Characterization of Transformation Temperatures with the Bend and Free Recovery Technique: Parameters and Effects

This study was conducted to investigate factors such as the deformation strain, straining temperature, and the number of testing cycles on the measurement of transformation temperatures using the bend and free recovery (BFR) technique. Ti-56.0%Ni wire with approximately 40% cold work and a 2 mm diameter was heat treated in an air furnace for 10 min at 490 °C to obtain an A_f of approximately 21 °C. Wire specimens were deformed with one of two mandrels to apply an outer fiber strain of 2.4 or 5.8%. Deformation was performed at one of four straining temperatures: 0, −30, −50, or −65 °C. Specimens were tested ten times to investigate the effect of repeated testing. The resulting BFR curves were analyzed to determine the trends in the R-phase start (\( R_{{\text{s}}}^{\prime} \)) and austenite finish (A_f) temperatures. For specimens strained at both 2.4 and 5.8% there was no detectable change in A_f resulting from changes in the deformation temperature. Increasing the deformation strain from 2.4 to 5.8% tended to increase the measured A_f by approximately 1 °C independent of deformation temperature. Repeat testing seemed to result in a slight increase in A_f but the intrinsic scatter of the BFR data made it impossible to conclusively identify a trend.