Atomistic characterization of the chemically heterogeneous Al–Pb solid–liquid interface

The chemically heterogeneous interface between solid Al and liquid Pb at 625 K is examined by using molecular-dynamics simulation. For the interfacial orientations (1 0 0), (1 1 0) and (1 1 1), we characterize the interface by calculating the density, potential energy, stress and diffusion constant profiles as well as a two-dimensional Fourier analysis of the interfacial layers. Our results are consistent with experimental observations [Acta Mater 2001;49:4259], based on the equilibrium shape of liquid Pb inclusions in solid Al just above the melting temperature of Pb, that the (1 1 1) interface is faceted, while the (1 0 0) and (1 1 0) interfaces are rough. We found that Al and Pb form immiscible two-dimensional domains within the interfacial layers, rather than an intermixed interfacial alloy, as was observed in recent simulations of the Cu–Pb interface [Acta Mater 2011;59:3137]. In addition, in contrast to earlier observations on the (1 1 1) Cu–Pb interface at this temperature, no prefreezing layers are found in Al–Pb interfaces for any of the orientations studied.     

Influence of Pb segregation on the deformation of nanocrystalline Al: Insights from molecular simulations

Molecular dynamics straining simulations using a two-dimensional columnar model were run for pure Al with grain sizes from 5 to 30 nm, and for 10 nm grain size Al–Pb alloys containing 1, 2 and 3 at.% Pb. Monte Carlo simulations showed that all the Pb atoms segregate to the grain boundaries. Pb segregation suppresses the nucleation of partial dislocations and twins during straining. At 3 at.% Pb, no dislocations or twins are observed throughout the straining history. It also appeared that Pb tends to segregate to the same locations in grain boundaries that were favorable for partial dislocation emission. Grain boundaries with Pb segregates were very robust against dissociation during straining compared to pure Al. The yield stress determined from stress–strain curves showed a decrease with increasing Pb content, supporting a similar observation for the hardness change measured on nanocrystalline Al–Pb alloys.     

The origin of high-mismatch orientation relationships for ultra-thin oxide overgrowths

A crystallographic orientation relationship (COR) of unusually high lattice mismatch (>15%) between an Al{1 0 0} substrate and its ultra-thin (<1 nm) oxide overgrowth is reported. This striking observation is in contrast with the general assumption that a COR with the lowest mismatch is preferred. However, as shown by thermodynamic model calculations, despite the relatively large energy contributions from residual strain and misfit dislocations, the high-mismatch overgrowth can be stabilized by a relatively low surface energy and a relatively high density of metal–oxygen bonds across the interface.     

Atomistic simulations of surface segregation of defects in solid oxide electrolytes

We performed atomistic simulations of yttria-stabilized zirconia (YSZ) and gadolinia-doped ceria (GDC) to study the segregation of point defects near (1 0 0) surfaces. A hybrid Monte Carlo–molecular dynamics algorithm was developed to sample the equilibrium distributions of dopant cations and oxygen vacancies. The simulations predict an increase of dopant concentration near the surface, which is consistent with experimental observations. Oxygen vacancies are also found to segregate in the first anion layer beneath the surface and to be depleted in the subsequent anion layers. While the ionic size mismatch between dopant and host cations has been considered as a driving force for dopant segregation to the surface, our simulations show that the correlation between individual point defects plays a dominant role in determining their equilibrium distributions. This correlation effect leads to more pronounced dopant segregation in GDC than in YSZ, even though the size mismatch between dopant and host cations is much greater in YSZ than in GDC.     

Multiscale simulation of onset plasticity during nanoindentation of Al (0 0 1) surface

The onset of plasticity in crystalline materials is important to the fundamental understanding of plastic deformation and the development of precision devices. Dislocation nucleation and interactions at the onset of plasticity are investigated here using a multiscale quasi-continuum (QC) method for the nanoindentation of the (0 0 1) surface of a single crystal aluminium (Al) of 200 × 100 nm² with infinite thickness. Deformation twinning was noted to occur during the nanoindentation of Al. We used unrelaxed and relaxed QC simulations with three embedded atom potentials of Al to evaluate the generalized planar fault (GPF) energies. The energy barrier for initial dislocation nucleation is much higher than that for subsequent nucleation events adjacent to the pre-existing defect. This mechanism promotes deformation twinning when some of the available slip systems are constrained. Dislocation initiation causes a minor load drop in the load–displacement curve, whereas major displacement excursion from experimental observations is the result of collective dislocation activities. (Some figures in this article are in color only in the on-line version.)     

