Ilaksh Adlakha

This study focuses on investigating alternative computationally efficient techniques for numerically estimating the mesoscale (grain and sub-grain scales) stress and strain in volume elements within an elastic constitutive framework. The underlying principle here lies in developing approximations for the localization tensor that relates the stress and strain fields at the component level to the mesoscale, using low rank approximations. The study proposes two methods to build low rank approximations of localization tensor using different mathematical principles. Numerical results are presented to discuss the relative merits of low rank approximation vis-a-vis full scale simulations across various metals.

Hydrogen embrittlement limits the service life of various metallic components by causing a transition from a ductile to a brittle failure of inherently ductile alloys. In this work, using first-principles calculations, the effect of interstitial hydrogen on the ideal shear strength across various metals (Al, Ni, Fe, Nb, Ti, and Zr) and its implications on plasticity are discussed. The presence of hydrogen led to a volumetric expansion, which in turn had a key role in the observed shear strength response of cubic metals. However, in the case of HCP metals, the chemical contributions also have a significant part in the observed shear strength response. The interstitial hydrogen atom interacts strongly with valence d orbital metals (Ni, Fe, Nb, Ti, and Zr). Based on the Peierls-Nabarro framework, the presence of interstitial hydrogen reduces the Peierls stress across all the metals examined here. Finally, these findings provide insights to comprehensively understand hydrogen embrittlement.

Magnesium alloys have drawn considerable attention for several engineering applications, owing to their excellent properties like low density and high specific strength. 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 properties of the alloy. However, the secondary intermetallic phases in Mg alloys are typically of sub-micron size; thus, accurate electrochemical characterization is a challenging issue. Using first-principles calculations, the electrochemical behavior of various Mg intermetallics was comprehensively quantified. The electrochemical polarization behavior of the intermetallics was strongly dependent on surface-mediated properties and chemical bonding characteristics. Finally, the computational framework provides an accurate screening tool that can assist in alloy design and development of coatings.

Shell structure in core@shell nanoalloys is studied where the core comprises of smaller atoms and covered by a thin shell with larger atoms. Mismatch strain, due to the size difference between core and shell atoms, plays a key role in determining the shell arrangement. Binary alloy systems having a wide range of lattice mismatch are considered which include Ni-Ag, Co-Ag, Cu-Ag, Co-Pt, Ni-Pd, Rh-Au, and Ni-Cu. Beginning from very small sizes, transformations in the shell structure are sketched out up to large sizes of ∼ 12 nm. These changes are accompanied by reconstruction of {100} facets in the shell to pseudo hexagonal (p-Hex) surfaces. Results show that p-Hex reconstruction occurs in specific size windows. The stability regime and the fraction of p-Hex surfaces is strongly dependent on the lattice mismatch. Comparison of p-Hex and {111} surfaces reveal significant atomic pressure differences. Finally, shells that are thicker than a monolayer are considered and it is found that p-Hex reconstruction is favored in thicker shells as well.

Crack growth kinetics in crystalline materials is examined both from the point of continuum mechanics and discrete dislocation dynamics. Kinetics ranging from the Griffith crack to continuous elastic-plastic cracks are analyzed. Initiation and propagation of incipient cracks require very high stresses and appropriate stress gradients. These can be obtained either by pre-existing notches, as is done in a typical American Society of Testing and Materials (ASTM) fatigue and fracture tests, or by in situ generated stress concentrations via dislocation pile-ups. Crack growth kinetics are also examined using the modified Kitagawa–Takahashi diagram to show the role of internal stresses and their gradients needed to sustain continuous crack growth. Incipient crack initiation and growth are also examined using discrete dislocation modeling. The analysis is supported by the experimental data available in the literature.

Here the electrochemical and mechanical behavior for different Mg based intermetallics (Mg17Al12, MgZn2, Mg3Nd, Mg2Si, Mg24Y5, Mg2Ca, Mg12Ce, Mg12La, Mg2Cu, and Mg2Sn) was comprehensively quantified. First, a robust thermodynamic framework was developed that utilized first-principles calculations to accurately predict the electrochemical polarization behavior of the Mg based intermetallics. Based on the predicted corrosion potential, apart from Mg2Ca which behaves as an anode to the Mg matrix, the rest of the Mg based intermetallics act as a cathode. The electrochemical polarization behavior of the intermetallics was strongly dependent on surface mediated properties (surface energy and work function) and chemical bonding characteristics. Furthermore, the electrochemical behavior was sensitive to the atomic arrangement on the surface. Based on Bader analysis, it was found that the direction of electron flow between the constituent elements of the intermetallic (towards or away from Mg) strongly influenced the electrochemical behavior. The accurate quantifications of elastic constants for the Mg based intermetallics conclusively clarified the mechanical behavior of Mg2Ca and Mg2Cu. Finally, the computational framework provides an accurate screening tool that can assist in alloy design and development of coatings.

