Improving Mechanical Properties of CoNiAl(-Fe) Shape Memory Alloys through Controlled γ-Phase Evolution

Polycrystalline CoNiAl-based shape memory alloys offer promising functional properties but are limited by poor mechanical properties. This study investigates the influence of alloy composition and processing history on γ-phase formation, spatial distribution, and mechanical response in CoNiAl(-Fe) alloys. Six compositions with varying Fe contents were analyzed in the as-cast and homogenized condition. Despite substantial variations in alloy composition, elemental partitioning between the γ and B2 phases remained essentially consistent. In contrast, the γ -phase fraction and spatial distribution varied substantially with alloy composition. γ -phase fractions below 0.2% were associated with low ductility and compressive strengths below 500 MPa, yet relatively small γ -phase fractions enabled pronounced deformation when favorably distributed along grain boundaries. However, increasing the γ -phase fraction did not inherently improve the mechanical response. Co 35 Ni 35 Al 28 Fe 2 was selected for detailed investigations on processing-induced γ-phase evolution. Variations in solidification rate, homogeneity, and aging substantially modified the γ -phase fraction and spatial distribution while maintaining largely unchanged γ -phase compositions. Aging at 1100 °C for 0.5 h promoted preferential γ -phase formation along grain boundaries while limiting intragranular precipitation and provided the most favorable combination of strength and ductility. The results demonstrate that controlling γ -phase distribution enables tailoring the mechanical response without requiring high overall γ -phase fractions.