NbMoTa refractory medium-entropy alloy (MEA) was fabricated by laser metal deposition (LMD) and vacuum arc melting (VAM) respectively. The crystal structures of NbMoTa MEA under two processes are all single-phase solid solution of BCC structure. Compared with the MEA formed by VAM, the NbMoTa MEA formed by LMD has smaller grain size and component microscopic segregation. Due to the difference in cooling rate, cellular and columnar substructures are demonstrated within the grain of LMDed MEA while the substructure within the grain of VAMed alloy is typical dendritic. However, the refinement of the grains in LMDed MEA does not lead to improvements in mechanical properties. In this study, the theoretical yield strength of NbMoTa MEA is calculated through solid solution strengthening (SSS) theory. The theoretical value is consistent with the experimental value of VAMed MEA, which is higher than the experimental value of LMDed MEA. The main reason for this result is that there are some metallurgical defects like porosities and intergranular cracks appeared in the LMDed MEA. The EDS test showed there is no elemental segregation seen at both sides of the crack. The reason for the intergranular cracks can be attributed to the high residual thermal stress caused by the rapid solidification characteristic of LMD process.

分别通过激光金属沉积（LMD）和真空电弧熔炼（VAM）制备了NbMoTa难熔中熵合金（MEA）。NbMoTa MEA在两个过程中的晶体结构均为BCC结构的单相固溶体。与VAM形成的MEA相比，LMD形成的NbMoTa MEA具有较小的晶粒尺寸和组分微观偏析。由于冷却速率的差异，在LMDed MEA晶粒内证实了蜂窝状和柱状亚结构，而在VAMed合金晶粒内则呈现出典型的树枝状结构。但是，LMDed MEA中晶粒的细化不会导致机械性能的提高。在这项研究中，NbMoTa MEA的理论屈服强度是通过固溶强化（SSS）理论计算的。理论值与VAMed MEA的实验值一致，高于LMDed MEA的实验值。该结果的主要原因是，LMDed MEA中出现了一些冶金缺陷，如气孔和晶间裂纹。EDS测试表明，在裂纹的两侧都没有发现元素偏析。晶间裂纹的原因可归因于LMD工艺的快速凝固特性引起的高残留热应力。