The effects of carbon contents (0-0.4 w/o) on the low temperature tensile behavior and strain-controlled fatigue properties in austenitic Fe-30Mn-1.2Al-C alloys have been studied. Tensile tests were performed from room temperature to 77K, while total strain-controlled fatigue tests were performed at room temperature and 77K under annealed condition(1123K, 1hr.).
Fe-30Mn-1.2Al alloy showed a continuous increase of elongation with increasing strength from room temperature to 77K. On the other hand, the addition of carbon to Fe-30Mn-1.2Al alloys greatly changed the tensile elongation behavior. Fe-30Mn-1.2Al-C alloys exhibited an elongation peak in the temperature range tested. The increase of carbon content from 0 to 0.4 w/o in Fe-30Mn8-1.2Al-C alloys shifted the elongation peak toward higher temperature above 77K, while the increase of aluminum content from 1.2 to 5 w/o to Fe-30Mn-Al-0.3C alloy shifted the elongation peak toward lower temperature below 77K. An increase of carbon content also gave rise to an increase of heights in the maximum elongation.
It has been found that the strain-induced deformation twinning plays a key role in the temperature dependence of elongation by varying work hardening characteristics. For exhibiting a large elongation, the formation rate of deformation twins must be great enough to prevent the onset of necking, but not too lare, as too high a rate result in early fracture before necking. The strain-induced deformation twinning is desired to be generated little by little at optimum rate during deformation up to failure. It is concluded in this Fe-30Mn-1.2Al-C alloys that total amount of deformation twins was not the controlling factor, but the rate and its temperature sensitivity of deformation twin formation played important roles in elongation behavior.
The Neel temperature ($T_N$) of Fe-30Mn-1.2Al-C alloys decreased with increasing carbon content from 0 to 0.3%, indicating less stability of austenite with increasing carbon content in the Fe-30Mn-1.2Al-C alloys. Therefore, the increasing carbon content favored the formation of deformation twins at elevated temperature, thereby shifting the elongation peak toward higher temperature in the alloys.
The new flow equation, $δ=Kε^Nexp(Mε)$, was applied to Fe-30Mn-Al-C alloys to calculate uniform elongation. They were in good agreement with the measured values unless the premature failure occurred due to the excessive high formation rate of deformation twins.
Total-strain-controlled fatigue tests showed that the fatigue resistance at 77K was superior to that at room temperature in the entire fatigue life range tested ($10^2-10^5$ cycles). The absolute values of fatigue strength and ductility exponents increased somewhat with decreasing temperature from room temperature to 77K. However, a pronounced increase in the fatigue ductility coefficient at 77K occurred as the result of the inverse ductility behavior in the alloys. The superior low cycle fatigue resistance at 77K was attributed to the significant increase in fatigue ductility coefficient and strength due to the formation of deformation twins.
