Browsing by Author "Murali, Palla"

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Atomic Scale Fluctuations Govern Brittle Fracture and Cavitation Behavior in Metallic Glasses
(APS, 2011-11) Murali, Palla
We perform atomistic simulations on the fracture behavior of two typical metallic glasses, one brittle (FeP) and the other ductile (CuZr), and show that brittle fracture in the FeP glass is governed by an intrinsic cavitation mechanism near crack tips in contrast to extensive shear banding in the ductile CuZr glass. We show that a high degree of atomic scale spatial fluctuations in the local properties is the main reason for the observed cavitation behavior in the brittle metallic glass. Our study corroborates with recent experimental observations of nanoscale cavity nucleation found on the brittle fracture surfaces of metallic glasses and provides important insights into the root cause of the ductile versus brittle behavior in such materials.
Cavitation in materials with distributed weak zones: Implications on the origin of brittle fracture in metallic glasses
(Elsevier, 2013-04) Murali, Palla
Several experimental studies have shown that fracture surfaces in brittle metallic glasses (MGs) generally exhibit nanoscale corrugations which may be attributed to the nucleation and coalescence of nanovoids during crack propagation. Recent atomistic simulations suggest that this phenomenon is due to large spatial fluctuations in material properties in a brittle MG, which leads to void nucleation in regions of low atomic density and then catastrophic fracture through void coalescence. To explain this behavior, we propose a model of a heterogeneous solid containing a distribution of weak zones to represent a brittle MG. Plane strain continuum finite element analysis of cavitation in such an elastic-plastic solid is performed with the weak zones idealized as periodically distributed regions having lower yield strength than the background material. It is found that the presence of weak zones can significantly reduce the critical hydrostatic stress for the onset of cavitation which is controlled uniquely by the local yield properties of these zones. Also, the presence of weak zones diminishes the sensitivity of the cavitation stress to the volume fraction of a preexisting void. These results provide plausible explanations for the observations reported in recent atomistic simulations of brittle MGs. An analytical solution for a composite, incompressible elastic-plastic solid with a weak inner core is used to investigate the effect of volume fraction and yield strength of the core on the nature of cavitation bifurcation. It is shown that snap-cavitation may occur, giving rise to sudden formation of voids with finite size, which does not happen in a homogeneous plastic solid.
Crack propagation in staggered structures of biological and biomimetic composites
(Elsevier, 2017-01) Murali, Palla
A phase field model is used to study crack propagation in staggered structures that are commonly found in several biological and biomimetic composites. The composite is modelled by creating an elastic mismatch between the two phases, ‘mineral’ and ‘organic’ which form into a staggered brick and mortar type micro-structure. The huge disparity in the stiffness of the two constituent phases gives rise to a non-uniform stress field near crack tips in these materials. Depending on the arrangement of the mineral platelets, different mechanisms of crack propagation may be observed. We find that cracks propagate straight when the aspect ratio of the mineral platelets is higher than a critical value. For lower values of aspect ratio, the cracks tend to exhibit a tortuous crack path in which fracture predominantly occurs in the soft organic phase. This critical aspect ratio is found to be a function of the mineral volume fraction as well as the elastic modulus mismatch. For some configurations, micro cracking in regions close to the crack tips is also observed. A simple theory is presented to analyse the observed crack paths in staggered composites.
Ductile to brittle transition in the Zr41.2Ti13.75Cu12.5Ni10Be22.5 bulk metallic glass
(Elsevier, 2006-09) Murali, Palla
The variation of impact toughness, Γ, of a Zr41.2Ti13.75Cu12.5Ni10Be22.5 (Vitreloy-1) bulk metallic glass (BMG) within the temperature range of 123–423 K was evaluated by using an instrumented Charpy impact testing machine, in order to examine if the BMGs exhibit ductile-to-brittle transition (DBT) that is seen in rapidly quenched glasses. Results show an abrupt reduction in Γ when the testing temperature is lowered to below 150 K, implying that the BMGs are also prone to the DBT. Fractographic observations indicate a transition in the fracture mode; from ductile vein-like morphology above DBT to a cleavage-dominant fracture mode below it. Complimentary Vickers indentation measurements show no variation in hardness with temperature. However, the shear banded plastic regions that are typically seen around the indents were observed to be completely absent around the indents that were made at low temperatures, indicating that the inhomogeneous plasticity mediated by shear bands becomes inoperative below a critical temperature resulting in the DBT. This observation suggests that the minimum amount of free volume required for extensive plasticity (and hence high toughness) in metallic glasses is strongly dependent on the temperature. Testing of the structurally relaxed samples (through annealing at 530 K for 2.5 h that induces severe embrittlement at room temperature) at 423 K reveal almost complete recovery of Γ, supporting this hypothesis.
