Fatigue Crack Growth Mechanisms in High-Pressure Die-Cast Magnesium Alloys
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WEIGHT reduction is considered a primary design metric for increasing fuel efficiency and reducing automotive and aircraft component costs.[1] High-strength and lightweight aluminum alloys, exemplified by the 6000-series grades, have been used successfully as structural automotive components. However, additional significant weight and process cost reductions could be achieved if magnesium alloys become the foremost replacement for heavier materials.[2] Contemporary Mg alloys possess the highest strengthto-weight ratio among all structural metals, with a density of 1.74 Mg/m3. One ongoing major barrier to the industrialization of Mg alloys in automotive applications is their relatively low resistance to fatigue and corrosion service conditions.[3–6] These low fatigue and corrosion resistances of Mg alloys are mainly due to the material’s physical properties. The Mg alloys are remarkably reactive and exceptionally susceptible to forming microstructural singularities during shaping processes.[3] To overcome the detrimental effects of these microstructural singularities, current material selection, manufacturing, and design optimization methods resort to overdesigning. The resulting increased safety factors counteract the original attractive attributes of Mg alloys that have a high stiffness-to-weight ratio. To fully achieve the broad benefit of these magnesium alloys, the mechanisms of fatigue and corrosion must be rigorously linked to microstructural properties. Corrosion resistance has been the primary concern in the recent research work topics.[4–6] However, too little effort has been deployed in identifying microstructural HAITHAM EL KADIRI, Assistant Research Professor, M.F. HORSTEMEYER, Chair Professor in Computational Solid Mechanics, J.B. JORDON, Postdoctoral Candidate, and YIBIN XUE, Assistant Research Professor, are with the Center for Advanced Vehicular Systems, Mississippi State University, Starkville, MS 39762-9627, USA. Contact e-mail: [email protected] Manuscript submitted February 3, 2007. Article published online December 4, 2007 190—VOLUME 39A, JANUARY 2008
explanations for fatigue crack growth mechanisms. Moreover, the results reported by various authors have sometimes been controversial and only treated Mgbased alloys separately.[7–17] A study of the fatigue crack growth mechanisms of the AM50 Mg alloy was recently reported by the present authors.[17] In this article, we quantify the microstructural defects to fatigue crack incubation, microstructurally small cracks (MSC), physically small cracks (PSC), and long cracks (LC) for four commercially dominant highpressure die-cast (HPDC) magnesium alloys: AM50, AM60, AZ91, and AE44. A full description of the incubation, MSC, PSC, and LC regimes was reported in Reference 18. We performed scanning electron microscope (SEM) fractographic analyses on specimens tested in a fully-reversed strain control condition at a single strain amplitude. The microstructural defects relevant to fatigue crack growth are identified and analyzed using backscatter ele
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