Effect of B2 austenite grain size and aging time on microstructure and transformation behavior of thermomechanically treated titanium nickelide

Abstract The size and morphology of Ti3Ni4 precipitates, as well as their transformation behavior after isothermal aging, were studied in a Ti-50.7 аt.%Ni shape memory alloy with various B2 austenite grain sizes. A 0.3 mm thick band with an accumulated strain of e = 44% (e = 0.6) obtained by cold rolling was used. The samples were then solution-treated at 600–800°С for 0.3–1 h with subsequent quenching in water to obtain the structure with different grain size (GS) from 5 to 15 μm. The samples were then subjected to isothermal annealing at 430 °C for 1, 3 and 10 h. The SEM observations were carried out using a JSM-6460LV. The microstructure was studied using a JEOL 2100 TEM. Characteristic temperatures were measured using a “Mettler Toledo” DSC. The results of the microstructure study prove that the GS strongly affects Ti3Ni4 particle size and morphology, as well as the degree of microscale microstructure heterogeneity, transformation kinetics and transformation sequences. This influence is non-unique and depends on the duration of isothermal annealing. After 1 h of aging, the particle thickness and diameter maintain the nanometer range in the whole GS range of 5–15 μm. The increase of aging time to 3 and 10 h leads to localization of nanosized precipitates in the grain-boundary region, with the width of these regions decreasing with increasing GS. After aging for 1 h, the samples with the GS of 5 μm show the B2→R transformation. The increase in the GS to 15 μm and aging duration to 10 h is accompanied by multiplication of martensitic transformations to four stages. The study explains the influence of GS on the precipitated particle size and morphology, transformation kinetics and staging.