Effect of deposition orientations on dimensional and mechanical properties of the thin-walled structure fabricated by tungsten inert gas (TIG) welding-based additive manufacturing process
(Springer, 2020-02) Murali, Palla; Kala, Prateek
Welding-based additive manufacturing can potentially produce a cost-effective process for the production of dense metallic parts. Tungsten inert gas (TIG) welding-based additive manufacturing process uses wire as a filler material and offers a high deposition rate with low spattering. In this study, different orientations of wire feeding nozzle and TIG welding torch, such as front wire feeding (FWF), back wire feeding (BWF), and side wire feeding (SWF), were investigated for thin-walled metal deposition with enhanced dimensional accuracy and mechanical properties. The dimensional accuracy of thin-walls deposited at four different orientations were investigated in terms of deposition height and deposition width. The FWF orientation with higher wire feeding angle and SWF orientation produced poor dimensional accuracy in the deposition. FWF orientation with normal wire feeding angle and BWF orientation provided a decent dimensional accuracy and surface appearance. The deposited samples exhibited a similar trend for Vickers microhardness, residual stress, and microstructure for the four different wire feeding orientations.
Factors influencing deformation stability of binary glasses
(Wiley, 2008-03) Murali, Palla
A possible mechanism of strain accommodation in large deformation of glasses is crystallization; deformation stability is a measure of the resistance of glasses to crystallization. We study the effect of atomic size ratio and atomic stiffness parameter (related to the curvature of the interatomic potential) on deformation stability of binary glasses using molecular static simulations. The deformation stability of a glass is found to increase with increasing atomic size ratio and magnitude of the atomic stiffness, which is proportional to the bulk modulus of the pure crystalline system, as well as the ratio of atomic stiffnesses of constituent atoms. To understand the role of the above parameters on deformation stability, misfit energies of randomly substituted solid solution fcc crystals and glasses are compared for various atomic size ratios and atomic stiffness values. Unlike in fcc solid solution, the misfit energy of binary glasses is found to be insensitive to the atomic size ratio. It is also found that the packing fraction of glasses is insensitive to the atomic size ratio, consistent with the above result. Beyond a critical atomic size ratio, the misfit energy of fcc solid solution exceeds the energy of the glass, thus making the amorphous state completely stable to deformation induced crystallization. Our analysis shows that critical atomic size ratio decreases with increasing atomic stiffness which leads to an increase in the deformation stability of glasses.
Failure and toughness of bio-inspired composites: Insights from phase field modelling
(Elsevier, 2014-12) Murali, Palla
Using a phase field model we explore crack propagation in bio-inspired composites in which the mineral and organic phases are arranged in a layered fashion. We show how the crack paths can be drastically altered by varying the elastic modulus mismatch between the organic and mineral layers, and by changing the thickness of the organic layer. Depending on the modulus mismatch and the thickness of the organic layer, the crack can either propagate straight, can branch inside organic layer or can get deflected along the interface, leading to delamination. The mechanism that governs the crack trajectories are analysed in terms of energy distribution near the crack tip. The critical energy release rate of the composite is also analysed as a function of the thickness of the organic layer and the modulus mismatch. A considerable enhancement is achieved when the ratio of the elastic modulus of the organic to mineral phase is less than 0.2. In such cases, for a given modulus mismatch, the critical energy release rate attains a maximum only for an optimal thickness of the organic phase. The origin of the optimal thickness is also investigated.
Influence of Cooling Rate on the Enthalpy Relaxation and Fragility of a Metallic Glass
(Springer, 2007-08) Murali, Palla
Structural relaxation behavior of a rapidly quenched (RQ) and a slowly cooled Pd40Cu30Ni10P20 metallic glass was investigated and compared. Differential scanning calorimetry was employed to monitor the relaxation enthalpies at the glass transition temperature, T g , and the Kolrausch–Williams–Watts (KWW) stretched exponential function was used to describe its variation with annealing time. It was found that the rate of enthalpy recovery is higher in the ribbon, implying that the bulk is more resistant to relaxation at low temperatures of annealing. This was attributed to the possibility of cooling rate affecting the locations where the glasses get trapped within the potential energy landscape. The RQ process traps a larger amount of free volume, resulting in higher fragility, and in turn relaxes at the slightest thermal excitation (annealing). The slowly cooled bulk metallic glass (BMG), on the other hand, entraps lower free volume and has more short-range ordering, hence requiring a large amount of perturbation to access lower energy basins.
