M₆C carbides depletes the solute elements and reduces the stability of the γ matrix. Moreover, the presence of large carbides at grain boundaries tends to introduce stress concentrations and lead to premature failure of the alloys [51,52]. Thus, the uniform distribution of small precipitates within grains is preferred to help maintain work hardening and uniform elongation [51,53]. Exhilaratingly, the Zr-modified Haynes 230 manufactured by LPBF can achieve this goal. As shown in Fig. 5b, the nanoprecipitates are uniformly distributed within the grains, and the continuous network of the Ni₁₁Zr₉ phase is completely eliminated in Zr-modified Haynes 230 after heat treatment. In addition, as shown in the TEM image (Fig. 5c), nanoprecipitates with an average size of 130 ± 10 nm are uniformly distributed at the cell and grain boundaries. The EDS maps (Fig. 5e) show that these nanoprecipitates are enriched with Zr, Mo and C, while Ni, W and Cr are not observed. The SADE pattern further confirms that the nanoprecipitate is ZrC (MC) carbide (Fig. 5f). Moreover, it should be noted that no M₆C carbides are observed in the Zr-modified Haynes 230 sample after the heat treatment. Previously, it has been demonstrated that Zr is effective in decreasing grain boundary diffusion and refining the carbide dimensions [54]. Herein, the dissolution of the Ni₁₁Zr₉ phase releases a large number of Zr atoms at the cell and grain boundaries. Such Zr enrichment can effectively annihilate grain boundary vacancies and reduce the interfacial energy, which plays an important role in decreasing the diffusion of solute atoms at the grain boundaries [55]. On the other hand, the diffusion of Zr atoms from the cell/grain boundaries into the γ matrix can play a role in hindering the outward diffusion of W atoms. Since Zr is a strong MC carbide former that preferentially reacts with C atoms at the cell/grain boundaries to form MC, MC replaces M₆C in the heat-treated Haynes 230 alloy. This M₆C-to-MC transition increases the lattice disregistry between the γ matrix and the carbide from 2% to 21% (see the Supplementary Material for further details), which promotes the formation of dislocations around the MC phase, as shown in Fig. 5c.
基于上述观点，应考虑 M₀C 碳化物的粗化会消耗合金元素并降低γ基体的稳定性。此外，晶界处存在的大碳化物往往会引入应力集中，导致合金的早期失效[51,52]。因此，晶粒内小析出物的均匀分布更有利于维持加工硬化和均匀延伸[51,53]。令人振奋的是，通过激光粉末床熔化（LPBF）制造的 Zr 改性 Haynes 230 合金能够实现这一目标。如图 5b 所示，纳米析出物在晶粒内均匀分布，且热处理后 Zr 改性 Haynes 230 中 Ni₁Zr₂相的连续网络被完全消除。此外，如透射电子显微镜（TEM）图像（图 5c）所示，平均尺寸为 130 ± 10 nm 的纳米析出物均匀分布在细胞和晶界处。能谱图（EDS，图 5e）显示这些纳米析出物富集 Zr、Mo 和 C，而未观察到 Ni、W 和 Cr。选区电子衍射（SAED）图样进一步确认该纳米析出物为 ZrC（MC）碳化物（图 5f）。此外，应注意在热处理后的 Zr 改性 Haynes 230 样品中未观察到 M₃C 碳化物。此前已有研究表明，Zr 能有效降低晶界扩散并细化碳化物尺寸[54]。在此，Ni₄Zr₅相的溶解在晶胞和晶界释放出大量 Zr 原子。这种 Zr 富集能有效消除晶界空位并降低界面能，这在减少晶界处溶质原子的扩散中起着重要作用[55]。另一方面，Zr 原子从晶胞/晶界向γ基体的扩散可以阻碍 W 原子的向外扩散。由于 Zr 是强 MC 碳化物形成元素，优先与晶胞/晶界处的 C 原子反应形成 MC，MC 取代了热处理后 Haynes 230 合金中的 M₆C。这种 M₇C 向 MC 的转变使γ基体与碳化物之间的晶格错配从 2%增加到 21%（详见补充材料），促进了如图 5c 所示 MC 相周围位错的形成。

