Research Highlight

Prof. Solanki wins the 2013 Air Force Office of Scientific Research Young Investigator Research Award

Congratulations to Mr. Mehul Bhatia, who received the 2013 TMS Student Travel Grant

Prof. Solanki wins the 2013 TMS LMD Young Leader Professional Development Award

The 2011 TMS Light Metals Magnesium Best Paper Award

 
Mechanical behavior of nanocrystalline alloys – A combined experimental and modeling approach
Nanocrystalline (NC) metals often have enhanced mechanical properties (high strength, increased wear resistance) compared to coarse-grained (CG) metals and hence are very attractive from an engineering perspective. However, to date, one difficulty in interpreting the results and drawing general conclusions from literature on the deformation and failure mechanisms of NC metals is the lack of consistency between the experimental conditions from all these studies (material, specimen geometry, testing conditions, grain size distribution, texture, GB character, impurity level, environment, etc.). Furthermore, there have been remarkably few systematic studies exploring how the interplay between microstructural (grain size and associated heterogeneities) and interfacially segregating solutes influences the stability of “bulk” NC metals and the associated deformation mechanics under dynamic conditions. In this project, we aim to address this knowledge gap through a synergistic combination of material processing, novel high strain rate testing setup, digital image correlation measurements, and transmission electron microscopy (see figure below).
TEM OIM IPF and grain size distributions of (a) as received, and (b) dynamically strained (rate = 104 S-1) Cu-10 at.% Ta sample. Majority of the grains are in the nano-regime with an average size of 68 nm and 74 nm, respectively. These are consistent with the XRD analysis.
 
XRD patterns  with indexed peaks of  (a) as received, and (b) dynamically strained (rate =104 S-1) Cu-10 at.% Ta sample. Peak broadening is observed  for both Cu and Ta indicating a change in grain size. However, the estimated increase in grain size for  Cu and Ta is approximately 12 nm and 5 nm between the unstrained and strained sample
 
Effects of oxygen on prismatic faults in Ti: A combined quantum mechanics/molecular mechanics study
Material strengthening and embrittlement are controlled by intrinsic interactions between defects, such as dislocations and oxygen that alter the observed deformation and failure mechanisms. In this work, the interaction of oxygen with prismatic faults in Ti was investigated through a coupled quantum and molecular mechanics approach. We show that oxygen increases the Peierls stress (by 400% for 1/6 monolayer oxygen) and reduces the core width (by 18%). The observed effect of oxygen on plasticity is consistent with the experimental observation.
a) GSFE as a function of shear displacement along the [12-10] direction of Ti with and without 1/6 ML oxygen on the slip plane. b) A 3D iso-surface plot for charge density showing the effect of O at the prismatic stacking fault. The orange and black atoms represent oxygen and Ti atoms, respectively. The yellow and cyan iso-surfaces represent charge accumulation and depletion, respectively.
 
Hydrogen embrittlement in α-Iron
Hydrogen embrittlement (HE) is a phenomenon that affects both the physical and chemical properties of many intrinsically ductile metals, including nickel (Ni), iron (Fe), and aluminum (Al). Consequently, understanding the mechanisms behind HE has been of particular interest in both experimental and simulation research. However, discrepancies between experimental observations and modeling results have led to multiple hypotheses to explain HE mechanisms, i.e. hydrogen enhanced localized plasticity (HELP), adsorption-induced dislocation emission (AIDE) and hydrogen enhanced decohesion (HED). The central ideas behind most of the experiments and models that attempt to explain the HE mechanisms are that hydrogen can 1) be absorbed at the surface or grain boundaries, causing HED; or 2) increase dislocation mobility, leading to plastic flow localization (HELP) (see Figure below). Understanding the fundamental issues involved in what is truly the interplay between HELP and HED is still a work in progress. For example, whether HE occurs due to dislocation starvation, a significant increase in dislocation density due to an increase in dislocation mobility, or reduction in surface/cohesive energies, is still not understood. In fact, the indeterminate understanding of the mechanistic origin of HE hinders our ability to satisfactorily address HE in important technologies, including nuclear power plants and other large-scale, industrial infrastructure (e.g., gas pipelines)
Schematic illustrating various hydrogen transport/embrittlement phenomena in iron.
 
Exploring deformation modes in titanium: Atomistic simulations
It is well accepted that both the dislocation core structure and its mobility are directly related to mechanical properties of materials. In materials with high stacking fault energy such as Ti, it is relatively difficult to investigate dislocation core structure through experiments. In this work, we used Zope and Mishin’s EAM potential to atomistically simulate dislocation core structures on the basal and the prismatic planes. Figure shows the core structure of the basal plane and the prismatic–edge dislocations after a minimization of the potential energy using a conjugate gradient relaxation algorithm. The core of the basal edge dislocation dissociated into two Shockley partials, bounds an intrinsic fault according to the reaction 1/3[2-1-10] → 1/3 [10-10] + 1/3 [1-100]. The core structure of the prismatic edge dislocation is not dissociated but spreads in the (10-10) plane.
 
Generalized Framework for Interatomic Potential Design Map
Radiation damage phenomena plays an important role in the lifetime of structural materials for future fusion power reactors. Developing predictive multiscale models for material behavior under irradiation conditions in a fusion reactor requires understanding the mechanisms associated with radiation damage phenomena, the He interaction with microstructures, and quantifying the associated uncertainties. Nanoscale simulations and interatomic potentials play an important role in exploring the physics of nanoscale structures. However, while interatomic potentials are designed for a specific purpose, they are often used for studying mechanisms outside of the intended purpose. Hence, a generalized framework for interatomic potential design is designed such that it can allow a researcher to tailor an interatomic potential towards specific properties. This methodology produces an interatomic potential design map, which contains multiple interatomic potentials and is capable of exploring different nanoscale phenomena observed in experiments. This methodology is efficient and provides the means to assess uncertainties in nanostructure properties due to the interatomic potential fitting process. As an initial example with relevance to fusion reactors, an Fe–He interatomic potential design map is developed using this framework to show its profound effect [1] [2].
 
NanoScale Defect Interaction Mechanisms
Molecular dynamics calculations were performed using embedded atom method (EAM) potentials to study the localization of inelastic flow and crack initiation in fcc single crystal copper and nickel. We compared the atomic scale anisotropic inelastic response of the copper single crystals from EAM to the results of [Philos. Mag. 78(5) (1998) 1151] (experiments and finite element results using single crystal plasticity). Hollow circular cylinders of single crystals were loaded radially with a constant average velocity at a strain rate of 10^9s−1, inducing the collapse of the cylinder. Various initial orientations of the lattice are examined to study the localization of flow and crack initiation. Comparisons between EAM, experiments, and finite element simulations were in good agreement with each other illustrating that kinematic and localization effects are invariant to extremely large spatial and temporal regimes. Finally, similar dislocation nucleation patterns, localization sites, and crack initiation sites were observed when comparing copper to nickel.
  
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