The effect of Al contents (0-5 wt%) and Cr contents (5, 13 wt%) on the stability of the austenite and low temperature tensile behavior have been studied. In particular, the role of Al was investigated in detail for the Fe-19Mn-(5,13)Cr-Al-0.2C-0.5Si in order to establish relations between mechanical property and formation of deformation twinning and/or strain-induced martensitic transformations.
The aluminum addition to Fe-19Mn-5Cr-0.2C-0.5Si alloy produced phase transformations from austenite plus ε martensite to the full austenite for 3wt% Al and from austenite plus ε martensite to austenite plus δ-ferrite for the 5wt% Al. It was verified again that Al played an important role for stabilizing the austenitic phase. The ability of Cr to stabilize austenite was inferior to Al because the parameter, $ΔG_cr^{(r-＞ε)}$ representing the effect of Cr on the phase stability was low compared with that of Al.
Thermodynamic analysis of the intrinsic stacking fault energy proposed by G.B Olson was applied to achieve the relationship between austenitic stability and tensile behavior of the alloy systems. Calculated stacking fault energy based on a regular solution approach showed that austenitic Fe-Mn-Cr-Al-C-Si alloys which have the stacking fault energy larger about 20mJ/㎡ favored the deformation twinning leading to the increase the ductility at low temperature.
Fe-19Mn-5Cr-3Al-0.2C-0.5Si alloy showed a very high elongation exceeding 74% in the temperature range tested(Room Temp., -40℃, -100℃, -196℃). It was found that deformation twinning and strain-induced α′ martensite played key roles for increasing elongations at all temperatures(Room Temp., -40℃, -100℃, -196℃) examined. But, austenitic Fe-19Mn-13Cr-0.2C-0.5Si alloy without Al decreased elongation from 66.2% at room temperature to 23.3% at the liquid nitrogen temperature. Poor ductility of Fe-19Mn-13Cr-0.2C-0.5Si alloy at the liquid nitrogen temp. was due to rapid formation of strain-induced α′ and ε martensites. It is confirmed tensile elongation is not controlled by the total amount of strain-induced phase, but rather the rate and temperature sensitivity of strain-induced phase formation.
The effect of Al on the impact toughness of Fe-19Mn-(5,13)Cr-Al-0.2C-0.5Si was also investigated. Impact toughness was closely dependent on crystal structure, interstitial element content, and precipitation of carbides at grain boundary etc.. Fe-19Mn-5Cr-3Al-0.2C-0.5Si alloy exhibited a very high impact toughness of 159J even at the liquid nitrogen temperature, which is twice higher than that of commercial 9% Ni steel. The dependence of impact toughness was related to the change of crystal structure induced by Al addition.
