Growth dynamics study of the martensitic transformation in Fe-30 pct Ni alloys: Part I. Quantitative measurements of growth velocity

Abstract

Previous studies of the growth velocity in athermal martensitic transformations were of an indirect nature in which the size of individual martensite plates could not be determined with precision. The present study focuses on the quantitative analyses of martensitic growth events in an Fe-30 pct Ni alloy in order to determine the growth velocity and the final size of single transfor-mation events directly and simultaneously. Both magnetic induction and acoustic emission (AE) techniques have been employed, and the corresponding analytical methods have been developed. While efforts have been made to minimize possible distortions of transformation signals caused by detectors and signal processing apparatus, it is recognized that the media in which the signals propagate impose a significant distortion on the signals. We have proposed methods to cope with the effect of eddy currents in the magnetic measurements and the frequency-dependent media attenuation in the AE measurements. Two independent experiments in our study have shown a consistent trend of the growth velocity and the final size of martensitic transformation events as a function of temperature. The growth velocity ranges from 0.25 to 0.65 of the shear wave velocity in the material. Several possibilities responsible for the velocity scatter are dis-cussed, and it is suggested that the varying strain conditions under which the martensitic growth events take place play a decisive role in determining the actual growth velocity.