Fe-22 mass%Mn合金のγ→εマルテンサイト変態と機械的性質に及ぼす結晶粒径の影響

抄録

In an Fe-22 mass%Mn alloy which undergoes the athermal transformation from austenite(γ) to epsilon martensite(ε), the effects of the γ grain size on the γ→ε transformation and the mechanical properties of (γ+ε) structures have been investigated by means of optical microscopy, scanning electron microscopy, transmission electron microscopy, and X-ray diffraction analysis. The γ grain size was altered from 10 to 100 μm by recrystallization. With grain refining, the formation of ε is suppressed and both of the elongation and the ultimate tensile strength are greatly enlarged. The 0.2% proof stress, however, does not depend on the γ grain size, because the yield stress is determined by the onset of γ→ε stress-induced transformation.
The fracture modes of this alloy depend on the microstructures of ε plates. When the γ grain size is as large as 100 μm, a significant stress concentration takes place at the intersections of ε plates within γ grains during the deformation. Such a stress concentration causes a microcrack initiation at the intersections of ε plates and this leads to the onset of quasi-cleavage fracture along thick athermal ε plates. When the γ grain size is refined to 10 μm, however, the quasi-cleavage fracture is completely suppressed because ε plates become difficult to intersect each other within small γ grains. Consequently, γ grain refining gives favorable effects as follows: (1) change in the morphology of athermal ε, (2) reduction in the stress concentration at grain boundaries, and (3) uniform dispersion of deformation strain. In terms of the suppression of quasi-cleavage fracture by γ grain refining, the effect (1) is a most contributory factor in the Fe-22 mass%Mn alloy.