The effect of aluminum contents (0, 0.2, 1, and 4 w/o) on the microstructures, low temperature tensile behaviors, and strain-controlled fatigue properties in austenitic Fe-26Mn-Al alloys were studied. Tensile tests were performed from RT to 4K, while total strain-controlled fatigue tests were performed at RT and 77K on the recrystallized (1123K, 1hr.) plates.
The addition of Al to the Fe-26Mn-Al alloys stabilized the austenitic structure by suppressing the formation of epsilon martensite, which indicated the increase of stacking fault energy with the Al additions to Fe-26Mn-Al matrix. The amounts of thermally induced epsilon martensite increased with decreasing temperature and lowering aluminum contents. The volume fractions of epsilon martensite in Fe-26Mn, Fe-26Mn-0.2Al, and Fe-26Mn8-1Al increased further after tensile deformations from RT to 4K. No deformation induced epsilon martensites were formed in the Fe-26Mn-4Al even at 4K. However, TEM study showed the formation of deformation twinning after tests in the Fe-26Mn-4Al alloy.
Characteristic of the strain-induced phases whether it was epsilon martensite or deformation twinning had greatly affected tensile deformation behavior and the strain-controlled fatigue behavior in Fe-26Mn-Al alloys. Uniform elongation of Fe-26Mn-4Al increased from 44% at RT to 80% at 77K, followed by decreasing to 71% at 4K by the formation of deformation twinning; twin density continuously increased from 11 at RT to 59 at 4K in the Fe-26Mn-4Al. On the other hand, the uniform tensile elongation of Fe-26Mn decreased gradually from 36\% at RT to 22% at 4K. The major reason for the increase in elongation with decreasing temperature from RT to 77K in Fe-26Mn-4Al was due to the less work hardening rate with strain with the formation of twinning, compared with that of the epsilon martensite formation in Fe-26Mn. It was found in this work that the formation of deformation twinning instead of epsilon martensite was very beneficial in enhancing low temperature tensile elongation in austenitic Fe-26Mn-Al alloys. However, the controlling factor for having maximum elongation was not the total amount of straininduced phases, but the optimum rate of the work hardening with the formation of the strain-induced phases.
In-situ TEM examination wa conducted to observe the formation of deformation twinning at RT and 77K for FE-26Mn-4Al alloy. The strain-induced deformation twinning was preferentially formed at the crack tip, thereby inhibiting the further crack propagation. The amount of deformation twin formed at RT was smaller compared to that at 77K, which was not enough to prevent the localized necking at RT. The increased elongation with decreasing temperature was a result of suppressing a local necking by a gradual formation of deformation twinning and the difficulty of microcrack propagation by the formation of strain-induced mechanical twinning.
The new flow equation, $\alpha = K\epsilon^Nexp(M\in)$, was applied to calculate uniform elongations in these Fe-26Mn-Al alloys. The calculated values were in good agreement with the measured values if no premature fractures occurred.
Total strain-controlled ($\triangle \epsilon_t : \pm 3.0%$) fatigue tests were performed at RT and 77K in the fatigue life range from $10^2$ to $10^4$ cycles for Fe-26Mn-4Al alloys. Both alloys displayed higher fatigue lives at 77K than those at RT. The increased fatigue life of Fe-26Mn-alloy was primarily due to the significant increase in strength by thermally transformed epsilon martensite. On the other hand, the increased fatigue resistance of Fe-26Mn-4Al was from the great increase in the fatigue ductility coefficient, $\epsilon_f', from 10 at RT to 46 at 77K by the formation of strain-induced deformation twinning besides the increase in strength with decreasing temperature. The transition fatigue life($2N_t$) shifted from 68 cycles in Fe-26Mn at 77K to 445cycles in Fe-26Mn-4Al at 77K. The total strain fatigue was mainly controlled by elastic mode in the Fe-26Mn with increased amount of epsilon martensite, while the plastic component became dominant with the increasing strain amplitude in Fe-26Mn-4Al.
