Crystalline Metallic Materials Research Articles

Effect of casting temperature on the solidification process and (micro)structure of Zr-based metallic glasses

Bulk metallic glasses (BMGs) bear versatile advantages compared to traditional crystalline metallic materials. However, preparation of large size BMG components remains a great challenge due to the lack of understanding the casting dynamics of BMGs. The present work investigates the dynamics of the solidification process during casting of a Zr41.2Ti13.8Cu12.5Ni10Be22.5 BMG by measuring the time-dependent temperature evolution and constructing distance–time curves showing the progress of solidification. The experimentally probed temperature evolution proves that a higher casting temperature causes a lower cooling rate and induces more pronounced nanocrystal phase formation in the final BMG casting. Thus, optimization of the casting temperature is crucial for casting BMG components. The solidification coefficient K of the studied Zr-based BMG in copper mold was determined. The solidification time can be calculated with model: τ=M2K2. The solidification of the Zr-based BMG occurs in a narrow transition layer region following a “layer-by-layer” solidification mode. This feature is vital for designing the casting process appropriately to avoid the formation of casting defects in BMG components. The present work sheds new light on the solidification mechanism of bulk glass-forming liquids and underlines the importance of optimizing the technology and processing conditions for fabrication of BMG components with practical applications.

On the origins of fatigue strength in crystalline metallic materials.

Metallic materials experience irreversible deformation with increasing applied stress, manifested in localized slip events that result in fatigue failure upon repeated cycling. We discerned the physical origins of fatigue strength in a large set of face-centered cubic, hexagonal close-packed, and body-centered cubic metallic materials by considering cyclic deformation processes at nanometer resolution over large volumes of individual materials at the earliest stages of cycling. We identified quantitative relations between the yield strength and the ultimate tensile strength, fatigue strength, and physical characteristics of early slip localization events. The fatigue strength of metallic alloys that deform by slip could be predicted by the amplitude of slip localization during the first cycle of loading. Our observations provide a physical basis for well-known empirical fatigue laws and enable a rapid method of predicting fatigue strength as reflected by measurement of slip localization amplitude.

Some Thermomagnetic and Mechanical Properties of Amorphous Fe75Zr4Ti3Cu1B17 Ribbons.

The microstructure, revealed by X-ray diffraction and transmission Mössbauer spectroscopy, magnetization versus temperature, external magnetizing field induction and mechanical hardness of the as-quenched Fe75Zr4Ti3Cu1B17 amorphous alloy with two refractory metals (Zr, Ti) have been measured. The X-ray diffraction is consistent with the Mössbauer spectra and is characteristic of a single-phase amorphous ferromagnet. The Curie point of the alloy is about 455 K, and the peak value of the isothermal magnetic entropy change, derived from the magnetization versus external magnetizing field induction curves, equals 1.7 J·kg−1·K−1. The refrigerant capacity of this alloy exhibits the linear dependence on the maximum magnetizing induction (Bm) and reaches a value of 110 J·kg−1 at Bm = 2 T. The average value of the instrumental hardness (HVIT) is about 14.5 GPa and is superior to other crystalline Fe-based metallic materials measured under the same conditions. HVIT does not change drastically, and the only statistically acceptable changes are visibly proving the single-phase character of the material.

Enhancement of Interfacial Hydrogen Interactions with Nanoporous Gold-Containing Metallic Glass.

Contrary to the electrochemical energy storage in Pd nanofilms challenged by diffusion limitations, extensive metal-hydrogen interactions in Pd-based metallic glasses result from their grain-free structure and presence of free volume. This contribution investigates the kinetics of hydrogen-metal interactions in gold-containing Pd-based metallic glass (MG) and crystalline Pd nanofilms for two different pore architectures and nonporous substrates. Fully amorphous MGs obtained by physical vapor deposition (PVD) co-sputtering are electrochemically hydrogenated by chronoamperometry. High-resolution (scanning) transmission electron microscopy and corresponding energy-dispersive X-ray analysis after hydrogenation corroborate the existence of several nanometer-sized crystals homogeneously dispersed throughout the matrix. These nanocrystals are induced by PdHx formation, which was confirmed by depth-resolved X-ray photoelectron spectroscopy, indicating an oxide-free inner layer of the nanofilm. With a larger pore diameter and spacing in the substrate (Pore40), the MG attains a frequency-independent impedance at low frequencies (∼500 Hz) with very high Bode magnitude stability accounting for enhanced ionic diffusion. On the contrary, on a substrate with a smaller pore diameter and spacing (Pore25), the MG shows a larger low-frequency (0.1 Hz) capacitance, linked to enhanced ionic transfer in the near-DC region. Hence, the nanoporosity of amorphous and crystalline metallic materials can be systematically adjusted depending on AC- and DC-type applications.

