Microstructure and mechanical properties of Co-added and Al-added austenitic stainless steels

The effects of Co and Co + Al addition on the microstructure, deformation-induced processes, and mechanical properties of a wrought Fe-14Cr-9Ni-3Mn-0.3C austenitic stainless steel were investigated. Whereas Al contributes to a reduced density and has a noticeable impact on the microstructure formation, Co addition could be regarded as a strategy to replace Ni, for instance to avoid Ni allergy. During tensile deformation at 300 ?, dynamic strain aging occurred in all steels but the associated increase in the work hardening rate was most obvious for the steel containing Co + Al. As the tensile test temperature was decreased to-196 ?, the yield strength of all steels increased. Furthermore, planar glide features such as deformation-induced alpha '-martensitic transformation became more dominant. Co and Co + Al additions reduced the M-d(gamma -> 5%alpha)' temperature and the amount of deformation-induced alpha '-martensite. The extent to which deformation-induced alpha '-martensite formation was suppressed was much greater upon Co + Al addition than upon Co addition alone. Accordingly, the work hardening rate at a given tensile test temperature was highest for the reference steel and lowest for the Co + Al-added steel with the most stable austenite of all steels. The highest tensile elongation of steels was reached at temperatures below their respective M-d(gamma -> 5%alpha)' temperature. For the reference steel, the interval between the temperature associated with maximum elongation and M-d(gamma -> 5%alpha)' temperature was the smallest. For the other two steels, in particular the one containing both Co and Al, the preceding interval was noticeable. This observation is likely related to the non-uniform spatial distribution of alloying elements and local variations in deformation mechanisms.