A multi-scale study was carried out to quantify the effect of interstitial hydrogen concentration on plasticity in α-Fe. In this work, the influence of hydrogen on the screw dislocation glide behavior was examined across several length-scales. The insights obtained were integrated to provide an accurate continuum description for the effect of hydrogen on the dislocation based plasticity in polycrystalline α-Fe. At the outset of this work, a new Fe[sbnd]H interatomic potential was formulated that enhanced the atomistic estimation of the variation in dislocation glide behavior in presence of hydrogen. Next, the dislocation core reconstruction observed due to the addition of hydrogen using atomistic simulations was validated with the help of large-scale DFT calculations based on the DFT-FE framework. Several atomistic simulations were carried out to comprehensively quantify the effect of hydrogen on the non-Schmid behavior exhibited during the dislocation glide in α-Fe. Finally, crystal plasticity simulations were carried out to understand the effect of hydrogen on the meso-scale deformation behavior of polycrystalline α-Fe.

A comprehensive assessment of interstitial hydrogen on the elastic behavior across different metals (Al, Ni, Fe, Nb, Ti, and Zr) was carried out using first-principles calculations. The volumetric strain introduced by interstitial hydrogen had a key role in the observed variation in elastic constants. However, in Nb, Ti, and Zr, the host and hydrogen atoms interact strongly which had a significant contribution towards the variation in elastic response due to the presence of hydrogen. The addition of hydrogen reduced the resistance to shear deformation along respective active slip systems for all the metals, except Nb. Similarly, the homogenized macroscopic approximation of Young's and shear moduli also demonstrated a drop with increasing hydrogen concentration across all the metals, apart from Nb. Finally, these findings accurately quantify the variation in elastic behavior of various metals when exposed to a hydrogen rich environment.

Cathodic plasma electrolytic nitriding (c-PEN) technique has been utilized to modify a low-alloy ferritic steel (2.25Cr-1Mo) surface and assess the effect of the c-PEN layer on corrosion, hydrogen permeation, and tribological behavior of the steel. The surface morphology and phase composition of the c-PEN-treated surface were analyzed, and it was found that the surface exhibits a globular network morphology of iron nitride and expanded ferrite. The potentiodynamic polarization results showed that the c-PEN treatment created an electrochemically noble surface compared to the untreated steel. Next, electrochemical hydrogen permeation experiments carried out on the nitrided surface exhibited a noticeable drop in hydrogen permeability, diffusivity, and reversible trap density of the steel. Furthermore, based on nanomechanical and tribological characterization, the c-PEN treatment was found to create a noticeably harder and wear-resistant surface. Overall, these findings demonstrate the applicability of c-PEN treatment to create a multi-functional coating for low-alloy steels that can assist in mitigating the effect of various harsh environments.

In this work, cerium conversion coating (CeCC) was deposited on AZ91D Mg alloy using potentiostatic polarization method combined with phosphate pore-sealing treatment. Initially, the optimum deposition parameters to obtain a crack-free surface were found. The characterization of coating revealed the presence of a nodular morphology of cerium oxide deposits. Next, the electrochemical behavior of the coated surface was assessed using potentiodynamic polarization and electrochemical impedance spectroscopy in 3.5 wt% NaCl solution. Based on electrochemical characterization, the coating exhibited a fivefold increase in the charge transfer resistance and a corresponding 76% reduction in corrosion rate, when compared to the bare surface. Furthermore, the conversion coating exhibited improved corrosion resistance when evaluated using the immersion test. Therefore, these findings demonstrate the feasibility of the potentiostatic method for creating nearly crack-free CeCC on Mg alloys, unlike conventional conversion coatings. Moreover, this approach holds great potential for effectively mitigating the corrosion issues in Mg alloys.