Scientific Events

Heterogeneous deformation in metallic polycrystals arises from several factors, including anisotropy in elastic properties and plastic slip. The ability to accurately simulate heterogeneous deformation requires physically based models of slip that includes grain boundary properties, as grain boundaries are usually barriers to slip. As slip transfer across boundaries occurs in some boundaries, grain boundary properties have been installed in a dislocation density based crystal plasticity model to enable slip transfer, and used to examine idealized bicrystal tensile samples. This code will be used to simulate deformation of annealed pure aluminum foil multicrystal experiments, in order to examine thresholds for slip transfer. An analysis of slip transfer events indicates that for near-cube oriented grains, the threshold is higher than observed in hexagonal materials, and potential reasons for this will be discussed. Secondly, as computational simulations of polycrystals normally assume a zero-stress initial condition, this assumption is questionable in non-cubic metals where the coefficient of thermal expansion (CTE) is anisotropic. To assess the effect of the anisotropic CTE on initial stress states, two pure titanium samples with different textures were examined using in-situ high energy x-ray diffraction microscopy to measure the evolution of the internal stresses in each grain during heating and cooling. These data show a significant change in expansion rates in the <a> and <c> directions at about 700 C. A simulation of this experiment shows good agreement with experimentally measured data, indicating that it is possible to start a simulation with a good estimate of the internal stress state arising from the anisotropic CTE. This work was supported by grants from US DOE/BES and the Community of Madrid [more]

Deformation in metallic glasses occurs by initiation and propagation of multiple thin shear bands. This mode is rather difficult to analyse since generally, a single band soon propagates to a large extent in the specimen leading to a catastrophic failure. Exceptions are for example in creep tests under very low stress and moderate temperature or in confined deformation tests. We used instrumented nano-indentations to perform series of independent experiments at room temperature on a Mg65Cu12.5Ni12.5(Ce75La25)10 metallic glass. Loading part of the curves shows serrations which size and duration were measured using an automatic procedure. To make analyses consistent, data were considered only in the domain with similar strain rates, in the range of 1 to 0.3 s-1. Times between successive serrations follow a normal distribution characterizing a random occurrence of deformation burst in the glass. It was then conjectured, first that serration occurs through activation of appropriate zone in the glass that should naturally scale with a multiple of an elementary domain size characterizing the deformation mechanism. Second, as activated zones leading to serration are very few in the glass, the model should be described by the Poisson statistics. Data analyses reveals that serration size are well fitted by a Poisson distribution. The model predict an elementary size which scale with that of the activation volume of 3 atoms, measured from creep test at constant load in the same series of experiments. Eventually, energy dissipated during serration is analyzed as to define shear bands dynamics characteristics.Depending on time, I shall present the use of nano-indention for investigating dynamics of nanoporous metallic materials deformation. N. Thurieau, L. Perriere, M. Laurent-Brocq, Y. Champion, J. Appl. Phys., 118 (2015) 204302. [more]