Effect of cooling rate on hardness of FeAl and NiAl
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hardness values after annealing NiAI at 673 K were similar to those after annealing at 923 K so that, again, only the 673 K values are reported. Figure 1 shows the microhardness v s aluminum concentration for various cooling rates in FeA1. The microhardness clearly depends strongly on the heat treatment. Several points are worth noting. First, on annealing even the slowly cooled samples (50 K / h ) at 673 K, the hardness decreases further, indicating that even a slow cool cannot completely eliminate vacancies. This effect is most pronounced for Fe-50A1. Second, the air-cooled samples have similar hardness values to the water-quenched sampies. For example, the hardness of Fe-45A1 after either water-quenching or air-cooling is 80 pct greater than after annealing at 673 K. Third, in the iron-rich alloys, the difference between furnace-cooling and air-cooling increases with increasing aluminum content, in agreement with earlier work indicating that the concentration of vacancies increases with aluminum content. [2j Fourth, Fe-50AI can retain a large concentration of vacancies, even after slow-cooling at 50 K/h. This is consistent with the observation that vacancies, as measured by positron annihilation, are retained in the equiatomic alloy, even after slow cooling, tat Fifth, the microhardness data from the samples annealed at 673 K, where most retained thermal vacancies have been removed, show that increasing aluminum concentration produces very little increase in strength from 34 to 45 at. pct AI, contrary to previous results. [~2[ Figure 2 shows microhardness as a function of aluminum concentration for various cooling rates in NiA1. Although equiatomic NiAI can retain a large concentration of vacancies, [61 the difference in microhardness
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P. NAGPAL, Ph.D. Student, and I. BAKER, Associate Professor, are with the Thayer School of Engineering, Dartmouth College, Hanover, NH 03755. Manuscript submitted September 22, 1989.
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at.% AI Fig. 1- - G r a p h of microhardness v s at. pct AI for FeAI after various heat treatments. VOLUME 21A, AUGUST 1990--2281
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1. J. Rieu and C. Goux: Mere. Sci. Rev. Metall., 1969, vol. 65, pp. 869-80. 2. D. Pads and P. Lesbats: J. Nucl. Mater., 1978, vol. 69, pp. 628-32. 3. J.P. Rivi/:re, H. Zonon, and J. Grilh6: Phys. Status Solidi A, 1973, vol. 16, pp. 545-52. 4. D. Weber, M. Meurtin, D. Paris, A. Fourdeux, and P. Lesbats: J. Phys. C7, 1977, vol. 38, pp. 332-36. 5. A. Fourdeux and P. Lesbats: Phil. Mag., 1982, vol. 45, pp. 81-93. 6. A. Ball and R.E. Smallman: Acta Metall., 1968, vol. 16, pp. 233-41. 7. J.E. Eibner, H.J. Engell, H. Schultz, H. Jacobi, and G. Schlatte: Phil. Mag. A, 1975, vol. 31, pp. 739-42. 8
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