Influence of applying external stress during aging on martensitic transformation and the superelastic behavior of a Ni-rich NiTi alloy

Superelastic properties and martensitic transition temperatures of Ni50.9Ti shape memory wires were studied after aging under different applied stresses at 450 °C for 45 min. The results indicate that applying external stresses during aging of Ni50.9Ti shape memory wires changes the strain value and stress level of plateaus related to stress induced martensitic transformation. Furthermore, martensitic transformation sequences change after applying external stresses but transformation temperatures measured with DSC remain constant in samples aged under stresses up to 120 MPa. Applying stress of 60 MPa during aging enhances the superelastic properties by increasing the strain of plateaus during loading and unloading cycles. Applying stresses more than 60 MPa, decreases the strain of the plateau during unloading.

► Applying external stress during aging increases the strain value of the plateaus.
► Best superelastisity is achieved by applying 60 MPa on aging at 450 °C for 45 min.
► Aging under stress may change the sequence of martensitic transformation.
► Aging under stress affect both the internal stress and kinetic of precipitation.