Relaxation and recovery of stress during martensite transformation for sputtered shape memory TiNi film

Abstract

Residual stress, relaxation and recovery of stress during martensite transformation of the sputtering-deposited TiNi films were studied in this paper. Results showed that a wide range of residual stress levels (both tensile and compressive) were obtained with the change of Ti contents in the deposited films. After film deposition, the thermal and intrinsic stress in the film could be very large, but due to the differences in phase transformation stress and transformation temperatures, the real stress values under room temperature, and stress evolution with temperature during phase transformation were quite different. For the film with a Ti content of 50.2%, due to the significant relaxation of stress caused by the perfect martensite transformation, the residual stress under room temperature was very low (−20 MPa) and recovery stress during heating was as high as 750 MPa. With both decrease and increase in Ti content in the deposited films, the tensile residual stress increased significantly. For the films with a Ti content of 51.3%, a two-step transformation was observed among martensite, R-phase and austenite. The recovery stress and average increase rate of stress during both heating and cooling were lower than those of film with 50.2% Ti. For the films with a Ti content of 47.3 and 53%, only partial relaxation of stress occurred due to the existence of R-phase transformation at room temperature. Also, because of non-equilibrium contents of Ni and Ti in these films, shape memory effect and the recovery of stress due to the phase transformation was not so significant. High residual stress in the thin films could cause non-recoverable deformation during phase transformation.