Simulations à l'échelle atomique de mécanismes locaux de plasticité dans des métaux et alliages métalliques – ATOPLAST

The plasticity of metals and alloys mainly proceeds by motion of dislocations, linear defects carrying an elemental displacement, their Burgers vector (<1nm). In generating long-range stress and strain fields, the dislocations interact with defects giving rise to hardening effects and hiding local interactions which are thus not well known. Local interactions are in addition difficult to investigate for two main reasons (i) the lack of appropriate techniques (imaging and/or spectroscopic) enabling a reliable analysis of nanometric defects in small amounts, (ii) the short-range interactions taking place in crystal volumes where linear elasticity is at its limit of validity. This is where the present project originates from. It is aimed at consolidating an ongoing joint program started in 2007 between two (LEM and IMR) of the three Partners. There is a significant basic research effort at IMR in support of several development projects of Ti-based alloys and intermetallics. The program is largely founded on the great potential of molecular dynamics (MD) to explore local deformation processes. Limited in the past by computing power, progress in this domain is rapidly growing. As shown by significant differences between atomistic simulations and predictions based on linear elasticity, local deformation processes are not clearly understood. In the last two years, the IMR and LEM groups have acquired a significant expertise in this domain. They first tackled and solved the property that, sheared in the pseudo-twinning direction, TiAl engenders true twins whereas a L10 to L11 phase transformation is expected. Simulations were further pursued on dislocation dipoles and showed that the transformations of narrow dislocation edge dipoles depend on two main factors, dipole height and temperature ? let alone composition which itself sets the crystal elastic properties and the stacking fault energy. Small-height dipoles (1 to 2 atomic planes) turn into vacancy tubes, very stable at low and moderate temperatures, but forming complex debris at high temperatures. The debris include stacking fault tetrahedra (SFT) whose formation proceeds from a diffusion-mediated mechanism uncovered in this work. The importance of SFT in controlling the strength of irradiated fcc alloys is well-known motivzating active basic investigations worldwide including MD. At low temperature, 3d to 8d high dipoles self-organize by ordering extended vacancies in their core. Thermal agitation aiding, the dipoles transform into zigzagged configurations comprising portions of faulted dipoles themselves almost indestructible. These properties are box-size independent. The nucleation of subsequent propagation of dislocations in sheared TiAl, investigated in 2-million-atom simulations under periodic boundary conditions, results in narrow dipoles of the two kinds, vacancy and interstitials, which in turn yield numerous point defects and their clusters such as loops. Unexpectedly, interstitial loops are not prismatic but contained in the operative slip plane under the form of interstitials crowdions agglomerated side by side. We intend to pursue this work in several directions. Our main objectives will be (i) to make use of our methods and experience to conduct atomic-scale simulations of processes taking place in the transformation and/or annihilation of dislocation dipoles of varied character in face-centered cubic metals and some intermetallic compounds, in order to determine the impact of these transformations on the dislocation mobility and deformation behavior of the metals ? (ii) to employ saddle-point search methods and global optimizers to study the long-term evolution of defect structures produced during molecular dynamics simulations, which is one of the main reasons for the incorporation of the SIMaP group into the project, another is its expertise in MD simulations of lattice defects ?(iii) to design a potential for alpha-Ti capable of investigating the motion of dislocations, their mutual interactions, and their interactions with alloying elements ?(ii) to test twinning models for ordered compounds. NI3Al, the L12 phase of Ni superalloys cannot be twinned by partial dislocations unless some atomic rearrangement is allowed, yet Ni3Al precipitates can be extensively twinned in superalloys crept a elevated temperatures, calling for simulations to specifically analyse the conjunction of shearing events and possible diffusion.