Online Lecture by Marcel H.F. Sluiter & Kai Liu

Anisotropy, Heterogeneity and Stress: Grain Boundaries under Tension and Elastic and Plastic behaviour of Pearlite

Department of Materials Science & Engineering

Delft University of Technology, Delft, the Netherlands

Title:
Anisotropy, Heterogeneity and Stress: Grain Boundaries under Tension and Elastic and Plastic behaviour of Pearlite.

Abstract:
A material is most likely to fail where it is loaded most heavily. On the macroscopic scale, when designing with materials, often great care is taken to minimize concentration of stress. On the microscale such stress concentrations occur also. But perhaps surprisingly, have received less attention. The microstructure provides various sources of local stress concentrations and failure initiation sites. A material under tension generally develops a stress difference across grain boundaries (GBs). On the scale of individual crystals but few materials are elastically isotropic. Therefore, under load, the crystals on either side of a GB have generally different compliances. As the two grains, when loaded elastically, remain fully connected at the boundary, it means that the stress tensor differs in the two grains. Thus, the stress tensor inside the material can differ substantially from the externally applied stress. Under linear elasticity the internal stress as function of the applied external stress has recently been analyzed in detail.

In spite of its complexity some general results have been derived. E.g. for all cubic crystals the highest stress discontinuity occurs at (111)//(100) grain boundaries, with little dependence of on rotation around the normal of the GB plane.

Another case where stress can much deviate from the average, is where dissimilar phases are intricately joined, such as in pearlite. Pearlite is long been used to produce wires that approach the theoretical strength limit. The behavior of pearlite with the Bagaryatskii orientation relationship under uniaxial stress within the lamellar plane has been explored both in the elastic and plastic regime. Within the lamellar plane a range of uniaxial loading directions has been considered. Schmid factors, interfacial dislocations, and crystallographic aspects all together cause a variety of deformation behaviors as function of the in-plane loading direction. For many directions deformation is initiated at interfacial dislocations at the ferrite side. Surprisingly, it is found that loading directions exist where slip begins in the cementite layers rather than in the ferrite layers. The extended range or work hardening and other features of pearlite are explored through atomistic modelling.

These observations might contribute to the design of other high-performance materials with lamellar microstructures deriving from eutectic or eutectoid transformations.

This session will be chaired by prof. Leo Kestens (Ghent University)

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