The effects of Al level(5,7w/o) on microstructure and low temperature mechanical properties (from-196$^\circ{C}$ to RT) in the cryogenic Fe-30Mn-Al-0.15C were studied. Both alloys exhibited inverse ductility behavior with decreasing temperature, but the 5 Al alloy showed higher ductility than 7 Al. Tensile strengthes of 7 Al were higher than those of 5 Al alloy. However, the impact energies of 7 Al alloy were inferior to those of 5 Al alloy due to the existence of σ-ferrite in FCC matrix compared with fully austenitic 5 Al alloy.
It has been found that Al plays a critical role in the inverse ductility behavior below room temperature in Fe-30Mn-Al 5Cr-0.15C. The Al-free alloy of Fe-30Mn-5Cr-0.15C steel maintained the elongation about 50% between RT and -196$^\circ{C}$, but the Al added Fe-30Mn-2Al-5Cr-0.15C exhibited significant increase in elongation with decreasing temperature; 30% elongation at RT, compared with 55% at -196$^\circ{C}$. An interesting observation in the behavior of elongations from RT to 500$^\circ{C}$ in the Al-free alloy (Fe-30Mn-5Cr-0.15C) was the appearance of the ductility peak at 350$^\circ{C}$, which was due to TRIP by formation of ε martensite. The addition of 2 Al to Fe-30Mn-5Cr-0.15C suppressed Ms temperature of martensite and stabilized the austenite structure below room temperature. It is proposed in this work that the inverse ductility behavior in this Fe-30Mn-2Al-5Cr-0.15C is associated with strain-induced transformation of austenite to martensite. It should be also pointed out by other works that the formation of microtwinning with decreasing temperature in Al-containing alloy also contribute significantly to the increased ductility with decreasing temperature.
It was also found that the addition of Cr(5 w/o) to the Fe-30Mn-5Al-0.15C improved the corrosion resistance, but deteriated mechanical properties.
