Abstract
The thermoelastic martensite transformation from fcc to fct exsits prior to the non-thermoelastic transformation to bct in Fe-Pd alloys containing about 30 at% palladium. The structure of the fcc austenite is a disordered one, similar to In-Tl and Mn-Cu alloys. The temperature hysteresis and volume change in the fcc-fct transformation are small, and the tetragonality of fct martensite phase increases on cooling. The behavior is like the second order transformation. In this study, the specimens with the Ms temperatures between 300 K and 160 K were prepared, and the internal structures of the fcc austenite and fat martensite were precisely examined before and after the fcc-fct transformation by X-ray analysis and optical and electron microscopy. The results are summarized as follows: (1) {110} internal twins were not introduced into a fct martensite in the beginning of the transformation, but rapidly appeared as the tetragonality of a fct martensite increased. (2) Microtwins observed in the in situ electron microscopy were identified as internal twins rather than thin fct martensite crystals. (3) A remarkable tweed pattern was observed in the fcc austenite and the fct martensite phase by electron microscopy. The tweed contrast was closely connected with the internal twins, in such a way that the former grew to the latter during the transformation. (4) The directional arrangement of the tweed pattern was perfectly broken by heating up to 200 K above Ms temperature. This change in the tweed pattern was reversible for cyclic heatings and coolings.
