Acoustic Emission During R-Phase and Martensitic Transformations in aTi_50.2Ni_48.3Fe_1.5 Alloy

Department of Materials Science and Resource Engineering, Kumanoto University, 2-39-1 Kurokami, Kumanoto 860, Japan

Abstract

Acoustic emission (AE) signals generated during phase transformations in a Ti_50.2Ni_48.3Fe_1.5 shape memory alloy have been measured, and the AE parameters have been correlated with the phase transformation events. The AE count rate curve during cooling of the specimen was found to have two distinct peaks at temperatures of approximately 8 and -30 °C. These peaks were confirmed by both optical microscopy and differential scanning calorimetry to correspond to the B2 to R phase transformation (at 8 °C) and the R to B19' transformation (at -30 °C) respectively. This is the first detection of the AE events associated with the R-phase transformation in Ti-Ni shape memory alloys. Although the amplitude distributions during both transformations were almost identical, both the duration and the rise time of AE events during the B2 to R phase transformation were larger than those during the R to B19' transformation. These observations suggest that the transformation velocity of the R-phase transformation is slower than that of the martensitic transformation, and are consistent with the nature of both these transformations. It is concluded that the AE technique can be applied to the determination of transformation temperatures of Ti-Ni alloys on cooling as well as DSC and electrical resistivity measurement.