Duplex-precipitation hardening in Al-Zn-Mg alloys highly super-saturated with Zr
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where A, B, and C can be considered constant for small amounts of crack growth. In the spirit of Ritchie, et a P the initial value of the tearing modulus would be determined by the attainment of a critical strain at a fixed distance ahead of the growing crack. It is therefore suggested that the analysis of Ritchie, et a l 3 can be extended to small amounts of crack growth provided the proper strain singularity is used and that the same metallurgical factors that control K~c also control T (c.f. Eq. [21 above with Eq. [10] of Ref, 3). I a m grateful to the Army Research Office for support of this research under Project P15689-MS and to I.-H. Lin for helpful discussions. 1. P. C. Paris, H. Tada, A. Zahoor, and H. Ernst: ASTM STP 666, 1979, pp. 5-36. 2. M. F. Kanninen: private communication, Battelle-Columbus, 1980. 3. R. O. Ritchie, W. L. Server, and R. A. Wullaert: Met. Trans. A, 1979, vol. 10A, pp. 1557-70. 4. I. H. Lin: private communication, Ohio State University, 1980. 5. C. W. Marschall: private communication, Battelle-Columbus 1980. 6. J. R. Rice: Mechantcs and Mechanisms of Crack Growth, pp. 14-39, British Steel Corporation, 1974. 7. L. Hermann and J. R. Rice: Met. Sct. (in press). 8. J. R. Rice, W. J. Drugan, and T. L. Sham: ASTM STP 700, pp. 189-221, 1980.
Duplex-Precipitation Hardening in Al-Zn-Mg Alloys Highly SuperSaturated with Zr T. O H A S H I A N D R. I C H I K A W A It is well-known that the solid solubility of Zr in AI can be extended extremely over the equilibrium value by rapid solidification and reaches up to about 1 wt pct at a cooling rate of the order of 103 ~ which is ordinarily obtained by using a water-cooled copper m o l d ) A characteristic precipitation hardening in the T. OHASHI is Assistant Professor, and R. ICHIKAWA is Professor, the Department of Metallurgical Engineering, Nagoya Institute of Technology, Nagoya, Japan. Manuscript submitted June 24, 1980. 546--VOLUME 12A, MARCH 1981
rapidly solidified AI- 1 wt pct Zr alloys is also shown by artificial aging at 350--450 ~ 1,2 and the optimum aging temperature is apparently higher than that applied to commercial age hardenable aluminum alloys. On the other hand, an A1-Zn-Mg alloy, which is the base of 7000 commercial aluminum alloys and has a definite age hardening property, has a solutionizing temperature appropriate to the precipitation of Zr. In the present investigation, an attempt has been made to utilize such a difference in the aging temperatures between those two alloys: i.e., it is to add newly the precipitation hardening by Zr to A1-Zn-Mg alloys. This may be accomplished by producing an A1-ZnMg-Zr alloy which is highly supersaturated with Zr by rapid solidification. These quarternary alloys allow precipitation hardening in a duplex m a n n e r during the solutionizing and artificial aging treatments because of the slow precipitation rate c" Zr comparing with those of Zn and Mg. The one is due to the precipitation of the metastable phases of A13Zr3 in solutionizing and the other G P zones a n d / o r ~/' precipi
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