Crystal Plasticity Phase-Field Model with Crack Tip Enhancement Through a Concurrent Atomistic-Continuum Model

This paper develops a method for physics-based augmentation of the Helmholtz free energy density functionals, used in coupled crystal plasticity phase-field finite element (CP-PF FE) models of fracture in crystalline metallic materials. Specifically, the defect and crack surface energy components are enhanced with terms that mechanistically account for the presence of atomic-scale, crack-tip nucleated dislocations. The additional terms in the free energy representation are motivated and calibrated by energy equivalence between a concurrent atomistic–continuum scale model and the CP-PF FE model. The atomistic domain of the concurrent model incorporates a time-accelerated, molecular dynamics (MD) LAMMPS code, while the continuum domain is modeled by a dislocation-density crystal plasticity FE model. The concurrent model transfers and transforms discrete dislocations in the atomistic domain to dislocation densities in the crystal plasticity domain. Dislocation densities are transported in the continuum domain by solving the advection equation using a particle-based reproducing kernel particle method with collocation. A new form of the defect energy density is proposed by considering the effect of crack-tip nucleated dislocations. Parameters in the augmented defect and surface energies are evaluated by comparing with the concurrent model results. A comparison of crack propagation with and without contributions from the nucleated dislocations demonstrates a significant effect of nucleated dislocations on the evolution of the crack.

A strain rate dependent thermo-elasto-plastic constitutive model for crystalline metallic materials

The strain rate and temperature effects on the deformation behavior of crystalline metal materials have always been a research hotspot. In this paper, a strain rate dependent thermo-elasto-plastic constitutive model was established to investigate the deformation behavior of crystalline metal materials. Firstly, the deformation gradient was re-decomposed into three parts: thermal part, elastic part and plastic part. Then, the thermal strain was introduced into the total strain and the thermo-elastic constitutive equation was established. For the plastic behavior, a new relation between stress and plastic strain was proposed to describe the strain rate and temperature effects on the flow stress and work-hardening. The stress–strain curves were calculated over wide ranges of strain rates (10–6–6000 s−1) and temperatures (233–730 K) for three kinds of crystalline metal materials with different crystal structure: oxygen free high conductivity copper for face centered cubic metals, Tantalum for body centered cubic metals and Ti–6Al–4V alloy for two phase crystal metals. The comparisons between the calculation and experimental results reveal that the present model describes the deformation behavior of crystalline metal materials well. Also, it is concise and efficient for the practical application.

Mechanical, Electromagnetic, and Tribological Properties of Microspirals Made from Amorphous and Crystalline Metallic Materials

Mechanical, Electromagnetic, and Tribological Properties of Microspirals Made from Amorphous and Crystalline Metallic Materials

A concurrent atomistic-crystal plasticity multiscale model for crack propagation in crystalline metallic materials

This paper develops a coupled concurrent atomistic–continuum multiscale model for analyzing crack propagation and associated mechanisms in crystalline metallic materials. This modeling framework can be used to develop effective continuum-scale constitutive models for crack evolution in deforming domains characterized by crystal plasticity. The atomistic region is modeled by the molecular dynamics (MD) code LAMMPS, while the continuum region is modeled by a dislocation density crystal plasticity FE model. A novel method is developed to transfer discrete dislocations in the atomistic domains to dislocation densities in the continuum domain. Propagation of dislocation densities in the continuum domain is modeled by the advection equation of a conserved quantity using the reproducing kernel particle method (RKPM) in conjunction with the collocation method. Validation studies are conducted by comparing results of the concurrent model with those by MD for nickel single crystal specimen with an embedded crack. An analytical model of the crack tip nucleated dislocation density evolution is developed for inclusion in crystal plasticity models. The effect of applied strain-rate and temperature on the parameters of this model are studied.

