Session R70: Metals: Structural and Shape Memory

Predicting γ′-Phase Stability in Co-Based Superalloys

The discovery of a cobalt-based γ/γ′-forming superalloy in 2006 inspired our computational investigation of the Co-Al-W system. We use the Moment Tensor Potentials (MTP) active-learning framework to predict the total energy, forces, and stresses for nearly 300,000 derivative superstructures in the Co-Al-W system. We report several new structures on the convex hull with the ordered L1₂-Co₃(Al, W) crystal structure, which may correspond to the experimentally discovered γ′ phase. Small additions of tantalum, titanium, and vanadium lower the formation enthalpy of the L1₂ structure, which could act as γ′ stabilizers at high temperature. While the total cost of the MTP analysis is about 1/1000^th the cost of a similar result using DFT, the difference between calculated and predicted results is less than 3 meV/atom. MTP can be used for high-throughput phase stability analysis for many kinds of materials, such as high-temperature superalloys, multi-principle element alloys, high-T_c magnetic materials, superhard materials, and electrochemical materials.   

Elastic properties of the Ni-containing metallic glasses.

Several different acoustic techniques can be used to determine the elastic constants of a material. We use resonant ultrasound spectroscopy (RUS) and acoustic pulse-echo (APE) to investigate the elastic properties of Ni_71.5Cr_5.6Nb_3.4P_16.5B₃, a bulk metallic glass alloy (Ni-BMG). The sample was made by a counter-gravity suction casting technique. Modeling of the measured spectrum of acoustic resonances for the sample in its cylindrical geometry was done with finite element analysis (FEA). FEA gives a longitudinal modulus (C11) of 300 GPa, and a Youngs modulus (C44) of 50 GPa (+/-5%). We compare these values with literature values for other Ni-bearing bulk metallic glass alloys.
  

Undercooling of gold nanodroplets levitated in a quadrupole ion trap in high vacuum

A nanoparticle levitated in vacuum has minimal thermal contact with its surroundings and can be heated efficiently with a laser beam. Precise control of the temperature of a levitated particle could facilitate thermodynamic measurements of materials with very high melting points. To establish such a measurement technique using a well-characterized material, we levitate a charged 200 nm gold nanoparticle in an AC quadrupole electric field trap in a vacuum chamber and use a 532 nm laser to illuminate and heat it. Accurate measurements of the mass of the particle are used to deduce the internal temperature of the particle from the rate of mass erosion. We have observed undercooling of gold particles, a phenomenon in which a melted droplet remains liquid when cooled below the usual freezing temperature. In our preliminary measurements, particles have remained liquid to temperatures around 190 K below the freezing point and have persisted in this state in excess of two minutes without refreezing. We will discuss progress toward the precise control of the temperature of a gold nanoparticle and prospects for the measurement of other materials.
  

Thermodynamic theory of crystal plasticity – formulation and application to fcc copper

We present a thermodynamic description of crystal plasticity. Our formulation is based on the thermodynamic dislocation theory (TDT) of Langer et al., which asserts the fundamental importance of an effective temperature that describes the state of configurational disorder and therefore the dislocation density of the crystalline material. We extend the TDT description from isotropic plasticity to crystal plasticity with many slip systems. Finite-element simulations show favourable comparison with experiments on polycrystal fcc copper under uniaxial compression. The thermodynamic theory of crystal plasticity thus provides a thermodynamically consistent and physically rigorous description of dislocation motion in crystals. We also discuss new insights about the interaction of dislocations belonging to different slip systems.   

Ab initio studies of TiPd-M (M= Os, Ru, Co) high temperature shape memory alloys

The TiPd alloy has a potential for high temperature shape memory applications due to its martensitic transformation capability from B2 to B19 at 823 K. Previous studies indicated that B2 TiPd is unstable displaying a negative C at room temperature. In order to improve its properties, the effects of partial substitution of Pd with either Os, Ru or Co were investigated using density functional theory. The calculations are carried out within PBE-GGA for the exchange correlation functional. It was found that the heats of formation decrease with an increase in Os and Ru concentration and increases with Co addition. The ductility of TiPd-M (M= Os, Ru and Co) is predicted using Pugh‘s ratio, Poisson‘s ratio and Cauchy pressure. It was found that the structures showed positive C above 18.75 at. % Os, 20 at. % Ru and 31 at. % Co, displaying condition of stability. The calculated moduli confirm that alloying with Os effectively increases hardness and ductility better compared to Ru and Co. Partial substitution of Pd with Os and Ru was found more effective as a strengthening elements and may enhance the martensitic transformation temperature of B2 TiPd alloy.   

