Abstract
The formation of coherent precipitates is often accompanied by large elastic mismatch stresses, which suppress phase separation. We discuss the presence of interfaces as a mechanism for stress relaxation, which can lead to preferred zones of precipitation. In particular, we discuss the proximity of free surfaces and shear-coupled grain boundaries, for which we can obtain a substantial local energy reduction and predict the influence on the local precipitation thermodynamics. The latter case is accompanied by morphological changes of the grain boundary, which are less suitable for large-scale descriptions. For that purpose, we develop an effective description through an elastic softening inside the grain boundary and map the microscopic grain boundary relaxation to a mesoscopic elastic and phase field model, which also allows generalizing the description to multi-phase situations.
Highlights
Precipitation is a common phenomenon in alloys and can be used to influence the mechanical properties, e.g., of steels via precipitation hardening
We develop an effective description using an elastic softening of a grain boundary layer in Section 4, which can be tuned to reproduce the shear-coupled motion results
The preceding investigations raise the question of how the precipitation thermodynamics may be influenced by internal interfaces inside materials, in particular grain boundaries
Summary
Precipitation is a common phenomenon in alloys and can be used to influence the mechanical properties, e.g., of steels via precipitation hardening. Precipitation is influenced by kinetic effects, which can, e.g., lead to precipitate-free zones near grain boundaries, which act as sinks for impurities and can generate a corridor of reduced supersaturation around a grain boundary, where the thermodynamic driving force for precipitate formation is reduced [1] This interplay of thermodynamic and kinetic aspects is prominent in bainitic steels, where carbide precipitation can occur both in the the austenitic matrix and bainitic ferrite with a low solubility limit for carbon. The key idea of the following discussion is the assumption that a precipitate introduces a mechanical deformation both inside the precipitate and in the surrounding matrix This is accompanied by a modification of the bulk elastic energy, which influences the thermodynamics of phase coexistence. We show the way to generalize the modeling approach to multi-phase situations and the consideration of other near interface stress relaxation mechanisms as a coherent-incoherent transition in the concluding Section 5
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