Department of Mechanical engineering
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Item Deformation and boundary motion analysis of a faceted twin grain boundary(Elsevier, 2024-05) Mishra, Radha RamanIn this article, molecular dynamics simulations are used to understand how a nickel bicrystal with faceted incoherent Σ3 grain boundaries responds to uniaxial tensile loading. The deformation response is studied over a wide range of temperatures (100 – 900 K) and strain rates (107 – 1010 s−1). The dislocation extraction algorithm and common neighbor analysis are employed to identify the deformation mechanisms. Our results reveal that the yield stress decreases with temperature and increases with strain rate; whereas the elastic modulus decreases with temperature and is independent of strain rate. Furthermore, incipient plasticity is detected ahead of the yield point at lower temperatures and lower strain rates. Interestingly, the incoherent twin grain boundaries are quite mobile under the uniaxial tensile loading at lower temperatures and lower strain rates. But this mobility decreased at higher temperatures and higher strain rates, thereby, confirming this faceted grain boundary's non-Arrhenius (anti-thermal) migration behavior even under mechanical loading. From a deformation perspective, the incoherent twin facet of the grain boundary served as the major source for stacking fault formation at lower temperatures and higher strain rates. However, with the increase in temperature, the stacking faults became shorter and originated from both the incoherent twin facet and the tips of coherent twin facet. These results are in qualitative agreement with the experimental results documented in the literature.Item A close-packed sphere model for characterising porous networks in atomistic simulations and its application in energy storage and conversion(Elsevier, 2024-05) Belgamwar, Sachin U.; Mishra, Radha Raman; Roy, TribeniHierarchical (micro, meso & macro) porosity in materials plays a crucial role in influencing the movement of ions which governs the energy and power density during energy storage and conversion. The extant available methods to characterise porosity across scales (nano to meso to macro) lacks rigour and accuracy. Having accurate assessment of the porosity in materials can unlock new designs of electrodes for energy efficient energy storage and conversion devices such as batteries, supercapacitors and fuel cells. Through this work, we report the systematic development of a method to fully characterise the carbon porous networks using a molecular dynamics simulation testbed. Our work entails modelling and simulation of porous carbon structures using quenched molecular dynamics (QMD) simulations using Gaussian Approximation potential (GAP) and benchmarking the results with prior literature. This modelling technique can reliably be used for quantitative characterisation of the interconnectivity in porous structures to study ionic movements and charge transfer mechanisms. A new parameter, namely nearest neighbour search (NNS) coefficient was introduced to quantify homogeneity and networking in the porous structures. NNS coefficient increased from 1.62 to 1.92 with decrease of the annealing temperature from 8000 K to 4000 K in carbon. The procedure outlined was although tested on porous carbon networks, but adaptable to study any other material system at multi-length scales.Item Synergistic effects of temperature and strain rate on tensile properties of simulated Ni-6Cu alloy with Σ3 non-Arrhenius grain boundary(Taylor & Francis, 2023) Mishra, Radha RamanComprehending the mechanical response of materials on an atomic level is pivotal in the optimisation of advanced materials with superior mechanical properties. This research article utilises the atomistic-scale based molecular dynamics simulations to report the uniaxial tensile behaviour of bicrystalline Ni-6Cu (Nickel – 94% and Copper – 6%) alloy incorporated with pre-existing faceted Σ3 [111] 60° {11 8 5} grain boundaries. The primary aim of this investigation is to comprehend the performance of bicrystalline Ni-6Cu alloy under varying thermodynamic conditions and to assess the influence of pre-existing faceted grain boundaries on its tensile behaviour. This work encompasses a range of strain rates (108 to 1010 1/s) and temperatures (spanning from 100 to 900 K) for the uniaxial tensile deformation simulations. The outcomes unveil that the Young’s modulus of Ni-6Cu alloy (with pre-existing faceted grain boundaries embedded in its domain) was inversely proportional to temperature and constant with respect to strain rate. For the same configuration, yield stress was inversely and directly proportional to temperature and strain rate, respectively. Interestingly, incipient plasticity in the tensile stress–strain response was observed at lower temperature and lower strain rate. From the microstructural point of view, at lower temperatures, the incoherent twin boundary served as a source for the nucleation of stacking faults; however, as the temperature increased, both the incoherent twin boundary and the tips of coherent twin boundary function as the source for stacking faults formation. Our simulations also verified the GB’s anti-thermal (or non-Arrhenius) migration behaviour even under tensile load.Item Exploring deformation