Recoverability of large strains and deformation twinning in martensite during tensile deformation of NiTi shape memory alloy polycrystals

Superelastic NiTi wires were deformed in tension up to gradually increasing total strains at various temperatures, recoverable strains were evaluated and lattice defects left in the microstructure of deformed wires were analyzed by TEM. The recoverable strains evaluated in tensile tests: i) are surprisingly large - exceed 10% strain at low temperatures T ≤ 20 °C, ii) are not significantly restricted by large plastic deformation up to 55% at low temperatures, iii) display local maxima in dependence on total strain in tensile tests at low temperatures, iv) decrease with increasing test temperatures and iv) increase with increasing total strain in tensile tests at high temperatures T ≥ 50 °C. Besides slip dislocations, key lattice defects created by the tensile deformation beyond the martensite yield point are deformation bands containing {114} austenite twins or R-phase. No other austenite twins were found statistically relevant. It is concluded that the plastic deformation of NiTi wires is initiated by deformation twinning in oriented martensite, accompanied by dislocation slip in austenite and/or martensite in extent depending on the test temperature and total strain. The deformation twinning is found to play an ambiguous role in NiTi deformation. When it proceeds at low temperatures (T ≤ 20 °C) beyond the yield point, it raises recoverable strain up to 13%, while at high temperatures (T ≥ 50 °C) when it proceeds already within the plateau range as intermediate step of the stress induced B2=>B19'=>B2^T martensitic transformation, it restricts the recoverability of transformation strains. The repeated deformation twinning in oriented martensite enables low temperature deformation of NiTi wires with specific microstructure up to ∼50% strain and causes severe microstructure refinement during the mechanical and thermomechanical processing of NiTi.