Origin of shape memory effect in Co–Ni alloys undergoing fcc⇌hcp martensitic transformation

It is not clear about which of the coalescence of thermal hcp martensite and stress-induced hcp martensite is responsible for shape memory effect (SME) in Co–Ni alloys undergoing fcc⇌hcp martensitic transformation. To clarify this problem, the relationship was re-examined between the SME and the amount of thermal hcp martensite in a Co–30.3Ni alloy obtained by deep cooling. The relationship between the SME and the amount of stress-induced hcp martensite was also investigated in the Co–30.3Ni alloy and in a Co–30.8Ni–2.99Si alloy without thermal hcp martensite. The results show that the SME in the Co–Ni alloys results from the stress-induced hcp martensite, not from the coalescence of the thermal hcp martensite. The low yield strength leads to their poor SME. The increase in the SME with the rise in the amount of thermal hcp marteniste results from its strengthening of matrix and promoting of stress-induced hcp martenite. The SME is much better in the Co–30.8Ni–2.99Si alloy than in the Co–30.3Ni alloy because much more stress-induced hcp martensite were produced in the Co–30.8Ni–2.99Si alloy. Strengthening of matrix by other methods will improve the SME in Co–Ni alloys undergoing fcc⇌hcp martensitic transformation.