IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Crack growth kinetics in crystalline materials is examined both from the point of continuum mechanics and discrete dislocation dynamics. Kinetics ranging from the Griffith crack to continuous elastic-plastic cracks are analyzed. Initiation and propagation of incipient cracks require very high stresses and appropriate stress gradients. These can be obtained either by pre-existing notches, as is done in a typical American Society of Testing and Materials (ASTM) fatigue and fracture tests, or by in situ generated stress concentrations via dislocation pile-ups. Crack growth kinetics are also examined using the modified Kitagawa–Takahashi diagram to show the role of internal stresses and their gradients needed to sustain continuous crack growth. Incipient crack initiation and growth are also examined using discrete dislocation modeling. The analysis is supported by the experimental data available in the literature. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.

Department of Applied Mechanics

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Crystals

Analysis of the crack initiation and growth in crystalline materials using discrete dislocations and the modified kitagawa–takahashi diagram

An immiscible nanocrystalline (NC) copper-tantalum (Cu-Ta) alloy is shown to exhibit a stable microstructure under thermo-mechanical loading conditions with exceptional mechanical strength (i.e., 1200 MPa strength at 298 K) indicating anomalous deformation mechanisms as compared to microstructurally unstable nanocrystalline materials. Therefore, in this work, various aspects of strength partitioning in such NC Cu-Ta alloys are discussed and the role of tantalum nanoclusters on the dominant deformation mechanism is presented as a function of temperature. Toward this, initially, the mechanical responses of NC Cu-Ta alloy were measured under uniaxial compression experiments at various temperatures. Later, atomistic simulations were performed along with the high-resolution electron microscopy to identify and validate the rate limiting mechanism behind the plastic deformation in NC Cu-Ta alloys. In general, the observed trend through experiments and simulations identify a transition from a dislocation – nanocluster interaction mediated deformation mechanism to one controlled by grain boundary strengthening as the temperature increases. The former mechanism is shown here to have a crucial role in the observed strengthening behavior of microstructurally stable NC materials. Overall, the paper demonstrates that through effective nano-engineering techniques, it is expected to extend the scope of nanocrystalline materials to a number of engineering design applications. © 2018

Materials and Design

Thermo-mechanical strengthening mechanisms in a stable nanocrystalline binary alloy – A combined experimental and modeling study

Despite significant gains on understanding strengthening mechanisms in precipitate strengthened materials, such as aluminum alloys, there persists a sizeable gap in the atomistic understanding of how different precipitate types and their morphology along with dislocation character affects the hardening mechanisms. Toward this, the paper examines nature of precipitation strengthening behavior observed in the Al-Cu alloys using atomistic simulations. Specifically, the critical resolved shear stress is quantified across a wide range of dislocation-precipitate interactions scenarios for both θ′ and θ phase of Al2Cu. Overall, the simulations reveal that the dislocation character (edge or screw) plays a key role in determining the predominant hardening mechanism (shearing vs. Orowan looping) employed to overcome the θ′ Al2Cu precipitate. Furthermore, the critical shear stress and mechanism to overcome the precipitate is sensitivity to the position of the glide plane with respect to the precipitate and its orientation. Interestingly in our findings, the θ Al2Cu precipitate conventionally regarded as un-shearable particle was overcome by shear cutting mechanism for small equivalent precipitate radius, which agrees with recent TEM observations. These findings provide necessary information for the development of atomistically informed precipitate hardening models for the traditional continuum scale modeling efforts. © 2019 Elsevier B.V.

Journal of Alloys and Compounds

Revealing the atomistic nature of dislocation-precipitate interactions in Al-Cu alloys

Solute addition is an effective way to enhance mechanical properties, especially in magnesium based alloys due to the limited number of slip systems available for deformation at the room temperature. Hence, the effects of various alloying elements on ideal shear resistance (ISR) across different slip systems of Mg were investigated using first-principles calculations. The addition of a Ce, Y, or Zr solute atom was found to decrease ISR, whereas the substitution of a Sn, Li, Al, or Zn atom increased the ISR of Mg. The most active slip system in Mg changed from the basal partial (0001) to prismatic upon substitution of a Ce, Y, or Zr solute atom, whereas the addition of Sn, Li, Al, or Zn solute atom had negligible effect on the plastic anisotropy. Furthermore, the electronic density of states and valence charge transfer provides a quantum insight into the underlying factors influencing the observed softening/strengthening behavior. For instance, the electronic density of states calculation shows that the contribution from d states of Ce, Y, and Zr solute atoms decreases the electronic structure stability of their respective solid solution, thereby enhancing slip activities. Theoretical analyses were also performed, and a shearability parameter was introduced to understand the implications of the observed variation in ideal shear resistance on the macroscopic behavior of Mg alloys. © 2019, The Minerals, Metals & Materials Society.

Minerals, Metals and Materials Series

First-Principles Investigation of the Effect of Solutes on the Ideal Shear Resistance and Electronic Properties of Magnesium

Niobium provides the basis for all superconducting radio frequency (SRF) cavities in use, however, hydrogen is readily absorbed by niobium during cavity fabrication and subsequent niobium hydride precipitation when cooled to cryogenic temperatures degrades its superconducting properties. In the last few years the addition of dopant elements such as nitrogen has been experimentally shown to significantly improve the quality factor of niobium SRF cavities. One of the contributors to Q degradation can be presence of hydrides; however, the underlying mechanisms associated with the kinetics of hydrogen and the thermodynamic stability of hydride precipitates in the presence of dopants are not well known. Using first-principles calculations, the effects of nitrogen on the energetic preference for hydrogen to occupy interstitial sites and hydride stability are examined. In particular, the presence of nitrogen significantly increased the energy barrier for hydrogen diffusion from one tetrahedral site to another interstitial site. Furthermore, the beta niobium hydride precipitate became energetically unstable upon addition of nitrogen in the niobium matrix. Through electronic density of states and valence charge transfer calculations, nitrogen showed a strong tendency to accumulate charge around itself, thereby decreasing the strength of covalent bonds between niobium and hydrogen atoms leading to a very unstable state for hydrogen and hydrides. These calculations show that the presence of nitrogen during processing plays a critical role in controlling hydride precipitation and subsequent SRF properties. © 2018 IOP Publishing Ltd.

