Creep deformation mechanisms in high-pressure die-cast magnesium-aluminum-base alloys
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TRODUCTION
IN the literature on Mg alloys, creep at constant stress is usually kept separate from inelastic deformation at constant rate. The present contribution emphasizes the connections between the two and proposes a unique description in terms of microstructural mechanisms. A creep test may start at any point of a stress-strain curve by switching from conditions of constant mechanical strain rate ˙ mech to constant stress . Usually, the starting point lies at small inelastic strains inel and relatively low flow stress, even below the yield stress for deformation at ˙ mech 104 s1. Under these conditions, the elastic strain rate ˙ el is still a significant fraction of the mechanical strain rate ˙ mech ˙ el ˙ inel. As ˙ el ˙ /Eeff (Eeff: effective elastic modulus of the specimen-machine system) is switched off by keeping the stress constant, the mechanical strain rate ˙ mech drops to ˙ inel and creep begins. Generally, the creep rate falls in the first part of the test called primary creep due to continued work hardening. The relatively low creep rate leads to slow progression of inelastic deformation. With the test results being traditionally displayed as strain ˙ mech vs time t, the minimum creep rate is difficult to measure. Near its minimum, the creep rate is constant for long time in a small strain interval. While there is no problem measuring the absolute minimum of ˙ inel at the end of a test, the relative minimum of creep rate may not be detected within the W. BLUM, Professor, and Y.J. LI and X.H. ZENG, Graduate Students, are with the Inst. f. Werkstoffwissenschaften. LS1, 91058 Erlangen, Germany. Contact e-mail: [email protected]. P. ZHANG, Dr.-Ing., formerly with the Inst. f. Werkstoffwiss. LS1, is now with BTU Cottbus. B. VON GROßMANN, Professor, formerly at Audi AG, 85045, Ingolstadt, Germany, is now with Georg-Simon-Ohm, Fachhochschule, 90486 Nürnberg, Germany. C. HABERLING, Dipl.-Ing., is with Audi AG. This article is based on a presentation made in the symposium entitled “Phase Transformations and Deformation in Magnesium Alloys,” which occurred during the Spring TMS meeting, March 14–17, 2004, in Charlotte, NC, under the auspices of the ASM-MSCTS Phase Transformations Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
time of testing because strain is too small.[1,2] If the absolute minimum is misinterpreted as relative minimum of creep rate, systematic errors result, as has been pointed out previously.[1,2,3] These systematic errors are easily avoided by considering the creep rate as a function of strain. In the following, the two old alloys AZ91 and AS21 are compared to the newly developed alloy AJ52[4] with high creep resistance. On the basis of results on evolution of microstructure (dislocations, twins, precipitates), a composite model of deformation is proposed, which allows one to simulate the creep behaviors of different alloys. II. EXPERIMENTAL The present work deals with the alloys AZ91hp (approximate composition in mass pct: 9Al, 1Zn), AS21hp (2Al
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