The engineering stress-strain curves for the original and Zr-modified Haynes 230 samples are shown in Fig. 6a, and the associated yield strength, ultimate strength, and elongation to failure are summarized in Table 1. For the as-printed (AP) sample, the yield strength of the AP-Haynes 230 sample was determined to be 488 ± 12 MPa and its elongation was only 2.5 ± 0.5% owing to the occurrence of high-density hot cracking. In comparison, the Zr-modified sample was found to possess an ultrahigh yield strength (812 ± 10 MPa), and its elongation reached 24 ± 1%. This increase in elongation indirectly suggests that hot cracking was effectively suppressed in the sample. It is also worth noting that the yield strength of our Zr-modified sample was improved by more than 50% compared with that of previously reported LPBF manufactured Haynes 230 alloys [31,42,56], as shown in Fig. 6b. After heat treatment (HT), the yield strength of the Zr-modified sample was found to drop to 621 ± 8 MPa due to the dissolution of Ni₁₁Zr₉ precipitates; however, this yield strength remained significantly higher than that of the HT-Haynes 230 sample (462 ± 14 MPa). More importantly, elongation of the HT-Haynes 230+Zr sample was significantly enhanced to 35 ± 2%, which is a significant improvement compared to the corresponding values for the previously reported LPBF manufactured Haynes 230 alloy. In addition, the HT-Haynes 230+Zr sample exhibited a notable improvement in strength compared to the fully dense wrought samples [57], [58], [59], [60], as shown in Fig. 6b.
原始和掺锆改性 Haynes 230 样品的工程应力-应变曲线如图 6a 所示，相关的屈服强度、极限强度和断后伸长率汇总于表 1。对于打印后（AP）样品，AP-Haynes 230 样品的屈服强度测定为 488 ± 12 MPa，断后伸长率仅为 2.5 ± 0.5%，这是由于高密度热裂纹的发生所致。相比之下，掺锆样品表现出超高的屈服强度（812 ± 10 MPa），其断后伸长率达到 24 ± 1%。伸长率的增加间接表明样品中的热裂纹得到了有效抑制。值得注意的是，如图 6b 所示，我们的掺锆样品的屈服强度较先前报道的激光选区熔化（LPBF）制造的 Haynes 230 合金[31,42,56]提高了 50%以上。经过热处理（HT）后，掺锆样品的屈服强度因 Ni ₁₁ Zr ₉ 析出相的溶解而下降至 621 ± 8 MPa；然而，该屈服强度仍显著高于 HT-Haynes 230 样品的 462 ± 14 MPa。更重要的是，HT-Haynes 230+Zr 样品的延伸率显著提高至 35 ± 2%，这相比之前报道的 LPBF 制造的 Haynes 230 合金的相应数值有了显著提升。此外，如图 6b 所示，HT-Haynes 230+Zr 样品的强度相比完全致密的锻造样品[57]，[58]，[59]，[60]也有显著改善。

Table 1. Summary of the mechanical properties of the original and Zr-modified Haynes 230 samples before and after heat treatment.
表 1. 原始和掺锆 Haynes 230 样品在热处理前后的机械性能总结。

Sample 样品	Yield strength (MPa) 屈服强度（兆帕）	Ultimate strength (MPa) 极限强度（兆帕）	Elongation (%) 伸长率（%）
AP-Haynes 230	488 ± 12 488 ± 12	526 ± 7 526 ± 7	2.5 ± 0.5 2.5 ± 0.5
AP-Haynes 230 + Zr AP-Haynes 230 + Zr	812 ± 10 812 ± 10	1084 ± 5 1084 ± 5	24 ± 1 24 ± 1
HT-Haynes 230	462 ± 14 462 ± 14	575 ± 9 575 ± 9	3.5 ± 0.5 3.5 ± 0.5
HT-Haynes 230 + Zr HT-Haynes 230 + Zr	621 ± 8 621 ± 8	1013 ± 8 1013 ± 8	35 ± 2 35 ± 2

