Effects of high rates of loading on the deformation behavior and failure mechanisms of hexagonal close-packed metals and
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I. INTRODUCTION
THE increasing structural applications of hcp metals and alloys (particularly titanium and its alloys) are largely a result of the significantly improved specific strength of these materials. However, broader application of these materials has resulted in an increased likelihood that they will be used in applications involving high rates of loading. Examples include impact-resistant structures, concerns about the crashworthiness of aircraft, foreign-object damage in turbines, and terminal ballistics in military applications. In addition, the drive for lower-cost structures leads to the use of various high-speed manufacturing processes (such as high-speed machining), which involve high rates of loading. Further, even in applications that involve slow conventional loadings, one may observe dynamic failure after accumulated quasistatic deformations (consider fast crack growth after critical size is achieved). For all of these reasons, it has become important to achieve a fundamental understanding of the deformation and failure mechanisms that are associated with high rates of loading in hcp metals and alloys. At the fundamental science level, the rate dependence of the deformation mechanisms in these low-symmetry systems is of interest (particularly the influence of twinning at high rates of loading). There are 23 elemental metals with the hcp structure (at room temperature), including those in the lanthanide series of rare earths. For reasons associated largely with abundance and toxicity, our current knowledge of the mechanical properties at high rates of deformation is limited primarily to magnesium, titanium, zinc, zirconium, and hafnium. There K.T. RAMESH, Professor and Chair, is with the Whiting School of Engineering, Mechanical Engineering, Johns Hopkins University, Baltimore, MD 21218-2684. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
is a comparatively small amount of published work on the rate dependence of polycrystalline cadmium,[1] rhenium,[2] and beryllium (mostly with regard to shock,[3–6] but also at high rates[7–10]). This article presents an overview of current work on highrate loading and the corresponding deformation and failure mechanisms in magnesium, titanium, zinc, zirconium, and hafnium (and, to a lesser extent, their alloys). A relatively recent review of refractory hcp metals that covers some of these topics is presented by Subhash.[11] In this article, mechanical behaviors at high strain rates and the associated deformation mechanisms are treated first, followed by a discussion of the f
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