mechanisms in a refractory high entropy alloy (MoNbTaW)(Elsevier, 2025-02) Mishra, Radha RamanUnderstanding the deformation behaviour of refractory high-entropy alloy (rHEA) at elevated temperatures are crucial due to their potential for high-temperature applications. In this study, molecular dynamics simulations were employed using a highly accurate machine learning- based forcefield to investigate the deformation behaviour of MoNbTaW rHEA under uniaxial tensile and compressive loading. Additionally, the dependency of deformation behaviour on the applied strain rates (5e8, 1e9, 5e9 and 1e10 s−1) and temperatures (300, 800, 1000 and 1200 K) was investigated. The yield strength of MoNbTaW rHEA increased by two-fold during compressive loading when compared to tensile loading. During tensile deformation, the BCC-FCC-other atom transition resulted in the formation of stripe-like twinning along the {112} plane. On the contrary, during compressive loading, BCC directly transitioned into other atoms, forming twinning that later acted as the nucleation sites for dislocations. These findings further demonstrate that the deformation mechanism during tensile loading is governed by the twinning mechanism, whereas during compressive loading, dislocation-induced plasticity plays a vital role.Item Analysis of material removal mechanism in nanoscale machining of copper(Sage, 2024-01) Sharma, Anuj; Roy, TribeniIn the current study, molecular dynamics modeling and simulation are carried out to analyse mechanisms in tool-workpiece interaction in nanoscale cutting. Various combinations of a/r ratios for constant r (r = tool edge radius and a = uncut chip thickness) are considered and different crystal orientations of the workpiece specimen are employed in the nanoscale cutting model. From the simulation, material anisotropy behavior is observed during the nanoscale cutting of copper material. Analysis at the molecular scale reveals that the crystal orientations family {1 1 0}<1 0 0> is hard to machine and the family of crystal planes {1 1 1}<1 1 0> is easiest to cut. While comparing in different crystal planes and directions, it was noticed that the material deformation in nanoscale machining takes place only in slip directions, that is, <1 1 0> family of directions. It is also found that as the uncut chip thickness is decreased, the cutting mechanism changes from shear plane cutting to plowing to sliding in Cu.Item A molecular dynamics simulation of wear mechanism of diamond tool in nanoscale cutting of copper beryllium(Springer, 2019-01) Sharma, AnujIn the present study, molecular dynamics simulation (MDS) is employed to study the wear mechanism of single crystal diamond tool during nanocutting of copper beryllium (CuBe). Two edge configurations, i.e., both sharp and worn out tools, are chosen to study the tool and workpiece interaction during the nanocutting of CuBe. Further, the study involves the experimental characterization techniques viz. scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and Raman spectroscopy to confirm the simulation results. The results of the molecular dynamics simulation (MDS) show that the presence of Be as a hard particle in workpiece material influences the cutting forces which subsequently causes degradation of the sharp edge of the diamond tool. Furthermore, the carbon (C) atoms removed from the tool react with Be particles and as a result, it causes the formation of beryllium carbide (BeC). Beryllium interaction with the blunt edge configuration of the tool causes amorphization at the tool edge. Raman spectroscopy of the used diamond tool on CuBe reveals the similar phenomena of amorphization of the diamond at the tool edge. Moreover, surface generation is dependent on the tool edge condition as blunt edge tool leads to smoother surface compared to the surface generated by sharp edge configuration.Item Molecular dynamics simulation to investigate the orientation effects on nanoscale cutting of single crystal copper(Elsevier, 2018-10) Sharma, AnujThe present study investigates the effect of six different crystal orientations on the nanoscale cutting operation carried out on single crystal copper (Cu) at various ratios of uncut chip thickness (a) to cutting edge radius (r). The study is focused on various aspects of cutting operation which include the material deformation mechanism, subsurface deformation, cutting forces, specific cutting energy, ploughing effect and surface roughness. Molecular dynamics simulation (MDS) was performed for the six orientations at five different a/r ratios varying from 0.1 to 2. The MDS results reveal that the material removal and shear deformation mechanisms are distinct for different crystal orientations. 