Session R04: Mechanical, Structural and Thermal Behavior of Metals

Physical and Thermal Properties of Iron Meteorites below 300 K

The iron meteorites Canyon Diablo [IAB-MG] and Agoudal [IIAB] are primarily kamacite or taenite based Fe-Ni polycrystalline alloys that solidify from planetary cores over a timespan of t > 10⁶ years via thermal radiation into the vacuum of space. Depending on their Ni composition and cooling rates, iron meteorites crystallize into a fine octahedrite, coarse octahedrite, or hexahedrite crystal structure.

Using resonant ultrasound spectroscopy, the elastic constants (c₁₁, c₂₂, c₄₄) and Young’s modulus were measured as a function of temperature from 10-300 K. These measurements allow a comparison of how Ni percentage, crystallographic structure, and impact history effect the mechanical behavior of these meteorites. Specific heat capacity and thermal expansion measurements permit an analysis of the Grüneisen parameter as a function of the same temperature range.   

First-principles study for the enhancement of the stability of the precipitates in the Cu alloys

Cu is widely used as an electronic circuit component because of its high conductivity. However, in order to be used as connector materials, high strength as well as the high conductivity is required to maintain the desired shape. The Cu-Ti alloy is one of precipitation hardening alloys with the tensile strength of 800~1200MPa and the conductivity of 10~20%IACS. In general, the precipitation hardening alloys can be strengthened more by additional cold working after aging. However, we confirmed that these precipitates dissolve in Cu matrix during cold working, lowering the conductivity considerably. To obtain high conductivity and high strength simultaneously, it is essential to solve this problem. In this study, we tried to find a way to increase the stability of the precipitates in order to avoid dissolution of them during cold working. Using the density functional theory(DFT) calculations, we analyzed the characteristics of the precipitates in Cu-Ti alloy. To find additional elements to enhance the stability of the precipitates, we investigated the properties of them doped with 3d transition metals. Based on DFT results, we found a proper element stabilizing the precipitate and confirmed that the dissolution was inhibited after cold working in Cu-Ti alloys experimentally.   

Metallic Refractory Titanium Nitride: An alternative stable metal with tunable optical properties for high temperature plasmonic applications

The increasing demands of high temperature plasmonic applications require a refractory plasmonic material that is more stable than commonly used metals such as gold (Au). Although a dielectric, under different deposition conditions titanium nitride (TiN) can be converted into a metal-like material that is stable at high temperatures and with plasmonic performance similar to that of Au. We produce TiN films with tunable optical properties ranging from those characteristic of a dielectric to strong metallic behavior with high negative real part of its frequency dependent dielectric function. Various nanostructures formed from both Au and TiN were characterized and compared, using Fourier Transform Infrared Spectroscopy and Scanning Near Field Optical Microscopy. TiN nano-antennas show tunable and distinct localized surface plasmon resonances that are comparable to those of Au. Moreover, the TiN nano-antennas exhibit a significantly more robust optical response after being annealed at high-temperatures, demonstrating the suitability of TiN to act as a stable metal for high temperature plasmonic applications.   

First-Principles Molecular Dynamics Simulation of a Liquid Li-Sn as a Plasma-Facing Component

Liquid metals, with inherent disorder, have long been suggested as plasma-facing components (PFC) in fusion reactors due to their imperviousness to mechanical damage, thereby allowing for a self-healing and self-replenishing surface. The most promising candidates for liquid PFCs are lithium (Li) and the lithium-tin alloys (Li₃₀Sn₇₀ and Li₂₀Sn₈₀) due to their low melting points and evaporation rates. Although liquid Li properties have been studied extensively, further research into the behavior of LiSn is needed.

We present results of first-principles molecular dynamics simulations of liquid Li₃₀Sn₇₀ and its interaction with various concentrations of deuterium from the melting point up to 970 K. Static and dynamic properties such as pair distribution functions, diffusion coefficients, and viscosities are evaluated. Moreover, deuterium retention in the liquid Li-Sn alloy is compared to that of pure liquid Li. Overall, we provide insight into the atomic-scale behavior of Li₃₀Sn₇₀, which will be useful for further research into PFCs for fusion reactors.   

High Strength Al Alloy Thin Films: A Nanoscale Analysis of Microstructure and Interface Segregation

Aluminum alloy (AA) 7075 is commonly used in industries for manufacturing highly stressed structural materials including various parts of commercial aircraft, aerospace and transportation equipment because of its extremely high strength to density ratio. This yield strength relies primarily on the microstructural properties, particularly the structure (types, frequency, and connectivity), geometry (inclination and dihedral angle), and chemistry (segregation and precipitation) of the grain boundaries. Herein, experimental results on the microstructural features and atomic scale Cu grain boundary segregation of high strength Al alloy 7075 thin films are analyzed in detail using advanced microscopic characterization techniques. Two distinct types (point and parallel array) of Cu grain boundary segregation behavior are demonstrated for the Al alloy GBs for the first time; GBs misorientated by less than 28^o displayed point (highly segregated atomic column surrounded by low segregated columns) segregation behavior whereas more than the 28^o displayed parallel array (two highly segregated columns opposite to each other across the interface surrounded by low segregated columns) segregation behavior.   

