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

Abstract

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 °C. The deformed microstructures of the Al-free and Al-added steels at −140 °C consisted of planar glide features such as deformation-induced α′-martensite and deformation twins, with a larger fraction of α′-martensite in the Al-free steel. The formation of α′-martensite was mediated by deformation twins. The delayed kinetics of α′-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 °C. The work hardening rates at 300 °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 Mdγ→α′ temperature, namely an immediate deterioration of tensile elongation occurred upon the α′-martensite formation. For the Al-added steel, on the other hand, the maximum tensile elongation was achieved at a temperature well below its respective Mdγ→α′ temperature. This indicates an enhancement of tensile elongation in spite of the deformation-induced α′-martensite formation. Since deformation-induced micro-mechanisms in both steels were quite similar, the difference in the sensitivity of steels to the deformation-induced α′-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.