Some Observations of Ausageing Effects on the Martensitic Transformation in an Fe–Ni–Ti Alloy

Abstract

The martensitic transformation in an Fe-30.0Ni-3.8Ti (massy.) alloy ausaged at 973K for various periods has been examined by means of optical as well as transmission electron microscopy, electron as well as X-ray diffraction and electrical resistivity-temperature measurements. While the martensites in the unaged specimens were lenticular in shape as those in Fe-Ni binary alloys, those in the specimens ausaged for longer periods were of thin plate to a great extent, when formed at temperatures near the Ms, though the Ms temperature became considerably higher than that of the unaged specimens. Some of the thin plate martensites grew in the sidewise as well as the lengthwise directions and eventually became lenticular upon cooling to lower sub-zero temperature, the newly produced martensites during the sub-zero cooling being also lenticular. In addition, the thin plate martensites were so unique that twinned regions were hardly observed inside. The crystal structure of the martensite was tetragonal with c/a=1.02 in the unaged state. The tetragonality rapidly increased a little to 1.03 after short ausageings, but it diminished to almost unity after prolonged ausageings.The morphological change of martensites associated with ausageing and sub-zero cooling was discussed together with the peculiar austenite stabilization phenomenon previously reported.

Electron Microscope Study on Fine Structures of the Austenite in an Ausaged Fe–Ni–Al Alloy

“…Meanwhile, Gun'ko et al (3) explained the satellites as a cross-sectional view of diffuse streaks along <110>* directions in reciprocal space cut by the Ewald sphere, and ascribed the origin to coherent shear strains produced by the concurrent formation of another precipitate named a' with the D03 type ordered structure. Ausageing effects in the former Fe-Ni-Ti alloys have so far been examined in some detail(4)- (9), but those in the latter Fe-Ni-Al alloys have not so much(1)- (3). Hence, the nature of the diffuse streaks and the corresponding striations in the austenite of the latter alloys have not been well understood yet.…”