Modeling dislocation evolution around precipitates in superalloy

In this paper we discuss a new approach to correlate the dynamic mechanical properties of Nickel based superalloys to their precipitate driven mechanism of dislocation evolution. These precipitates are responsible for arresting defects or dislocations and enhancing the life of a structural component. The mechanism of dislocation evolution across grain or phase boundaries give complex patterns whose initial stages can be correlated to strain rate dependent damping parameters. We show here molecular dynamic simulation of dislocation evolution under cyclic loading of Ni3Al precipitates in Ni matrix and make an attempt to relate the growth rate parameters to the size of the precipitate and damping constants. The mechanism of deformation under cyclic normal and shear loading are investigated. This is an ongoing effort to extend our understanding to more complex and realistic situation involving temperature effects and microstructure of the superalloys.
  

Atomistic simulation of dislocation-assisted γ-precipitate nucleation in Mg-Al alloys

Magnesium has drawn increasing interests as a lightweight material for applications in transportation and aerospace industries. Mechanical processing methods like equal channel angular extrusion (ECAE) have shown potential in controlling the morphology of the precipitate particles for enhanced precipitation hardening performance. Understanding how deformation affects the precipitation process is crucial for processing condition optimization, property prediction, and materials design.

We study the dislocation-assisted precipitate nucleation of Mg₁₇Al₁₂ precipitates in Mg-Al alloys. We determine the critical nucleus size under different composition and stress conditions. The critical nucleus size is very sensitive to the solute concentration. The local stress introduced by dislocations can significantly change the nucleation barrier and the nucleation path. The local stress not only affects the misfit strain energy, but also significantly alters the relative stability of the phases. As a result, although γ precipitate is denser than the α matrix, counter-intuitively nucleation is favored under tension conditions rather than compression conditions. We incorporate our results into continuum model and compare with recent ECAE experiments.   

Stabilizing the Simple Hexagonal Structure -- Alloys of Tin

Approximately thirty elements have ground state or room-temperature structures with only one atom in the unit cell: fcc (Cu, Ir, Pb, etc.), bct (Cr, Ta, W, etc.), bct (In), rhombohedral (Hg), and even simple cubic (Po) lattices. The simple hexagonal (sh) lattice is never seen, even though it has a higher symmetry than rhombohedral and the same number of near neighbors as the bcc lattice. Only element, tin, has even a relatively low-energy sh structure.

Experimentally, the sh structure is found in mercury-tin alloys. To understand this, we performed a series of Density Functional calculations on Hg_xSn_1-x and Cd_xSn_1-x structures by the virtual crystal approximation (VCA); supercell calculations with ordered placement of the Cd/Hg atom (e.g., one impurity atom per unit cell); and using high-throughput AFLOW calculations to sample larger supercells with random impurities.

Both Cd and Hg stabilize the simple hexagonal structure against α- and β-Sn. The success of the VCA suggests that this is a density of states effect, and we examine the effect of alloying on the density of states using all of our techniques.   

Phase Transformations in Cu-Zr-X High-Temperature Shape Memory Alloys Studied Using Nanocalorimetry and Materials Simulations

Integrating computational and experimental methods can speed up the discovery process of shape memory alloys and provide a better understanding of the martensitic transformation mechanisms. We use density functional theory (DFT), molecular dynamics (MD), and high-throughput nanocalorimetry to study the Cu-Zr-X high-temperature shape memory alloy system (X= Ni/Co/Hf). Nanocalorimetry shows the martensite-austenite transformation temperature and stability on thermal cycling as a function of composition. DFT and MD are used to map out the martensitic transformation pathways. We found that the martensite-austenite energy difference and transformation temperature are positively correlated, while the martensite-austenite lattice mismatch and transformation hysteresis are negatively correlated. Our results provide a map of the shape-memory properties of Cu-Zr-X, and can be extrapolated to a larger compositional space to search for novel shape memory alloys.   

Thermal Characterization of Nickel Titanium Shape Memory Alloys via Frequency Domain Thermoreflectance

Shape Memory Alloys (SMAs) have a wide range of applications due to their unique thermal-mechanical properties caused by the austenitic-martensitic phase transformation. One of the most promising applications is in elastocaloric cooling, which has the potential to become a small, environmentally friendly, and efficient alternative to vapor-compression systems. The SMA with the most promise is Nickel Titanium (NiTi) due to its stability and thermal-mechanical performance. The mechanisms that govern the thermal properties of NiTi are not well characterized and may be affected grain size, strain, and temperature. Using the frequency-domain thermoreflectance (FDTR) method, the thermal properties of NiTi SMAs with different grain sizes will be determined as a function of strain and temperature. FDTR is a pump-probe non-contact optical technique that can accurately determine the thermal properties such as heat capacity, thermal conductivity, and thermal boundary conductance of thin films and bulk materials. It is expected that measuring the thermal properties of NiTi will help to determine its viability as a candidate material for strain-induced elastocaloric cooling.   

