IMX Seminar Series - Crystal plasticity of body-centered cubic transition metals at the atomic scale

Plasticity of crystal materials is mainly controlled by the motion of dislocations, i.e. line defects carrying a shear increment defined by their Burgers vector. In transition metals with a body-centered cubic (BCC) crystallographic structure like iron, tungsten or chromium, these dislocations with a 1/2 <111> Burgers vector tend to align along their screw orientation, the line direction parallel to the Burgers vector. The glide properties of these screw dislocations are responsible of particular features of plasticity in BCC metals, among them a strong temperature dependence and deviations from the Schmid law with the yield criterion depending not only on the resolved shear stress in the slip plane but also on other stress components and on their signs. Using atomistic simulations and ab initio calculations, we show how plasticity in BCC pure metals can be understood at low temperature from core properties of screw dislocations, i.e. from the region in the immediate vicinity of the line-defect where the perturbation of the crystal is too high to be described by elasticity and where an atomic description is needed.
In presence of impurities like carbon, a strong friction also appears at temperatures high enough to allow for impurity diffusion. Ab initio calculations show that this friction arises from an attractive interaction of impurities, in particular C, with screw dislocation, leading to an important segregation of carbon in the dislocation core, even in very pure metals. The dislocation glide motion of screw dislocation becomes then controlled by impurity migration. A good agreement is obtained between the mechanism deduced from atomic modeling and from experimental observations by transmission electron microscopy during in situ tensile tests.
Bio: Emmanuel Clouet is a research director in the physical metallurgy laboratory at the French Alternative Energies and Atomic Energy Commission (CEA). Before joining CEA in 2004, Emmanuel was a visiting scientist at the University of Texas at Austin and he received his PhD in Physics in 2004 from École Centrale Paris, France for research performed at CEA and Pechiney (now Constellium). He spent two years on secondment to Lille University, France between 2007 and 2009. Emmanuel's research focuses on understanding the physical mechanisms controlling microstructure evolution and mechanical properties in metals and alloys used as structural materials, with a particular emphasis on crystal plasticity, irradiation effects and precipitation. His work relies on numerical simulations and modeling ranging from atomic to mesoscopic scale, in close connection with experiments. He received the Jean Rist's award of the French Society for Metallurgy and Materials in 2008. He is editor at Acta and Scripta Materialia.