Research Highlight

Our paper on "Anomalous mechanical behavior of nanocrystalline binary alloys under extreme conditions" is published in Nature Communications (2018)

MultiPhysics Lab recieves a grant from NSF on creep of nanocrystalline materials (Sep 2018)

Congratulation former students: Mehul Bhatia and Ilaksh Adlakha: Mehul joined WPI as an Assistant Teaching Professor and Ilaksh as an Assistant Professor in IIT-Madras

Congratulation students: Chaitanya, Scott and Mansa for the TMS student best paper award (2018)

MultiPhysics Lab recieves a grant from NSF on fatigue of nanocrystalline materials (Apr 2017)

Mansa receive the outstanding graduate research (Materials Science & Engineering) award for her dissertation (Apr 2017)

Our paper on creep of nanocrystalline Cu-Ta is published in Nature (Sep 2016)

 
Design and synthesis of advanced multifunctional materials
A problem central to designing the next generation of advanced multifunctional materials for various applications is that the time required for development of commercial product and their components far outpaces the time required for new material design. Thus, a grand challenge in materials engineering is to drastically reduce the development time for new materials and processes, while preserving or exceeding current safety benchmarks. Indeed, accelerating the pace of discovery, development, manufacturing, and deployment of advanced materials systems is the principal objective of the “Materials Genome Initiative.” In this regard, integrated computational materials engineering (ICME) provides a conceptual roadmap to new design materials; however, critical knowledge gaps currently preclude the application of ICME for the development of advanced materials such as nanocrystalline (NC) alloys. To address this need, our group in collaboration with the ARL, and George Mason University, are using a multiscale modeling approach along with detailed experimental and advanced manufacturing processes to push the design boundary closer to the theoretical limit.

Extreme creep resistance in a microstructurally stable nanocrystalline alloy

Nanocrystalline metals, with a mean grain size of less than 100 nanometres, have greater room-temperature strength than their coarse-grained equivalents, in part owing to a large reduction in grain size1. However, this high strength generally comes with substantial losses in other mechanical properties, such as creep resistance, which limits their practical utility; for example, creep rates in nanocrystalline copper are about four orders of magnitude higher than those in typical coarse-grained copper. The degradation of creep resistance in nanocrystalline materials is in part due to an increase in the volume fraction of grain boundaries, which lack long-range crystalline order and lead to processes such as diffusional creep, sliding and rotation3. Here we show that nanocrystalline copper–tantalum alloys possess an unprecedented combination of properties: high strength combined with extremely high-temperature creep resistance, while maintaining mechanical and thermal stability. Precursory work on this family of immiscible alloys has previously highlighted their thermo-mechanical stability and strength, which has motivated their study under more extreme conditions, such as creep. We find a steady-state creep rate of less than 10−6 per second—six to eight orders of magnitude lower than most nanocrystalline metals—at various temperatures between 0.5 and 0.64 times the melting temperature of the matrix (1,356 kelvin) under an applied stress ranging from 0.85 per cent to 1.2 per cent of the shear modulus. The unusual combination of properties in our nanocrystalline alloy is achieved via a processing route that creates distinct nanoclusters of atoms that pin grain boundaries within the alloy. This pinning improves the kinetic stability of the grains by increasing the energy barrier for grain-boundary sliding and rotation and by inhibiting grain coarsening, under extremely long-term creep conditions. Our processing approach should enable the development of microstructurally stable structural alloys with high strength and creep resistance for various high-temperature applications, including in the aerospace, naval, civilian infrastructure and energy sectors. see

Compressive creep response of nanocrystalline Cu–10 at% Ta. (a) Conventional creep strain versus time for various applied temperatures and constant-stress conditions and (b)Map of the theoretical creep deformation mechanisms including values for Cu-10 at% Ta.

Anomalous mechanical behavior of nanocrystalline binary alloys under extreme conditions

Fundamentally, material flow-stress increases exponentially at deformation rates exceeding, typically, ~103 s-1, resulting in brittle failure. The origin of such behavior derives from the dislocation-motion causing non-Arrhenius deformation at higher-strain-rates due to drag-forces from phonon interactions. Here, we discover that this assumption is prevented from manifesting when microstructural length is stabilized at an extremely fine size (nanoscale regime). This divergent strain-rate insensitive behavior is attributed to a unique microstructure that alters the average dislocation velocity, and distance traveled, preventing/delaying dislocation interaction with phonons until higher strain-rates than observed in known systems; thus enabling constant flow-stress response even at extreme conditions. Previously, these extreme loading conditions were unattainable in nanocrystalline materials due to thermal and mechanical instability of their microstructures; thus, these anomalies have never been observed in any other material. Finally, the unique stability leads to high temperature strength maintained up to 80% of the melting point (~1356 K). also, see

Yield strength and flow strength for NC-Cu-10at.%Ta’s data along with NC and UFG data reported in the literature at (a) quasi-static and (b) high strain rates

Cryogenic behavior of nanocrystalline binary alloys

The work looks to investigate the mechanisms at play in a stable NC alloy by mechanically testing stabilized NC-Cu-3Ta (at.%) over a range of cryogenic-temperatures from 273 to 113 K. The NC-Cu-3Ta represents an optimized Ta concentration; consequently, this work will focus on this alloy composition. The tested specimens will then have their microstructures characterized via transmission electron microscopy (TEM) to elucidate which deformation mechanisms are operating at different temperatures and address the microstructural stability.

Normalized yield stress for Cu-3Ta (at.%) plotted as a function of Zener-Hollomon parameter at various cryogenic and high temperatures and compared with that of pure Cu 
   
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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