Abstract Austenitic, ferritic, and duplex Fe–17Cr–6Mn–0.45C–xNi–yAl (3Ni alloy: x=3, y=4 and 9Ni series: x=9, y=0,4,7) cast stainless steels were obtained by adjusting the Al and Ni concentrations. The Al-free steel containing 9wt% Ni exhibited an austenitic microstructure. The addition of 4wt% Al to the 9Ni series of alloys retained the microstructure austenitic. Increasing the Al content to 7wt%, however, led to the development of a fully ferritic matrix with a high hardness and poor ductility. The alloy containing 4wt% Al but a reduced Ni content of 3wt% exhibited a duplex microstructure consisting of nearly 20% ferrite in an austenitic matrix. Dilatometry indicated unusually high thermal expansion coefficients for the 7wt% Al ferritic steel at temperatures above the Curie temperature of the alloy. This anomaly was interpreted in terms of the formation of high concentrations of thermal vacancies possibly due to the presence of intermetallic compounds. The tensile tests conducted at room temperature indicated a concurrent enhancement of tensile strength and total elongation of the Al-free austenitic alloy by the addition of 4wt% Al. This advantage is magnified by the lower density of the Al-alloyed variant. The extended elongation and the high strength were achieved in spite of the decreasing work hardening rate of the Al-alloyed steel variant at high strains, interpreted as a gradual transition from planar to wavy glide mode of dislocations. The high glide planarity at low strains of the Al-alloyed austenitic alloy was confirmed by the TEM observation of dislocation pile-up arrangements. Substantial strengthening compared to austenitic steels was achieved in the duplex steel. This strengthening was associated with a marginal loss of ductility as 20% ferrite was introduced in the microstructure.