In situ experimental evidence on R-phase related deformation processes in activated NiTi wires

Although many commercial NiTi wires commonly exhibit sequential B2 ↔ R ↔ B19′ transformations, their unique stress–strain–temperature behaviors are readily comprehended and modeled as being solely due to the B2 ↔ B19′ martensitic transformation and/or reorientation/twinning processes B19′ ↔ B19′ in the martensite phase. Omitting the R-phase is commonly advocated by the fact that the B2 ↔ R transformation strains are much smaller and not all NiTi alloys exhibit R-phase transformation. However, small strains are commonly utilized in engineering applications and the R-phase is known to be promoted by thermal and/or mechanical cycling. The activity of the R-phase related deformation processes is moreover difficult to detect experimentally and hence the investigators do not have to be always aware of it. Two newly developed experimental techniques (in situ neutron diffraction and in situ combined ultrasonic and electric resistance measurement) were used to investigate the deformation/transformation processes in the activated NiTi wires related with the R-phase. Activity of individual B2 ↔ R transformation, R ↔ R reorientation and R ⇒ distortion processes during thermomechanical loads on binary NiTi wires was detected and distinguished from the B2 ↔ B19′ transformation or B19′ ↔ B19′ reorientation processes. It is claimed that the involvement of R-phase related processes in thermomechanical behaviors of common binary superelastic NiTi wires is more important than commonly acknowledged.