High-strain-rate nanoindentation behavior of fine-grained magnesium alloys
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Christopher A. Schuhb) Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (Received 10 November 2011; accepted 30 January 2012)
The effects of temperature and alloying elements on deformation in the high-strain-rate regime were investigated by testing fine-grained magnesium alloys with an average grain size of 2 ; 3 lm by a nanoindentation technique. The dynamic hardness measurements aligned well with existing quasistatic data, together spanning a wide range of strain rates, 103 ; 150/s. The high-rate hardness was influenced by various alloying elements (Al, Li, Y and Zn) to different degrees, consistent with expectations based on solid solution strengthening. Transmission electron microscopy observations of the indented region revealed no evidence for deformation twins for any alloying elements, despite the high strain-rate. The activation energy for deformation in the present alloys was found to be 85 ; 300 kJ/mol within the temperature range of 298 ; 373 K, corresponding to a dominant deformation mechanism of dislocation glide. I. INTRODUCTION
Magnesium alloys have high potential for application as structural materials, since they are the lightest, and have the highest natural abundance, among all the conventional structural alloys. The need for magnesium alloys with an improved suite of mechanical properties, including e.g., strength, toughness and fatigue resistance, is driven by reliability and safety requirements, and two strategies in alloy design are being widely considered to address these. Specifically, grain refinement1–4 and the addition of solute elements5–10 are both active topics in the magnesium research field. Although much research emphasizes the properties noted above, it is important to recognize that there are secondary properties that may also be affected by grain refinement and alloying additions, and these can also be critical for many applications. Rate dependence and high-rate mechanical properties fall into this category. In many loading situations of practical relevance for structural materials, the local strain-rates are quite high, especially for automotive and transportation applications involving high-rate impacts, or for localized regions of geometries nominally under low Address all correspondence to this author. a) e-mail: [email protected] b) e-mail: [email protected] This author was an editor of this journal during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http://www.mrs. org/jmr-editor-manuscripts/ DOI: 10.1557/jmr.2012.52 J. Mater. Res., Vol. 27, No. 9, May 14, 2012
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loading rates. For example, the strain-rate in the crack-tip region during fracture has been reported to be at least 100 times higher than the global rate.11 The mechanical properties and deformation behaviors of magnesium and magnesium alloys are unique compared with those of more conventional metallic
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