Microstructure characterisation of a low density steel by transmission electron microscopic study

The microstructure including precipitates in a low density steel containing 3.3Mn-6.6Al-0.18C (wt%) was investigated using transmission electron microscopy under two heat treatment conditions of annealing (850 °C, 1 min.) and annealing+ageing (400 °C, 10 min.). The phases identified by electron diffraction patterns were ferrite (α), austenite (γ), κ-carbides and M3C type compounds. Ferrite was the matrix of the microstructures. The κ-carbides were found to contain on average 9.9 wt% Mn and have the following orientation relationship with ferrite: (2 2 0)α∥(2 0 0)κ with [1 1 2]α∥[0 1 1]κ and (1 1 0)α∥(1 1 1)κ with [1 1 2]α∥[0 1 1]κ. Coarsening of κ-carbides was observed in the annealed+aged condition as compared to the annealed condition. The solubility of Al and Mn in bcc ferrite and fcc austenite were determined. The κ-carbides consisted of a wide compositional range of Mn and Al.