Inclination dependence of grain boundary energy and its impact on the faceting and kinetics of tilt grain boundaries in aluminum

The shape evolution and migration of <1 0 0> and <1 1 1> tilt grain boundaries with rotation angles θ in the range between 6° and 24° were investigated in situ in a scanning electron microscope at elevated temperatures. The results revealed that boundaries with misorientation θ < 15° did not attain a continuously curved shape in the entire temperature range up to the melting point and, thus, did not move under a capillary driving force. Instead, they remained straight or formed several facets which were inclined to the initial boundary orientation. Molecular statics simulations suggest that the observed behavior of low-angle boundaries is due to the anisotropy of grain boundary energy with respect to boundary inclination. This anisotropy diminishes with increasing misorientation angle, and high-angle boundaries assume a continuously curved shape and move steadily under the curvature driving force.     

Proximity effect controlled superconducting behavior of novel biphasic Pb–Sn nanoparticles embedded in an Al matrix

We report the synthesis of biphasic Pb (46 at.%)–Sn (54 at.%) nanoparticles dispersed in an aluminum matrix and explore the nature of the superconducting transition in these particles. The nanoscaled Pb–Sn alloy particles were dispersed in Al by rapid solidification and the two-phase nature of these particles was characterized by transmission electron imaging, diffraction and composition mapping. A weak superconducting transition occurs at 3.1 K in these alloys, which is much lower than the T_C expected for a Pb₄₆Sn₅₄ alloy or that due to the proximity effect between Pb and Sn. We show that it is the superconducting Al matrix with T_C = 1.2 K that plays a major role in determining the effective transition temperature of the system.     

Laser shock-induced spalling and fragmentation in vanadium

Polycrystalline and monocrystalline (〈1 0 0〉 and 〈1 1 0〉) vanadium was subjected to shock compression followed by tensile wave release to study spall and fragmentation behavior. The shock pulse was generated by a direct laser drive at energy levels ranging from 11 to 440 J mm^–2 (laser beam irradiated area 1.12 mm²) and initial pulse durations of 3 and 8 ns (approximate initial pressures between 10 and 250 GPa). Glass and polycarbonate shields placed at a specific distance behind the vanadium targets were used to collect and analyze the ejected fragments in order to evaluate and quantify the extent of damage. The effects of target thickness, laser energy, polycrystallinity and pulse duration were studied. Calculations show melting at a pressure threshold of ～150 GPa, which corresponds to a laser energy level of ～180 J mm^–2. Consistent with the analytical predictions, the recovered specimens and fragments show evidence of melting at the higher energy levels. Spalling in the polycrystals occurred by a ductile tearing mechanism that favored grain boundaries. In the monocrystals it occurred by a mixture of cleavage fracture along the {0 1 0} planes and ductile dimple fracture. This lower spall strength in polycrystals contradicts predictions from the Hall–Petch equation. Experimentally obtained fragment sizes were compared with predictions from the Grady–Kipp model. The spall strength of vanadium under laser loading conditions was calculated from both VISAR pull-back signals and using the spall thickness. It was found to be considerably higher than predictions from gas gun experiments, the monocrystals showing a higher value than polycrystals. This higher spall strength is suggestive of a strong time dependence of the phenomenon, consistent with the nucleation and growth kinetics of voids and the strain rate sensitivity embedded in the Grady theory.     

