We have studied localization of tensile deformation in thermomechanically loaded thin superelastic NiTi wires with different microstructures tested at various temperatures reaching up to 400°C by employing 1D-DIC and in situ electric resistance methods. At different temperatures, the tensioned wires deform by various deformation mechanisms ranging from martensite reorientation at lowest temperatures over stress induced B2 – (R) – B19’ martensitic transformation at intermediate temperatures and stress induced B2 - B19’- B2T at elevated temperatures to conventional plasticity by dislocation slip at highest temperatures.
The 1D-DIC method enables to obtain a record of the homogeneity/heterogeneity of the tensile deformation of the wire. The record presented in a single figure provides complete information on the conditions under which the localized deformation starts and proceeds - where and when the bands of localized deformation nucleate; how many band fronts move or marge and when and where disappear when the band covers the wire completely, what is the magnitude of strain in the band and where necking leading to final failure occurs.
While hardened NiTi wires deform in a localized manner up to approx. 220°C and fail by localization of inelastic deformation of stress induced martensite via necking at ~13% strain, the NiTi wires with softer microstructure however, deform in a localized manner up to ~350°C and further deform beyond the plateau end up to ~ 60% strain where they fail again by necking. The necking, as another strain localization phenomenon additional to Lüders like deformation, appears to be essential for tensile deformation of NiTi wires. We found untypical coupling of this localized transformation proceeding in specific temperature window forming together with plastic deformation transformation strains reaching 20%.