arXiv subjects

G. Sainath

Publications and source records attributed to G. Sainath.

At least 19 recordsLinked to original sources

arXiv2023

Size effects on dislocation starvation in Cu nanopillars : A molecular dynamics simulations study

Size plays an important role on the deformation mechanism of nanopillars. With decreasing size, many FCC nanopillars exhibit dislocation starvation state which is responsible for their high strength. However, many details about the dislocation starvation mechanism like how often it occurs, and how much is its contribution to the total plastic strain, are still elusive. Similarly, the size below which the dislocation starvation occurs in the nanopillars is not clearly established. In this context, the atomistic simulations have been performed on the compressive deformation of $<$110$>$ Cu nanopillars with size (d) ranging from 5 to 21.5 nm. The molecular dynamics (MD) simulation results indicate that the nanopillars deform by the slip of extended dislocations and exhibit dislocation starvation mainly at small sizes ( $<$ 20 nm). The frequency of the occurrence of dislocation starvation is highest in small size nanowires and it decreases with increasing size. Above the nanopillar size of 20 nm, no dislocation starvation has been observed. Further, we define the dislocation starvation strain and based on this, it has been shown that, the contribution of dislocation starvation state to the total plastic strain decreases from 70\% in small size nanopillars to below 5\% in large size pillars. The present results suggest that the dislocation starvation is a dominant phenomena in small size nanopillars.

cond-mat.mtrl-sci↗

arXiv2023

Effect of shape on mechanical properties and deformation behavior of Cu nanowires: An atomistic simulations study

We study the effect of nanowire shape on mechanical properties and deformation behaviour of Cu nanowires using atomistic simulations. Simulations were carried out on $[100]$ nanowires with different shapes such as triangular, square, pentagon, hexagon and circular.Results indicate yield strength is different for different shapes. In both cases, triangular nanowire exhibit the lowest yield strength, while circular nanowire is the strongest. Deformation in all the nanowires is dominated by partials slip and twinning. Due to twinning, different shapes expose different surfaces at the twinned region. All nanowires show ductile failure and square nanowire exhibits the highest failure strain, while it is lowest for triangular nanowire.

cond-mat.mtrl-sci↗

arXiv2022

Twin boundary reversibility characteristics in α-Fe

Understanding the grain boundary deformation dynamics is very crucial to designing materials with stable microstructures. With this quest, the deformation behavior of coherent twin boundary under cyclic shear loading has been studied in α-Fe using molecular dynamics simulations to understand the influence of strain amplitude and temperature. Twin boundary exhibited shear coupled migration along with cyclic irreversibility character at lower temperatures and gained almost perfect reversibility at higher temperatures of around 1500 K. TB exhibited more sliding than migration with an increase in temperature. The stress associated with the migration of twins was observed to fluctuate around its average value. This was caused by layer by layer propagation of twin boundary through the activity of 1/6 <111> = 1/12 <111> + 1/12 <111> type edge partial dislocations on "\{"112"\}" twin plane. Complete twin reversibility and complex TB migration were noticed in the presence of multiple parallel twin nanowires. Twin migration was observed at the larger size nanowires with a proper combination of boundary conditions and strain rates; otherwise, it was absent. The influence of shear strain amplitude in offsetting the twin boundary was found to be minimal.

cond-mat.mtrl-sci↗

arXiv2022

Twin interaction with $Σ$11 tilt grain boundaries in BCC Fe : Formation of new grain boundaries

It is well known that the twinning is an important mode of plastic deformation in nanocrystalline materials. As a result, it is expected that the twin can interact with different grain boundaries (GBs) during the plastic deformation. Understanding these twin-GB interactions is crucial for our understanding of mechanical behavior of materials. In this work, the twin interaction with different $Σ$11 symmetric and asymmetric tilt GBs has been investigated in BCC Fe using molecular dynamics (MD) simulations. The results indicate that twin nucleate from the crack or GB and, its interaction with $Σ$11 asymmetric tilt GBs leads to the formation of a new GB. This new GB consist of $<$100$>$ Cottrell type immobile dislocations. The detailed atomistic mechanisms responsible for this new GB formation have been revealed using atomistic simulations. Interestingly, the new GB formation has not been observed in the case of twin interaction with $Σ$11 symmetric tilt GBs.

cond-mat.mtrl-sci↗

arXiv2021

Atomistic simulations of twin boundary effect on the crack growth behaviour in BCC Fe

