Magnetic monitoring of the disproportionation behavior in Sm-Fe-based materials

Two Sm-Fe-based alloys, with compositions Sm/sub 13.7/--Fe/sub 86.3/ and Sm/sub 13.8/--Fe/sub 82.2/--Ta/sub 4.0/, were investigated in order to determine their disproportionation reaction kinetics. The alloys were first heated in hydrogen at a rate of 4/spl deg/C/min to a temperature where the disproportionation reaction starts. At this onset temperature, which is different for the individual material, the alloy begins to decompose into a rare-earth hydride and /spl alpha/Fe. The disproportionation behavior of the systems can be explained in terms of the Johnson-Mehl-Avrami (JMA) theory of phase transformations. The ternary alloy with Ta satisfies the JMA conditions and has Avrami constants of /spl les/0.5, which implies diffusion-controlled one-dimensional growth, i.e., the thickening of particles with an appreciable initial volume. On the other hand, the growth of a newly formed iron phase in the binary alloy can only be explained in terms of mixed-transformation process involving one-dimensional and two-dimensional growth.