Larger recovery strains in a CoNiSi alloy due to enhanced reversibility of FCC⇌HCP martensitic transformation

Abstract

It is not clear how coherent ordered nanoparticles affect characteristics of FCC⇌HCP martensitic transformation in disordered alloys. In this paper, the precipitations of coherent ordered nanoparticles and their effects on the FCC⇌HCP martensitic transformation were systematically investigated in a solution-treated Co24Ni6Si shape memory alloy. The results showed that both the coherent ordered L12 structured and L11 structured nanoparticles could precipitate even in the solution-treated Co24Ni6Si alloy followed by water quenching. The L12 structured nanoparticles strongly hindered the occurrence of stress-induced HCP martensitic transformation and its reverse transformation, but their hindrance dramatically reduced with the decrease of size when below 1.5 nm. The coherent ordered nanoparticles did not convert the non-thermoelastic FCC⇌HCP martensitic transformation to the thermoelastic ones owing to the absence of group-subgroup relation between the parent phase and martensite. A maximal recovery strain of 6.7 % was obtained after only aging at 1073 K for 1 min. This value was 1∼2 % higher than the reported maximal ones in conventional FeMnSi-based alloys subjected to complicated thermomechanical treatments. The enhanced reversibility of HCP martensitic transformation could account for this larger recovery strain by the ultrafine coherent ordered L12 structured nanoparticles.

Keywords

• Shape memory alloys
• Shape memory effect
• Martensitic transformation
• Nanoparticles
• L12
• Reversibility