Phase transformations of mechanically alloyed Fe-Cr-P-C powders

Fe77Cr4P8C11 alloy was prepared by mechanical alloying (MA) of elemental Fe, Cr, P and C (graphite) powders in a planetary ball mill type Fritsch P7 under argon atmosphere. Morphological changes, microstructural and structural evolutions during ball milling were followed by scanning electron microscopy (SEM), X-ray diffraction (XRD) and 57Fe Mössbauer spectrometry (MS) as a function of the milling time. The crystallite size refinement against the milling time is accompanied by an increase of the atomic level strain. After 6 h of milling, the dissolution of phosphorous into the α-Fe matrix is evidenced by the formation of a small amount (∼4%) of the paramagnetic Fe2P phase as revealed by Mössbauer spectrometry. The complete mixing of all the elemental powders at the atomic level is achieved at 12 h of milling and results, after 24 h, in an amorphous matrix where nanocrystalline phosphides and carbides with nearly equal crystallite sizes are embedded. Further milling time up to 190 h gives rise to the formation of both the orthorhombic and the hexagonal (FeCr)7C3 carbide as well as the superparamagnetic ɛ′-Fe2.2C carbide through the recrystallisation of the amorphous phase.