Mechanical Disordering of Cubic Intermetallics By Unconventional Methods of Cold-Work and Their Reordering Upon Annealin
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at.%Al iron aluminide (B2 Fe-45A1) were induction melted from pure elements as described elsewhere [1,4,5]. Some other FeAl iron aluminides with different Al concentrations and minor alloying elements such as Zr, Cr and B, employed for shock-wave loading, were fabricated as described in Ref.[2]. The details of the ball-milling and shock-wave loading operations were exhaustively reported in Refs.[1,2,5]. The relative long-range order (LRO) parameter S, was estimated from X-ray diffraction scans by a comparison of the relative intensities of superlattice (s) - fundamental (f) pairs of peaks: (100,) and (200f) and (1 10/220f) for L12 titanium trialuminide, and ( 1 0 0s/ 2 0 0 f) for B2 iron aluminide, according to the equation S = [(I/If)di/(I/If)hr]!/2 [61 (error±0.02) where dis- is the disordered and hr- is the homogenized reference material, respectively. Lattice parameter, a, was calculated against an extrapolation function 1/2(cos2 0/sinO+cos 0/6) [7](error±3.13310nm for step size KK5.31.1 Mat. Res. Soc. Symp. Proc. Vol. 552 © 1999 Materials Research Society
0.020). The software TRACES (Diffraction Technology Ltd., Canberra, Australia) was used for X-ray diffraction calculations and all the pertinent details can be found in Refs. [1,5,8].
RESULTS AND DISCUSSION Ball-Milled Powders Figure 1 shows the relative long-range order (LRO) parameter S and Fig.2 the lattice constant of milled powders as a function of milling time. The S parameter of cubic titanium trialuminide decreases rather slowly with milling time and is still measurable after 160 h of milling (Fig.la), i.e. material is only partially disordered. Superlattice peaks were no longer detectable at the XRD pattern of powder milled for 208 h. Thus, it seems that the disordering process upon ball milling of titanium trialuminide terminates somewhere between 160 and 208 h of milling. However, as discussed by Bakker et al.[6] a complete disappearance of superlattice peaks in a XRD pattern may not necessarily be an evidence of a complete disordering. For example, for the milled B2 CoGa powders the superlattice peaks disappeared but magnetization and lattice constant measurements showed that the intermetallic was still in the ordered B2 structure. It is interesting to note that the S parameter of L12 A13Ti(Mn) calculated from the (110s/ 2 2 0f) pair has always been systematically greater than that from the (100s/ 2 0 0 f) pair (Fig. la) also for annealed powders. This structural anisotropy of (1 10s/ 2 2 0f) vs. (100S/ 2 0 0 f) was taken as an evidence of a tetragonal arrangement for the third atomic species (e.g. Mn), even though the crystal retains the ordered L12 structure [9]. In contrast, the LRO parameter of FeAl decreases rapidly with milling time as manifested by a complete disappearance of superlattice peaks (assuming S-0) barely after 23 h of milling (Fig. l b). The mechanical disorder by milling of FeA1 occurs indeed rapidly. Clavaguera-Mora et al. [10] reported that the LRO parameter of the Fe-50A1 powders milled in a vibratory mill decreased monoto
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