Superplastic behaviour of two extruded gamma TiAl(Mo,Si) materials

The superplastic properties of two intermetallic Ti–46.8Al–1.2(Mo,Si) and Ti–46Al–1.5(Mo,Si) (at.%) materials produced by arc melting and processed by hot extrusion in the temperature range between 1200 and 1250 °C were studied. The materials exhibited an equiaxic near γ microstructure with γ grains finer than 1 μm and some band like region of γ grains with a size ranging from 5 to 20 μm. The finer grained zone contained a volume fraction of about 12 vol% in the 46.8Al material and about 25 vol% in the 46Al material of finely dispersed α₂-Ti₃Al particles. Mechanical tests performed on both materials at strain rates ranging from 4.6×10⁻⁴ to 10⁻² s⁻¹ in the temperature range of 975–1050 °C showed strain rate sensitivity exponents of about 0.5 at most strain rates. A maximum elongation to failure of about 300% was obtained for the 46.8Al material while about 900% was recorded for the 46Al material at 1050 °C at a relatively high strain rate of 8×10⁻³ s⁻¹. This difference is attributed to the larger volume fraction of α₂-phase particles in the 46Al material that leads to a decrease of the number and size of band like regions of coarse γ grains. The microstructure in the fine-grained areas of both materials remains essentially constant during deformation. The mechanical behavior at high temperature of these superplastic materials can be explained by considering grain boundary sliding as the dominant deformation mechanism.     

Coarsening of γ (Ni–Al solid solution) precipitates in a γ′ (Ni3Al) matrix

The coarsening behavior of Ni–Al solid–solution precipitates in an Ni₃Al matrix was investigated in alloys containing 22.0–22.8 at.% Al aged at 650–800 °C for times exceeding 1800 h. The rate constant for coarsening increases with equilibrium volume fraction as predicted by the MLSW theory. The activation energy for coarsening, 314.1 ± 16.6 kJ mol⁻¹, agrees very well with results from conventional diffusion experiments. The particle size distributions are not in very good agreement with the predictions of any theory; possible reasons are discussed. The particles become more spherical with decreasing elastic self-energy. The results are consistent with the premise that a strong volume fraction effect is observed so long as diffusion in the matrix phase, and not through the precipitate–matrix interface, controls the kinetics.     

Study on behaviors of tungsten powders in radio frequency thermal plasma

This paper describes an investigational study of the spheroidization of refractory metal tungsten powders by radio frequency thermal plasma, with emphasis on the melting, solidification and growth behavior of the tungsten powder particles during the spheroidization process. The flight time and melting time of tungsten powder particles in the plasma were estimated, and the growth behavior of the tungsten powder particles was analyzed in detail by investigating the change in the average particle size before and after plasma spheroidization. The morphology and spheroidization rate were analyzed using field emission scanning electron microscopy. The flight time and melting time for tungsten powder particles with radius of 7.8 μm were calculated to be 9.3 ms and 2.9 ms, respectively. The change in powder particle size during the process showed that the growth of tungsten powder particles was mainly caused by the coalescence of droplets in the thermal plasma system. The experimental results demonstrated that the spheroidization rate can reach up to 95% under the operating conditions used in this work.     

Molecular dynamics study of diffusional creep in nanocrystalline UO2

We present the results of molecular dynamics (MD) simulations to study high-temperature deformation of nanocrystalline UO₂. In qualitative agreement with experimental observations, the oxygen sublattice undergoes a structural transition at a temperature of about 2200 K (i.e. well below the melting point of 3450 K of our model system), whereas the uranium sublattice remains unchanged all the way up to melting. At temperatures well above this structural transition, columnar nanocrystalline model microstructures with a uniform grain size and grain shape were subjected to constant-stress loading at levels low enough to avoid microcracking and dislocation nucleation from the grain boundaries (GBs). Our simulations reveal that in the absence of grain growth, the material deforms via GB diffusion creep (also known as Coble creep). Analysis of the underlying self-diffusion behavior in undeformed nanocrystalline UO₂ reveals that, on our MD timescale, the uranium ions diffuse only via the GBs, whereas the much faster moving oxygen ions diffuse through both the lattice and the GBs. As expected for the Coble-creep mechanism, the creep activation energy agrees well with that for GB diffusion of the slowest-moving species, i.e. the uranium ions.     

Thermal explosion synthesis of ZrC particles and their mechanism of formation from Al–Zr–C elemental powders

ZrC particles were fabricated by thermal explosion (TE) from mixture of Al, Zr and C elemental powders. Without the addition of Al, the synthesized ZrC particles had irregular shape of ~ 4.0 μm in average. Increasing Al content up to 30 wt.%, however, refined significantly them down to < 0.2 μm with regularly square morphology. The Al effect of reaction mechanism promoted the ZrC formation as diluents in the course of TE, which was clarified using differential thermal analysis and X-ray diffraction technique. The melting of Al favored the reaction with Zr to generate ZrAl₃, and then the dissolution of C into the Al–Zr liquid resulted in precipitation of ZrC. Meanwhile, the exothermic effect prompted C atoms dissolving into Zr–Al liquid and eventually led to precipitation of ZrC out of the supersaturated liquid. The Al addition inhibited particle growth, but also promoted the TE reaction.     

