Durcissement des superalliages monocristallins: des mécanismes physiques à la modélisation continue - Strength of single crystal superalloys: from dislocation mechanisms to continuum micromechanics

The present work deals with crystal plasticity of single crystal nickel-base superalloys. In this context, a scale transition of transferring information from mesoscale towards a physically justified micromechanical model is shown. A numerical coupling between dislocation dynamic simulations and the finite element method, the socalled Discrete-Continuous Model (DCM) is used in order to take into account the mutual interactions between dislocations and precipitates. In a first application, the dependence of the mechanical properties on the channel width is investigated. The relationship between simulated microstructures of dislocation, flow stress and plastic strain is then analysed. A second set of calculations addresses the anisotropic mechanical response of single crystal superalloys. Analyses of dislocation interactions show the crucial role of one active slip system and its collinear system in the strain localisation in the form of slip bands. Furthermore, screening of long-range elastic interactions associated to the interfacial dislocation network explain the origin of the low hardening rate observed in 〈111〉-oriented specimens at high temperatures. From these interpretations at the dislocation scale, two different modeling approaches are developed. On the one hand, one model uses the geometrically necessary dislocations in a hardening law. Both the formation and the evolution of the dislocation microstructures are analysed: comparison with results obtained by the DCM shows some short comings of this continuous approach. Then, a second micromechanical model based on a homogenisation procedure is justified. Its global response is determined by the microstructure and the mechanical interactions between its subdomains. In this micromechanical model, the local mechanisms are physically described and the constitutive laws are written in terms of mobile dislocation densities. It has been identified at 850 and 950◦C, and successfully validated on the single crystal superalloy CMSX-4.