Effect of Al addition on structural evolution and mechanical properties of the AlxHfNbTiZr high-entropy alloys

Compared to conventional high temperature alloys, refractory high-entropy alloys (HEAs) have shown superior mechanical properties at room and elevated temperatures. Nevertheless, high density and consequent low specific strength restrict their engineering application. To address these issues, alloying light-weight Al can to some degree relieve these problems, and a series of AlxHfNbTiZr (x = 0-1.5 in mole ratio) HEAs were synthesized by arc melting technique. Alloying effects of Al on structural transition and mechanical properties were systematically investigated. The Al0HfNbTiZr HEA with excellent plasticity consists of a single random body-centered cubic (BCC) solid solution phase. When x reaches 0.75, new ordered BCC phase is formed, whose lattice constant is quite close to the random BCC phase. Under the premise of the small atomic size difference (similar to 4.9%), high negative mixing enthalpy (< -15.63) and the low valence electron concentration (< 4.08) induced by Al addition promote the ordered structural transformation. Phase transition results in the drastic increase of compressive yield strength and microhardness accompanied by an evident plasticity reduction. Encouragingly, the compressive specific yield strength is gradually enhanced form 82.5 (x = 0) to 253.4 (x = 1.5) MPa cm(3) g(-1), which is on account of the density reduction and distinct strengthening effect.