Session G33: Metal Physics: Structural and Mechanical Properties including Alloys and Superalloys

Bulk Modulus of Spherical Palladium Nanoparticles by Chen-Mobius Lattice Inversion Method

Palladium is a precious and rare element that belongs to the Platinum group metals (PGMS) with the lowest density and melting point. Numerous uses of Pd in dentistry, medicine and industrial applications attracted considerable investment. Preparation and characterization of palladium nanoparticles have been conducted by many researchers, but very little effort has taken place on the study of Pd physical properties, such as, mechanical, optical, and electrical. In this study, Chen-Mobius lattice inversion method is used to calculate the cohesive energy and modulus of palladium. The method was employed to calculate the cohesive energy by summing over all pairs of atoms within palladium spherical nanoparticles. The modulus is derived from the cohesive energy curve as a function of particles' sizes. The cohesive energy has been calculated using the potential energy function proposed by (Rose et al., 1981). The results are found to be comparable with previous predictions of metallic nanoparticles.   

Using magnetization measurements to detect small amounts of plutonium hydride formation in plutonium metal

We report the formation of plutonium hydride in 2 at {\%} Ga-stabilized $\delta $-Pu, with 1 atomic {\%} H charging. We show that magnetization measurements are a sensitive, quantitative measure of ferromagnetic plutonium hydride against the nonmagnetic background of plutonium. It was previously shown that at low hydrogen concentrations, hydrogen forms super-abundant vacancy complexes with plutonium, resulting in a bulk lattice contraction. Here we use magnetization, X-ray and neutron diffraction measurements to show that in addition to forming vacancy complexes, at least 30{\%} of the H atoms bond with Pu to precipitate PuH$_{\mathrm{x}}$, largely on the surface of the sample with x $\sim$ 1.9. We observe magnetic hysteresis loops below 40 K with magnetic remanence, consistent with precipitates of ferromagnetic PuH$_{1.9}$.   

Influence of van der Waals corrected \textit{xc}-functionals on the anisotropic mechanical properties of coinage metals

Current materials-related calculations employ the density-functional theory (DFT), commonly using the (semi-)local-density approximations for the exchange-correlation (\textit{xc}) functional. The accuracy to studying the electronic structure depends not only on the employed approximation to the \textit{xc} potential but also upon the system which is being investigated. The difficulties in arriving at a reasonable description of van der Waals (vdW) interactions by DFT-based models, is to date a big challenge. This stems from the well-known fact that vdW interaction is a non-local correlation effect which is not captured in the deployed (semi-)local \textit{xc} functionals. In this work, using various flavours of vdW-corrected DFT \textit{xc} functionals, we study the lattice and mechanical properties (including the elastic constants and anisotropic stress-strain curves) of the coinage metals (copper, silver, and gold), and critically assess the reliability of the different vdW-corrected DFT methods in describing their anisotropic mechanical properties which are less reported on in the literature.   

High-precision study of time- and temperature-dependence of the elastic properties of $^{239}$Pu

It is important to determine the origin of changes in elastic properties in $^{239}$Pu as a function of time. The measurement of mechanical resonance frequencies can be made with extreme precision and used to compute the elastic moduli without corrections giving important insight in this problem. The precision of these measurements enabled observation of changes in elastic properties of 1 part in 107 for measurements lasting hours up to several days. The most-likely source of these changes include a) ingrowth of radioactive decay products such as He and U, b) the introduction of radiation damage, c) phase instabilities associated with transformations to the delta phase or to Pu$_{3}$Ga. Using Resonant Ultrasound Spectroscopy, measurements were made of the mechanical resonance frequencies of 300mg cylinders of fine-grained polycrystalline alpha-phase $^{239}$Pu with about 600PPM Ga. We present the surprising result that at temperatures below 60K, there is a strong dependence on temperature of the rate of change of elastic moduli with time. Older results showed that the sign of this rate of change reverses at higher temperature. Such studies of nascent state are key to exploring damage evolution and its impact on specific volume and elastic moduli. Future studies will continue these measurements to above ambient temperatures.   

First-principles study of energy, structure and atomic solubility of twinning-associated boundaries in hexagonal metals

HCP metals are widely used as structural materials in many industries, ranging from transport and energy to biomedical applications due to their low density, high specific strength. Twining is one of the important plastic deformation modes in HCP metals. Understanding atomic structure and chemistry of twin-associated boundaries is very crucial to improve mechanical properties of these HCP metals. In this work, using first-principles density function theory, we study twinning-associated boundaries (TBs), \{10$\bar{1}$n\} coherent twin boundaries (CTBs) and coherent basal-prismatic boundary (CBP) in six hexagonal metals (Cd, Zn, Mg, Zr, Ti and Be), with a focus on structure and solute's solubility at twin boundaries. We find that the formation of TBs is associated with creation of an excess volume. All the six metals show positive excess volume associated with (10$\bar{ 1}$1) and (10$\bar{1}$3) CTBs, but the excess volume associated with (10$\bar{ 1}$2) CTBs and CBP can be positive or negative depending on metal. To understand solubility at TBs, we calculated solubility of solute atoms in Mg, Ti, and Zr for solute positions in bulk, (10$\bar{ 1}$2) CTB and CBP boundaries and show that, in general, solute atoms have better solubility at CTB and CBP than in bulk.   

