Detwinning process of martensite in Ni 58 Mn 25 Ga 17 as a high temperature shape memory alloy under uniaxial compression
Résumé
The Ni-rich Ni-Mn-Ga alloy as a high temperature shape memory alloy has been directionally solidified, which is consisted of non-modulated (NM) martensite with preferred orientation and γ precipitate with a three dimensional net structure. The detwinning process of Ni58Mn25Ga17 alloy was investigated under step-wise uniaxial compression with a cumulative strain of 2.5%, 5.5%, 12.9% and 30.2%, respectively. It has been captured how the hierarchically structured martensite variants reorient and the effect of the surrounding γ phase on detwinning process with the increase of compression strain. It can be found that the volume fraction of the original minor lamella gradually increases and then it occupies the whole plate. The detwinning of new microtwins is not observed during further compression. The results indicate that the move of the intra-plate boundaries is attributed to a higher Schmid factor of the nanotwins. The mobility of the new inter-plate boundary is mainly dependent on the preferred orientation of the lamella inside martensite variants. The present study provides useful insights for microstructure training in high temperature shape memory alloys including martensite and γ phase.