Elastic accommodation and anomalous tetragonality of thin-plate martensite in an Fe-31Ni-10Co-3Ti alloy revealed by neutron diffraction at cryogenic temperatures

山下 享介*; Harjo, S.   ; Gong, W.   ; 川崎 卓郎   ; 森戸 茂一*; 諸岡 聡  ; 藤井 英俊*; 友田 陽*

Yamashita, Takayuki*; Harjo, S.; Gong, W.; Kawasaki, Takuro; Morito, Shigekazu*; Morooka, Satoshi; Fujii, Hidetoshi*; Tomota, Yo*

In this study, neutron diffraction during cryogenic cooling was performed on Fe-31Ni-10Co-3Ti alloy to investigate changes in the austenite lattice constant and martensite tetragonality during thin-plate martensitic transformation. The austenite lattice constant showed only a slight change because of the Invar effect, but decreased below the Ms temperature due to elastic accommodation of transformation strain. Broadening of austenite diffraction peaks and EBSD analysis indicated localized plastic accommodation near intersecting martensite regions. For martensite, the a-axis decreased while the c-axis increased anomalously with decreasing temperature, leading to enhanced tetragonality below 35 K. During heating, the c-axis decreased and tetragonality was reduced. These behaviors are attributed to Bain strain during twin formation and possible interfacial stress with surrounding austenite.