On the coarsening of the modulated structure during aging of austenitic FeMnAlC alloys prepared by the rapid solidification process

The coarsening behaviour of the {100} modulated structure in austenitic Fe31Mn9AlxC alloys (x = 1.23–1.65 wt.%), prepared by the melt-spinning process, was investigated at 823–923 K by X-ray diffraction and transmission electron microscopy. In the initial stage of isothermal aging, the wavelength of the modulated structure increases insignificantly, but at later stages it increases rapidly. Finally, the modulated structure changes into a discrete two-phase structure, losing its spatial periodicity. In the later regime of the modulated structure, the change in the wavelength (λ) can simply be described by a power law, given by λn ≈ kt. However, the exponent n is observed to deviate from that expected from the capillarity-driven coarsening theory (Lifshitz-Slyozov-Wagner theory): it varies in the range of 3.3–5.5, depending primarily on the carbon content. The apparent activation energy for this coarsening process, as determined using an Arrhenius plot, is found to be about 307 kM mol−1. This is considerably higher than the result obtained in a previous study, and strongly indicates that coarsening involves the diffusion of substitutional elements which possess lower atomic mobility than interstitial carbon. Therefore decomposition in these experimental alloys, leading to the formation of the modulated structure, will accompany the redistribution of substitutional elements as well as interstitial carbon atoms.