Effect of Nb content on the mechanical properties of Nbx(MoTaW)(1-x) (x = 0.4, 0.55, and 0.7) refractory multicomponent alloys

The MoNbTaW refractory multicomponent alloy (RMCA) retains good yield strength up to 1873K, but limited plasticity and high density at room temperature impede its utility in practical applications. Thus, a unique alloy design approach based on composition modification was used to improve these properties. Among the four constituent elements of the MoNbTaW alloy, Nb has good plasticity and lower density; hence a series of alloys with varying Nb content, namely Nb _x(MoTaW) _(1-x) with x = 0.4, 0.55, and 0.7 (designated as Nb _0.4, Nb _0.55, and Nb _0.7 in this manuscript), were processed using vacuum arc melting. X-ray diffraction results showed that all the alloys formed as single-phase with the body-centered cubic crystal structure. The microstructures of these RMCAs are dendritic in nature with segregation of elements that decreased with heat treatment. The hardness values showed a decrease as the Nb content increased. The compressive yield strength values are 1251, 1055, and 945 MPa, and those of fractured strain are 10, 22, and 33 % for Nb _0.4(MoTaW) _0.6, Nb _0.55(MoTaW) _0.45, and Nb _0.7(MoTaW) _0.3 respectively. The increase in plastic strain and toughness is due to the increment in the Nb concentration, which is directly interpreted as an enhancement in degree of plasticity. The average grain sizes of Nb _0.4, Nb _0.55, and Nb _0.7 RMCAs are measured to be 86 μm, 106 μm and 145 μm respectively. It was observed that 36 % of the grains are above 100 μm for Nb _0.4 alloy, whereas this fraction is increased to 49 % and 60 % for Nb _0.55 and Nb _0.7 alloys respectively, which indicates an increase in average grain size as well as fraction of coarser grains as the Nb content increases. Hence, the improvement in plasticity is attributed to the increase in the average grain size and broader grain distribution. Fracture surface analysis of all the compositions showed quasi-cleavage behavior. This study demonstrates that increasing the Nb concentration in the MoNbTaW RMCA enhances plasticity while decreasing density. In comparison to conventional niobium alloys like C103 and FS-85, the alloys of the present study had higher yield strength at the temperatures where the tests were carried out. The specific yield strength values of these alloys are found to be higher than commercially available Nb alloys.