The large enhancement in strength observed for the AP-Haynes 230+Zr alloy was attributed to the continuous network of intermetallic Ni₁₁Zr₉ that can act as a “skeleton” and serve as a barrier to dislocation motion during plastic deformation. However, these dislocations cannot cut through the continuous intermetallic Ni₁₁Zr₉ precipitates, and as a result, they accumulate at the interface of Ni₁₁Zr₉ and the γ matrix, as shown in Fig. 7a. These precipitates are prone to decohesion and fracture, as well as crack propagation along the grain boundary. Upon observation of the fracture surface, it is apparent that the cracks mainly expand along the grain boundaries (Fig. 7c), and deformed cell structures can be observed on the fracture surface, as highlighted in the inset image. These observations indicate that the presence of Ni₁₁Zr₉ promotes crack extension along the cell and grain boundaries, which sacrifices partial elongation of the Zr-modified as-printed sample. After heat treatment, the intermetallic Ni₁₁Zr₉ was completely dissolved and the MC nanoparticles were uniformly distributed within the Zr-modified Haynes 230 sample (Suppl. Fig. 3b). As shown in Fig. 7d, high-density equiaxed dimples can be observed at the fracture surface, suggesting the highly ductile fracture behavior of the HT-Haynes 230+Zr sample. In addition, Fig. 7b shows a bright-field TEM image of the fractured tensile sample, in which high-density dislocations can be seen to be blocked by MC nanoparticles and stored in the γ matrix, which can help sustain work hardening and promote simultaneous improvements in the strength and the plasticity [51]. Furthermore, the increased dislocations and precipitation of the MC phases observed in the microstructure contribute to dislocation strengthening and precipitate strengthening. Ultimately, the HT-Haynes 230+Zr sample exhibits an excellent combination of strength and plasticity. The addition of Zr clearly enables the elimination of hot cracking from the Haynes 230 alloy manufactured by LPBF, whilst also optimizing the heat-treated microstructure, and leading to excellent mechanical properties at room temperature. However, the effects of Zr addition on the creep, oxidation, and high temperature plasticity of the Haynes 230 alloy under long-term service conditions have yet to be evaluated in detail, and so these points should be the subject of subsequent studies.
AP-Haynes 230+Zr 合金强度显著提升，归因于连续的金属间化合物 Ni-Zr 网络，该网络可作为“骨架”，在塑性变形过程中阻碍位错运动。然而，这些位错无法切穿连续的金属间化合物 Ni-Zr 析出相，因而在 Ni-Zr 和 γ 基体的界面处积累，如图 7a 所示。这些析出相易发生脱粘和断裂，并沿晶界扩展裂纹。观察断口表面可见，裂纹主要沿晶界扩展（图 7c），断口表面还可见变形的细胞结构，插图中有所突出显示。这些现象表明 Ni-Zr 的存在促进了裂纹沿细胞和晶界扩展，导致 Zr 改性打印样品的部分延伸率降低。热处理后，金属间化合物 Ni-Zr 完全溶解，MC 纳米颗粒均匀分布于 Zr 改性 Haynes 230 样品中（补充图 3b）。如图 7d 所示，在断口表面可以观察到高密度的等轴凹坑，表明 HT-Haynes 230+Zr 样品具有高度延展性的断裂行为。此外，图 7b 显示了断裂拉伸样品的明场透射电子显微镜（TEM）图像，其中可以看到高密度位错被 MC 纳米颗粒阻挡并储存在γ基体中，这有助于维持加工硬化并促进强度和塑性的同步提升[51]。此外，微观结构中观察到的位错增加和 MC 相的析出有助于位错强化和析出强化。最终，HT-Haynes 230+Zr 样品表现出优异的强度与塑性组合。添加 Zr 显著消除了由激光粉末床熔化（LPBF）制造的 Haynes 230 合金中的热裂纹，同时优化了热处理后的微观结构，并在室温下展现出优异的机械性能。然而，Zr 添加对 Haynes 230 合金在长期服役条件下的蠕变、氧化及高温塑性的影响尚未进行详细评估，因此这些方面应成为后续研究的重点。