First-Principles Study on the Role of Doped In in Fe–Pd–In Systems

Electronic states, for example, atomic positions, orbital hybridization, etc. determine crystal structures and these physical properties. The most stable crystal structure of binary Fe–Pd systems is known as a L1₂-type crystal structure[1, 2]. However, according to our latest experimental studies, a new Mille-feuille-like Fe–Pd structure is synthesized by adding a small amount of In, implying that In is the key to stabilizing the new system.
We performed first-principles calculations for the In-doped FePd₃ systems in order to understand the role of In in terms of its stability. There are two symmetrically different sites for Pd in the new system, which are labeled e and g, and In prefers the e site. This result is consistent with our recent experiments. From our analysis, we find that the distance between Fe, Pd, and In is the key factor in stabilizing the new system.
[1] T. B. Massalski et al., Binary Alloy Phase Diagrams (ASM International, ed. 2, 1990).
[2] R. V. Chepulskii et al., PRB 85, 144201 (2012).   

Molecular Dynamics (MD) Potential Development for Carbides

We utilized the reference-free (RF) MEAMfit code to generate many-body potentials of Embedded Atom Method (EAM) and Modified Embedded Atom Method (MEAM) for a variety of MC (M = Transition Metals, C = Carbon) systems with an emphasis on the M23C6 and MC carbide phases. These phases are the key ingredients to strengthen the Ni-based Superalloys at the grain boundary. By sampling of the results of energy, stress and force data from the DFT calculations, a set of transferable potentials can be produced and be utilized to model the mechanical properties of these carbides.   

Developing the Modified Embedded Interatomic Potential for Atomistic Simulations in Ni-Ti-Hf System

The NiTiHf alloy, owing to its superior mechanical behavior over the parent alloy NiTi (nitinol) has received a considerable attention in recent years as a promising alternative for shape memory and super-elastic applications. The substantial mechanical strength, and wide variability in martensitic transformation temperature of NiTiHf shape memory alloy have enabled its applications in numerous sectors ranging from bio-medical to aviation industries. However, the lack of atomic level understanding during its micro-structural engineering is inhibiting its further applicabilities. To unlock the atomic level information for an efficient shape memory application requires an interatomic potential to perform the atomistic simulations with a high predictive accuracy. Herein, we discuss the development of an optimized interatomic potential for NiTiHf system within the framework of modified embedded atom method (MEAM).
  

Accelerating the computational design of multi-principle element alloys

We present a metaheuristic hybrid Cuckoo-Search (CS) algorithm that overcomes NP-hard global optimization and produces ultrafast solutions for large-dimensional combinatorial problems, using Levy flights (global) and Monte Carlo (local) searches, which avoids local-minima traps that stagnate solutions. The hybrid-CS removes a roadblock to computational materials design of arbitrary MPEAs by enabling ``on-the-fly'' construction of optimized Super-Cell Random Approximates (SCRAPS) with extraordinary reduction in solution times, scaling linear with cell size and exhibiting strong scaling for parallel solution. For example, a 4-element, 128-atom cell [10⁷³⁺ space] in 45s or 5-element, 500-atom cell [10⁴¹⁵⁺ space] in 270s. For a 4-component 128-atom model, we find a factor of 12,600+ reduction in parallel [400+ in serial] execution over current limited strategies. SCRAPS has specified point and pair probabilities with proper Gaussian distributions. We present several example applications using electronic-structure-based energetics and phonons.   

