Deformation induced transformations and grain boundary thickness in nanocrystalline B2 FeAl
Precise measurement of fundamental Bragg peak shifts during milling of nanocrystalline ordered B2 Fe{sub 60}Al{sub 40} has allowed for the first time, the deconvolution of the 110 fundamental Bragg peak intensities of the b.c.c. ordered regions and of the ordered B2 regions from the start and before full disappearance of the latter. The evolution of the lattice parameter a{sub 0} of the b.c.c. solid solution with milling time shows two characteristics. First a jump to higher values from the initial a{sub 0} of the B2 phase, with a {Delta}a{sub 0} change of the order of 1% corresponding to a volume per atom {Delta}V{sub expansion} of about 3%. Subsequently, a{sub 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 a{sub 0} with further milling up to at least 180 min is attributable to a reduction of chemical 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 Moessbauer spectra indicated the presence of two ferromagnetic environments contributing to the broadened Moessbauer resonance sextet signal. The evolution of the second component of this Moessbauer 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{sub gb} of 1.25 nm determined by Fultz et al. for b.c.c. Fe-based alloys, the fraction of grain boundary atoms n{sub gb}/n{sub total} was estimated and found to be consistent with the fraction of Fe atoms contributing to the lower HF component of the Moessbauer sextet signal. The grain boundary atom count using both methods confirms that grain boundaries in materials nanocrystallized by heavy deformation are nearly as dense as in the bulk.
- Research Organization:
- Inst. National Polytechnique de Grenoble, St.Martin d'Heres (FR)
- OSTI ID:
- 20005410
- Journal Information:
- Acta Materialia, Vol. 47, Issue 18; Other Information: PBD: 10 Dec 1999; ISSN 1359-6454
- Country of Publication:
- United States
- Language:
- English
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