Anne Joulain

Dislocation analysis of Ti2AlN deformed at room temperature under confining pressure

Compression experiments of the brittle MAX phase Ti2AlN were performed under confining gas pressure at room temperature. Subsequently, a complete dislocation analysis was performed by transmission electron microscopy. In particular, the Burgers vectors and the dislocation lines were studied via the weak beam technique: dislocation reactions are reported for the first time in a MAX phase, as well as dipole interactions. Footprints of a high lattice friction were also observed. All these features point towards classical dislocation activity, eventually leading to hardening.

Revisiting the defect structure of MAX phases: the case of Ti4AlN3

Microstructural study of as-grown Ti4AlN3 MAX phase has been performed by transmission electron microscopy. Dislocation walls, dislocation nucleation sites and stacking faults are described. In particular, diffraction contrast analysis combined with high-resolution images give a new insight into the nature of the stacking faults: contrarily to what is usually postulated, it is shown that the stacking faults possess a shear component in the basal plane. The stacking faults are created by the insertion of MX layers in the lattice via diffusion mechanisms. Their possible role on the deformation mechanism of MAX phases is discussed.

Pressure-enforced plasticity in MAX phases: from single grain to polycrystal investigation

Ti4AlN3, Ti3AlC2 and Ti3Al0.8Sn0.2C2 MAX phases were plastically deformed at room temperature (RT) under gaseous confining pressure. Microstructures of as-grown and deformed samples are carefully analysed using scanning electron microscopy (SEM), atomic force microscopy (AFM) and transmission electron microscopy (TEM). It is demonstrated that high level of plastic deformation can be reached under confining gas pressure; the later suppresses the brittle failure at RT to the profit of plasticity. Multiscale characterization techniques are shown to provide a unique insight into all the scales of the plastic deformation; in particular, the effect of the mesoscale. Indeed, grain shape and orientation relative to the compression axis are shown to play a key role in the deformation process, intergranular stresses leading to a complex stress field in the polycrystalline samples. The TEM results show that dislocation activity highly depends on the grain orientation. The observation of dislocation entanglements unambiguously demonstrates that dislocations may be organized in such a configuration so that their glide in the basal plane can be hindered when deep plastic regime is reached.

Al-matrix composite materials reinforced by Al-Cu-Fe particles

Al-matrix material composites were produced using hot isostatic pressing technique, starting with pure Al and icosahedral (i) Al-Cu-Fe powders. Depending on the processing temperature, the final reinforcement particles are either still of the initial i-phase or transformed into the tetragonal ω-Al00.70Cu0.20Fe0.10 crystalline phase. Compression tests performed in the temperature range 293K − 823K on the two types of composite, i.e. Al/i and Al/ω, indicate that the flow stress of both composites is strongly temperature dependent and exhibit distinct regimes with increasing temperature. Differences exist between the two composites, in particul ar in yield stress values. In the low temperatureregime (T ≤ 570K), the yield stress of the Al/ω composite is nearly 75% higher than that of the Al/i composite, while for T > 570K both composites exhibit similar yield stress values. The results are interpreted in terms of load transfer contribution between the matrix and the reinforcement particles and elementary dislocation mechanisms in the Al matrix.

Evidence of dislocation cross-slip in MAX phase deformed at high temperature

Ti2AlN nanolayered ternary alloy has been plastically deformed under confining pressure at 900°C. The dislocation configurations of the deformed material have been analyzed by transmission electron microscopy. The results show a drastic evolution compared to the dislocation configurations observed in the Ti2AlN samples deformed at room temperature. In particular, they evidence out-of-basal-plane dislocations and interactions. Moreover numerous cross-slip events from basal plane to prismatic or pyramidal planes are observed. These original results are discussed in the context of the Brittle-to-Ductile Transition of the nanolayered ternary alloys.

