Anjana Talapatra

Stabilization of bcc Mg in Thin Films at Ambient Pressure: Experimental Evidence and ab initio Calculations

Recent experiments suggest that bcc Mg can be stabilized when grown together with bcc Nb in Mg/Nb multilayer films at ambient conditions. This finding is remarkable as (pure) bcc Mg has only been observed under very high pressures and is in fact (mechanically) unstable under room conditions. Density functional theory calculations were performed to gain insight into the stability of Mg in the bcc structure. Calculations of the thermodynamic, electronic and structural stability of bcc Mg show that this structure is in fact metastable under thin film conditions, when Mg grows epitaxially on bcc Nb, in agreement with experiments.

Real-time atomistic observation of structural phase transformations in individual hafnia nanorods

High-temperature phases of hafnium dioxide have exceptionally high dielectric constants and large bandgaps, but quenching them to room temperature remains a challenge. Scaling the bulk form to nanocrystals, while successful in stabilizing the tetragonal phase of isomorphous ZrO2, has produced nanorods with a twinned version of the room temperature monoclinic phase in HfO2. Here we use in situ heating in a scanning transmission electron microscope to observe the transformation of an HfO2 nanorod from monoclinic to tetragonal, with a transformation temperature suppressed by over 1000°C from bulk. When the nanorod is annealed, we observe with atomic-scale resolution the transformation from twinned-monoclinic to tetragonal, starting at a twin boundary and propagating via coherent transformation dislocation; the nanorod is reduced to hafnium on cooling. Unlike the bulk displacive transition, nanoscale size-confinement enables us to manipulate the transformation mechanism, and we observe discrete nucleation events and sigmoidal nucleation and growth kinetics.

Does aluminum play well with others? Intrinsic Al-A alloying behavior in 211/312 MAX phases

ABSTRACT The search for further control over the properties of MAX phases as well as the promise of discovering compounds with new functionalities has resulted in an increased interest in MAX solid solutions resulting from mixing in either the M, A, or X sublattices. The possibility of alloying MAX compounds not only enables finer tuning of their properties but can also be used to stabilize compounds that may otherwise be metastable in their pure state. In this letter, we present an ab initio-based investigation of the intrinsic alloying behavior in the A sublattice of Ti(Al,A)C, Zr(Al,A)C and Ti(Al,A)C MAX compounds. GRAPHICAL ABSTRACT IMPACT STATEMENT In this work, we present for the first time a comprehensive study of the intrinsic alloying tendencies in MAX solid solutions with (Al,A) mixing in the A sublattice.

On the stochastic phase stability of Ti2AlC-Cr2AlC

The quest towards expansion of the Mn+1AXn design space has been accelerated with the recent discovery of several solid solution and ordered phases involving at least two Mn+1AXn end members. Going beyond the nominal Mn+1AXn compounds enables not only fine tuning of existing properties but also entirely new functionality. This search, however, has been mostly done through painstaking experiments as knowledge of the phase stability of the relevant systems is rather scarce. In this work, we report the first attempt to evaluate the finite-temperature pseudo-binary phase diagram of the Ti2AlC-Cr2AlC via first-principles-guided Bayesian CALPHAD framework that accounts for uncertainties not only in ab initio calculations and thermodynamic models but also in synthesis conditions in reported experiments. The phase stability analyses are shown to have good agreement with previous experiments. The work points towards a promising way of investigating phase stability in other MAX Phase systems providing the knowledge necessary to elucidate possible synthesis routes for Mn+1AXn systems with unprecedented properties.

