Plasticity in bcc single crystals investigated by Laue diffraction during micro-compression

Body-centered cubic metals are of high technological interest: for example tungsten as potential plasma facing component in future fusion reactors, molybdenum employed in aircraft parts, niobium as superconducting magnets, etc. The characteristics of their mechanical behavior are therefore of prime importance from scientific, technical and economic points of view. The specific plastic deformation of bcc metals below the critical temperature raised numerous questions on the fundamentals of dislocations dynamic. Screw dislocations are anchored in the lattice because of a three dimensional core structure and they are consequently controlling the deformation. They have been observed to glide on both {110} and {112} planes, and the type of plane where the dislocations glide by elementary steps is still a matter of debate. Furthermore, deformation has been reported to occur sometimes with slip systems for which the resolved shear stress is very low, often called anomalous slip, and this is still not understood properly. Several models or theories have been developed to explain anomalous slip: some refer to the effect of stress components other than the shear stress in the slip direction, others to the role of dislocation-dislocation interactions. Most of our experimental knowledge comes from slip traces analysis or post-mortem transmission electron microscopy. Both methods give valuable insights, however incomplete what concerns the slip activity and dislocation dynamics. Here, in-situ Laue diffraction has been employed in order to follow non-destructively the dislocations dynamics during deformation of single crystal micro-pillars. Micro-compression experiments are performed during Laue diffraction at the microXAS beamline of the Swiss Light Source Synchrotron and allow following the sequence of activated slip systems in bcc pillars with diameters of 1-2 micron. Diffraction patterns are recorded continuously during the deformation in transmission geometry with a 5-23keV x-ray beam presenting a full-width at half maximum of approximately 1μm2. Complementary Laue scans are performed before and after the load to observe the local crystallographic orientations, activated dislocation slip systems and spatial distribution of strain gradients in the pillars. Complementary slip trace analysis is carried out in a scanning electron microscope. This type of experiments provides complementary insight into the deformation of bcc crystals. It has been applied on three different bcc metals having different critical temperatures (W, Mo and Nb). Various crystallographic orientations of the compression axes were probed to determine the influence of crystal orientation on the activation of slip systems. It is shown that when the highest resolved shear stress is on a {112} plane, slip can be described with elementary steps on {110} planes occurring by frequent alternation or by collective cross-slip, depending on the local stresses in the pillar and the mobility of the screw dislocations. It is furthermore shown that anomalous slip occurs in tungsten and molybdenum micro-pillars at high strains, suggesting the role of dislocation-dislocation interactions or at least the local stress variations induced by these interactions. The experiments also demonstrate the importance of the sample geometry when testing materials at the micron scale: stress concentrations and confinements provoked by specific geometries can lead to differences in slip system activation and/or early plastification.