An in-situ synchrotron study of the B2→B19′ phase transformation in a Ni-Ti alloy subjected to uniaxial monotonic tension

A cold-drawn and annealed 56Ni-44Ti wt% alloy was subjected to in-situ uniaxial monotonic tension in a synchrotron. Spatially resolved diffraction data was acquired along the gauge length by pausing the loading at five select macroscopic strains within the stress plateau region. This enables tracking localised transformation phenomena by sub-dividing the gauge length into transformation band, untransformed and apparently transformed regions. Within the macroscopic stress plateau region: (i) the highly strained B2 phase within the propagating transformation band and apparently transformed regions produces a relaxation of the B2 phase within the untransformed region. (ii) The newly formed B19′ grain families exhibit a transition in relative lattice strain values from the transformation band through to the apparently transformed region. (iii) The 〈111〉B2 fibre texture transforms to the [1̅20]B19′, [1̅30]B19′ and [010]B19′ such that the latter fibres continue to record increases in maximum intensity up to maximum load. Within the slowly rising macroscopic stress region and beyond a critical stress value of ~ 426 MPa: (i) the relative lattice strains of the (12̅0)B19′ and (020)B19′ grain families deviate from linearity along the axial and transverse directions, respectively and, (ii) the anisotropy in crystallite size and micro-strains in all B19′ grain families reduces markedly.