Mixed mode crack propagation in staggered biocomposites using phase field modelling
(Elsevier, 2020-01) Murali, Palla
Exceptional fracture resistance and specific strengths observed in several natural biocomposites have inspired many researchers to discern the underlying mechanisms responsible for their mechanical behavior. Staggering of stiff mineral platelets in the layers of organic phase akin to the brick and mortar configuration is understood to be one of the key factors contributing to their high elastic modulus and toughness. The elastic heterogeneties in these configurations are shown to cause crack branching and kinking, leading to the increased resistance to fracture. Most of the fracture mechanisms discussed in the literature intrinsically assume mode I fracture. The presence of mixed modes of deformation in staggered composites may give rise to new interesting fracture mechanisms. In this paper we study crack propagation in staggered composites under mixed mode conditions using a phase field method. We find four different crack trajectories which will depend on the elastic modulus mismatch, microstructure geometry and the mode mixity. For very high elastic moduli mismatch of organic matrix and the mineral, we find that the crack trajectories are nearly independent of the mode mixity and the cracks propagate without kinking. For moderate elastic modulus mismatch and high mode mixity ratio () we find that the cracks divert into the interface leading to interface delamination. The mechanism that controls the crack trajectories is analyzed in terms of maximum tangential stress and strain energy density criteria at the crack tip.
On factors influencing the ductile-to-brittle transition in a bulk metallic glass
(Elsevier, 2009-06) Murali, Palla
An experimental study to ascertain the ductile-to-brittle transition (DBT) in a bulk metallic glass (BMG) was conducted. Results of the impact toughness tests conducted at various temperatures on as-cast and structurally relaxed Zr-based BMG show a sharp DBT. The DBT temperature was found to be sensitive to the free-volume content in the alloy. Possible factors that result in the DBT were critically examined. It was found that the postulate of a critical free volume required for the amorphous alloy to exhibit good toughness cannot rationalize the experimental trends. Likewise, the Poisson’s ratio–toughness correlations, which suggest a critical Poisson’s ratio above which all glasses are tough, were found not to hold good. Viscoplasticity theories, developed using the concept of shear transformation zones and which describe the temperature and strain rate dependence of the crack-tip plasticity in BMGs, appear to be capable of capturing the essence of the experiments. Our results highlight the need for a more generalized theory to understand the origins of toughness in BMGs.
On the characteristic length scales associated with plastic deformation in metallic glasses
(AIP, 2012-05) Murali, Palla
Atomistic simulations revealed that the spatial correlations of plastic displacements in three metallic glasses, FeP, MgAl, and CuZr, follow an exponential law with a characteristic length scale ℓc that governs Poisson’s ratio ν⁠, shear band thickness tSB⁠, and fracture mode in these materials. Among the three glasses, FeP exhibits smallest ℓc⁠, thinnest tSB⁠, lowest ν⁠, and brittle fracture; CuZr exhibits largest ℓc⁠, thickest tSB⁠, highest ν⁠, and ductile fracture, while properties of MgAl lie in between those of FeP and CuZr. These findings corroborate well with existing experimental observations and suggest ℓc as a fundamental measure of the shear transformation zone size in metallic glasses.
Role of modulus mismatch on crack propagation and toughness enhancement in bioinspired composites
(APS, 2011-07) Murali, Palla
Natural materials such as nacre exhibit a high resistance to crack propagation, inspiring the development of artificial composites imitating the structure of these biological composites. We use a phase field approach to study the role played by the elastic modulus mismatch between stiff and soft layers on crack propagation in such bioinspired composites. Our simulations show that the introduction of a thin layer of a soft phase in a stiff matrix can lead to arrest of a propagating crack and can also lead to crack branching. The crack branching observed in the phase field model is analyzed using a cohesive zone approach. Further, we show that the toughness of such a composite can be substantially higher than that of its constituents.
Shear bands mediate cavitation in brittle metallic glasses
(Elsevier, 2013-04) Murali, Palla
Recent experimental studies have revealed nanoscale cavities and periodic corrugations on the fracture surfaces of brittle metallic glasses. How such cavitation in these materials leads to brittle failure remains unclear. Here we show, using atomistic and continuum finite element simulations, that a shear band can mediate cavity nucleation and coalescence owing to plastic flow confinement caused by material softening. This leads to brittle fracture as cavities nucleate and coalesce within a shear band, causing the crack to extend along it.
Strain accommodation in inelastic deformation of glasses
(APS, 2007-01) Murali, Palla
Motivated by recent experiments on metallic glasses, we examine the micromechanisms of strain accommodation including crystallization and void formation during inelastic deformation of glasses by employing molecular statics simulations. Our atomistic simulations with Lennard-Jones-like potentials suggests that a softer short range interaction between atoms favors crystallization. Compressive hydrostatic strain in the presence of a shear strain promotes crystallization whereas a tensile hydrostatic strain is found to induce voids. The deformation subsequent to the onset of crystallization includes partial reamorphization and recrystallization, suggesting important atomistic mechanisms of plastic dissipation in glasses.