Mixing and electronic entropy contributions to thermal energy storage in low melting point alloys

Melting of crystalline solids is associated with an increase in entropy due to an increase in configurational, rotational, and other degrees of freedom of a system. However, the magnitude of chemical mixing and electronic degrees of freedom, two significant contributions to the entropy of fusion, remain poorly constrained, even in simple 2 and 3 component systems. Here, we present experimentally measured entropies of fusion in the Sn-Pb-Bi and In-Sn-Bi ternary systems, and decouple mixing and electronic contributions. We demonstrate that electronic effects remain the dominant contribution to the entropy of fusion in multi-component post-transition metal and metalloid systems, and that excess entropy of mixing terms can be equal in magnitude to ideal mixing terms, causing regular solution approximations to be inadequate in the general case. Finally, we explore binary eutectic systems using mature thermodynamic databases, identifying eutectics containing at least one semiconducting intermetallic phase as promising candidates to exceed the entropy of fusion of monatomic endmembers, while simultaneously maintaining low melting points. These results have significant implications for engineering high-thermal conductivity metallic phase change materials to store thermal energy.Melting of crystalline solids is associated with an increase in entropy due to an increase in configurational, rotational, and other degrees of freedom of a system. However, the magnitude of chemical mixing and electronic degrees of freedom, two significant contributions to the entropy of fusion, remain poorly constrained, even in simple 2 and 3 component systems. Here, we present experimentally measured entropies of fusion in the Sn-Pb-Bi and In-Sn-Bi ternary systems, and decouple mixing and electronic contributions. We demonstrate that electronic effects remain the dominant contribution to the entropy of fusion in multi-component post-transition metal and metalloid systems, and that excess entropy of mixing terms can be equal in magnitude to ideal mixing terms, causing regular solution approximations to be inadequate in the general case. Finally, we explore binary eutectic systems using mature thermodynamic databases, identifying eutectics containing at least one semiconducting intermetallic phase as pr...

First-Principles and Monte Carlo Studies of Magnetocaloric Effects

We have performed \textit{ab initio} electronic structure calculations and Monte Carlo simulations of magnetically frustrated intermetallic materials where complex magnetic configurations and chemical disorder lead to rich phase diagrams. With lowering of temperature, we find for magnetic Heusler alloys a ferromagnetic phase which transforms to an antiferromagnetic phase at the magnetostructural phase transition and to a cluster spin glass at still lower temperatures. We discuss chemical bonding features of Ni

Martensitic Transformations of Ni–Mn–X Heusler Alloys with X = Ga, In and Sn

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Out-of-plane ordering in quaternary MAX alloys: an alloy theoretic perspective

MnGa and the giant magnetocaloric effec of Ni-Mn-In with Co and Cr substitution as well as the origin of the magnetostructural transition.The numerical simulations allow a complete characterization of the magnetically frustrated materials.

Minimal effect of stacking number on intrinsic cleavage and shear behavior of Tin+1AlCn and Tan+1AlCn MAX phases

Martensitic transformations of rapidly quenched and less rapidly cooled Heusler alloys of type Ni–Mn–X with X = Ga, In and Sn are investigated by ab initio calculations. For the rapidly cooled alloys, we obtain the magnetocaloric properties near the magnetocaloric transition. For the less rapidly quenched alloys these magnetocaloric properties start to change considerably. This shows that none of the Heulser alloys is in thermal equilibrium. Instead, each alloy transforms during temper-annealing into a dual-phase composite alloy. The two phases are identified to be cubic Ni–Mn–X and tetragonal NiMn.

Computational Thermodynamics and Kinetics-Based ICME Framework for High-Temperature Shape Memory Alloys

ABSTRACT This letter is motivated by an apparent paradox, in that some quaternary systems (particularly in the Ti–Zr–Al–C system) have been shown to exhibit M-site out-of-plane ordering, while prior work and calculations by the present authors suggest endothermic interactions between Zr and Ti. In this letter we provide a resolution to this issue and provide a more extended analysis on the out-of-plane and in-plane ordering in the M sites of quaternary MAX alloys. The results provide further insights to develop criteria to predict potential out-of-plane ordering tendencies in other MAX systems.fx1 GRAPHICAL ABSTRACT IMPACT STATEMENT In this work, we resolve a recently evident contradiction between theoretical predictions for phase separation in the Ti–Zr–Al–C 312 MAX system and experimental observations that indicate out-of-plane ordering.