Comparative study of microstructure, transformation behavior and functional properties of titanium nickelide with various grain size

The parameters of aging-induced microstructure, transformation behavior and functional properties (FP) under bending procedure were studied in the Ti-50.7 аt.%Ni shape memory alloy with various sizes of B2-austenite grain sizes (GS) from 5 to 15 mm. The SEM observations were carried out using a JSM-6460LV. The particle size and morphology was studied using a JEOL 2100 TEM; energy dispersive spectroscopy was carried out using the EDS stage. Characteristic temperatures were measured using a “Mettler Toledo”. The distribution of Ti₃Ni₄ particles is heterogeneous; the degree of heterogeneity depends on the aging time. It is weak after aging for 1 h and becomes pronounced with increasing aging time to 3 and 10 h. In the fine-grained structure, В19’ martensitic formation is suppressed; its formation becomes possible with GS growth above 11 μm and an increase in the degree of heterogeneity. The appearance of a В2-В19’ transformation is observed in the structure with the largest GS of 15 μm at the maximum aging time. This is associated with the loss of the coherency between coarse Ti₃Ni₄ precipitates (~350 nm) and the matrix.The maximum total recovery strain ~16% and recovery strain 15.5 alloy is provided by the structure with the GS 5 µm. Effect further aging at 430 ºС on FP is ambiguous and depends on the exposure time under aging and the GS. It is accompanied by their degradation in fine-grained alloy and improves them in the coarse-grained alloy.