Investigation of the ageing-induced R-phase and its impact on static/cyclic tensile failure in Ni50.9Ti49.1 wire

Abstract

The trade-off between the safety and effectiveness of NiTi wire in practical applications has consistently hindered its advancement. This underscores the significance of adjusting its microstructure via appropriate heat-treatment techniques. This study systematically investigates the evolution of the constitution phases of the matrix and the grain size as the aging time increases from 3 min to 3 h at 550 °C for Ni50.9Ti49.1 wire, and highlights the significant role of the R phase in static/cyclic tensile failure behavior. When aged for 30 min, the matrix consists mainly of the B2 phase, similar to the as-received wire, while it tends to transform into the R phase during loading (intermediate-R phase). In contrast, aging for 3 h leads to the formation of the R phase as the matrix phase (matrix-R phase). The two types of R phase are discussed and compared in detail with regard to their contribution to the static/cyclic tensile failure behaviour. The longest fatigue life was obtained in wires with the R phase as the matrix phase, which was due to the combined effect: (i) the rapid attainment of a stable state so as to minimise the extent of phase transformation; (ii) coordination deformation of twin and dislocation to alleviate stress concentration; (iii) and the strong bonding between Ti2Ni and the matrix that reduces the occurrence of the “stretching into the hole-coalescence-microcrack process”. The enhanced flexibility observed in the wire with matrix-R phase is due to its reduced elastic modulus and lower transformation plateau. The increase in plasticity is linked to changes in the wire's fracture behavior resulting from grain growth. Such microstructural changes allow the alloy wire to exhibit a more uniform plastic deformation under stress, which in turn postpones the onset of significant fracturing.