Influence of Interface Proximity on Precipitation Thermodynamics

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.

Modeling Bainitic Transformations during Press Hardening

“…For a transfer to larger scale descriptions, which do not require this detailed modeling, an effective approach ascribing reduced elastic constants to a “soft” grain boundary allows to capture this effect without spatially resolving the grain boundary contour, e.g., in simulations of bainitic microstructures [ 85 ]. A quantitative matching between the microscopic and mesoscopic descriptions, including a perspective to generalize the approach to precipitation near interfaces between different phases (e.g., carbides near interfaces between ferrite and austenite in bainitic steels) using phase field approaches is discussed in [ 86 ].…”

“…The entire micrograph is compiled from 9 high-resolution SEM backscattered electron images, where carbides are visible at the highest zoom level. More details of carbides in this microstructure can be found in Figure 1 of reference [ 86 ]. A layer of EBSD map (kernel average misorientation color coded) is superimposed on top of the SEM image.…”