The effect of nano-sized κ-carbides on the mechanical properties of an Fe-Mn-Al-C alloy

The strengthening via precipitation of nano-sized kappa-carbides leads to exceptional strength-ductility balance in low-density steels. During aging, such nanocarbides form through spinodal decomposition by fluctuations in the aluminum and carbon content in the austenite, followed by short-range ordering. At lower aging temperatures and short aging times, the kappa-carbides are very fine, coherent with the matrix, and homogeneously distributed. When the aging temperature increases, heterogeneous nucleation initiates on the grain boundaries, and the kappa-carbides become coarse and lose coherency with the austenite matrix, leading to the deterioration of the mechanical properties. This work studies a fully austenitic hot rolled Fe-29Mn-8.7Al-0.9C alloy after different aging treatments. Two aging treatments were selected for a detailed study of the microstructure based on the exceptional strength-ductility balance demonstrated by these conditions. The samples aged at 550 degree celsius for 8 h exhibited an ultimate tensile strength of 1141 MPa and strain at failure around 49%. The second aging treatment selected was aging at 600 degree celsius for 1 h, and these samples exhibited an ultimate tensile strength of 1084 MPa, with strain at failure 62%. The size and morphology of the austenite grains and the annealing twins were studied through EBSD. Additionally, the size, morphology, and volume fraction of the nano-sized kappa-carbides were studied using TEM. Both aging conditions led to microstructures consisting of a matrix formed by equiaxed austenite grains with homogeneously distributed intragranular kappa-carbides. The kappa-carbides were coherent with the matrix and showed globular morphology with a diameter between 3 and 6 nm and coherent with the austenite matrix. The interaction between gliding dislocations and kappa-carbides was analyzed. It was shown that the mechanical behavior of the studied material is characterized by very high sensitivity to the size of kappa-carbides and, therefore, can be tailored by appropriate aging treatment.