Ab initio typical medium theory of substitutional disorder

By merging single-site typical medium theory with density functional theory we introduce a self-consistent framework for electronic structure calculations of materials with substitutional disorder which takes into account Anderson localization. The scheme and details of the implementation are presented and applied to the hypothetical alloy Li_cBe_1-c, and the results are compared with those obtained with the coherent potential approximation. Furthermore we demonstrate that Anderson localization suppresses long-range magnetic order for a very low concentration of
(i) carbon impurities substituting oxygen in MgO_1-cC_c$, and
(ii) manganese impurities substituting magnesium in Mg_1-cMn_cO for the low-spin magnetic configuration.   

Prediction of Stability, Short-range Order, and Phase Selection in Multi-Principal-Element-Alloys

We present the capabilities of KKR-CPA-based stability and short-range order (SRO) prediction in multi-component solid-solution alloys for general lattices. The refractory-based Al_xTiZrHf and Cantor-type FeMnCoCr are chosen to showcase the powerful predictive capabilities and insights found from our electronic-structure-based thermodynamic linear-response method. For AlxTiZrHf (x=1) we discovered a vacancy-mediated mechanism for phase selection driven by vacancy-atom SRO. Our X-ray diffraction data finds a variant of gamma-brass with 4 vacancies per cell (not 2, as in typical brasses), which was indicated by the calculated SRO. For fcc FeMnCoCr, we monitor energy, stacking fault energy, and SRO and find quantitative agreement with single-crystal data.   

Synthesis of nanocrystalline Mg-based alloy powders by mechanical alloying and their microstructural characterization

Nanocrystalline Mg₆₅Ni₂₅Y₅M₅ (M= Si, B, Ag) alloys have been produced by mechanical alloying from mixtures of pure crystalline Mg, Ni, Y and M powders using a Spex Industries Mill Model 8000 in order to characterize the influence of Si, B and Ag on the microstructure of Mg-Ni-Y alloy system. Microstructural evolution, thermal behavior and morphological changes of the mechanically alloyed powders at different stages of milling have been investigated using a variety of analytical techniques including x-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrometry (EDX), differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). The present results revealed that the crystallite size of Mg₆₅Ni₂₅Y₅M₅ (M= Si, B, Ag) powders decreased with increasing milling time and it was determined in the range of 18–30 nm by TEM observation. After 65 h of milling, different intermetallic phases such as Mg₂₄Y₅, MgB₂, Mg₂Ni and Mg₂Ni₃Si were obtained. According to SEM images, particle size of the powders decreased during mechanical alloying and their shape and distribution became uniform. The compositional homogeneity of the Mg₆₅Ni₂₅Y₅M₅ (M= Si, B, Ag) alloys after mechanical alloying were confirmed by EDX analyses.   

Ab initio investigation of Pb-based solder replacements

Alloys have been the focus of a large amount of research across different disciplines due to the ability to manipulate inherent properties of different materials. Due to the tunable nature of these materials, industrial needs can be more readily met with materials that are designed towards a specific purpose, as opposed to bulk materials that may not fit a unique need. One specific area that can be satisfied through the alloy process is the removal of industrial components that are harmful to humans, or their environment, with alloys that are equally effective. In this work, we have used density functional theory (DFT) and Ab Initio Molecular Dynamics (AIMD) to model materials at temperatures below and above their melting points. From these simulations, we are able to calculate the temperature dependence of the mechanical properties of materials which increases the quantitative understanding of the physical nature of ductility and how these properties can be captured on short time scales using first principles techniques.   

The Temperature Evolution of Dislocation Dynamics in Aluminum

A material’s response to its surroundings depends on both its native properties and the imperfections (defects) in its structure. While techniques exist to probe material defects, they are mainly limited to surface measurements or rastered scans that cannot measure the dynamics of irreversible processes. Dark-field X-ray microscopy can now directly image defects in single- and poly-crystals, resolving the lattice tilt and inclination with high sensitivity over long length-scales. I extend this novel technique to time-resolved studies that measure real-time movies to visualize how dislocations evolve at thermal equilibrium. These movies resolve the creep dynamics of dislocations in high-purity single-crystal aluminum, directly measuring their mobility and interactions by measuring the strain with 10^-5 resolution over hundreds of micrometers. Our results directly measure the dislocation velocity as a function of temperature, covering the final 6% of temperatures for T < T_melt. These findings have important implications for dislocation models that have relied on multi-scale modeling and indirect measurements.