Microstructure And Mechanical Properties Of NbMoTaW(V) High-entropy Alloy Prepared By Mechanical Alloying

High entropy alloy(HEA) is a new type of alloy with a mount of elements and simple phase, which has broken though the design mode of traditional alloys. As high entropy alloyhas high strength and hardness, excellent temperature and corrosion resistant properties, it becomes a research hotspot of scholars both in China and abroad in recent years. The choice of elements in HEA is mainly concentrated on those transition metals with close atomic radius and the research system is always based on the Al-Co-Cr-Cu-Fe-Ni and it’s derivatives, while few research about refractory HEA system has been reported. As far the available fabricating method of refractory HEA is restricted to casting, which is difficult and needs high heating temperature, and thus greatly limits the development of the alloy system.In the present work, we fabricated bulk refractory HEA by means of the mechanical alloying and spark plasma sintering(SPS) with the raw materials of high purity elements powder Nb, Mo, Ta, W, V and equiatomic ratio compositions. The main research aspects are as follows:The alloying behavior during MA process and the phase transformation, mechanical properties as well as fracture mechanism of NbMoTaWV refractory HEA sintered under 1500℃ was studied. The powder was single BCC supersaturated solid solution after MA, while was turned to two more stable phases after SPS. They were Nb-Mo-W-rich BCC matrix and Ta-V-rich BCC precipitation, with the grain sizes of 2.7 μm and 0.5 μm, and the lattice parameters of 3.312 ? and 3.325 ?, respectively. The precipitation phase was distributed in the matrix as particles and its volume percentage was about 13.8 vol%. The alloy had excellent comprehensive mechanical properties at room temperature, with the hardness, compressive yield strength, fracture strength and plastic strain of 774 Hv, 2785 MPa, 3661 MPa and 12.4%, respectively. Fracture mechanism of the alloy consisted of intergranular and plastic fracture, while intergranular fracture is predominant fracture mode.The alloying process, phase composition, microstructure and mechanical properties of NbMoTaW and NbMoTaWV refractory HEA were compared. The results showed that alloying rate increased with the increasing entropy value, while the alloying order of the components was not changed. Single BCC solid solution formed after MA, however the phase composition changed from originalthree-phase to two-phase after SPS as the entropy increased, implying the increased phase stability at elevated temperature. NbMoTaWV alloy exhibited higher hardness, strength and compression ratio than NbMoTaW alloy at the same sintering temperature. In addition, the fracture mechanism of 5-component alloy changed from single brittle fracture to mixed fracture mode.The phase composition and transformation process, microstructure and mechanical properties of refractory HEA NbMoTaW under different sintering temperature were studied. The density and hardness of sintered samples decreased with increasing sintering temperature, while the fracture strength and compression ratio showed a decline after rising first. The comprehensive performance of the alloys reached their maximum at the sintering temperature of 1500 ℃. Bulk NbMoTaW alloy consisted of Mo-W-rich BCC phase, Nb-Ta-rich BCC precipitated phase and Nb-rich FCC precipitated phase with small amounts of Fe impurity. The grain size of matrix and precipitate phase, as well as the volume percentage of precipitation phase increased with increasing sintering temperature.