Parallel hybrid Monte Carlo / Molecular Statics for Simulation of Solute Segregation in Solids

A parallel hybrid Monte Carlo/molecular statics method is presented for studying segregation of interstitial atoms in the solid state. The method is based on the efficient use of virtual atoms as placeholders to find energetically favorable sites for interstitials in a distorted environment. MC trial moves perform an exchange between a randomly chosen virtual atom with a carbon atom followed by a short energy minimization via MS to relax the lattice distortion. The proposed hybrid method is capable of modeling solute segregation in deformed crystalline metallic materials with a moderate MC efficiency. To improve sampling efficiency, the scheme is extended towards a biased MC approach, which takes into account the history of successful trial moves in the system. Parallelization of the hybrid MC/MS method is achieved by a Manager-Worker model which applies a speculative execution of trial moves, which are asynchronously executed on the cores. The technique is applied to an Fe-C system including a dislocation as a symmetry breaking perturbation in the system.

Unified Model for Size-Dependent to Size-Independent Transition in Yield Strength of Crystalline Metallic Materials.

Size-dependent yield strength is a common feature observed in miniaturized crystalline metallic samples, and plenty of studies have been conducted in experiments and numerical simulations to explore the underlying mechanism. However, the transition in yield strength from bulklike to size-affected behavior has received less attention. Here a unified theoretical model is proposed to probe the yield strength of crystalline metallic materials with sample size from nanoscale to macroscale. We show that the transition in yield strength versus size can be fully explained by the competition between the stresses required for dislocation source activation and dislocation motion, which is regulated by dislocation density, irradiation defect, grain boundary, and so on. Based on various grain boundary densities, the extended Hall-Petch relation, incorporated into the unified model, captures the reverse size effect for polycrystalline samples. The proposed model predictions agree well with reported experimental measurements of various specimens, including the prestrained nickel, irradiated copper, ultrafine grain tungsten, and so on.

Large scale ab-initio simulations of dislocations

We present a novel methodology to compute relaxed dislocations core configurations, and their energies in crystalline metallic materials using large-scale ab-intio simulations. The approach is based on MacroDFT, a coarse-grained density functional theory method that accurately computes the electronic structure with sub-linear scaling resulting in a tremendous reduction in cost. Due to its implementation in real-space, MacroDFT has the ability to harness petascale resources to study materials and alloys through accurate ab-initio calculations. Thus, the proposed methodology can be used to investigate dislocation cores and other defects where long range elastic effects play an important role, such as in dislocation cores, grain boundaries and near precipitates in crystalline materials. We demonstrate the method by computing the relaxed dislocation cores in prismatic dislocation loops and dislocation segments in magnesium (Mg). We also study the interaction energy with a line of Aluminum (Al) solutes. Our simulations elucidate the essential coupling between the quantum mechanical aspects of the dislocation core and the long range elastic fields that they generate. In particular, our quantum mechanical simulations are able to describe the logarithmic divergence of the energy in the far field as is known from classical elastic theory. In order to reach such scaling, the number of atoms in the simulation cell has to be exceedingly large, and cannot be achieved with the state-of-the-art density functional theory implementations.

Effect of the stress ratio on the fatigue behavior of Zr55Al10Ni5Cu30 bulk metallic glass part I — Analysis of the fatigue resistance

