Creep and microstructure of magnesium-aluminum-calcium based alloys
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resistant magnesium die-casting alloy. Creep mechanisms for different temperature/stress regimes are proposed. A ternary intermetallic phase, (Mg,Al)2Ca, was identified in the microstructure of the ACX alloys and is proposed to be responsible for the improved creep resistance of the alloys.
I. INTRODUCTION MAGNESIUM, one of the most promising lightweight materials, has made significant inroads in automotive interior and other room- or near-room-temperature applications in the last decade. The most visible magnesium applications in North America have been instrument panel beams, transfer cases, valve/cam covers, steering components, and various housings and brackets.[1] The poor creep resistance and high costs of commercial magnesium alloys have prevented magnesium applications in major powertrain components, such as engine blocks and automatic transmission cases, where the operating temperatures can be as high as 250 ⬚C and 175 ⬚C, respectively. Recent efforts to develop creep-resistant alloys for such applications have resulted in a number of experimental alloys. Most of these alloys are based on the Mg-Al-Ca system, which has been recently reviewed by Luo.[1] Recently, a new series of Mg-Al-Ca alloys containing small additions of strontium and/or silicon (designated as ACX alloys where A stands for aluminum; C for calcium; and X for strontium or silicon) was developed at General Motors Research and Development Center (Warren, MI).[2,3] The new ACX alloys offer excellent creep resistance (under both tensile and compressive stresses) and low cost that meet the materials requirements for automotive powertrain applications. This article presents creep properties and microstructural characterization of the ACX alloys. An indepth analysis was also conducted to provide a further understanding of the microstructure and the creep mechanisms of the new alloys. II. CREEP IN MAGNESIUM ALLOYS There are generally three stages for creep deformation in metals and alloys, i.e., primary creep, secondary creep, and ALAN A. LUO, Staff Research Engineer, and BOB R. POWELL, Staff Research Scientist, Materials and Processes Lab., and MICHAEL P. BALOGH, Staff Research Scientist, Chemical and Environmental Sciences Lab., are with General Motors Research and Development Center, Warren, MI 48090-9055. Manuscript submitted March 30, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
tertiary creep.[4] It is generally, agreed[5–14] that the steadystate secondary creep rate, ˙ , of magnesium and Mg-Al– based alloys is described by a power-law equation in the stress ( ) and temperature (T ) ranges of interest to automotive applications ( ⫽ 20 to 100 MPa and T ⫽ 100 ⬚C to 250 ⬚C): ˙ ⫽ A n exp (⫺Q/RT )
[1]
where A is a material-related constant, R is the gas constant, Q is the apparent activation energy for creep, and n is known as the stress exponent. Based on Eq. [1], the slope of the log ˙ vs log plot at a given temperature is the stress exponent, n, and an Arrhenius plot (ln ˙ vs 1/T ) at a specific stress level will yield the ap
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