MICROSTRUCTURAL CHANGE IN AUSTENITIC 30.0WT-PERCENT-MN-7.8WT-PERCENT-AL-1.3WT-PERCENT-C INITIATED BY SPINODAL DECOMPOSITION AND ITS INFLUENCE ON MECHANICAL-PROPERTIES

The microstructural change of supersaturated austenitic Fe-30.0wt%Mn-7 .8wt%Al-1.3wt%C alloy on ageing at 823 K has been investigated by tran smission electron microscopy (TEM) and X-ray diffraction. Efforts to c orrelate the modulation wavelength behavior with mechanical proper tie s in the process of spinodal decomposition leading to the cubic kappa' -carbide (Fe,Mn)(3)AlCx formation have been made. In parallel with the modulation wavelength coarsening behavior, both the microhardness and the tensile strength increase in two stages during ageing. The first stage strengthening coincides wi th the slow growth spinodal decomposi tion, while the second occurs at the Lifshitz-Slyozov-Wagner (LSW) gro wth stage of ordered kappa' particles. By the optimal ageing treatment of the alloy, a high yield strength up to 1080 MPa with an excellent 31.5% elongation can be attained. After further ageing, the alloy soft ened rapidly, and its elongation deteriorated drastically due to the f ormation of the grain boundary lamellar colonies composed of discontin uously coarsened kappa carbide and transformed alpha ferrite phases. ( C) 1997 Acta Metallurgica Inc.