Slip (materials science) | EPFL Graph Search

In materials science, slip is the large displacement of one part of a crystal relative to another part along crystallographic planes and directions. Slip occurs by the passage of dislocations on close/packed planes, which are planes containing the greatest number of atoms per area and in close-packed directions (most atoms per length). Close-packed planes are known as slip or glide planes. A slip system describes the set of symmetrically identical slip planes and associated family of slip directions for which dislocation motion can easily occur and lead to plastic deformation. The magnitude and direction of slip are represented by the Burgers vector, b. An external force makes parts of the crystal lattice glide along each other, changing the material's geometry. A critical resolved shear stress is required to initiate a slip. Slip in face centered cubic (fcc) crystals occurs along the close packed plane. Specifically, the slip plane is of type {111}, and the direction is of type . In the diagram on the right, the specific plane and direction are (111) and [10], respectively. Given the permutations of the slip plane types and direction types, fcc crystals have 12 slip systems. In the fcc lattice, the norm of the Burgers vector, b, can be calculated using the following equation: Where a is the lattice constant of the unit cell. Slip in body-centered cubic (bcc) crystals occurs along the plane of shortest Burgers vector as well; however, unlike fcc, there are no truly close-packed planes in the bcc crystal structure. Thus, a slip system in bcc requires heat to activate. Some bcc materials (e.g. α-Fe) can contain up to 48 slip systems. There are six slip planes of type {110}, each with two directions (12 systems). There are 24 {123} and 12 {112} planes each with one direction (36 systems, for a total of 48). Although the number of possible slip systems is much higher in bcc crystals than fcc crystals, the ductility is not necessarily higher due to increased lattice friction stresses.

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

Cécile Renée Maria Marichal

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.

EPFL2013

In-situ Laue Diffraction During Compression of Directionally Solidified Mo Micropillars

Julien René André Zimmermann

With recent developments in micro and nano-technologies, mechanical components such as those used in medicine or electronics tend to be miniaturized, requiring a new set of testing techniques to study their reliability and performance. One of the new methods is micro-pillar compression testing, which allows the study of mechanical properties of confined volumes precisely shaped by focused ion beam (FIB) milling from sometimes complex microstructures. This technique has garnered a great deal of interest in recent years; on one hand because of its apparent simplicity, on the other hand because it revealed an unexpected size dependence in the flow stress of single crystals when the pillar diameter was reduced below a few tenths of a micrometre. The flow stress of FIB milled face centred cubic (fcc) micro-pillars appears to follow a power law as a function of their size. In contrast, non-FIB milled, body centred cubic (bcc) eutectic Molybdenum (Mo) alloy pillars prepared by directional growth exhibit, in their pristine state, whisker-like behaviour without a size effect. Furthermore, after FIB milling or moderate pre-deformation, the whisker character is lost and the typical scatter of the flow stress usually observed for FIB milled pillars is reproduced. After large pre-deformation the scatter vanishes and the pillars exhibit no size effect. These findings show that 1) the deterministic parameter in the size effect can be correlated with the initial microstructure of the pillar, and 2) FIB milling influences this microstructure. This study aims to link the deformation modes occurring in small-scale pillars with their initial microstructure by studying directionally solidified eutectic Mo-alloy pillars with a diameter of 1 micrometre. In-situ Laue diffraction experiments during the compression of such defect-free pillars show that with readily-available dislocation sources from a concave pillar top, the {112}[111] slip system with the highest Schmid factor is initially activated and not the allegedly assumed {110}[111] slip system that is typical for bcc metals. Post-mortem slip trace analysis and TEM observations confirm this finding. Moreover, 3D atom probe measurements and TEM images show that this behaviour can be linked to the presence of both alloying elements and fcc nano-precipitates found in the bcc Mo-alloy. Both can lead to a decrease of the critical temperature and the observation of an fcc-like deformation. Additionally, it is shown that both FIB milling and pre-deformation introduce defects in the initially dislocation-free pillars. In addition to this, pillars prepared under similar conditions yield quite different microstructures. The angle between the focused ion beam and the crystal orientation is found to play an important role in the creation of defects. It can also be shown that pillars with high initial dislocation density deform bulk-like and exhibit strain hardening, while pillars with low initial dislocation content demonstrate a large scatter in the flow and yield stress and exhibit large strain burst at later deformation stages. Furthermore several slip systems are activated before the flow stress is reached for pillars containing initially low dislocation content. Interestingly the first activated slip system can be linked to the initial streaking direction; slip is observed on the {112} plane that contains excess dislocations. For both the as-prepared pillars and the pillars with low initial dislocation content, the yield seems to be source limited, which is not the case for pillars with high initial dislocation content.

EPFL2011