DEFORMATION INDUCED TRANSFORMATIONS AND GRAIN BOUNDARY THICKNESS IN NANOCRYSTALLINE B2 FeAl

摘要

Precise measurement of fundamental Bragg peak shifts during milling of nanocrystalline ordered B2 Fe_60Al_40 has allowed for the first time, the deconvolution of the 110 fundamental Bragg peak intensities of the b.c.c. disordered regions and of the ordered B2 regions from the start and before full disappearance of the latter. The evolution of the lattice parameter α_0 of the b.c.c. solid solution with milling time shows two characteristics. First a jump to higher values from the initial α_0 of the B2 phase, with a Δα_0 change of the order of 1 corresponding to a volume per atom Δ V_expansion of about 3. Subsequently, a_0 continues to increase slowly with further milling at constant grain size D and in the absence of any B2 phase. This continuing change of α_0 with further milling up to at least 180 min is attributable to a reduction of chemi- cal short-range order (CSRO) or the number of Al-Fe heteroatomic “bonds”. The appearance of two well- defined maxima in the hyperfine field (HF) distributions derived from the Mossbauer spectra indicated the presence of two ferromagnetic environments contributing to the broadened Mossbauer resonance sextet sig- nal. The evolution of the second component of this Mossbauer signal scales with the grain size. Using the mean grain size D derived from X-ray peak profiles and TEM pictures together with the grain boundary thickness d_gb of 1.25 nm determined by Fultz et al. (J. appl. Phys., 1996, 7, 127) for b.c.c. Fe-based alloys, the fraction of grain boundary atoms n_gb_total was estimated and found to be consistent with the fra