In this paper, the effect of twin boundaries on the crack growth behaviour of single crystal BCC Fe has been investigated using molecular dynamics simulations. The growth of an atomically sharp crack with an orientation of (111)$<$110$>$ (crack plane/crack front) has been studied under mode-I loading at constant strain rate. In order to study the influence of twin boundaries on the crack growth behaviour, single and multiple twin boundaries were introduced perpendicular to crack growth direction. The results indicate that the (111)$<$110$>$ crack in single crystal BCC Fe grows in brittle manner. However, following the introduction of twin boundaries, a noticeable plastic deformation has been observed at the crack tip. Further, increasing the number of twin boundaries increased the amount of plastic deformation leading to better crack resistance and high failure strains. Finally, an interesting relationship has been observed between the crack growth rate and flow stress.

cond-mat.mtrl-sci↗

arXiv2021

Role of twin boundary position on the yield strength of Cu nanopillars

It is well known that the twin boundary (TB) spacing plays an important role in controlling the strength of twinned metallic nanopillars. One of the reasons attributed to this strengthening behaviour is the force exerted by the TBs on dislocations. Since the TBs exert repulsive force on dislocations and the plasticity in nanopillars is surface controlled, it is interesting to know whether the TB position from the nanowire surface has any effect on the strength of twinned nanopillars. Using atomistic simulations, here we show that the TB position significantly influences the strength of twinned nanopillars. Atomistic simulations have been performed on nanopillar containing one and two TBs and their position is varied within nanopillar from the center to the surface. The results indicate that in nanopillar containing a single TB, the strength is maximum when the TB is located at the center of the nanopillar and it decreases as the TB is shifted towards the nanopillar surface. On the other hand in nanopillar containing two TBs, the maximum strength is observed when the twin boundaries are placed at distance of one fourth or one fifth the pillar size from the surfaces and it decreases when TBs are moved on either side. The present study demonstrates that the mechanical properties of the twinned nanopillars can be controlled by carefully tailoring the position of the TBs within the nanopillars.

cond-mat.mtrl-sci↗

arXiv2020

Atomistic mechanisms of twin-twin interactions in Cu nanopillars

Twinning is an important mode of plastic deformation in metallic nanopillars. When twinning occurs on multiple systems, it is possible that twins belonging to different twin systems interact and forms a complex twin-twin junctions. Revealing the atomistic mechanisms of how twin-twin interactions lead to different twin junctions is crucial for our understanding of mechanical behaviour of materials. In this paper, we report the atomistic mechanisms responsible for the formation of two different twin-twin interactions/junctions in Cu nanopillars using atomistic simulations. One junction contains two twin boundaries along with one $Σ$9 boundary, while the other contains five twin boundaries (five-fold twin). These junctions were observed during the tensile deformation of [100] and $[1\bar1 0]$ Cu nanopillars, respectively.

cond-mat.mtrl-sci↗

arXiv2019

Role of axial twin boundaries on deformation mechanisms in Cu nanopillars

In recent years, twinned nanopillars have attracted tremendous attention for research due to their superior mechanical properties. However, most of the studies were focused on nanopillars with twin boundaries (TBs) perpendicular to loading direction. Nanopillars with TBs parallel to loading direction have received minimal interest. In this backdrop, the present study is aimed at understanding the role of axial TBs on strength and deformation behaviour of Cu nanopillars using atomistic simulations. Tensile and compression tests have been performed on $<$112$>$ nanopillars with and without TBs. Twinned nanopillars with twin boundary spacing in the range 1.6-5 nm were considered. The results indicate that, under both tension and compression, yield strength increases with decreasing twin boundary spacing and is always higher than that of perfect nanopillars. Under compression, the deformation in $<$112$>$ perfect as well as twinned nanopillars proceeds by the slip of extended dislocations. In twinned nanopillars, an extensive cross-slip by way of Friedel-Escaig and Fleischer mechanisms has been observed in compression. On the other hand, under tensile loading, the deformation in perfect nanopillars occurs by partial slip/twinning, while in twinned nanopillars, it proceeds by the slip of extended dislocations. This extended dislocation activity is facilitated by stair-rod formation and its dissociation on the twin boundary. Similar to compressive loading, the extended dislocations under tensile loading also exhibit cross-slip activity in twinned nanopillars. However, this cross-slip activity occurs only through Fleischer mechanism and no Friedel-Escaig mechanism of cross-slip has been observed under tensile loading.

cond-mat.mtrl-sci↗

arXiv2019

Effect of size, temperature and strain rate on dislocation density and deformation mechanisms in Cu nanowires

In the present study, molecular dynamics (MD) simulations have been performed to understand the effect of nanowire size, temperature and strain rate on the variations in dislocation density and deformation mechanisms in $<$100$>$ Cu nanowires. The nanowire size has been varied in the range 1.446-43.38 nm with a constant length of 21.69 nm. Different temperatures varying from 10 K to 700 K and strain rates in the range of $5 \times 10^7$ - $1 \times 10^9$ s$^{-1}$ have been considered. For all the conditions, the variations in dislocation density $(ρ)$ has been calculated as a function of strain. The results indicate that the variations in dislocation density exhibits two stages irrespective of the conditions: (i) dislocation exhaustion at small strains followed by (ii) dislocation starvation at high strains. However, with decreasing size and increasing temperature, the rate of dislocation exhaustion increases, which results in early transition from dislocation exhaustion stage to dislocation starvation stage. Similarly, with increasing strain rate, the rate of dislocation exhaustion and also the transition strain increases.