Ti$_{1-x}$Au$_x$ Alloys: Hard Biocompatible Metals and Their Possible Applications

The search for new hard materials is often challenging from both theoretical and experimental points of view. Furthermore, using materials for biomedical applications calls for alloys with high biocompatibility which are even more sparse. The Ti$_{1-x}$Au$_x$ ($0.22 \leq x \leq 0.8$) exhibit extreme hardness and strength values, elevated melting temperatures (compared to those of constituent elements), reduced density compared to Au, high malleability, bulk metallicity, high biocompatibility, low wear, reduced friction, potentially high radio opacity, as well as osseointegration. All these properties render the Ti$_{1-x}$Au$_x$ alloys particularly useful for orthopedic, dental, and prosthetic applications, where they could be used as both permanent and temporary components. Additionally, the ability of Ti$_{1-x}$Au$_x$ alloys to adhere to ceramic parts could reduce the weight and cost of these components.   

Ab initio phase stability at high temperatures and pressures in the V-Cr system

Vanadium metal has seen a surge in research, experimental and theoretical, driven mainly by its importance in applications but also because of its surprising destabilization of the body-centered cubic (bcc) ground-state phase close to 60 GPa. The phase stability of vanadium metal and vanadium-chromium alloys at high temperatures and pressures is explored by means of first-principles electronic-structure calculations. Utilizing the self-consistent \textit{ab initio} lattice dynamics approach in conjunction with density-functional theory, we show that pressure-induced mechanical instability of body-centered cubic vanadium metal, which results in formation of a rhombohedral phase at around 60 GPa at room temperatures, will prevail significant heating and compression. Furthermore, alloying with chromium decreases the temperature at which stabilization of the body-centered cubic phase occurs at elevated pressure. Computing support for this work came from the LLNL Computing Grand Challenge program. This work performed under the auspices of the U.S. DOE by LLNL under Contract DE-AC52-07NA27344 and funded by the Laboratory Directed Research and Development Program at LLNL under project tracking code 11-ER-033.   

Structure and dynamics of bulk liquid iron near melting. A first principles study

First principles molecular dynamics simulations, based on the density functional theory and the projector augmented wave technique, have been performed in order to study several static and dynamic properties of bulk liquid Fe at a thermodynamic state near melting. As for the static properties, the obtained results for the pair distribution function and the static structure factor show a good agreement with the available X-ray and neutron diffraction data. The calculated dynamical structure reveals collective density excitations with an associated dispersion relation which closely follows recent experimental data; moreover, its slope at the long-wavelength limit provides an estimate for the velocity of sound. The dynamical structure factors have been calculated and they are compared with their experimental counterparts, which have recently been measured by inelastic X-ray diffraction experiments. Finally, results are also reported for some transport coefficients.   

Evidence for reentrant strain glass behavior in a ferroelastic-martensitic system Ti-Pd-V

The concept of strain glass (SG) has recently received much attention because of its unique properties such as shape memory effect, superelasticity, and stress-tuned intelligent damping behavior. Recent in-situ TEM experiments have proved that, only local-symmetry change in the crystal structure has been observed during the SG transition, but the macroscopic symmetry or average structure is still keeping unchanged. In this presentation, we report the discovery of reentrant-strain-glass (RSG) behavior in a ferroelastic-martensitic system Ti50Pd50-xVx (x is from 6 to 12). Unlike the SG, with decreasing of temperature, the RSG system first undergoes a macroscopic martensitic transition and then the martensite variants further transforms to a frustrated glassy state below a critical temperature. The X-ray diffraction and high resolution TEM results further indicate the RSG no longer remains the average structure of the high-temperature parent phase, but rather low-temperature martensitic phase. This new discovery may open a new research field and may lead to new insights into a range of possible applications of this unique class of materials.   

Studying the enhanced ductility in bimodal nanocrystalline metals using a model with tunable crystallinity and crystallite stiffness

We propose a polycrystalline model composed of small and large particles, where crystallinity and stiffness of each crystallite can be separately tuned by varying the number ratio of small/large particles [H. Shiba et al., Phys. Rev. E. 81, 051501 (2010)], and implementing a pairwise interparticle n-6 Lennard-Jones (L-J) potential [Z. Shi et al. J. Chem. Phys. 135, 084513 (2011)], respectively. This simple model resembles the molecular structure of bimodal nanocrystalline metals containing crystallites of two sizes, where crystallite stiffness inversely proportional to its size. We conduct 2D molecular dynamics (MD) simulations to study the shear deformation of the model consist of stiff crystallites, soft crystallites, or stiff and soft crystallites. We find the flow stress increases monotonically with increasing crystallite stiffness, and it can be systematically adjusted via changing the ratio of soft/stiff crystallites. We address applying the results of our study to explain the enhanced ductility found in bimodal nanocrystalline metals, where shear localization and propagation are presumably weakened due to crystallites of two properties.   

