The influence of imposed strain on the development of microstructure and transformation characteristics of Ni–Ti shape memory alloys

Presented paper describes the influence of imposed strain on the development of microstructure (substructure) in Ni–Ti materials during the application of ECAE (equal channel angular extrusion). The interrelationship between imposed strain and resulting transformation characteristics is in the main point of interest while the chemical composition, namely the occurrence of secondary phases such as TiC and Ti4Ni2O resulting from the technology used for the sample alloy preparation is also investigated. The alloy Ni50.6–Ti (at.%) was prepared by melting in the HF vacuum induction furnace using a graphite crucible. The deformation was carried out via ECAE combined with prior rotary forging/swaging. ECAE was carried out at 280 °C. The imposed strain had the value ∼2. The alloy was processed by deformation route Bc. It was established the high dislocation density generated by the imposed strain stabilized the B2 and R phases and shifted the transformation R ↔ B19′ towards the lower temperature region. Direct effect of the deformation stress was demonstrated in samples after the second pass when the occurrence of the deformation induced transformation B2 → B19′ was confirmed. Deformation induced transformation also represents contribution to the overall deformation of the samples during the process.

Research highlights▶ SMA–Ni50.6–Ti (at.%) was formed by rotary forging and subsequent ECAE pressing (imposed strain ∼2). ▶ High dislocation density created by the imposed strain stabilized the B2 and R phases and shifted the transformation R ↔ B19′ to low temperatures. ▶ After the second pass a direct influence of deformation stress was shown. ▶ The deformation-induced transformation B2 → B19′ was confirmed.