Computational Investigation of the Mechanical Properties of Alloys

The nature of alloying is to combine other elements with a metal in order to make a unique material that possesses desired properties that are lacking in the separate constituents. However, with these novel materials there are new and exciting physical implications that need to be studied in order to collectively understand the alloy process; which is also crucial for their use in industrial applications. We have used DFT to calculate the mechanical properties of several bulk materials to gain a quantitative understanding of the nature of ductility. Specifically, using the VASP code, we have been able to calculate the elastic tensor of different materials, which allows for the calculation of elastic properties: including the bulk, shear, and Young’s moduli, Poisson’s ratio, and the Pugh ratio. We have applied this process to previously unstudied alloy materials with the goal of offering a further insight into the nature of ductility and a means to quantify it. Through Bader charge analysis, and calculation of the density of states, we have also worked to understand the significance of ductility as it relates to bonding. We anticipate that the proposed technique will help guide experimentalists in the development of ductile materials, in the interest of replacing Pb based solders.   

Glass transition and primary crystallization of Al86Ni6Y4.5Co2La1.5 metallic glass at heating rates spanning over six orders of magnitude

The glass transition and primary crystallization of melt-spun Al₈₆Ni₆Y_4.5Co₂La_1.5 metallic glass have been investigated at continuous heating covering more than six orders of magnitude of heating rates (0.083 K/s to 40,000 K/s). Differential fast scanning calorimetry (DFSC) and conventional differential scanning calorimetry (DSC) were employed, by which the glass transition kinetics was analyzed. In particular, it is shown that the heating rate dependence of the glass transition temperature can be described by a generalization of the Bartenev-Ritland equation utilizing the Vogel-Fulcher-Tammann equation for the description of the viscosity and the Maxwellian relaxation time. This result is confirmed by measurements of primary crystallization performed also over a wide range of scanning rates. The results reveal that the kinetics of crystallization follows a non-Arrhenius behavior. In the analysis a theoretical model is employed which takes into account the effect of decoupling of viscosity and diffusion, i.e., the breakdown of the Stokes-Einstein equation. In this way, a modified description of the crystal growth kinetics in the whole temperature range is obtained.   

Role of Fe and Co addition in the phase stabilization and magnetic properties of NiMnGa Magnetic Shape Memory Alloys

Magnetic Shape Memory Alloys (MSMAs) are a group of active materials that undergo phase transitions (resulting in large recoverable mechanical deformations) induced by temperature, stress and/or magnetic fields. Their superfast response and high energy density makes them ideal candidates to be implemented in the field of sensors and actuators. The magnetic properties of MSMAs depend on the magnetic coupling between atoms that, in turn, depends on the atomic positions within the lattice. We have performed powder neutron diffraction experiments in order to elucidate the crystal structure of the martensite phases of Ni₅₁Mn_28-xGa₂₁Y_x MSMAs, where the doping element is Y=Co,Fe and x=0,1,3,5. The analysis of the diffractograms obtained in the experiment shows that the x=0 alloy has a mixture of modulated orthorhombic and tetragonal phases; the Co-rich samples tend to stabilize the tetragonal phase towards a non-modulated one; and the Fe-rich samples tend to stabilize the orthorhombic phase. We have determined the atomic site occupancies in both the martensite and austenite phases of all the alloys studied, and explored their relation with the changes on the magnetic properties of the alloys.   

Spherical Nanomechanical Characterization of Novel Nanocrystalline Cu Cold Spray Manufactured Materials

The novel mechanical, microstructural, and functional properties associated with nanocrystalline metal materials has brought about interdisciplinary interest and curiosity. Nanostructured metals can be manufactured by way of multiple methods; however, during the course of this research the method studied herein is employs severe plastic deformation processing via cold spray powder deposition. TEM and SEM analysis confirmed the noteworthy grain refinement brought about by cold spray for two copper feed stocks. More specifically, the two types of copper powders were that of conventional gas atomized Cu as well as nanostructured Cu powders produced by way of spray drying and agglomeration. As such, the classical nanoindentation properties of hardness and Elastic modulus are experimentally determined using both static and dynamic nanomechanical testing. Dynamic spherical nanoindentation methodologies are employed to unveil the cold spray consolidated coating’s stress-strain response and flow behavior. The mechanical flow curves extracted herein suggest that the protocol refinements proposed by Leitner, Maier-Kiener, and Kiener may have fundamentally advanced spherical nanoindentation testing at large.