Enhancing the adiabatic shear resistance of Ti-Zr-Nb-Al high entropy alloy via metastability engineering

Dual-phase Ti-Zr based high entropy alloys are lightweight structural materials with high specific strength, which are promising for the application in warheads and munitions. However, they usually experience adiabatic shear under impact, resulting in early failure. Here, we demonstrate an effective method to enhance the adiabatic shear resistance, benefiting from the transformation-induced plasticity (TRIP) effect. The composition of Ti-Zr-Nb-Al is designed based on a combination of lattice distortion enthalpy, mixing enthalpy, and Bo-Md criteria, to enable a metastable solution structure. The alloy is directly water quenched from 900 degrees C to maximally keep the metastable BCC phase (MA), while an annealed alloy with the BCC+HCP +HCP dual-phase structure (PA) is also fabricated for comparison. The metastable MA alloy exhibits a doubled fracture strain and 79 % enhancement of impact energy compared with the dual-phase PA alloy, while its dynamic ultimate strength remains almost unchanged. Microstructural analyses at different dynamic stain levels reveal a BCC-alpha" phase transformation in MA alloy, which delays the generation of phase boundaries, avoids the premature stress concentration, and thereby delays the occurrence of adiabatic shear.