High-entropy alloy (HEA) has the advantages of high strength, high hardness, high temperature resistance, radiation resistance, and strong corrosion resistance. The refractory HEA system composed of high melting point elements such as Nb, Mo, Ta, W, V, Re, and Hf has become a material of great research value in the future development of aviation industry and military. However, the refractory HEA composed of ultra-high melting point elements such as Nb, Mo, Ta, and W has poor plasticity at room temperature, and the formed samples are prone to cracks. Therefore, in this study the forming plasticity of WNbMoTa HEA is increased by adding plastic elements to form a special HEA powder material for additive manufacturing. The crack-free RHEA01 HEA is formed by laser selective melting. The structure, morphology and properties of the RHEA01 HEA are analyzed, and the HEA samples with excellent mechanical properties are obtained.
In the experiment, the Nb, Mo, Ta, Ti, and Ni powders are mixed in an equal or near equal atomic ratio. After the powder proportioning is completed, the selective laser melting (SLM) equipment independently developed by Xi’an Jiaotong University is used for RHEA01 SLM additive manufacturing forming. The GeminiSEM 500 electron microscope is used for the morphological EDS and EBSD analysis. The Bruker D8 ADVANCE XRD equipment is used for the crystal structural analysis. The HX-1000TM equipment is used for the room temperature hardness testing. The mechanical property testing machines are used for the room temperature and high temperature compression performance test. The Hopkinson pressure bar is used to conduct the dynamic compression mechanical performance test experiment.
It can be seen from the BSD image (Fig. 2) that Ti and Ni, which are quite plastic during the solidification of the alloy, supplement the shrinkage of the high melting point elements after solidification. The refractory HEA RHEA01 formed in this study has no cracks and good formability. As shown in Fig. 3, the refractory HEA RHEA01 has a single-phase BCC structure. Note that there are some miscellaneous peaks around the peak of RHEA01. These peaks are a small amount of new phase structures formed after Ti and Ni elements fed in the later stage of solidification. It can be seen from the surface energy spectrum in Fig. 4 that the elements in RHEA01 formed by SLM are uniformly distributed without macro-segregation. As shown in Fig. 5, the average grain size of the RHEA01 alloy is about 8.5 μm. RHEA01 has an obvious grain boundary filling phenomenon due to the small amount of Ni and Ti elements from the BSD diagram. From the perspective of grain size, RHEA01 has a relatively good theoretical strength due to the effect of grain refinement. At the same time, because it has a single-phase BCC structure composed of a large number of high melting point elements, it has good high temperature resistance.
SLM is used to form crack-free RHEA01 with added plastic elements, which solves the crack defects in the process of SLM forming of NbMoTa HEA and greatly increases the formability of NbMoTa HEA. The average grain size of RHEA01 is 8.5 μm and it is a single-phase BCC structure. Compared with NbMoTa HEA formed by SLM, the addition of plastic elements refines the grains. The yield strength and normal temperature compressive strength of RHEA01 are 1277.35 MPa and 1597.62 MPa, respectively, 27.9% and 36.9% higher than those of NbMoTa HEA (VAM). The hardness is 511.76 HV. The high temperature resistance of RHEA01 has good retention. The compressive strength at 1000 ℃ is as high as 993.84 MPa, only 37.8% lower than that at normal temperature. The dynamic compressive strength at 1400 ℃ (temperature)and 2000 s-1 (strain rate) is as high as 1015 MPa. The comprehensive performance of RHEA01, the high temperature compression resistance, and the normal temperature compression resistance are all at the world’s leading level.