In the field of innovative material processing, researches dealing with the mechanical properties of Bulk Metallic Glasses (BMGs) are incontestably one of the most topical issues in the latest years. Indeed, this type of materials presents some very interesting properties, such as a high yield stress, a noticeable elongation at fracture and a good corrosion resistance. Thus, it is though that, in a near future, metallic glasses would be able to fulfill structural material applications, which are currently restricted to conventional crystalline metallic materials. However, it is already known that BMGs usually show poor fatigue properties compared to its crystalline counterparts, reducing the actual use of BMGs to non-structural applications.Thus, such promising usages require a better understanding of the fatigue properties of BMGs, explaining the numerous works carried out to overcome this difficulty. Surprisingly, only a few literature reports are focused on the effect of the stress ratio R (= σmin/σmax) on the high-cycle fatigue behavior of BMGs. In the present work, the effect of the stress ratio, up to R = 0.7, on both the fatigue properties and fatigue crack propagation behavior of Zr55Al10Ni5Cu30 BMG have been studied. This first part is dealing with the fundamental fatigue resistance investigated at three distinct stress ratios of R = 0.1, 0.4 and 0.7. In addition, some discussions related to fracture observation will be discussed, revealing an irregularity of the fatigue crack propagation mechanism at R = 0.7. Finally, relative influences of both stress range and maximum stress level were analyzed using a phenomenological law.

Physics‐based multiscale damage criterion for fatigue crack prediction in aluminium alloy

ABSTRACTIn this paper, a physics‐based multiscale approach is introduced to predict the fatigue life of crystalline metallic materials. An energy‐based and slip‐based damage criterion is developed to model two important stages of fatigue crack initiation: the nucleation and the coalescence of microcracks. At the microscale, a damage index is developed on the basis of plastic strain energy to represent the growing rate of a nucleated microcrack. A statistical volume element model with high computational efficiency is developed at the mesoscale to represent the microstructure of the material. Also, the formation of a major crack is captured by a coalescence criterion at mesoscale. At the macroscale, a finite element analysis of selected test articles including lug joint and cruciform is conducted with the statistical volume element model bridging two scale meshes. A comparison between experimental and simulation results shows that the multiscale damage criterion is capable of capturing crack initiation and predicting fatigue life.

Influence of Grain Boundary Mobility on Microstructure Evolution during Recrystallisation

The paper introduces first investigations on how low angle grain boundaries can influence the recrystallisation behaviour of crystalline metallic materials. For this purpose a three-dimensional cellular automaton model was used. The approach in this study is to allow even low angle grain boundaries to move during recrystallisation. The effect of this non-zero mobility of low angle grain boundaries will be analysed for the recrystallisation of deformed Al single crystals with Cube orientation. It will be shown that low angle grain boundaries indeed influence the kinetics as well as the texture evolution of metallic materials during recrystallisation.

Single-crystal SnO2 nanoshuttles: shape-controlled synthesis, perfect flexibility and high-performance fieldemission

Vertically aligned single-crystalSnO2 nanoshuttle arrays with uniform morphology and a relatively high aspect ratio weresynthesized by a simple hot-wall chemical vapor deposition (CVD) method. It was foundthat regulating the growth temperature gradient could change the shape of theSnO2 nanostructure from nanoshuttles to nanochisels and nanoneedles, and aself-catalyzing growth process was responsible for tunable morphologies ofSnO2 nanostructures. Theas-synthesized SnO2 nanoshuttles showed ultrahigh flexibility and strong toughness with a large elastic strain of ∼ 6.2, which is much higher than reported for Si and ZnO nanowire as wellas most crystalline metallic materials. The field emitter fabricated usingSnO2 nanoshuttle arrays has a low turn-on electric field of around0.6 V µm − 1, and a high field emission current density of above10 mA cm − 2, which is comparable with the highest emission current density of carbon nanotube andnanowire field emitters.

Sub-Micron Feature Patterning of Thermoplastics using Multi-Scale BMG Tooling

ABSTRACTOne potential application for Bulk Metallic Glasses (BMGs) is in dies with micro- and nano-sized features. Three basic characteristic sets inherent to BMGs make them ideal materials for micro/nano-tooling applications: (1) excellent compressive strength, wear and corrosion resistance; (2) amorphous structure which presents no microstructural length scale limitation to cutting and forming operations; (3) the presence of a glass transition temperature above which they can be easily formed. There are many potential applications for multi-scale BMG tooling, including in production of microfluidic and other precision biomedical devices. In the current work, discs were cut from 5 mm diameter cylindrical specimens of Zr44Cu40Al8Ag8 BMG produced via arc melting and casting into water-cooled copper molds. The cylindrical specimens were then thermoplastically formed into thin coin-like disc samples. The thin disc-shaped plates were then ground and polished to create a smooth flat surface. Sub-micron-sized features were patterned into the plates via a focused ion beam. We demonstrated that such feature sizes are not achievable in conventional crystalline metallic tool materials. The patterned BMG tools were then set in a compression press where the platen temperature was precisely controlled and a series of load-controlled embossing trials were carried out in which the features of the BMG tooling were replicated in poly(methyl methacrylate) (PMMA) sheet. An exercise in mapping out the size limitation of such a multi-scale embossing operation is reported.