Superconductor Science and Technology

Revealing the role of nitrogen on hydride nucleation and stability in pure niobium using first-principles calculations

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). © 2018 The Author(s).

Nature Communications

Anomalous mechanical behavior of nanocrystalline binary alloys under extreme conditions

Solution strengthening or softening is an effective way to enhance mechanical properties, especially in magnesium based alloys due to their inability to activate adequate non-basal deformation mechanisms at the room temperature. Hence, using first-principles calculations, the effects of several different alloying elements on the ideal shear resistance across various slip systems of Mg were investigated. The results reveal that the addition of a Ce or Zr solute atom decreases the ideal shear resistance (softening); whereas, the substitution of a Sn, Li or Zn atom increases the ideal shear resistance of Mg (strengthening). The dominant slip system in Mg was found to change from the basal partial (0001)[101¯0] to prismatic (101¯0)[112¯0] with the addition of a Ce or Zr solute atom; whereas, the addition of a Sn, Li or Zn solute atom had negligible effect on the plastic anisotropy. Furthermore, the electronic density of states and valence charge transfer, which provides a quantum mechanical insight into the underlying factors influencing the observed softening/strengthening behavior, was probed. For instance, the electronic density of states calculations show that the contribution from d states of Ce and Zr solute atoms decreases the electronic structure stability of their respective solid solution, thereby enhancing slip activities. In the end, theoretical analyses were performed and a shearability parameter was introduced to understand the implications of the observed variation in ideal shear resistance on the macroscopic behavior of Mg alloys. © 2018 Acta Materialia Inc.

Acta Materialia

Effect of solutes on ideal shear resistance and electronic properties of magnesium: A first-principles study

Role of hydrogen on the incipient crack tip deformation behavior in α-Fe: An atomistic perspective

Effect of mechanical loading on the galvanic corrosion behavior of a magnesium-steel structural joint

The physics of embrittlement is dictated by the various interactions between the impurities/defects and the local structure in polycrystalline material systems. In this study, a physically motivated model that describes the degree of interaction of hydrogen (H) defects on the segregation behavior to α-Fe grain boundaries (GBs) is developed. Molecular statics simulations were performed to quantify the segregation behavior of 1–2 H atoms at various interstitial sites around the <100>, <110>, <111>, and <112> symmetric tilt GBs. The results provide insights into the concentration profile of hydrogen defects along different GBs. Furthermore, the model accurately links the intrinsic GB character by quantifying the segregation length scale for the individual GBs based on the segregation behavior of defects. Finally, the metrics provided in this work are essential to comprehensively understanding the effect of hydrogen on the macroscopic behavior of α-Fe. © 2017, The Minerals, Metals & Materials Society.

Department Of Civil Engineering

Energetics of Hydrogen Segregation to α-Fe Grain Boundaries for Modeling Stress Corrosion Cracking

In this article, a numerical investigation examining the influence of various mechanical loading scenarios on the corrosion behavior of a galvanic joint formed between a magnesium alloy and mild steel was carried out. A wide spectrum of mechanical conditions were examined to facilitate the development of a comprehensive understanding on the effect of mechanical deformation during galvanic corrosion. In general, we found that the onset of nonlinear deformation drastically accelerates the localized corrosion. Furthermore, subjecting the galvanic joint to alternating corrosion and fatigue loading was found to increase the observed localized corrosion drastically. Finally, the simulation results are discussed with respect to the identification and description of the detrimental physical phenomena that undermine the structural integrity of joints subjected to various mechanical loading scenarios. © 2017, The Minerals, Metals & Materials Society.

Role of Static and Cyclic Deformation on the Corrosion Behavior of a Magnesium-Steel Structural Joint

Dislocation core properties of tin (β-Sn) were investigated using the semi-discrete variational Peierls-Nabarro (SVPN) model. The SVPN model, which connects the continuum elasticity treatment of the long-range strain field around a dislocation with an approximate treatment of the dislocation core, was employed to calculate various core properties, including the core energetics, widths, and Peierls stresses for different dislocation structures. The role of core energetics and properties on dislocation character and subsequent slip behavior in β-Sn was investigated. For instance, this work shows that a widely spread dislocation core on the 110 plane as compared to dislocations on the 100 and 101 planes. Physically, the narrowing or widening of the core will significantly affect the mobility of dislocations as the Peierls stress is exponentially related to the dislocation core width in β-Sn. In general, the Peierls stress for the screw dislocation was found to be orders of magnitude higher than the edge dislocation, i.e., the more the edge component of a mixed dislocation, the greater the dislocation mobility (lower the Peierls stress). The largest Peierls stress observed was 365 MPa for the dislocation on the 101 plane. Furthermore, from the density plot, we see a double peak for the 0° (screw) and 30° dislocations which suggests the dissociation of dislocations along these planes. Thus, for the 101 〈101〉 slip system, we observed dislocation dissociation into three partials with metastable states. Overall, this work provides qualitative insights that aid in understanding the plastic deformation in β-Sn. © 2017 IOP Publishing Ltd.