Determining recoverable and irrecoverable contributions to accumulated strain in a NiTiPd high-temperature shape memory alloy during thermomechanical cycling

When Ni29.5Ti50.5Pd30 shape memory alloy is thermally cycled under stress, significant strain can accumulate due to elasticity, remnant oriented martensite and plasticity. The strain due to remnant martensite can be recovered by further thermal cycling under 0 MPa until the original transformation-induced volume change and martensite coefficient of thermal expansion are obtained. Using this technique, it was determined that the 8.15% total accumulated strain after cycling under 200 MPa consisted of 0.38%, 3.97% and 3.87% for elasticity, remnant oriented martensite and creep/plasticity, respectively.

► Discovered a methodology to find oriented martensite contribution to the irrecoverable strain. ► This methodology allows separation of irreversible creep and plastic strain from oriented martensite. ► Retained oriented martensite recovers upon heating to TUC ranging from 370 to 500 °C. ► Noticeable creep occurs above 450 °C during thermal cycling under load. ► Oriented martensite in NiTiPd SMAs shows negative coefficient of thermal expansion.