©lmushongera 2019

Current Research Projects:

Modeling microstructural mechanisms of cyclic fatigue deformation in FCC polycrystals

A glance at the available literature on mechanisms determining the local characteristics of persistent slip bands (PSB) reveals that the subject has many unresolved issues. The volumetric swelling of PSBs is one area yet to to be fully establish.  We use a phase-field model of fatigue-crack initiation in nominally defect-free pure f.c.c metals. The scale of the envisioned study is that of primary (111) PSBs intersecting polycrystalline grains and interacting with the free surfaces. The PSBs are regarded as a result of spontaneous evolution of localized dissipative structures in the form of solitary static regions. Basic building blocks of the theory are: a model of cyclic plasticity tailored to PSBs, coupled to a model of vacancy diffusion.

An integrated computational approach to predict microstructure-property relationships in additive manufactured alloys

This work will provide a generic computation-based path towards the prediction of microstructure property relations in AM metals and alloys that can be extended to other alloy systems. The framework will allow for the correlation of non-equilibrium features and defects that form at the solidification-scale to mechanical behavior. The idea is to relate crucial aspects of solidification (e.g., primary dendritic arm spacing microsegregation, microporosity, intermetallic phases) to mechanical behavior. The objectives of this work are: (i) Determine the driving forces for non-equilibrium phase and feature formation during rapid solidification. (ii) Investigate the effects of process parameters on microstructure development and develop a computational approach to obtain the process dependent microstructures in ranges guided by the experiments. (iii) Correlate microstructure and microstructural defects to mechanical properties using crystal plasticity finite element modeling.(iv) Develop a transferable simulation tool to relate microstructures of AM components to the mechanical properties.

Microstructural-sensitive fatigue modeling of additively manufactured polycrystalline materials

Fatigue failure of polycrystals is often dominated by crack initiation processes, which are strongly influenced by salient features and inherent defects existing in the microstructure. Internal microstructural heterogeneities such as twins, grain boundaries act as stress concentration sites and preferred crack initiation zones during cyclic loading, leading to premature rupture. In this work, we use the crystal plasticity finite element approach to analyze the influence of microstructural heterogeneities on the low cycle fatigue of large-grained polycrystals. A set of microstructurally differing statistically equivalent microstructures is subjected to strain-controlled fatigue to analyze transgranular and intergranular crack paths in dependence on microstructural characteristics. The morphology and texture characterized using EBSD is utilized to construct representative statistically equivalent microstructures

Modeling of phase transformations in steel

A literature review on the mechanisms of eutectoid transformation in ternary steels reveals that the subject remains fascinating, with many unresolved issues and disparate observations. Cooperative growth of pearlitic lamellae and the factors that engender transition to divergent eutectoid are areas where stipulated bridging between theory and experiments is yet to establish. We use a grand-chemical potential model that uses thermodynamic information from the CALPHAD to explore the conditions that stimulate fascinating morphological transitions as the eutectoid transformation proceeds in Fe-Mn-C steel.  Meaningful insights on the mechanisms of eutectoid transformation are derived based on synergies established between computational and experimental data.