The effects of Al contents on the phase transformation and low temperature deformation characteristics of Fe-25Mn-12Cr-(2,3,4,5)Al -0.6C and Fe-30Mn-12Cr-(0,1,2,3)Al-0.4C alloys were investigated. The tensile tests, impact tests, X-ray diffraction analysis and microstructure observations were performed at temperature ranged from room temperature to 77K. The stacking fault energy was calculated from the known thermodynamic data.
The addition of Al in Fe-25Mn-12Cr-xAl-0.6C and Fe-30Mn-12Cr-xAl-0.4C cause the transition of phases from single $\gamma$ to $\gamma$ plus δ-ferrite. This result was verified that Al played an important role for the precipitation of δ phase. The precipitation of carbides were observed and they were identified as $(Cr,Fe)_{23}C_6$ type carbides.
In Fe-25Mn-12Cr-xAl-0.6C alloys, the tensile elongation decreased with increasing Al contents from 2 to 5 wt% due to the formation of γ-ferrite. The tensile elongation of Fe-25Mn-12Cr-xAl-0.6C alloys decreased with decreasing temperature from 298K to 77K as no strain induced transformed phase was formed. While, in Fe-30Mn-12Cr-xAl-0.4C alloys, the deformation twinning was observed in deformed structure except Fe-30Mn-12Cr-2Al-0.4C alloy. The Fe-30Mn-xAl-12Cr-0.4C alloys showed peak elongation at temperatures lower than 298K and this variation with temperature is related with the formation of deformation twins. The Fe-30Mn-12Cr-0.4C alloy showed ε-martensite in addition to deformation twinning at 77K, since stacking fault energy decreases to 18.9mJ/㎡ at 77K, which is favorable for the formation of ε-martensite.
The impact energies of the melted Fe-Mn-Cr-Al-C alloys decreased with decreasing temperature. The impact energy of Fe-25Mn-12Cr-xAl-0.6C alloys decreased with increasing Al content from 2 to 5wt% from the embrittling $\gamma$-ferrite and carbides, while the impact energy of Fe-30Mn-12Cr-xAl-0.4C alloys were insensitive to Al content.
