Thermally induced porosity in Ti-6Al-4V prealloyed powder compacts
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CEMENTITE AUSTENITE
Fig. 6--Austenite formation during low temperature intercritical annealing of ferrite-pearlite microstructure. (a) Ferrite-pearlite starting structure, (b) spheroidization of cementite and coarsening of cementite particles located on a / a boundaries. (c) nucleation of austenite at cementite particles located on a / a boundaries, and (d) nucleation of austenite on cementite particles within the spheroidized pearlite colony and growth of austenite on a/ot boundaries.
Figure 6 summarizes schematically the stages of austenite formation during low temperature intercritical annealing of specimens with normalized ferrite-pearlite microstructures. A key intermediate stage of the process is the spheroidization of the cementite within the pearlite colonies. Austenite forms on and around the spheroidized carbide particles, and the initial growth of the austenite is accomplished by dissolution of the carbide particles and the diffusion of carbon from the particles through the austenite to the austeniteferrite interfaces. This process directly determines the distribution of austenite as grain boundary allotriomorphs and intragranular idomorphs in specimens intercritically annealed at low temperatures. Higher intercritical annealing temperatures form austenite which occupies larger volume fractions of the microstructure than the volume fraction of the original pearlite colonies. 4'5 Thus, low temperature (just above Act) intercritical annealing, where small volume fractions of austenite are formed and where spheroidization results in a fine distribution of the austenite, may provide a viable approach to producing the fine dispersions of martensite which produce good combinations of strength and ductility in dual-phase steels, s'9
This work was supported by the Metallurgy Program, Division of Materials Research, National Science Foundation. The Inland Steel Company provided the steel. REFERENCES 1. G. R. Speich, V. A. Demarest, and R. L. Miller: Metall. Trans. A, 1981, vol. 12A, pp. 1419-28. 2. C.I. Garcia and A.J. DeArdo: Metall. Trans. A, 1981, vol. 12A, pp. 521-30. 3. P.A. Wycliffe, G.R. Purdy, and J.D. Embury: in Fundamentals of Dual-Phase Steels, R.A. Kot and B.L. Bramfitt, eds., TMS-AIME, Warrendale, PA, 1981, pp. 59-83. 4. R.D. Lawson, D.K. Matlock, and G. 'Krauss: Metallography, 1980, vol. 13, pp. 71-87. 5. D.K. Matlock, E Zia-Ebrahimi, and G. Krauss: in Deformation, Processing, and Structure, G. Krauss, ed., ASM Metals Park, OH, 1984, pp. 47-87. 6. D.Z. Yang, E.L. Brown, D.K. Matlock, and G. Krauss: Metall. Trans. A, 1985, vol. 16A, pp. 1385-92. 7. L. Zwell, L.A. Gorman, and S. Weissmann: Trans. ASM, 1966, vol. 59, pp. 491-504. 1526--VOLUME 16A, AUGUST I985
8. E Zia-Ebrahimi: unpublished research, Colorado School of Mines, Golden, CO, June 1983. 9. N.K. Balliger and T. Gladman: Metal Science, 1981, vol. 15, pp. 95-108.
Thermally Induced Porosity in Ti-6AI-4V Prealloyed Powder Compacts D. EYLON, S. W. SCHWENKER, and E H. FROES Hot isostatic pressing (HIP) of preall
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