Lightweight magnesium alloys have attracted increasing interest in recent years for potential applications in the automotive industry primarily due to their high strength-to-weight ratio. Development of lightweight materials with lower production costs that lead to higher fuel efficiencies and longer component lifetimes are key to a competitive advantage [1-5]. Alloy AZ91 (Mg-9Al-0.8Zn-0.2Mn) is the most favored magnesium alloy, being used in approximately 90% of all magnesium cast products [3]. The widespread use of magnesium alloys in automobiles has been inhibited, however, by inherent limitations of the material's physical properties [5], not the least of which is its very high reactivity. In fact, the tendency to succumb to galvanic corrosion and the low strength and poor creep resistance at temperatures in excess of 120°C limit the range of applications (e.g., components in automobile engines) for this alloy [3,4]. Therefore, understanding the properties of the structural and compositional phases and constituents is required for the development of new alloys that will possess improved physical properties. Previous studies of the many diverse properties of thin film Mg-Al alloys include the following: 1) The examination of the structure of vacancy clusters, their size and density [10]. 2) The investigation of their electronic properties from collective excitations which revealed behavior of both bulk-like and surface plasmons in these alloys [11]. 3) The study of this alloy's deformation behavior with in-situ nanoindentation in a transmission electron microscope [12]. 4) The behavior of hydrogen in these alloys after plasma hydriding under high-flux ion irradiation [13]. Up to 12 at.% Al can be dissolved in solid solution in binary Mg-Al alloys at about 430°C, while only about 1 at.% at 100°C [10]. This suggests that a large volume fraction of strengthening precipitates may be produced with appropriate thermal processing. Unfortunately, heat treatment in the temperature range 100 to 300°C produces relatively coarse precipitates of the equilibrium β (Mg_(17)Al_(12)) phase. The role of these intermetallic phases has been addressed in a number of papers [11-13]. The β phase has a body-centered cubic structure [14] and has been reported [15] to form as plates on (0001) Al. The orientation relationship between β and the matrix phase was determined based on selected area diffraction patterns [16]. An in-depth review of the characterization of the structure, morphology and orientation of fine-scale, strengthening precipitate phases in selected magnesium alloys using transmission electron microscopy and microdiffraction has been given [17].
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