Phase decomposition of rapidly solidified Fe-Mn-Al-C austenitic alloys

Abstract

The phase decomposition of two (Fe_0.65Mn_0.35)_0.83Al_0.17-xC (x=3 and 4 at. pct orx=0.74 and 0.98 wt pct) austenitic alloys prepared by rapid solidification (RS) has been investigated on aging at 823 and 923 K by means of X-ray diffraction, transmission electron microscopy, and scanning electron microscopy. Under low bulk carbon supersaturation conditions (823 K aging of low carbon alloy and 923 K aging of high carbon alloy), zones formed preferentially at the cellular boundaries and in the bands in the {100} planes, giving rise to line broadening in the X-ray diffraction patterns. On the other hand, the initial aging under high, carbon supersaturation condition (823 K aging of high carbon alloy) resulted in the sideband formation, resulting from homogeneous structural modulation in the <100>_γ directions throughout the grain. The bulk carbon supersaturation dependence of initial decomposition modes indicates that carbon atom fluctuations are crucial in the initial state of phase decomposition, and that the observed {100} modulated structure corresponds to a structure consisting of alternate carbon-rich and carbon-poor zones. Together with the interstitial clustering process, an fcc-based substitutional ordering reaction concurrently took place. Later on these zones were replaced by a coherent metastable phase in the matrix, which was finally transformed into the cubic carbide (κ carbide) of (Fe, Mn)₃AlC_x chemical formula with the L'1₂ structure. However, at the end, a combined heterogeneous β-Mn and κ carbide precipitation seemed to finalize the decomposition process over the matrix κ carbide precipitation.