Shape Memory Effect by FCC-HCP Transformation in High-Entropy Alloys

Central concept of high-entropy alloys (HEAs) is to maximize configurational entropy and to stabilize a solid-solution with respect to ordered phases. Extensive investigation has been conducted to search for HEA with superior mechanical properties; however, much less effort has been paid to explore their functional properties. Present study focuses on martensitic transformation and shape memory effect in fcc HEAs.

Alloy ingots of Cr₂₀Mn₂₀Fe₂₀Co_40-xNi_x (x=0~20) were prepared by high-frequency melting. Final heat treatment was done at 1373 K for 1 h followed by water quench. DSC traces for Ni0 and Ni5 alloys show clear transformation peaks while those of Ni content more than 10 mol% did not show any transformation peaks in the measured temperature range. Low- and high temperature XRD confirmed these peaks are FCC-HCP martensitic transformation. Martensitic transformation temperatures were listed in the inset table. An increase of Ni content leads to a decrease in transformation temperatures, which agrees well with the CALPHAD analysis. Shape memory effect was investigated by bending tests. It was revealed that the Ni5 alloy exhibit the recovery strain of about 2% when pre-strained at 293 K, which is much larger than the one in Fe-Mn-C alloys (~0.3%) and is comparable to that in Fe-Cr-Mn-Si alloys. Observed small volume change on FCC-HCP transformation (~ 1.1%) and high yield strength (~200 MPa) due to the enhanced solid-solution hardening in HEAs contributes to improved shape memory properties.