〈1 1 0〉 is the slip direction along which the dislocations propagate predominantly. Maximum material removal occurs for the (0 0 1)[ 0] orientation which is 45% higher than its minimum counterpart in the (1 1 1)[1 0] crystal orientation at a/r equals to 2. Results of cutting forces as well as specific cutting energy plots show that the crystal orientation {1 1 0}〈0 0 1〉 is difficult to cut whereas crystal orientation {1 1 1}〈1 1 0〉 is easier to cut in nanoscale cutting of Cu. Based on the surface roughness calculation, it is observed that surface quality is highest for the crystal orientation while cutting on (0 0 1)[ 0] and lowest for the crystal orientation ()[1 0 0]. Nanocutting of Cu with larger dimensions used to reduce the size effect shows that there is an increase of ∼275% in the dislocation density for the maximum dislocated crystal orientation (0 0 1)[ 0] when the tool edge sharpness is increased from 2 nm to 5 nm.Item Molecular dynamics simulation of single discharge and dimensionless correlation with actual material removal in micro electrical discharge machining(Taylor & Francis, 2019-06) Roy, Tribeni; Sharma, AnujShorter time pulses and length scales in micro EDM (MEDM) makes it difficult to observe the material removal phenomena taking place at very small zone during discharge. Since numerical or analytical model considers material to be uniform/isotropic, it cannot predict the discrete effects viz. crystal structure distortion due to discharge. Also, little attention has been given to the mechanism of material removal in MEDM at the atomistic level. Therefore, to understand the behaviour of material removal and crystal structure distortion due to single spark in MEDM, MD simulations have been carried out. Based on MD simulations, it was found that the percentage of material removed by vaporisation (∼40%) was higher at spark energies used in this model as compared to spark energies used by Wong et al. (20% at 23.50 µJ). Conversion from FCC crystal structure to amorphous was observed at the top surface of crater in both cases; the one with higher spark energy has marginally higher distortion indicating higher amorphisation. A new method is proposed based on which a dimensionless correlation was obtained between results of MDS and experiments which relates the ratio of specific material removal at higher spark energy to that at lower energy for both MDS and experimentsItem Comparative analysis of mechanical properties for mono and poly-crystalline copper under nanoindentation – Insights from molecular dynamics simulations(Elsevier, 2022-02) Roy, Tribeni; Sharma, AnujCrystallographic orientation and grain size for monocrystalline and polycrystalline materials respectively play a critical role in defining their mechanical behaviour under nanoindentation. To understand their effects on mechanical properties, molecular dynamics (MD) simulations help in revealing the underlying physical phenomena governing the nanoindentation behaviour. This paper attempts to comparatively analyse and study the effects of crystallographic orientations of monocrystalline copper {(100), (110) and (111)} and critical grain size of polycrystalline copper on the nanoindentation response using MD simulations. The results obtained for indentation load vs. depth curve, hardness, dislocations, and elastic recovery were analysed for comparison. Cu(111) exhibited an average hardness of 12.62 GPa, which is 18.27% more than that of Cupoly. The pile-ups of 8 Å size were observed in Cupoly; and this was higher than any of copper system studied here. The dislocation extraction algorithm (DXA) analysis revealed that the total dislocations in Cu(111) was 34.23% and 153.8% lower than that of Cu(110) and Cupoly, respectively. Cu(111) comprised of highest Stair-rod dislocation along with LC and Hirth locks. Furthermore, a prismatic loop comprised of sessile dislocations also appeared in Cu(111). The elastic depth recovery rate for Cu(100) was 52.75%, 41.60% and 40.66% higher than that of Cu(110), Cu(111) and Cupoly, respectively. This study revealed that the nanoindentation based mechanical performances of monocrystalline copper systems, specifically Cu(111) were superior to any other copper systems.Item Atomistic analysis of the effect of cholesterol on cancerous membrane protein system: unfolding and associated resistance stresses under strain(Taylor & Francis, 2023-05) Rao, Venkatesh K.P.; Belgamwar, Sachin U.The low-cholesterol cancerous environment can affect the biophysical behaviour of transmembrane proteins. It is difficult to experiment and measure the dynamics of membrane protein systems when cholesterol concentration is decreasing. In this work, atomistic approach is adopted to investigate the transmembrane protein behaviour during lipid-bilayer separation under strain at different cholesterol concentrations. Finding shows that the decreasing cholesterol across membrane protein system leads to an increase in area-per-lipid and average tilt angle by 6.4% and 62.6%, respectively with decreased order parameter. This observation indicates that the decreased cholesterol concentration in a cancerous environment hinders the bonding and compactness of membrane protein system. Stretching and unfolding of protein were observed during bilayer separation and the resistance stresses decreased by 68.01% for decreasing cholesterol. The cholesterol molecules observed to be bonded with proteins. The investigation revealed that the cholesterol is an important constituent of membrane that impedes the diffusion and resist the detachment of protein at high concentration. Thereby, the transmembrane proteins can retain end terminals positions across the membrane and resist functional failure. This study showed that decreased cholesterol concentration causes significant changes in the biophysical behaviour of the membrane protein system that could trigger the mechanosensitivity of transmembrane proteins under mechanical perturbation.