In situ neutron diffraction study on lattice parameter changes of austenite and martensite during transformation in Fe-18Ni alloy

Gong, W.   ; Harjo, S.   ; 友田 陽*; 諸岡 聡  ; 川崎 卓郎   ; 柴田 曉伸*; 辻 伸泰*

Gong, W.; Harjo, S.; Tomota, Yo*; Morooka, Satoshi; Kawasaki, Takuro; Shibata, Akinobu*; Tsuji, Nobuhiro*

Austenite to martensite transformation in steels is a volume expansion phase transformation. However, the puzzling results have still been reported that a hydrostatic compressive stress was observed in austenite as opposite to the Eshelby's inclusion theory. In present study, we carried out neutron diffraction experiments having higher penetration ability than above methods to investigate the lattice parameter changes in real time during martensitic transformation. A high-purity Fe-18Ni binary alloy was selected to avoid the chemical contribution from the interstitial element. Several measuring processes were carefully designed and performed by in-situ neutron diffraction. Thermal phase stresses in martensite and austenite phases were separated from those stemmed from the face-centered cubic to body-centered cubic transformation strains and lattice defects. As results, the plastic accommodation is suggested to affect the lattice parameter of austenite and then its phase stress.