The ability to predict damage nucleation and evaluate whether it will lead to a fatal flaw is one of the major goals of computational plasticity. However, most modeling of damage is based upon the assumption of pre-existing flaws or cracks, and the modeling approaches developed so far predict the growth rather than the nucleation of damage. While heterogeneous deformation is understood to be a precursor to damage nucleation, the step between heterogeneous deformation and damage nucleation is not clearly understood. If heterogeneous deformation is not modeled accurately, then it is unlikely that damage nucleation and subsequent damage growth can be confidently predicted. From a review of current understanding of heterogeneous deformation and deformation transfer at grain boundaries, identification of mechanisms of damage nucleation will require quantitative knowledge of (i) the orientations of crystals on either side of the interface, (ii) the boundary orientation and structure (energy), (iii) the activated deformation systems on either side of the boundary, and (iv) the stress-strain gradient history in the grains on either side of an interface. This research examines metals and alloys with simple microstructures that have an intrinsically low ductility (in this case titanium and dual phase steels) and thus provide the best opportunity to gain the slip-system-based information needed to clearly identify damage nucleation mechanisms.
The goals of this research program are: (1) Identify fundamental rules for identifying strong and weak boundaries in the context of a deformation path (2) Express rules quantitatively in the form of models that track boundary strength as a function of local stress and strain history. (3) Implement grain boundary strength rules into computational models of mesoscale deformation. This is accomplished in a joint research project involving Michigan State University (MSU) and Max Planck Institut fr Eisenforschung (MPIE) in Dsseldorf, Germany, where mutually useful skills are present which can reach the above goals when integrated into an international cooperative research program. The research is based upon obtaining sufficiently detailed data sets so that damage nucleation mechanisms in polycrystal boundaries can be clearly identified. State of the art methodologies are used, which permit comparisons of multiple experimental methodologies that provide complimentary information. Comparison of data sets helps quantify the resolution and credibility of orientation imaging microscopy, serial sectioning, three-dimensional x-ray mapping, electron channeling contrast imaging (ECCI) and backscattered electron imaging. These rich data sets facilitate model development that is needed before damage nucleation can be predicted with confidence in computational models. The work is carried out by three Ph.D. students under the guidance of Profs Bieler and Crimp at MSU, and Drs. Philip Eisenlohr and Franz Roters at the Max Planck Institut fr Eisenforschung. Extensive exchanges between the two laboratories occur in order to integrate experimental and analytical methods to reach these goals. Multi-scale modeling of sophisticated processing and service conditions of advanced materials and components is needed to support all technological innovations that drive the world economy. It is difficult to think of an aspect of material processing that affects society more than being able to reliably predict damage nucleation. Success in implementing credible models of damage nucleation in design environments will lead to reduced waste, accelerated time to market for highly value-added manufactured goods, improved safety and economy in all aspects of technology and engineering.
This award is co-funded with the Office of International Science and Engineering.