Magnetic and Structural Properties in Non-Stoichiometric Gallium Deficient Ni$_2$MnGa$_{1-x}$ Heusler Alloys

Magnetic data show that off-stoichiometric gallium deficient Heusler alloys of the form Ni$_2$MnGa$_{1-x}$ have martensite transition temperatures that increase strongly with $x$, while their ferromagnetic Curie temperatures remain nearly unchanged. The martensite transition approaches room temperature for $x=0.13$. Within the tetragonal martensite phase, bulk magnetic properties depend strongly on stresses within the sample. These effects were investigated using post annealing, thermal cycling, and grinding. These treatments effect the bulk coercivity but do not move the transition temperatures. As the martensite forms, lattice elongations of $>3\%$ are observed using XRD. Domain properties are reported, for both structural grains and magnetic ones, within the martensite phase, from optical and MFM imaging.   

A search for new cobalt-based high temperature superalloys

The discovery of a high temperature Co$_3$(Al,W) [1] superalloy has provided a promising avenue for further search of other Co-based superalloys. The L1$_2$ Co$_3$(Al,W) system is found to have higher strength and melting temperature than common Ni-based alloys. The high strength of super alloys is generally attributed to the stable or metastable austentic face-centered cubic crystal structure. We performed an extensive series of ab-initio calculations to search for stable or metastable Co-based ternary alloys of the form Co$_3$(A$_{0.5}$B$_{0.5}$). A 32 atom cell special quasi random structure (SQS-32) is considered to mimic the properties of the alloy at high temperatures. The results from the DFT calculations for over 780 different Co-based ternary systems and the potential candidates of the future high temperature super alloys is presented.\\[4pt] [1] Sato $et$, $al$. ``Cobalt-base high temperature alloys. Science 2006; 312 (5770):90-1.''   

Ab Initio Investigation of He Bubbles at the Y2Ti2O7-Fe Interface in Nanostructured Ferritic Alloys

Nanostructured ferritic alloys are promising materials candidates for the next generation of nuclear reactors due to their ability to withstand high temperatures and pressures, high neutron flux and especially, the presence of high concentrations of transmutation product helium. ~As helium diffuses through the matrix, large number densities of complex oxide nanoclusters act as trapping sites for individual helium atoms and helium clusters. Consequently, there is a significant decrease in the amount of helium that reaches grain boundaries, mitigating the threat of pressurized bubble formation and embrittlement. In order to understand the trapping phenomena of the oxides, the interface between the nanoclusters and the iron matrix must be modeled. ~We present results obtained using density functional theory on the structural and thermodynamic properties of the Y2Ti2O7-Fe interface containing helium. ~In addition, helium bubbles of varying sizes have been introduced in order to observe the effects of a growing helium bubble   

First-principles investigation on mechanical properties of $\zeta$-Ta$_4$C$_{3-x}$

As a group of transition metal carbides, tantalum carbides are of great interest due to their high melting temperature and high strength. Among different tantalum carbide phases of different space group and C/Ta atom ratio, the trigonal phase $\zeta$-Ta$_4$C$_{3-x}$ attracts special attention as high volume fraction of the $\zeta$ phase is reported to increase the fracture toughness of a tantalum carbide matrix. Using first-principles method, the structural and mechanical properties of $\zeta$-Ta$_4$C$_{3-x}$ have been investigated. The calculation results show that the weak bonding between Ta atoms in Ta$_4$C$_3$ is further weakened when structural vacancies occupy the carbon sub-lattice in $\zeta$-Ta$_4$C$_{3-x}$. The (0 0 1) Ta surface is prominent to appear from surface energy and stacking fault energy calculations, consistent with the observed lamellar substructure during indentation process in fracture toughness measurements. The theoretical fracture toughness is derived from the Griffith relation, in comparison with the experimental results, to explain outstanding questions pertaining to mechanical properties of $\zeta$-Ta$_4$C$_{3-x}$.   

Structure Defect Property Relationships in Binary Intermetallics

Ordered intermetallics are light weight materials with technologically useful high temperature properties such as creep resistance. Knowledge of constitutional and thermal defects is required to understand these properties. Vacancies and antisites are the dominant defects in the intermetallics and their concentrations and formation enthalpies could be computed by using first principles density functional theory and thermodynamic formalisms such as dilute solution method. Previously many properties of the intermetallics such as melting temperatures and formation enthalpies were statistically analyzed for large number of intermetallics using structure maps and data mining approaches. We undertook a similar exercise to establish the dependence of the defect properties in binary intermetallics on the underlying structural and chemical composition. For more than 200 binary intermetallics comprising of AB, AB2 and AB3 structures, we computed the concentrations and formation enthalpies of vacancies and antisites in a small range of stoichiometries deviating from ideal stoichiometry. The calculated defect properties were datamined to gain predictive capabilities of defect properties as well as to classify the intermetallics for their suitability in high-T applications.