Plastic deformation of hafnium under uniaxial compression
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I.
INTRODUCTION
HAFNIUM (Hf) is a refractory metal with a hexagonal close-packed (hcp) crystal structure and an axial ratio (c/a) of 1.581, which is less than the ideal value of 1.633. It, along with titanium (Ti) and zirconium (Zr), forms the Group IVA elements of the Periodic Table. Hafnium has several useful structural and engineering properties such as high strength, good ductility, and excellent resistance to corrosion, irradiation, and mechanical damage. For example, it has been used in nuclear reactors as a control material due to its high capture cross-sectional area for thermal neutrons. Hafnium is also widely used as a solid solution strengthener in columbium (niobium) alloys, C-103 (Cb10Hf-lTi) and C-129Y (Cb-10W-10Hf-0.07Y), which have wide applications in jet engines and missile systems. Due to its high density and favorable thermomechanical properties, Hf has been identified as a potential matrix material in developing a new generation of tungsten heavy alloys for advanced defense applications.[1–4] Recently, there have been a number of investigations on the plastic response of hcp metals with emphasis on computer simulations of defect properties,[5–9] mechanisms of plastic flow,[10–22] and the importance of deformation twinning to plastic flow.[23–29] The majority of this literature has focused on Ti, Zr, and their alloys. In contrast, studies on less commonly used hcp metals, such as Hf, are more limited. Das and Mitchell[30] have examined the deformation mechanisms in single crystal Hf, while Goodwin and Trout[31] have examined the effect of oxygen impurity on the mechanical properties of polycrystalline Hf. Thus, in spite of its potential for use in many structural applications, little is known about the plastic behavior of Hf and its ability to undergo localized shear deformation. Accordingly, the objective of the present investigation was to study the plastic response of polycrystalline Hf over G. SUBHASH, Assistant Professor, Mechanical EngineeringEngineering Mechanics Department, and B.J. PLETKA, Professor, Metallurgical and Materials Engineering Department, are with Michigan Technological University, Houghton, MI 49931. G. RAVICHANDRAN, Associate Professor, is with the Graduate Aeronautical Laboratories, California Institute of Technology, Pasadena, CA 91125. Manuscript submitted August 6, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
a range of strain rates under uniaxial compression and to identify the micromechanisms of deformation. More specifically, the study was aimed at examining the contributions of slip and twinning to the observed overall plastic behavior. The temperature rise during the dynamic deformation of Hf was also investigated, and the effect of this temperature rise on the flow and fracture behavior is discussed. II.
EXPERIMENTAL PROCEDURE
Commercially available polycrystalline hafnium (grain size ; 1.0 mm) was obtained in two separate batches from Teledyne Wah Chang (Huntsville, AL) in the form of 50mm-diameter rods. Some physical and mechanical properties of Hf, w
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