Effect of Initial Pre-Aging Conditions on the Isothermal ω-Phase Formation in Metastable Beta Ti-Nb-Zr SMA

The effect of pre-aging conditions, including initial grain size, residual stress level, and cooling/heating rates prior to isothermal holding on the β→ω_iso transformation is studied in Ti-22Nb-6Zr shape memory alloy, since the formation of the ω-phase significantly impacts functional behavior of this group of alloys. Different thermomechanical treatments to create fine-grained (~5 μm) and coarse-grained (~50 μm) initial microstructures and subsequently, three distinct cooling-heating routes were employed before isothermal aging at 300°C or 375°C were applied. The evolutions of phase and structure states were studied using OM, EBSD, TEM, and XRD. To compare diffusion effects on β→ω_iso transformation, diffusion coefficients and paths were calculated for all alloy components, and to evaluate mechanical properties tensile tests were performed. The "initial" stresses formed prior to the ageing process are the most important driving force for the ω_iso-phase precipitation and accelerate the precipitation process dramatically. Therefore, difference in the initial pre-aging conditions leads to the significant difference in the β→ω_iso transformation features. The initial structural state can enhance or suppress the ω_iso-phase formation by the shear mechanism: the fine-grained state has a higher level of residual stresses, which plays a role of “initial” stresses for the ω_iso-phase formation and accelerates the β→ω_iso transformation. The thermal route conditional conditions, such as cooling-heating rates and temperatures, can only accelerate the ω_iso-phase formation by the implementation of diffusion mechanism.