Effect of Al–Zr and Si–Zr atomic pairs on phases, microstructure and mechanical properties of Si-alloyed (Ti28Zr40Al20Nb12)100-xSix(x=1, 3, 5, 10) high entropy alloys

Abstract

The phases, microstructure, and mechanical properties of Si-alloyed (Ti28Zr40Al20Nb12)100-xSix(x = 1, 3, 5, 10) high entropy alloys are studied. Although silicides were not detected in the as-cast 1 at. % Si-alloyed sample, after annealing all the four alloys have the same phase composition: the Zr5Al3-type phase, Zr5Si3-type silicides, and the BCC/B2 dual-phase matrix, indicating that 1 at. % Si addition is adequate for the formation of silicides in equilibrium. The phase morphologies show a dependence on the Si content and the cooling rate, which changed from the occurrence of columnar grain zones together with coarse dendrites of the as-cast 1 at. % Si-alloyed alloy to four different cooling-rate-related morphologies in the other three as-cast alloys. After heat treatment, differently shaped precipitates tend to aggregate and spherize, and a three-step-shaped phase morphology is observed when the Si content is above 5 at. %. All precipitates are rich in Zr due to either the strong Si–Zr pair or Al–Zr pair, but heterogeneous distributions of Si and Al exist. An increase of the Si content led to a gradual decrease of the compressive strength in both as-cast and annealed samples, yet the total fracture strain does not show monotonously decrease trend. 5 at. % Si-alloyed samples possessed a higher total fracture strain than the 3 at. % Si-alloyed samples.