Cubic alni compound dispersed mg-based amorphous matrix composites prepared by rapid solidification
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Figure 1 shows X-ray diffraction patterns of meltquenched Mg95-xAlsNix (patterns a, b, c, and d) and crystalline phases obtained by annealing at 673 K of melt-quenched MgToA15Ni25(pattern e). In binary Mg-Ni system, the amorphous phase was found to be formed in the composition close to Mgs0Ni20. At X = 10, the asquenched structure is amorphous with a small amount of hcp Mg. At X = 15, 20, and 25, the hcp Mg disappeared, a c-A1Ni appeared, and the peaks of c-A1Ni became more intensive with increasing Ni content, as shown in Figures l(b) through (d). Although the amorphous MgToA15Niz5alloy crystallized to a mixed structure of Mg and Mg2Ni, the diffraction peaks from c-AINi were still observed. This indicates the possibility that the A1Ni pre-existed in the melt before rapid solidification. The lattice parameter of the c-A1Ni was measured to be a0 = 0.292 - 0.294 nm for all the as-quenched and annealed samples. Compared with ao = 0.2887 nm of pure AiNi compound, tl~ the cubic A1Ni in Mg-AI-Ni alloys has an a0 value much larger than that of the pure AINi. The difference is interpreted to result from the dissolution of the Mg atom with the largest atomic size into the c-AiNi phase. Figure 2 reveals the scanning electron micrographs of melt-quenched (a) MgaoA15Nil5, (b) MgTsAlsNi20, and (c) MgToAlsNi25. Bright particles of the c-A1Ni with an average size of about 5 /zm are homogeneously dispersed in the amorphous matrix. The density of particles with a square shape, sharp edges, and corners increased with increasing Ni contents. The volume fraction (VI) of c-A1Ni particles is estimated from SEM micrographs at - 2 . 0 vol pct for MgsoAlsNils, - 8 . 5 vol pet for Mg75AlsNi2o, and - 1 5 . 0 vol pct for MgToA15Ni25. Compositions of the amorphous matrix and dispersed particles have been examined by EDX analysis and are listed in Table I. There are two features which could be highlighted from the results of EDX analysis. First, the compositions of the dispersed c-A1Ni particles are similar for three alloys, i.e., independent of alloy composition. Second, no appreciable A1 (A1 < 0.5 at. pet) remains in the amorphous matrix. This means that all the A1 atoms in the Mg-AI-Ni alloys were compensated to form the c-A1Ni. The formation of c-A1Ni in the Mg-based amorphous matrix is surprising but logical, because the heat of mixing, AHx, in melt is - 4 9 kJ/mol (at 1773 K) for AI-Ni, - 1 4 kJ/mol (973 - 1423 K) for Mg-Ni, and - 0 . 4 kJ/mol for Mg-A1. tll,12] At sufficiently high temperatures, all three elements are homogeneously distributed in the melt, but the A1Ni will crystallize first during cooling. This can be inferred from extreme differences in melting temperature, Tin, of the compound among three binary systems. The values of Tm are 1911, 1033, and 723 K, respectively, for AINi, Mg2Ni, and Mg3A12. The micro-Vickers hardness variation with Ni content for melt-quenched Mg95_~AlsNix is shown in Figure 3. Hardness enhances with increasing Ni owing to the increase in AINi particle density. The tensile fracture strength (o?) and fr
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