Dislocation modelling in Ti2AlN MAX phase based on the Peierls–Nabarro model

In this study, we determined the core structure and the Peierls stress of dislocations in Ti2AlN MAX phase. We use a generalized Peierls–Nabarro model, called Peierls–Nabarro–Galerkin (PNG), coupled with first principles calculations of generalized stacking fault (GSF). The GSF calculations show that dislocation glide in the basal plane will occur preferentially between M (here Ti) and A (here Al) planes. Additionally, the results of PNG calculations demonstrate that whatever the dislocation character, dislocations are dissociated in the basal plane, with a dissociation distance below the experimental resolution of transmission electron microscopy observations. Finally, the Peierls stress calculations show that the edge and screw characters are the easiest characters to glide in the basal plane.

Effect of microstructure anisotropy on the deformation of MAX polycrystals studied by in-situ compression combined with neutron diffraction

In situ compression tests combined with neutron diffraction were performed on Ti2AlN MAX polycrystals with lamellar anisotropic microstructure: the diffraction peak evolution (position and profile) with applied stress reveals that lamellar grains parallel to compression axis remain elastic while lamellar grains perpendicular to compression plastify, both families being subjected to strong variations of heterogeneous strains (types II and III). We demonstrate that this behavior originates from the complex response of the very anisotropic lamellar microstructure and explains the observation of reversible hysteretic loops when cycling MAX polycrystals even in the elastic regime.

Al–Pd–Mn icosahedral quasicrystal: deformation mechanisms in the brittle domain

The extreme brittleness of Al–Pd–Mn quasi-crystalline alloys over a wide range of temperatures drastically restricts investigation of their plastic deformation mechanisms over a small high-temperature regime. Recently, plastic deformation of Al–Pd–Mn quasicrystal has been achieved in the brittle domain (20 ≤ T ≤ 690°C) using specific deformation devices, which combined a uniaxial compression deformation or a shear deformation with a hydrostatic pressure confinement (0.35–5 GPa). Results of these experimental techniques, which provide various deformation conditions giving rise to a range of Al–Pd–Mn plastic features in the brittle domain, are discussed. On this basis, we propose that low and intermediate temperature plastic properties of Al–Pd–Mn are controlled by non-planar dislocation core extensions specific to the non-periodic structure.

Nanoindentation-induced deformation in Al–Pd–Mn single quasicrystals

Nanoindentation experiments were performed at room temperature on Al–Pd–Mn single quasicrystals to induce plastic deformation in very localized areas and to examine microscopic mechanisms taking place in the bulk. Nanoindentation imprints investigated by transmission electron microscopy revealed the presence of grains and crystalline phases, but did not provide any evidence of dislocation activity. Pop-in events observed on the nanoindentation curves support the idea of grain formation and phase transformation just beneath the indenter, as shown by transmission electron microscopy.

Pinning in

Bulk MgB₂- and YBaCuO-based materials are competitive candidates for applications. The properties of both compounds can be significantly improved by high temperature-high pressure preparation methods. The transformation of grain boundary pinning to point pinning in MgB₂-based materials with increasing manufacturing temperature from 800 to 1050^°C under pressures from 0.1 MPa to 2 GPa correlates well with an increase in critical current density in low and intermediate magnetic fields and with the redistribution of boron and oxygen in the material structure. As the manufacturing temperature increases (to 2 GPa), the discontinuous oxygen-enriched layers transform into distinct Mg-B-O inclusions, and the size and amount of inclusions of higher borides MgB_X (X>;2) are reduced. The effect of oxygen and boron redistribution can be enhanced by Ti or SiC addition. The oxygenation of melt-textured YBa₂Cu₃O_{7 - δ} (MT-YBaCuO) under oxygen pressure (16 MPa) allows one to increase the oxygenation temperature from 440°C to 700-800°C, which leads to an increase of the twin density in the Y123 matrix and to a decrease of dislocations, stacking faults, and the density of microcracks, and as a result, to an increase of the critical current density, <i>J</i>_c, and the trapped magnetic field. In MT-YBaCuO, practically free form dislocations and stacking faults and with a twin density of 22-35 μm^-1, <i>J</i>_c of 100 kA/cm² (at 77 K, 0 T) has been achieved, and the importance of twins in Y123 for pinning was demonstrated experimentally.