Analysis of flow stress and size effect on polycrystalline metallic materials in tension

A model describing the size effect on the tensile strength of crystalline metallic materials is investigated. Using the nonhomogeneity of polycrystalline materials and free surface effect, the density of the geometrically necessary dislocations during tension is derived. Using the proposed model, an analysis of the effect of both specimen size and grain size on the tensile strength of the polycrystalline metallic materials is carried out.

Our thoughts are ours, their ends none of our own: Are there ways to synthesize materials beyond the limitations of today?

Historically, the following three approaches to synthesize materials with new atomic structures and, thus, new properties may be distinguished. In the first period – beginning with the discovery of metals about 5000 years ago – the atomic structure of crystalline metallic materials was modified by introducing crystal defects, e.g. by hammering or rolling. However, even at the maximum defect density achievable by this approach (about 10 12 dislocations per cm 2) only about 1% of the atoms are located in the cores of the crystal defects where the atomic structure deviates significantly from the one in the perfect crystal lattice. In other words, this approach does not permit the generation of crystalline materials, the atomic arrangements of which deviate significantly (i.e. in a large volume fraction of the material) from the atomic arrangement in perfect crystals with the same chemical composition. Many crystalline materials used commercially today are based on this approach. In the second approach, the way to a class of materials with new atomic structures was opened about 35 years ago by having up to 50% of the atoms located in cores of grain boundaries and/or interphase boundaries. Materials of this kind were obtained by reducing the crystal size of polycrystals to a few nanometers. These materials were called nanocrystalline or nanostructured materials. As the atomic arrangements in the cores of grain and/or interphase boundaries differ from the ones in perfect crystals, this approach led to materials with new properties (in comparison to the chemically identical single crystals). In the most recent, third approach, a new class of materials with a glassy structure is synthesized. Their novel feature is that the atomic structure throughout the entire volume of the material as well as the density of the entire material can be tuned. Materials of this kind are called nanoglasses. They are generated by introducing interfaces into metallic glasses on a nanometer scale. These interfaces delocalize upon annealing, so that the free volume associated with the interfaces spreads throughout the volume of the glass. This delocalization changes the atomic structure and density of the glass throughout the volume. In fact, by controlling the spacing between the interfaces introduced into the glass as well as their degree of delocalization (by modifying the annealing time and/or annealing temperature), the atomic structures as well as the density (and hence all structure/density-dependent properties) of nanoglasses may be controlled. A reduction of the density by up to 15% seems to be possible. A comparable tuning of the atomic structure/density of crystalline materials is not conceivable because defects in crystals (grain boundaries, dislocations, etc.) do not delocalize upon annealing.

Influence of nonmetallic inclusions on the fatigue strength of metals

A model of growth of short fatigue cracks in the vicinity of nonmetallic inclusions in crystalline metallic materials is proposed. We establish the influence of inclusions on the fatigue strength of materials of this type. A conclusion is made that the inclusions whose typical size is smaller than the grain size exert practically no influence on the fatigue strength of the material.

Welding technologies of bulk metallic glasses

We have succeeded in joining bulk metallic glasses (BMGs) to the same ones, different ones or commercial crystalline metallic materials by friction and electron-beam welding methods. No crystallization and no visible defects were observed in the interface. Metallurgical bonding of BMGs to BMGs and crystalline metals were obtained. The tensile strength of the welded BMGs was the same as that of the parent BMGs or crystalline metallic materials. The successful results of the friction and electron-beam welding are expected to push forward the application of BMGs.