Abstract:To address the role of the HCP martensite in CoAl and CoNi shape memory alloys, the relationship between the shape memory effect (SME) and the content of the thermal and stress-induced HCP martensite was investigated in the solution-treated CoAl and CoNi alloys. In-situ optical observations were employed to investigate the contents of thermal HCP martensite before and after deep cooling and its influence on the stress-induced HCP martensite transformation and SME. The results show that the SME in both the CoAl and the CoNi alloys results from the stress-induced HCP martensite. The role of the thermal HCP martensite in both of them is the strengthening of the matrix. The much higher yield strength in the solution-treated CoAl alloy due to solution strengthening of Al is responsible for its better SME compared with the CoNi alloy.

Key words: CoAl alloy; CoNi alloy; shape memory effect; thermal HCP martensite; stress-induced HCP martensite; solution strengthening