MultiScale Mechanics Lab
Indian Institute of Technology Madras
Research Overview
The thermomechanical properties of metallic alloys are greatly influenced by the underlying microstructural features (such as grain boundaries, triple junctions, dislocations and precipitate/second phase particle distribution etc.) which spans across several length scales. Therefore, uncovering the role of these microstructural constituents can aid in the bid to design energy efficient and safer structural components. However, conventional research approaches are inadequate in accelerating the development of advanced materials. Thus, there is a need to formulate multiscale approach, where complimentary experimental and modelling efforts can aid in the deeper understanding of the role microstructural features have on the overall mechanical behavior. Therefore, we @ MML strive to develop a research program that will focus on addressing these challenges by combining experimental and modelling techniques.
Past Research Projects
Multi-scale Assessment of the Effect of Hydrogen on Plasticity of α-Fe
The quantitative effects of interstitial hydrogen atoms on the plastic anisotropy of the body-centered cubic (BCC) phase of steels is unclear. An accurate experimental characterization of this behavior presents several challenges. Therefore, the current study employs a multi-scale approach, combining DFT, MD, and crystal plasticity techniques, to comprehensively investigate the impact of hydrogen concentration on dislocation-based plasticity in α-Fe, revealing a decrease in critical resolved shear stress and potential shear localization in a hydrogen-rich environment. The findings are incorporated into a non-Schmid crystal plasticity model for accurate assessment of meso-scale plastic deformation in polycrystalline α-Fe.
Electrochemical Polarization and Mechanical Behavior of Mg based Intermetallics
The room temperature ductility and mechanical properties of Mg are usually enhanced by alloying additions. Based on the thermomechanical processing, the presence of critical concentration of alloying element typically leads to the formation of stable binary intermetallic phases with Mg thereby, distinctly altering the microscopic electrochemical and mechanical properties of the alloy. However, the secondary intermetallic phases in Mg alloys are typically of sub-micron size, thus accurate electrochemical and mechanical characterization is a challenging issue. Using first-principles calculations, the electrochemical and mechanical behavior of various Mg intermetallics was comprehensively quantified. Overall, the computational framework provides an accurate screening tool that can assist in alloy design and development of coatings.
Effect of Hydrogen on Plastic Response of Metals
Among the various non-metallic solutes which comes from environment, hydrogen plays a significant role in determining macroscopic plastic behavior of broad class of metallic systems. In order to understand the role of hydrogen on the plastic deformation of pure metallic density functional theory calculations were utilized to evaluate ideal shear strength and elastic constants of several structural metals which are key input for deriving Peierls stress of dislocation using classical Peierls-Nabaro framework. Further several experimentals and DFT calculations were utilized to develop a classical multibody interatomic potential for Fe-H system.
Effect of Mechanical Deformation on Corrosion Behavior
In general, there is a lack of fundamental understanding on the role of mechanical deformation plays in the corrosion behavior. In this work, the role of mechanical deformation on the galvanic corrosion behavior was examined across a wide range of mechanical and electrochemical conditions using a novel modeling framework. A noticeable increase in the peak pit depth was observed due to the onset of plastic deformation. Furthermore, the presence of tensile loads was found to increase the tendency for localized corrosion. Overall, the findings presented here highlight the complex interactions that occur between the mechanical and electrochemical processes during stress assisted corrosion of galvanic joints.
Hydrogen Embrittlement
Hydrogen embrittlement (HE) is a phenomenon that affects both the physical and chemical properties of several intrinsically ductile metals. In this work, the effects of hydrogen on the defect interaction during mechanical deformation was examined using a multiscale perspective. For instance, the grain boundaries (GBs) act as preferential site for hydrogen aggregation eventually leading to intergranular failure. Therefore, we thoroughly examined a large database GBs to identify GBs that were immune to hydrogen segregation. GBs present an effective barrier to dislocation motion, thereby strengthening the material. The understanding of the interactions between the GBs and dislocations in a hydrogen rich environment is critical to gain insights into the events that lead to intergranular crack initiation. Therefore, we employed molecular dynamics to study the interactions between screw dislocations and several <111> tilt GBs in Fe. The segregation of adequate hydrogen along the GB was found to increase the resistance offered by the GBs to dislocation motion, thereby increasing the dislocation pile up size that can finally lead to an accumulation of sufficient strain energy to cause intergranular crack initiation.
The Role of Triple Junctions on the Structural Stability of NC materials
Nanocrystalline (NC) metals (mean grain sizes (d) ≤100 nm) have enhanced mechanical strength as compared to coarse-grained metals (d ≥ 1 mm), thus are a promising alternative as structural materials for future high energy nuclear reactors. However, during extreme conditions, the NC microstructure has been found to be thermodynamically unstable, thereby limiting its applicability. For small grain sizes (< 10 nm), the triple junctions (TJs) have been observed to have a significant contribution on the material behavior. Using atomistic simulations, we demonstrated that the strain energy evolution around the TJ can provide insights into the distinct segregation behavior of point defects and solute atoms. Next, the activation energy for vacancy diffusion was found to linearly increase for thermodynamically stable TJs, i.e. the TJs that had lower resolved line tension. Interestingly, the examination of solute binding behavior revealed a localized region of stable sites around the TJs aids in accommodation of high solute concentration at high temperatures.
Improving the Formability of Mg Alloys
In wrought magnesium alloys, the room-temperature plasticity is largely controlled by the limited number of active slip systems such as basal slip and extension twinning. The insufficient number of active slip systems therefore limits the broader structural applicability of Mg-alloys. Using first-principles calculations, we investigate the effects of several different alloying elements on the ideal shear resistance across various slip systems of Mg. The results reveal that the addition of a Ce, Y or Zr solute atom, decrease the ideal shear resistance; whereas, a substitution of a Sn, Li, Al or Zn atom, respectively, increases the ideal shear resistance of Mg. The electronic density of states and valence charge transfer calculations can explain the profound effect of various solutes on the shear resistance of Mg. Finally, this understanding could enable the development of Mg alloys with improved room temperature formability for structural applications.
The Role of Nitrogen on Hydride Formation in pure Nb
The absorption of hydrogen and subsequent hydride precipitation degrades the superconducting properties of Nb. The addition of various dopant elements, particularly nitrogen have shown to cause a decrease in hydride concentration. Nonetheless, the underlying mechanisms associated with kinetics of hydrogen and the thermodynamic stability of hydride precipitates are not well known. the presence of nitrogen significantly increased the energy barrier for hydrogen diffusion from one tetrahedral site to another interstitial site.