cond-mat.mtrl-sci↗

arXiv2018

Effect of orientation and mode of loading on deformation behaviour of Cu nanowires

Molecular dynamics simulations have been performed to understand the variations in deformation mechanisms of Cu nanowires as a function of orientation and loading mode (tension or compression). Cu nanowires of different crystallographic orientations distributed uniformly on the standard stereographic triangle have been considered under tensile and compressive loading. The simulation results indicate that under compressive loading, the orientations close to $<$100$>$ corner deform by twinning mechanism, while the remaining orientations deform by dislocation slip. On the other hand, all the nanowires deform by twinning mechanism under tensile loading. Further, the orientations close to $<$110$>$ and $<$111$>$ corner exhibit tension-compression asymmetry in deformation mechanisms. In addition to deformation mechanisms, Cu nanowires also display tension-compression asymmetry in yield stress. The orientations close to $<$001$>$ corner exhibits higher yield stress in tension than in compression, while the opposite behaviour (higher yield stress in compression than in tension) has been observed in orientations close to $<$110$>$ and $<$111$>$ corners. For the specific orientation of $<$102$>$, the yield stress asymmetry has not been observed. The tension-compression asymmetry in deformation mechanisms has been explained based on the parameter $α_M$, defined as the ratio of Schmid factors for leading and trailing partial dislocations. Similarly, the asymmetry in yield stress values has been attributed to the different Schmid factor values for leading partial dislocations under tensile and compressive loading.

cond-mat.mtrl-sci↗

arXiv2018

Twinning to slip transition in ultrathin BCC Fe nanowires

We report twinning to slip transition with decreasing size and increasing temperature in ultrathin $<$100$>$ BCC Fe nanowires. Molecular dynamics simulations have been performed on different nanowire size in the range 0.404-3.634 nm at temperatures ranging from 10 to 900 K. The results indicate that slip mode dominates at low sizes and high temperatures, while deformation twinning is promoted at high sizes and low temperatures. The temperature, at which the nanowires show twinning to slip transition, increases with increasing size. The different modes of deformation are also reflected appropriately in the respective stress-strain behaviour of the nanowires.

cond-mat.mtrl-sci↗

arXiv2017

Atomistic simulations on ductile-brittle transition in <111> BCC Fe nanowires

Molecular dynamics simulations have been performed to understand the influence of temperature on the tensile deformation and fracture behavior of $<$111$>$ BCC Fe nanowires. The simulations have been carried out at different temperatures in the range 10-1000 K employing a constant strain rate of $1\times$ $10^8$ $s^{-1}$. The results indicate that at low temperatures (10-375 K), the nanowires yield through the nucleation of a sharp crack and fails in brittle manner. On the other hand, nucleation of multiple 1/2$<$111$>$ dislocations at yielding followed by significant plastic deformation leading to ductile failure has been observed at high temperatures in the range 450-1000 K. At the intermediate temperature of 400 K, the nanowire yields through nucleation of crack associated with many mobile 1/2$<$111$>$ and immobile $<$100$>$ dislocations at the crack tip and fails in ductile manner. The ductile-brittle transition observed in $<$111$>$ BCC Fe nanowires is appropriately reflected in the stress-strain behavior and plastic strain at failure. The ductile-brittle transition increases with increasing nanowire size. The change in fracture behavior has been discussed in terms of the relative variations in yield and fracture stresses and change in slip behavior with respect to temperature. Further, the dislocation multiplication mechanism assisted by the kink nucleation from the nanowire surface observed at high temperatures has been presented.

cond-mat.mtrl-sci↗

arXiv2016

Formation of pentagonal atomic chains in BCC Fe nanowires

For the first time, we report the formation of pentagonal atomic chains during tensile deformation of ultra thin BCC Fe nanowires. Extensive molecular dynamics simulations have been performed on $<$100$>$/{110} BCC Fe nanowires with different cross section width varying from 0.404 to 3.634 nm at temperatures ranging from 10 to 900 K. The results indicate that above certain temperature, long and stable pentagonal atomic chains form in BCC Fe nanowires with cross section width less than 2.83 nm. The temperature, above which the pentagonal chains form, increases with increase in nanowire size. The pentagonal chains have been observed to be highly stable over large plastic strains and contribute to high ductility in Fe nanowires.

