Plastic deformation of materials under pressure
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Introduction Plastic deformation is generally considered primarily a distortion that occurs without volume changes. As such, it should only be sensitive to deviatoric stresses (deviation from hydrostatic) and not to pressure. Pioneering experiments performed following the developments in high-pressure techniques by Bridgman showed that hydrostatic pressure loading significantly affects the mechanical properties of solids. Both yield strength and ductility of metals1 and rocks2 increase with applied confining pressure. The pressures involved were initially relatively small (1–3 GPa), and the increase in ductility was mostly due to the inhibiting effect of pressure on propagation of preexisting microcracks. The development of diamond anvil cells has allowed the application of static pressures reaching and exceeding 100 GPa. Evaluation of shear stresses sustained by samples compressed between the diamond anvils has provided compelling evidence that the yield strength is pressure dependent when several tens of GPa are applied (see Figure 1).3–13 Using radial x-ray diffraction, the polycrystalline texture can be analyzed in situ providing further information on the deformation history and mechanisms of the samples.14 Higher pressures can be reached using dynamic compression. Standard shock wave techniques use explosives or gas guns to accelerate a flyer plate to several km s–1, which impacts a sample. Multiterapascal conditions are now experimentally accessible following the development
of powerful pulsed lasers and magnetic pulsed power facilities. Besides very high pressures, these experiments also involve extremely high rate loadings on the order of 106 to 1010 s–1. At the turn of the century, new deformation apparatus have been developed that can perform rheological measurements at relatively high pressures (up to 10–20 GPa) and high temperatures (up to 1000–2000 K)—including the rotational Drickamer apparatus15 and deformation-DIA.16 Compared to the diamond anvil cell or shock experiments, the P, T range is smaller in these, but deviatoric stress (in compression or in torsion) can be applied at typical strain rates of 10–5 s–1 and controlled independently from the confining pressure. Also, in situ strain and stress measurements during high-pressure deformation experiments are possible using these pieces of apparatus when coupled with synchrotron x-rays. At present, we have a better understanding of the elementary mechanisms of plasticity influenced by pressure. The main results are reviewed in this article.
Bonds under pressure The first indication of the influence of pressure on mechanical properties of solids is obtained from elasticity. Since the pressures considered here (from a few to a few hundreds of GPa) are comparable to most elastic moduli, one can expect pressure to have a profound influence on the electronic structure of solids. This influence is well illustrated by the
Philippe Carrez, University of Lille, France; [email protected] Patrick Cordier, University of Lille, France; patrick.c
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