Achieving synergetic enhancement of strength and ductility in zirconium alloys containing C15 laves phase via regulating aluminum concentration

Achieving a synergistic combination of high strength and adequate ductility in zirconium alloys containing strengthening Laves phases remains a significant challenge. This study systematically investigates the effect of aluminum content (x = 0 to 15 at.%) on the microstructural evolution, deformation mechanisms, and mechanical properties of hot-rolled Zr-1.8Cr-xAl alloys. It is revealed that Al plays a dual role governed by its solubility in α-Zr. At x ≤ 10 at.% Al, a supersaturated solid solution forms alongside C15 (ZrCr₂) Laves phases. This microstructure yields an excellent strength-ductility synergy in the Zr-1.8Cr-9Al alloy, with a yield strength of 795 MPa and a uniform elongation of 8.4%. The enhancement is attributed to solid-solution and grain-refinement strengthening, coupled with the activation of multiple slip systems and dislocation emission from grain boundaries, which promote uniform strain distribution. In contrast, at x ≥ 10 at.% Al, extensive precipitation of brittle Zr₃Al and Zr₂Al intermetallics occurs (at 12 at.% Al or more). While these precipitates provide substantial precipitation hardening, shifting the strengthening trend from linear to parabolic, they also induce severe stress concentrations at interfaces. This leads to premature crack initiation and a drastic drop in ductility, despite the activation of typically hard-to-activate slip systems (including both basal 〈a〉 and pyramidal <c + a>) driven by heterogeneous deformation-induced stress. Quantitative strengthening analysis confirms the deformation mechanism and strengthening model transition. This study elucidates the intricate interplay between Al content, phase structures, and deformation mechanisms, providing a guideline for designing strong and ductile Zr alloys containing Laves phases.