Effect of initial strengthening on recovery stress generation and isothermal relaxation processes in TiNi alloys

Abstract
Recovery stress (RS) generation and forced isothermal relaxation processes in Ti-50.5 at. % Ni and Ti-50.7 at. % Ni shape memory alloys were investigated using in situ X-ray structure analysis, X-ray texture analysis, transmission electron microscopy and mechanical tests. The alloys were subjected to thermomechanical treatments (TMT) including cold rolling followed by post-deformation heating in the range from 400 to 700°C for obtaining partially recovered, completely polygonized or recrystallized B2-austenite structures. Specimens containing partially recovered, polygonized or recrystallized structures obtained by thermomechanical treatment (TMT) were deformed in tension by 3 - 12 % strain inducing shape memory. When the austenite get low hardening by TMT or by deformation inducing SME, the RS generation is accompanied by elastic and plastic deformations. When the austenite get high hardening , the RS generation is accompanied by elastic deformation only. The isothermal RS relaxation is accompanied by elastic deformation of the austenite excluding the case of very high RS (> 1000 MPa) and recovery strain during relaxation (8 - 9 %) when plastic deformation also takes place. During RS generation the reverse martensite transformation by the orientation variant "directly backwards" is restricted by external counteraction and other orientation variants can partially realize.