Characterization of microstructural evolution in austenitic Fe-Mn-Al-C lightweight steels with Cr content

Abstract

The influence of Cr addition on the microstructures and tensile properties of Fe-20Mn-12Al-1.5C lightweight steels was investigated. Microstructural phases were thoroughly identified through scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), and x-ray diffraction (XRD). Tensile behaviors were characterized using a tensile test and observation around fractured surface. Fe-20Mn-12Al-1.5C lightweight steel without Cr consisted of austenite with fine intragranular κ-carbides, coarse intergranular κ-carbides, and a small amount of ferrite. As Cr content increased to 5 wt%, coarse κ-carbides around grain boundaries disappeared and the fraction of ferrite slightly decreased, while the fraction of austenite slightly increased, leading to a homogenized microstructure consisting of mostly austenite with fine intragranular κ-carbides and a very small amount of ferrite and ordered phase DO3. This results from the addition of Cr, which is a carbide-forming element, which suppresses the formation of κ-carbides and consequently, austenite retains more stability due to an increase in the amount of carbon inside austenite. When Cr content exceeded 5 wt%, the fraction of DO3 increased drastically, the fraction of austenite reduced sharply, and Cr-rich M7C3 carbides precipitated. This is attributed to the role of Cr in steels, which is a carbide former as well as a ferrite stabilizer. As for tensile properties, tensile strength declined, and elongation improved with increasing Cr, up to 5 wt%. The decrease in tensile strength originates from the reduced κ-carbide fraction and growth in grain size, and the improvement in elongation is due to the reduction in coarse κ-carbides which act as crack initiation sites. As Cr content exceeded 5 wt%, the tensile strength increased, but the elongation decreased dramatically, owing to the precipitation of M7C3 carbides which are vulnerable to cracking. The steel containing 5 wt% of Cr showed the best tensile properties due to microstructural homogeneity, namely, a simple microstructure containing austenite with fine κ-carbides inside austenite and the low fraction of ordered phase DO3.