Understanding interface decohesion in complex polycrystalline microstructures

Understanding interface decohesion in polycrystalline microstructures is the key aspect towards enhanced materials. In this project the interplay between dislocations and decohesion processes of grain and phase boundaries is studied by a modelling approach.

PhD Candidate: Franz Bormann
Supervisor: R.H.J. Peerlings
Promoter: M.G.D. Geers
Project Financing: M2i, STW
Project Period: June 2014 – June 2018

Multiphase metallic materials such as advanced high-strength steels are of great interest for high-tech applications because of their higher strengths compared to conventional forming steels. In the automotive industry, for instance, this higher strength enables lightweight, fuel-efficient designs which are nevertheless safer than those using conventional materials. However, the higher strength of multiphase materials is generally accompanied by a lower ductility, which implies a lower energy absorption during impact and thus may compromise safety.

The unique properties of multiphase materials originate from their microstructure. The companies producing these advanced materials have an excellent understanding of how detailed aspects of the microstructure determine the overall strength and hardening behaviour. However, their understanding of the failure mechanisms and the resulting limits to ductility, is much less developed.

Thus, in this project we study how decohesion of internal boundaries in advanced multiphase alloys nucleates, how it limits the ductility of these materials and how this process depends on the materials’ microstructures. We consider grain boundaries as well as phase boundaries and in particular study how local stress build-up associated with the accumulation of dislocations at such boundaries may result in the initiation and propagation of interface cracks (ref. Figure 1).

A modelling approach as in Figure 2 is followed in this project, which is however supported by experimental characterisation in a parallel project. The grain interiors are modelled by a novel plasticity approach. For grain and phase boundaries, dedicated interface models are to be formulated which combine the proposed plasticity model (accounting for the local stress state) with enriched cohesive zones (i.e. loss of the ability to relay any stress). In this project we aim to acquire fundamental insight for a broad class of materials and loading conditions.