Some remarks on the hardness and yield strength of aluminum alloy 7075 as a function of retrogression time
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Alloy Mg-10A1 (RSP)
Mg-12.5AI-I.5Si (RSP) Mg-9AI (I/M) (Reference 6) Mg-9AI-IZn (l/M) (Reference 7) Mg-8A1-0.5Zn (l/M) (Reference 12)
Solution Treatment 420 ~ 420 ~ 420 ~ 420 ~ 420 ~ 420 ~ 420 ~ 420 ~ 450 ~ 450 ~ 420 ~ 420 ~ 400 ~
Aging Treatment -100 ~ 2 pct stretch + 100 ~ 140 ~ 2 pct stretch + 140 ~ 180 ~ -180 ~ -175 ~ 140 ~ 180 ~ T5 temper
Yield Stress (MPa [ksi]) 233 [33.8] 284 [41.2] 353 [51.2] 331 [48.0] 351 [50.9] 318 [46.1] 287 [41.6] 362 [52.5] 165 [24] 228 [33] 303 [44] 262 [38] 275 [40]
Ultimate Tensile Stress (MPa [ksi]) 365 [52.9] 387 [56.2] 427 [62.0] 420 [61.0] 430 [62.3] 416 [60.3] 355 [51.4] 393 [57.0]
414 [60] 379 [55] 380 [55]
Elongation (Pct) 11.8 5.4 4.7 4.2 3.8 4.5 2.0 1.4 --7 5 7
yield stress of RSPMg-10.9A1 is 194 MPa cm 3 g-~, or 8 pet larger than that of aluminum alloy 7075-T6. RSP Mg-A1 alloys are thus attractive substitutes for high-strength aluminum alloys, provided that their corrosion resistances can be improved. Although the alloys in the present investigation were not formulated for corrosion resistance, Das and Chang 3 have reported an 85 pct reduction in corrosion rate in 3.5 pct NaC1 solution for an RSP Mg-A1-Zn-Si-Mn alloy compared with that of I/M ZK60A-Mg. It is concluded that RSP of Mg high-A1 alloys improves their strengths by a combination of grain refinement and increased density of (continuous or discontinuous) Mg17A112 precipitates relative to those in I/M Mg-A1 alloys. Grain refinement resulting from RSP promotes discontinuous precipitation at the expense of continuous precipitation, but insertion of a 2 pct stretch prior to aging increases the volume fraction of continuous precipitates and hence the yield stress, particularly at low aging temperatures. RSP Mg-A1 alloys have density-normalized yield stresses comparable to those of high-strength aluminum-base alloys.
6. J.B. Clark: Acta Metall., 1968, vol. 16, pp. 141-52. 7. B. Lagowski: Trans. American Foundryman's Soc., 1971, vol. 79, pp. 115-20. 8. A.E Crawley and B. Lagowski: Metall. Trans., 1974, vol. 5, pp. 949-50. 9. A.M. Talbot and J. 3". Norton: Trans. Am. Inst. Min. Eng., 1936, vol. 122, pp. 301-14. 10. E A. Fox and E. Lardner: J. Inst. Met., 1943, vol. 69, pp. 373-96. 11. T.E. Leontis and C.E. Nelson: Trans. Am. Inst. Min. Eng., 1951, vol. 191, pp. 120-24. 12. Metals Handbook, 9th ed., ASM, Metals Park, OH, 1979, vol. 2.
This research was performed under the McDonnell Douglas Corporation Independent Research and Development program. H. Hoke, a summer intern at McDonnell Douglas Research Laboratories, was responsible for alloy processing and operation of the roller-quenching apparatus.
E HABIBY, A. UL HAQ, F.H. HASHMI, and A. Q. KHAN
REFERENCES 1. S. Isserow and E J. Rizzitano: Int. J. Powd. Met. Powd. Tech., 1974, vol. 10, pp. 217-27. 2. P.J. Meschter and J.E. O'Neal: Metall. Trans. A, 1984, vol. 15A, pp. 237-40. 3. S. K. Das and C. E Chang: Rapidly Solidified Crystalline Alloys, S. K. Das, B.H. Kear, and C.M. Adam, eds., TMS-AIME, Warrendale, PA, 1985, pp. 137-56. 4. P.J. Meschter, R. J. Lederich
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