arXiv subjects

Karthik Guda Vishnu

Publications and source records attributed to Karthik Guda Vishnu.

4 recordsLinked to original sources

arXiv2020

Tuning martensitic transformations via coherent second phases in nanolaminates using free energy landscape engineering

We explore the possibilities and limitations of using a coherent second phase to engineer the thermo-mechanical properties of a martensitic alloy by modifying the underlying free energy landscape that controls the transformation. We use molecular dynamics simulations of a model atomistic system where the properties of a coherent, nanoscale second phase can be varied systematically. With a base martensitic material that undergoes a temperature-induced transformation from a cubic austenite to a monoclinic martensite, the simulations show a significant ability to engineer the transformation temperatures, from a ~50% reduction to a ~200% increase, with 50 at. % of the cubic second phase. We establish correlations between the properties of the second phase and the transformation characteristics and microstructure, via the free energy landscape of the two-phase systems. Coherency stresses have a strong influence on the martensitic variants observed and can even cause the non-martensitic second phase to undergo a transformation. Reducing the stiffness of second phase increases the transformation strain and modifies the martensitic microstructure, increasing the volume fraction of the transformed material. This increase in transformation strain is accompanied by a significant increase in the Af and thermal hysteresis, while the Ms remains unaltered. Our findings on the tunability of martensitic transformations can be used for informed searches of second phases to achieve desired material properties, such as achieving room temperature, lightweight shape memory alloys.

cond-mat.mtrl-sci↗

arXiv2020

Tunability of martensitic behavior through coherent nanoprecipitates and other nanostructures

Molecular dynamics simulations show that coherent precipitates can significantly affect the properties of martensitic transformations in Ni$_{63}$Al$_{37}$ alloys. The precipitates, consisting of non-martensitic Ni$_{50}$Al$_{50}$, modify the free energy landscape that governs the phase transformation and result in a significant reduction of the thermal hysteresis, at comparably minor expense of transformation strain, and modification of transformation temperatures. Importantly, this paper shows that free energy landscape engineering is possible with nanostructures potentially accessible through standard metallurgical processing routes. The atomistic-level nucleation and transformation mechanisms within the nanoprecipitate systems are explored and compared with epitaxial nanolaminates and nanowires. The simulations reveal three distinct regimes of transformation mechanisms and martensitic nanostructure as a function of volume fraction of the non-martensitic phase. Free energy landscape engineering is generally applicable and could contribute to the design of new shape memory alloys with novel properties, such as light weight alloys that operate at room temperature.

cond-mat.mtrl-sci↗

arXiv2020

Uncharacteristic second order martensitic transformation in metals via epitaxial stress fields

While most phase transformations, e.g. ferroelectric or ferromagnetic, can be first or second order depending on external applied fields, martensitic transformations in metallic alloys are nearly universally first order. We demonstrate that epitaxial stress originating from the incorporation of a tailored second phase can modify the free energy landscape that governs the phase transition and change its order from first to second. High-fidelity molecular dynamics simulations show a remarkable change in the character of the martensitic transformation in Ni-Al alloys near the critical point. We observe the continuous evolution of the transformation order parameter and scaling with power-law exponents comparable to those in other ferroic transitions exhibiting critical behavior. Our theoretical work provides a foundation to recent experimental and computational results on martensites near critical points.

cond-mat.mtrl-sci↗

arXiv2019

Ferroelectricity in free standing perovskite-based nanodots: A density functional theory study

We use density functional theory to investigate the possibility of polar and multiferroic states in free-standing, perovskite-based nanodots at their limit of miniaturization: single unit cell with termination to allow centrosymmetry. We consider both A-O and B-O2 terminations for three families of nanodots: i) A=Ba with B=Ti, Zr, and Hf; ii) A=Ca and Sr with B=Ti; and iii) A = Na and K with B=Nb. We find all A-O terminated dots to be non-polar and to exhibit cubic symmetry, regardless of the presence of ferroelectricity in the bulk. These dots all carry a net magnetic moment, except Ca8TiO6. On the other hand, all B-O2 terminated nanodots considered in this study relax to a non-cubic ground state. Rather surprisingly, a subset of these structures (BaTi8O12, BaHf8O12, BaZr8O12, SrTi8O12, CaTi8O12 and KNb8O12) exhibit polar ground states. We propose a new structural parameter, the cluster tolerance factor (CTF), to determine whether a particular combination of A and B will yield a polar ground state in the nanodot geometry, analogous to the Goldschmidt factor in bulk ferroelectrics. We also find that in all polar B-O2 terminated nanodots are also magnetic. The multiferroic state originates from separation between spin density in peripheral B atoms and polarity primarily caused by the off-center central A atom. Our findings stress that surface termination plays a crucial role in determining whether ferroelectricity is completely suppressed in perovskite-based materials at their limit of miniaturization.

cond-mat.mtrl-sci↗