4. Conclusion 4. 结论

In this work, a crack-fee Haynes 230 alloy was prepared by taking advantage of the segregation of Zr atoms and abundant cell boundaries to relieve the stress/strain concentration and coordinate grain deformation through the introduction of stable liquid backfilling and a networked intermetallic Ni₁₁Zr₉ phase. The increased quantity of the intercell Ni₁₁Zr₉ phase contributed to reducing the crack density, and it was found that the hot cracking was completely suppressed when the Zr content reached 1 wt.%. Furthermore, the continuous network of the intermetallic Ni₁₁Zr₉ phase was observed to act as a “skeleton” significantly improving the yield strength of the as-printed sample by more than 50%. Following subsequent heat treatment, the Zr-modified Haynes 230 alloy exhibited an extraordinary combination of strength and plasticity, which were attributed to the dissolution of intermetallic Ni₁₁Zr₉, reduced precipitation of the large-sized M₆C, and MC precipitation at the cell and grain boundaries. Overall, our work provides a new alloy design route for the laser additive manufacturing of crack-free alloys with excellent mechanical properties.
在本研究中，通过利用 Zr 原子的偏析和丰富的晶胞边界，结合引入稳定的液态回填和网络状的金属间化合物 Ni-Zr 相，制备出无裂纹的 Haynes 230 合金，从而缓解应力/应变集中并协调晶粒变形。金属间化合物 Ni-Zr 相数量的增加有助于降低裂纹密度，研究发现当 Zr 含量达到 1 wt.%时，热裂纹被完全抑制。此外，观察到连续的网络状金属间化合物 Ni-Zr 相作为“骨架”，显著提高了打印样品的屈服强度超过 50%。经过后续热处理，掺杂 Zr 的 Haynes 230 合金表现出卓越的强度与塑性组合，这归因于金属间化合物 Ni-Zr 的溶解、大尺寸 M₈C 的减少沉淀以及 MC 在晶胞和晶界的沉淀。总体而言，本研究为激光增材制造无裂纹且具优异机械性能的合金提供了一条新的合金设计途径。

Declaration of Competing Interest
利益冲突声明

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
作者声明其不存在任何已知的可能影响本文报告工作的竞争性财务利益或个人关系。

Acknowledgement 致谢

The authors are grateful for funding from the National Natural Science Foundation of China (Grant No. U22A20172 and 52171044).
作者感谢国家自然科学基金（项目编号 U22A20172 和 52171044）的资助。

Appendix. Supplementary materials
附录。补充材料

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A precipitation strengthened high entropy alloy with high (Al+Ti) content for laser powder bed fusion: Synergizing in trinsic hot cracking resistance and ultrahigh strength
2023, Acta Materialia
Additive manufacturing (AM) is a highly promising technique for producing near-net-shape high-value aerospace components with intricate geometries. However, due to the susceptibility to hot cracking, high-volume-fraction Ni₃(Al,Ti)-type precipitation-strengthened Ni-base superalloys typically used for these applications are usually difficult or even infeasible to manufacture via AM. Previous metallurgical methods to mitigate hot-cracking in AM usually compromise strength, resulting in a trade-off between hot-cracking resistance and strength. Here, we overcome this trade-off in AM of a multicomponent Ni-rich precipitation-strengthened high-entropy alloy (MNiHEA) Ni_46.23Co₂₃Cr₁₀Fe₅Al_8.5Ti₄W₂Mo₁C_0.15B_0.1Zr_0.02 (at%): synergizing remarkable hot-cracking resistance and ultrahigh strength. Crack-free MNiHEA is successfully fabricated via laser powder bed fusion (LPBF) without preheating, despite its high (Al+Ti) content of ∼7.4 wt%. This remarkable resistance to hot cracking arises from the comparatively low critical solidification range, the small average solidification cracking index, the suppression of the intermetallic phases in solidification, and the moderate hardening rate induced by the nanoprecipitates in aging, which are intrinsically imparted by thermodynamic and mechanical characteristics. The yield strength of the as-built-to-aged MNiHEA reaches ∼1.2 gigapascal, together with acceptable ductility. This ultrahigh strength outperforms its cast-to-aged counterpart by more than one-third and existing commercial superalloys CM247 and IN738LC. Such pronounced enhancement in strength is attained through multiple strengthening mechanisms, including solid-solution hardening, dislocation hardening, precipitation-hardening and grain-boundary strengthening. Our results demonstrate that intrinsic hot-cracking resistance can be utilized as a new metallurgical concept for mitigating hot-cracking without compromising strength in AM of high-temperature materials such as HEAs and superalloys.
Alloy design for laser powder bed fusion additive manufacturing: a critical review
2024, International Journal of Extreme Manufacturing

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1. Introduction 1. 引言