Microstructure and grain boundary relaxation in ultrafine-grained Al/Al oxide composites

The microstructure and grain boundary relaxation in ultrafine-grained Al/Al oxide composites were studied by electron microscopy observation and internal friction measurement, respectively. Both the microstructure and the internal friction behavior of the composites were strongly influenced by the thermomechanical treatment parameters. All the Al particles were still covered by the native amorphous oxide shells in those composites sintered at T < 823 K, and no indication of Al grain boundary relaxation was detected. Some Al oxide shells were cracked, resulting in the formation of a few Al–Al grain boundaries between adjacent particles in the sample sintered at 823 K, and one internal friction peak centered at ～440 K was detected. All the oxide shells were broken into small fragments in those samples sintered at T ? 843 K, and two internal friction peaks were detected, one prominent peak at ～440 K and one weak peak at ～540 K. A microstructure with a bimodal grain size distribution of Al was formed via partial recrystallization after thermomechanical treatment of the sample sintered at 893 K, and two internal friction peaks with comparable intensity were detected. The internal friction peaks were associated with the relaxation of Al grain boundary in the composites.     

Reinforcement architectures and thermal fatigue in diamond particle-reinforced aluminum

Aluminum reinforced by 60 vol.% diamond particles has been investigated as a potential heat sink material for high power electronics. Diamond (CD) is used as reinforcement contributing its high thermal conductivity (TC ≈ 1000 W mK^?1) and low coefficient thermal expansion (CTE ≈ 1 ppm K^?1). An Al matrix enables shaping and joining of the composite components. Interface bonding is improved by limited carbide formation induced by heat treatment and even more by SiC coating of diamond particles. An AlSi7 matrix forms an interpenetrating composite three-dimensional (3D) network of diamond particles linked by Si bridges percolated by a ductile α-Al matrix. Internal stresses are generated during temperature changes due to the CTE mismatch of the constituents. The stress evolution was determined in situ by neutron diffraction during thermal cycling between room temperature and 350 °C (soldering temperature). Tensile stresses build up in the Al/CD composites: during cooling <100 MPa in a pure Al matrix, but around 200 MPa in the Al in an AlSi7 matrix. Compressive stresses build up in Al during heating of the composite. The stress evolution causes changes in the void volume fraction and interface debonding by visco-plastic deformation of the Al matrix. Thermal fatigue damage has been revealed by high resolution synchrotron tomography. An interconnected diamond–Si 3D network formed with an AlSi7 matrix promises higher stability with respect to cycling temperature exposure.     

A molecular dynamics simulation study of the velocities,mobility and activation energy of an austenite–ferrite interface in pure Fe

Molecular dynamics simulations have been used to obtain the mobility, in pure Fe, of a face-centered cubic (fcc)–body-centered cubic (bcc) interphase boundary with an orientation given by (1 1 0)_bcc//(7 7 6)_fcc and [0 0 1]_bcc//[?1 1 0]_fcc. The interface is best described by a 4.04° rotation, about an axis lying in the boundary plane, from the Nishiyama–Wasserman orientation and the boundary consists of a parallel array of steps (disconnections). An embedded atom method interatomic potential was employed to model Fe, and the free energy difference as a function of temperature between the fcc and bcc phases, which provided the driving force for boundary motion, was determined by a thermodynamic integration procedure. Although the boundary was found to be very mobile, the transformation did not proceed by a martensite mechanism. The boundary mobility was obtained for several temperatures in the range 600–1400 K and Arrhenius behavior was found with an activation energy of 16.5 ± 2.7 kJ mol^?1 and a pre-exponential factor equal to 7.8( ± 0.9) × 10^?3 mmol J^?1 s^?1. The activation energy is much lower than that extracted from experiments on the massive transformation in Fe alloys and possible reasons for the discrepancy are discussed.     