cond-mat.mes-hall↗

arXiv2016

Influence of twist boundary on deformation behaviour of <100> BCC Fe nanowires

Molecular dynamics simulations revealed significant difference in deformation behaviour of $<$100$>$ BCC Fe nanowires with and without twist boundary. The plastic deformation in perfect $<$100$>$ BCC Fe nanowire was dominated by twinning and reorientation to $<$110$>$ followed by further deformation by slip mode. On the contrary, $<$100$>$ BCC Fe nanowire with a twist boundary deformed by slip at low plastic strains followed by twinning at high strains and absence of full reorientation. The results suggest that the deformation in $<$100$>$ BCC Fe nanowire by dislocation slip is preferred over twinning in the presence of initial dislocations or dislocation networks. The results also explain the absence of extensive twinning in bulk materials, which inherently contains large number of dislocations.

cond-mat.mtrl-sci↗

arXiv2016

Deformation behaviour of body centered cubic iron nanopillars containing coherent twin boundaries

Molecular dynamics simulations were performed to understand the role of twin boundaries on deformation behaviour of body-centred cubic (BCC) iron (Fe) nanopillars. The twin boundaries varying from one to five providing twin boundary spacing in the range 8.5 - 2.8 nm were introduced perpendicular to the loading direction. The simulation results indicated that the twin boundaries in BCC Fe play a contrasting role during deformation under tensile and compressive loadings. During tensile deformation, a large reduction in yield stress was observed in twinned nanopillars compared to perfect nanopillar. However, the yield stress exhibited only marginal variation with respect to twin boundary spacing. On the contrary, a decrease in yield stress with increase in twin boundary spacing was obtained during compressive deformation. This contrasting behaviour originates from difference in operating mechanisms during yielding and subsequent plastic deformation. It has been observed that the deformation under tensile loading was dominated mainly by twin growth mechanism, due to which the twin boundaries offers a negligible resistance to slip of twinning partials. This is reflected in the negligible variation of yield stress as a function of twin boundary spacing. On the other hand, the deformation was dominated by nucleation and slip of full dislocations under compressive loading. The twin boundaries offer a strong repulsive force on full dislocations resulting in the yield stress dependence on twin boundary spacing. Further, it has been observed that the curved twin boundary can acts as a source for full dislocation. The occurrence of twin-twin interaction during tensile deformation and dislocation-twin interaction during compressive deformation were presented and discussed.

cond-mat.mtrl-sci↗

arXiv2016

Directional Anisotropy of Crack Propagation Along $Σ$3 Grain Boundary in BCC Fe

Crack growth behaviour along the coherent twin boundary (CTB), i.e., $Σ$3{112} of BCC Fe is investigated using molecular dynamics (MD) simulations. The growth of an atomistically sharp crack with {112}$<$110$>$ orientation has been examined along the two opposite $<$111$>$ directions of CTB under mode-I loading at a constant strain rate. Separate MD simulations were carried out with crack inserted in the left side, right side and middle of the specimen model system. The results indicate that the crack grows differently along the two opposite $<$111$>$ directions. In case of a crack inserted in the left side, the crack grows in ductile manner, while it propagates in semi-brittle manner in the case of crack inserted in the right side. The directional dependence of crack growth along the CTB is also confirmed by the stress-strain behaviour. This anisotropy in crack growth behaviour has been attributed to the twinning-antitwinning asymmetry of 1/6$<$111$>$ partial dislocations on {112} planes.

cond-mat.mtrl-sci↗

arXiv2016

Fatigue Deformation of Polycrystalline Cu Using Molecular Dynamics Simulations

Molecular dynamics (MD) simulations have been performed to investigate the fatigue deformation behaviour of polycrystalline Cu with grain size of 5.4 nm. The samples were prepared using Voronoi algorithm with random grain orientations. Fatigue simulations were carried out by employing fully reversed, total strain controlled cyclic loading at strain amplitude of $\pm4$\% for 10 cycles. The MD simulation results indicated that the deformation behaviour under cyclic loading is dominated by the slip of partial dislocations enclosing the stacking faults. At higher number of cycles, the grain boundary migration leading to coarsening of larger grains at the expense of the smaller grains has been observed. The cyclic stress-strain behaviour, the deformation mechanisms and the variation of dislocation density as a function of cyclic deformation have been discussed.

cond-mat.mtrl-sci↗

arXiv2016

Molecular dynamics simulation of twin boundary effect on deformation of Cu nanopillars

Molecular dynamics simulations performed on <110> Cu nanopillars revealed significant difference in deformation behavior of nanopillars with and without twin boundary. The plastic deformation in single crystal Cu nanopillar without twin boundary was dominated by twinning, whereas the introduction of twin boundary changed the deformation mode from twinning to slip consisting of leading partial followed by trailing partial dislocations. This difference in deformation behavior has been attributed to the formation of stair-rod dislocation and its dissociation in the twinned nanopillars.

cond-mat.mtrl-sci↗