Mechanical behavior of zirconium and hafnium in tension and compression
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I. INTRODUCTION
ZIRCONIUM (Zr) and hafnium (Hf) are hexagonalclosed packed Group IVa metals, along with titanium (Ti). Both possess desirable engineering properties such as high strength, good ductility, and resistance to corrosion and irradiation. Recent structure/property research on the Group IVa elements has focused on Ti and Zr. In particular, the tensioncompression behavior of Zr has been studied in detail. This study is concerned with the tension-compression behavior of Hf and its comparison to Zr. In this article, differences in the tensile-compressive loading will be referred to as differences in stress sense. The mechanical behavior and microstructural evolution of Zr and Hf are strongly influenced by chemistry, texture, temperature, and strain rate.[1,2,3] Deformation at quasi-static strain rates in hcp metals is typically accommodated through a combination of slip and twinning.[1,4–6] The hcp metals deformed quasi-statically yield principally by slip and then work harden through a combination of slip and twinning.[6] Decreasing temperature and increasing strain rate have each been documented to increase yield stress and work-hardening rates as well as to increase the propensity of deformation twinning.[7] The hcp metals display rate- and temperaturedependent yielding due to Peierls stress contributions to defect motion. Increasing interstitial concentrations, particularly oxygen, lead to higher yield stresses but decreased volume fraction of twinned grains.[3,7,8] Texture has been found to display a significant effect on mechanical behavior of hcp metals.[1,2,3] Differences in the orientation of the c-axis with respect to the loading direction affect the yield stress, work-hardening behavior, and anisotropy of the macroscopic sample shape evolution during mechanical testing. The highest yield stresses and rates of work hardening, along with the lowest postmortem specimen cross-sectional LAURA B. ADDESSIO, Graduate Student, is with the Metallurgy and Materials Engineering Department, Colorado School of Mines, Golden, CO 80401. Contact e-mail: [email protected] ELLEN K. CERRETA and GEORGE T. GRAY, III, Technical Staff Members, are with the Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545. Manuscript submitted August 9, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
anisotropies, were found to correspond to specimens compressed along the c-axis.[3] In both Zr and Hf, relatively low yield stresses can be associated with easy activation of prism slip, which accommodates deformation most effectively for loading orientations that align within the plane of a rolled plate.[1,6,9] The most frequently observed slip systems in Zr and Hf are the prismatic slip system, ((1010), 1/3[1210]), and the two pyramidal slip systems, ((1101), 1/3 [1120] and (1122), 1/3 [112 3]).[1,3–6] It was found in Zr and Hf single-crystal studies that only prismatic slip was activated independent of the loading direction.[6,10,11] In textured polycrystalline Zr, Schmid factors have b
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