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Dive into the research topics where M. Rajagopalan is active.

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Featured researches published by M. Rajagopalan.


JOM | 2014

Grain Boundary Segregation of Interstitial and Substitutional Impurity Atoms in Alpha-Iron

M. Rajagopalan; Mark A. Tschopp; K.N. Solanki

The macroscopic behavior of polycrystalline materials is influenced by the local variation of properties caused by the presence of impurities and defects. The effect of these impurities at the atomic scale can either embrittle or strengthen grain boundaries (GBs) within. Thus, it is imperative to understand the energetics associated with segregation to design materials with desirable properties. In this study, molecular statics simulations were employed to analyze the energetics associated with the segregation of various elements (helium, hydrogen, carbon, phosphorous, and vanadium) to four 〈100〉 (Σ5 and Σ13 GBs) and six 〈110〉 (Σ3, Σ9, and Σ11 GBs) symmetric tilt grain boundaries in α-Fe. This knowledge is important for designing stable interfaces in harsh environments. Simulation results show that the local atomic arrangements within the GB region and the resulting structural units have a significant influence on the magnitude of binding energies of the impurity (interstitial and substitutional) atoms. These data also suggest that the site-to-site variation of energies within a boundary is substantial. Comparing the binding energies of all 10 boundaries shows that the Σ3(112) boundary possesses a much smaller binding energy for all interstitial and substitutional impurity atoms among the boundaries examined in this study. Additionally, based on the Rice–Wang model, our total energy calculations show that V has a significant beneficial effect on the Fe grain boundary cohesion, while P has a detrimental effect on grain boundary cohesion, much weaker than H and He. This is significant for applications where extreme environmental damage generates lattice defects and grain boundaries act as sinks for both interstitial and substitutional impurity atoms. This methodology provides us with a tool to effectively identify the local as well as the global segregation behavior that can influence the GB cohesion.


Nature | 2016

Extreme creep resistance in a microstructurally stable nanocrystalline alloy

Kristopher A. Darling; M. Rajagopalan; M. Komarasamy; M. A. Bhatia; B. C. Hornbuckle; Rajiv S. Mishra; K.N. Solanki

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 size. 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 rotation. 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.


Materials research letters | 2017

The role of Ta on twinnability in nanocrystalline Cu–Ta alloys

M. A. Bhatia; M. Rajagopalan; Kristopher A. Darling; Mark A. Tschopp; K.N. Solanki

ABSTRACT Nanostructured Cu–Ta alloys show promise as high-strength materials in part due to their limited grain growth. In the present study, we elucidate the role of Ta on the transition from deformation twinning to dislocation-mediated slip mechanisms in nanocrystalline Cu through atomistic simulations and transmission electron microscopy characterization. In particular, computed generalized stacking fault energy curves show that as Ta content increases there is a shift from twinning to slip-dominated deformation mechanisms. Furthermore, heterogeneous twinnability from microstructural defects decreases with an increase in Ta. The computed effect of Ta on plasticity is consistent with the HRTEM observations. GRAPHICAL ABSTRACT IMPACT STATEMENT We show for the first time using atomistic simulations and TEM that, similar to grain size, the Tanano-particles can be used to tailor the governing deformation mechanisms in NC-alloys.


Materials research letters | 2018

On the roles of stress-triaxiality and strain-rate on the deformation behavior of AZ31 magnesium alloys

C. Kale; M. Rajagopalan; S. Turnage; B. C. Hornbuckle; Kristopher A. Darling; Suveen N. Mathaudhu; K.N. Solanki

ABSTRACT The presence of complex states-of-stress and strain-rates directly influence the dominant deformation mechanisms operating in a given material under load. Mg alloys have shown limited ambient temperature formability due to the paucity of active slip-mechanisms, however, studies have focused on quasi-static strain-rates and/or simple loading conditions (primarily uniaxial or biaxial). For the first time, the influence of strain-rate and stress-triaxiality is utilized to unravel the active deformation mechanisms operating along the rolling, transverse- and normal-directions in wrought AZ31-alloy. It is discovered that the activation of various twin-mechanisms in the presence of multiaxial loading is governed by the energetics of the applied strain-rates. IMPACT STATEMENT It is shown for the first time that the higher deformation energy associated with dynamic strain-rates, coupled with high-triaxiality, promotes detwinning and texture evolution in HCP alloys with high c/a ratio. GRAPHICAL ABSTRACT


Nature Communications | 2018

Anomalous mechanical behavior of nanocrystalline binary alloys under extreme conditions

S. Turnage; M. Rajagopalan; Kristopher A. Darling; P. Garg; C. Kale; B. G. Bazehhour; I. Adlakha; B. C. Hornbuckle; C. L. Williams; P. Peralta; K.N. Solanki

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).Metals deformed at very high rates experience a rapid increase in flow stress due to dislocation drag. Here, the authors stabilise a nanocrystalline microstructure to suppress dislocation velocity and limit drag effects, conserving low strain-rate deformation mechanisms up to higher strain rates and temperatures.


International Symposium on Magnesium Technology, 2017 | 2017

Dynamic behavior of an AZ31 alloy under varying strain rates and stress triaxialities

C. Kale; M. Rajagopalan; S. Turnage; B. Hornbuckle; K. Darling; Suveen N. Mathaudhu; K.N. Solanki

Determination of microstructural and mechanical response to real-world loading conditions is imperative for the development of accurate models to predict the failure behavior of structural materials. The dynamic behavior of magnesium alloys is of particular interest to structural industries as lightweight materials must be able to withstand high impact loading. This study examines the influence of dynamic strain rate on the deformation behavior of a polycrystalline, hot-rolled AZ31 Mg alloy under varying stress triaxialities. The high strain rate testing results indicate that an increase in triaxiality leads to a transition in the deformation mechanisms. Subsequent characterization of microstructure and fracture surfaces were correlated to the mechanical response observed. Finally, these findings provide critical insights into the role of stress-state on dynamic behavior of an AZ31 alloy.


Acta Materialia | 2014

Atomic-scale analysis of liquid-gallium embrittlement of aluminum grain boundaries

M. Rajagopalan; M. A. Bhatia; Mark A. Tschopp; David J. Srolovitz; K.N. Solanki


JOM | 2015

Effect of Ta Solute Concentration on the Microstructural Evolution in Immiscible Cu-Ta Alloys

B. C. Hornbuckle; T. Rojhirunsakool; M. Rajagopalan; Talukder Alam; G. P. Purja Pun; Rajarshi Banerjee; K.N. Solanki; Y. Mishin; L. J. Kecskes; Kristopher A. Darling


Materials & Design | 2017

Microstructural evolution in a nanocrystalline Cu-Ta alloy: A combined in-situ TEM and atomistic study

M. Rajagopalan; Kristopher A. Darling; S. Turnage; R.K. Koju; B. C. Hornbuckle; Y. Mishin; K.N. Solanki


JOM | 2017

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

M. Rajagopalan; I. Adlakha; Mark A. Tschopp; K.N. Solanki

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K.N. Solanki

Arizona State University

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S. Turnage

Arizona State University

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M. A. Bhatia

Arizona State University

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C. Kale

Arizona State University

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Y. Mishin

George Mason University

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R.K. Koju

George Mason University

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