Frédéric Mompiou

Evidence of grain boundary dislocation step motion associated to shear-coupled grain boundary migration

Abstract The present work reports dynamical observations of the grain boundary (GB)-mediated plasticity during in situ transmission electron microscopy straining experiments at moderate temperature (400 C) both in a 76.4 bicrystalline and a polycrystalline Al sample. We show that the GB migration occurs by the lateral motion of elementary GB dislocation steps. The accumulation of GB dislocation steps eventually form macro-steps. This observation agrees with the idea that GB dislocation steps generally operate in high angle GBs similarly as in twinning or martensitic transformations. The coupling factor, i.e. the strain produced by the motion of the steps was measured using fiducial markers and image correlation. The migration process involves different types of GB dislocation steps, producing different amounts of strain both parallel (coupling factor) and perpendicular to the GB plane.

Dislocation climb in icosahedral quasicrystals

Abstract We discuss here some arguments in favor of climb being the dominant mode of dislocation motion responsible for the plastic deformation of icosahedral quasicrystals.

In situ TEM study of twin boundary migration in sub-micron Be fibers

Abstract Deformation twinning in hexagonal crystals is often considered as a way to palliate lack of independent slip systems. This mechanism might be either exacerbated or shut down in small-scale crystals whose mechanical behavior can significantly deviate from bulk materials. Here, we show that sub-micron beryllium fibers initially free of dislocation and tensile tested in situ in a transmission electron microscope (TEM) deform by a { 1 0 1 ¯ 2 } 〈 1 0 1 ¯ 0 〉 twin thickening. The propagation speed of the twin boundary seems to be entirely controlled by the nucleation of twinning dislocations directly from the surface. The shear produced is in agreement with the repeated lateral motion of twinning dislocations. We demonstrate that the activation volume ( V ) associated with the twin boundary propagation can be retrieved from the measure of the twin boundary speed as the stress decreases as in a classical relaxation mechanical test. The value of V ≈ 1.6 ± 0.4 × 10 - 28 m 3 is comparable to the value expected from surface nucleation.

In situ TEM observations of reverse dislocation motion upon unloading of tensile-deformed UFG aluminium

Loading–unloading cycles have been performed on ultrafine-grained (UFG) aluminium inside a transmission electron microscope (TEM). The interaction of dislocations with grain boundaries, which is supposed to be at the origin of the inelastic behaviour of this class of materials, differs according to the main character of the dislocation segments involved in pile-ups. Pile-ups are formed by spiral sources and lead to the incorporation of dislocations into grain boundaries (GBs) during loading. Upon unloading, partial re-emission of dislocations from GBs can be observed. Stress and strain measurements performed during these in situ TEM loading–unloading experiments are in agreement with the rather large inelastic reverse strains observed during unloading in loading–unloading tests on bulk macroscopic UFG aluminium specimens.

Study of quasicrystals obtained from a structural model of icosahedral phases of type F AlPdMn/AlCuFe

Abstract A recent structural model [1] for the F-type icosahedral phases i-AlPdMn and i-AlCuFe based on the KG cell decomposition [2] of the Mackay clusters has shown that they both enter in an unique model of general composition Al61.8(Pd Ι Cu)21.35(Mn Ι Fe)8.29(Al Ι Fe Ι Mn)4.28(Al Ι Cu)4.28 where (X Ι Y) means X or Y species. Three new alloys satisfying this composition rule, two ternaries Al66.08Cu21.35Mn12.57 and Al70.36Pd21.35Fe8.29 and one quaternary Al66.08Cu21.35Mn8.29Fe4.28 have been prepared by rapid quenching and annealing and studied by X-ray diffraction and electron microscopy. All three phases are quasicrystalline after quench: Al66.08Cu21.35Mn12.57 is decagonal, and the two others are F-icosahedral phases. After annealing, the quaternary alloy transforms into a decagonal phase. The present results are compared to those of previous studies in the litterature of samples with close compositions.

A TEM study of the crystallography of lath-shaped austenite precipitates in a duplex stainless steel

The morphology of austenite (fcc) precipitates in a duplex stainless steel (DSS) is dominated by rods distributed in a ferrite (bcc) matrix. Minority of austenite precipitates also exhibits a lath shape, a common morphology in fcc to bcc transformations rather than a bcc to fcc transformation in a DSS. While the rod-shaped austenite precipitates in a DSS have been interpreted in previous investigations, precipitates with a lath shape were not well understood. This study focused on the lath-shaped austenite by using transmission electron microscopy. The habit plane of lath-shaped austenite was observed to be free of dislocations, but one array of dislocations was observed in the major side facet with a spacing of 9.6xa0nm and Burgers vector of [110]f/2|[010]b. These observations of crystallographic features were interpreted consistently by an O-line and good matching site analysis. Different morphologies in a DSS and similar morphologies in fcc to bcc and bcc to fcc transformations are compared and discussed.

Tiny but mighty: Size effects on the strength of metals

Size effects on metals and alloys that have external dimensions on the micron scale and smaller have been questioning the scientific community for several years. We have shown that these effects can be rationalized in terms of defects structure that is both sampled or altered during the fabrication processes and hindered by internal or surface effects when plastic flow has to develop.

Low temperature plasticity of an AlPdMn quasicrystal

TEM observations have been performed in AlPdMn single-grain quasicrystal deformed at low temperature. They show that dislocation motion has occurred by climb associated with vacancy diffusion. At room temperature, deformation also occurs by crack followed by rehealing.