Impact of Al addition on deformation behavior of Fe-Cr-Ni-Mn-C austenitic stainless steel

The influence of Al on the temperature dependence of deformation mechanisms in metastable austenitic stainless steels was investigated by tensile tests in the temperature range of -196 - 300 degrees C. The deformed microstructures of the Al-free and Al-added steels at -140 degrees C consisted of planar glide features such as deformation-induced alpha'-martensite and deformation twins, with a larger fraction of alpha'-martensite in the Al-free steel. The formation of alpha'-martensite was mediated by deformation twins. The delayed kinetics of alpha'-martensite formation in the Al-added steel implies an increase in the stacking fault energy upon Al addition. This was also supported by the higher work hardening rate of the Al-free steel at deformation temperatures up to 150 degrees C. The work hardening rates at 300 degrees C were influenced by the dynamic strain aging. The more pronounced dynamic strain aging in the Al-added steel caused a higher work hardening rate compared to the Al-free steel. According to the tensile test results, the addition of Al caused a shift in the curves representing the temperature dependence of tensile elongation and tensile strength to lower temperatures. For the Al-free steel, the temperature associated with the maximum tensile elongation almost exactly coincided with the M-d(gamma -> a)' temperature, namely an immediate deterioration of tensile elongation occurred upon the alpha'-martensite formation. For the Al-added steel, on the other hand, the maximum tensile elongation was achieved at a temperature well below its respective M-d(gamma -> a)' temperature. This indicates an enhancement of tensile elongation in spite of the deformation-induced alpha'-martensite formation. Since deformation-induced micro-mechanisms in both steels were quite similar, the difference in the sensitivity of steels to the deformation-induced alpha'-martensite formation was interpreted in terms of an inhomogeneous distribution of alloying elements and local variations in the stacking fault energy and deformation-induced micro-mechanisms.