Thien Duong

Ab-initio aprroach to the electronic, structural, elastic, and finite-temperature thermodynamic properties of Ti2AX (A = Al or Ga and X = C or N)

In this work, the electronic, structural, elastic, and thermodynamic properties of Ti2AX MAX phases (A = Al or Ga, X = C or N) were investigated using density functional theory (DFT). It is shown that the calculations of the electronic, structural, and elastic properties of these structures, using local density approximation (LDA) and generalized gradient approximation (GGA) coupled with projected augmented-wave (PAW) pseudopotentials, agree well with experiments. A thermodynamic model, which considers the vibrational and electronic contributions to the total free energy of the system, was used to investigate the finite-temperature thermodynamic properties of Ti2AX. The vibrational contribution was calculated using the supercell method, whereas the electronic contribution resulted from one-dimensional integration of electronic density of states (DOSs). To verify the model, the specific heats of pure elements were calculated and compared to experimental data. The DFT-D2 technique was used to calculate the ...

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.

Thermodynamic and mechanical stabilities of α- and β-Ta4AlC3 via first-principles investigations

Recently, it has been predicted that Ta4AlC3 is likely to exhibit an (α–β) polymorphic transformation at temperatures above 1873 K. However, recent X-ray diffraction and transmission electron microscopy experiments suggest on the other hand that the α phase remains stable up to temperatures close to the limit of experimental capabilities and no transition has yet been observed. While the matter has already been settled experimentally, in this work, we re-investigate the phase stability problem in Ta4AlC3 by using first-principles methods. The study was carried out by considering both thermodynamic and mechanical stabilities of the Ta4AlC3 polymorphs. Particularly, finite-temperature Gibbs free energies and elastic properties of the polymorphs were calculated using density functional theory. Calculation results reveal that the α phase continue to be stable even at temperatures exceeding 1875 K, which is in agreement with experimental results reported in literature.

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

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.

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

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.

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

MAX phases are layered carbides or nitrides with the general formula Mn+1AXn, which exhibit a unique combination of ceramic- and metal-like properties. The effect of stacking a number (determined by n) remains to be elucidated and a priori is not clear whether, for a given chemistry, n significantly changes the intrinsic deformation behavior of these systems. In this work, we have studied the intrinsic deformation behavior of Tin+1AlCn and Tan+1AlCn (n = 1 … 5) using DFT-based calculations. Surprisingly, the results suggest that the stacking number tends to have a minimal effect on the intrinsic mechanical behavior of the systems studied.

On the Interfacial Phase Growth and Vacancy Evolution during Accelerated Electromigration in Cu/Sn/Cu Microjoints

Abstract In this work, we integrate different computational tools based on multi-phase-field simulations to account for the evolution of morphologies and crystallographic defects of Cu/Sn/Cu sandwich interconnect structures that are widely used in three dimensional integrated circuits (3DICs). Specifically, this work accounts for diffusion-driven formation and disappearance of multiple intermetallic phases during accelerated electromigration and takes into account the non-equilibrium formation of vacancies due to electromigration. The work compares nucleation, growth, and coalescence of intermetallic layers during transient liquid phase bonding and virtual joint structure evolution subjected to accelerated electromigration conditions at different temperatures. The changes in the rate of dissolution of Cu from intermetallics and the differences in the evolution of intermetallic layers depending on whether they act as cathodes or anodes are accounted for and are compared favorably with experiments. The model considers non-equilibrium evolution of vacancies that form due to differences in couplings between diffusing atoms and electron flows. The significance of this work lies in understanding the vacancy transport due to the difference in intrinsic diffusion of elements in different features of the microstructure, and the severe unidirectional convective flux of atoms due to the enforced electron wind that ultimately paves the road to study nucleation of microvoids.