257. IV.R. MECHANICAL PROPERTIES AND DEFECTS IN REFRACTORY METALS M.S. WECHSLER Institute for Atomic Research and Department of Metallurgy, Iowa State University, AMES, Iowa (U.S.A.) 1. INTRODUCTION 1. INTRODUCTION 2. TEMPERATURE DEPENDENCE OFPLASTIC DEFORMATION 3. EFFECTS OF RADIATION ON TEMPERATURE DEPENDENCE OF THE YIELD STRESS 4. RADIATION-ANNEAL HARDENING Progress in the understanding of mechanical aspects of defects in the refractory metals has not been rapid, although recent years have seen considerable improvement. The mechanical properties of the refractory metals at low defect concentrations had not received great attention until recently, and the vast difference between the behaviour of fee metals and the refractory metals has been a matter of some perplexity. Furthermore, we are only beginning to obtain a grasp of the nature and influence of defects in the refractory metals, which include single and aggregated vacancies and interstitials, dislocation loops, and impurity atoms. It is not surprising, therefore, that there is controversy and speculation concerning mechanisms underlying the mechanical properties of the'refractory metals. Despite the immature state of knowledge of mechanical properties and defects in the refractory metals, the papers at this conference give testimony to the exciting new ideas and results that are emerging.^Three main themes concerning yielding appear to predominate: the origin of the temperature and strain rate dependence of yielding (particularly as it relates to alloy softening), the effect of irradiation on the temperature ! dependence of yielding, and the phenomenon of radiation-anneal hardening. In all three, the role of defects, ' especially impurities, is being actively investigated and Jebate~dl In_additi6n, hew research is described in^ 'the discussion-JU-Iu-ef this-session-deal-ing-with 1 'internal friction in a particular hop refractory metal, rhenium (-see-Eaper-IVV+J. 2. TEMPERATURE DEPENDENCE OF PLASTIC DEFORMATION One of the most striking and significant aspects of the mechanical properties of bec and, to a lesser extent, hep metals is the large' increase in yield stress with decreasing test temperature and increasing strain rate. This was known'to be the^case for polyerystalliae metals quite a few years ago, as was shown, for example, by BECHTOLD- in 1955 OQ • Fi E- IV.R.I. shows the increase in yield stress with decreasing test temperature for the polycrystalline bec metals tantalum, tungsten, molybdenum and iron, but not for the fee metal nickel. The same temperature-dependent behaviour is observed for single crystals of bec metals, as is seen in Fig. IV.R.2. for niobium. The spread in values for the critical shear stress is due to differences in orientation, strain rate, and purity \2-b\ • It has become standard to regard the tempera:ure dependence of the yield stress in bec metals to be composed of two components, as shown schematically in rig. IV.R.3. taken from the paper by TAKEOCHI [7J. The high temperature asymptote, a., is the ai.hermal o.v internal stress, and the part that rises rapidly with decreasing temperature, <**, is the thermal or effective stress. The rise in yield stress at low temperatures suggests that yielding is partly controlled by overcoming of barriers by thermal activation. The point at issue in much of the current literature is the nature of the barrier to slip dislocation motion that is overcome to a greater or lesser extent depending on the test temperature. , The prevalent view - perhaps one might sey "the es1 ablishment view" - is that the temperature.dependence of yielding is due to the intrincio atomic arrangement of the bec crystal lattice, which exerts a restraining force on. the slip dislocation as it presses forward in response to an applied stress. This force is the Peierls-Nabarro force (or Feierls stress) illustrated schematically in FIR. IV.R.4. The illustration employs + Work performed at Ames Laboratory of the OSAEC under contract with Iowa State University. I , .
MECHANICAL PROPERTIES AND DEFECTS IN REFRACTORY METALS
Jun 23, 2023
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