Structural evolution and grain growth kinetics of the Fe–28Al elemental powder during mechanical alloying and annealing

The structural evolution and grain growth kinetics of the Fe–28Al (28 at.%) elemental powder during mechanical alloying and annealing were studied. Moreover, the alloying mechanism during milling the powder was also discussed. During mechanical alloying the Fe–28Al elemental powder, the solid state solution named Fe(Al) was formed. The lattice parameter of Fe(Al) increases and the grain size of Fe(Al) decreases with increasing milling time. The Fe and Al particles were first deformed, and then, the composite particles of the concentric circle-like layers were generated. Finally, the composite particles were substituted by the homogeneous Fe(Al) particles. The continuous diffusion mixing mechanism is followed, mainly by the diffusion of Al atoms into Fe. During annealing the milled Fe–28Al powder, the order transformation from Fe(Al) to DO₃-Fe₃Al and the grain growth of DO₃-Fe₃Al occurred. The grain growth kinetic constant, K = 1.58 × 10⁻⁹ exp(−540.48 × 10³/RT) m²/s.     

Quasicontinuum study of incipient plasticity under nanoscale contact in nanocrystalline aluminum

Atomistic simulations using the quasicontinuum method are performed to examine the mechanical behavior and underlying mechanisms of surface plasticity in nanocrystalline aluminum with a grain diameter of 7 nm deformed under wedge-like cylindrical contact. Two embedded-atom method potentials for Al, which mostly differ in their prediction of the generalized stacking and planar fault energies, and grain boundary (GB) energies, are used and characterized. The simulations are conducted on a randomly oriented microstructure with 〈1 1 0〉-tilt GBs. The contact pressure–displacement curves are found to display significant flow serration. We show that this effect is associated with highly localized shear deformation resulting from one of three possible mechanisms: (1) the emission of partial dislocations and twins emanating from the contact interface and GBs, along with their propagation and intersection through intragranular slip, (2) GB sliding and grain rotation and (3) stress-driven GB migration coupled to shear deformation. Marked differences in mechanical behavior are observed, however, as a function of the interatomic potential. We find that the propensity to localize the plastic deformation at GBs via interface sliding and coupled GB migration is greater in the Al material presenting the lowest predicted stacking fault energy and GB energy. This finding is qualitatively interpreted on the basis of impurity effects on plastic flow and GB-mediated deformation processes in Al.     

Polarization fatigue in Pb(In0.5Nb0.5)O3–Pb(Mg1/3Nb2/3)O3–PbTiO3 single crystals

Electric fatigue tests have been conducted on pure and manganese-modified Pb(In_0.5Nb_0.5)O₃–Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ (PIN–PMN–PT) single crystals along different crystallographic directions. Polarization degradation was observed to suddenly occur above 50–100 bipolar cycles in 〈1 1 0〉 oriented samples, while 〈0 0 1〉 oriented samples exhibited almost fatigue free characteristics. The fatigue behavior was investigated as a function of orientation, magnitude of the electric field and manganese dopant. It was found that 〈0 0 1〉 oriented PIN–PMN–PT crystals were fatigue free, due to its small domain size, being on the order of 1 μm. The 〈1 1 0〉 direction exhibited a strong electrical fatigue behavior due to mechanical degradation. Micro/macro cracks developed in fatigued 〈1 1 0〉 oriented single crystals. Fatigue and cracks were the result of strong anisotropic piezoelectric stress and non-180° domain switching, which completely locked the non-180° domains. Furthermore, manganese-modified PIN–PMN–PT crystals were found to show improved fatigue behavior due to an enhanced coercive field.     

High temperature micropillar compression of Al/SiC nanolaminates

The effect of the temperature on the compressive stress–strain behavior of Al/SiC nanoscale multilayers was studied by means of micropillar compression tests at 23 °C and 100 °C. The multilayers (composed of alternating layers of 60 nm in thickness of nanocrystalline Al and amorphous SiC) showed a very large hardening rate at 23 °C, which led to a flow stress of 3.1 ± 0.2 GPa at 8% strain. However, the flow stress (and the hardening rate) was reduced by 50% at 100 °C. Plastic deformation of the Al layers was the dominant deformation mechanism at both temperatures, but the Al layers were extruded out of the micropillar at 100 °C, while Al plastic flow was constrained by the SiC elastic layers at 23 °C. Finite element simulations of the micropillar compression test indicated the role played by different factors (flow stress of Al, interface strength and friction coefficient) on the mechanical behavior and were able to rationalize the differences in the stress–strain curves between 23 °C and 100 °C.