X-ray study of4He close-packed structures at high pressure

We determined the transition line between hcp and fcc helium up to 4.5 kbar using x-ray diffraction. The slopedP _tr/dT _tr increases rapidly with pressure as predicted recently. Unit cell dimensions are given for both structures at 3.33 kbar and 18.5 K.     

Pressure-induced lattice instability in fcc lanthanum at low temperature

Strong anomalies have been discovered in the dependence of the superconducting transition temperature T_c on pressure, the most prominent one being a sudden decrease of the slope dT_c/dP by a factor of four at 53 kbar. The electrical resistivity behaves anomalously in several respects as a function of temperature and pressure up to 200 kbar. The residual resistance shows a marked peak at 53 kbar. It is concluded that, at low temperatures, a subtle isostructural phase transformation (fcc fcc) occurs at 53 kbar. The existence of a critical point in the P-T phase diagram is expected at low temperature, not unlike the situation in cerium metal at elevated temperature. Another fcc fcc phase transformation may occur at 25 kbar and liquid helium temperature. The phase boundary will presumably also end in a critical point at higher temperature. There is hence some evidence for the P-T diagram of La being, in principle, isomorphous with the phase diagram of Ce. In the case of Ce, a second critical point might also exist, forming the endpoint of the / phase boundary at high temperature.Part of a thesis by H. Balster, Abteilung für Physik und Astronomie, Ruhr-Universität Bochum (1974).Paper presented at the 1974 Spring Meeting of the Deutsche Physikalische Gesellschaft in Freudenstadt [H. Balster and J. Wittig, Verhandl. DPG (VI)9, 817 (1974)].     

Crystallization of Spheres

Simplified computer models are used to gain insight into more complex real systems. In a reversion of this protocol, a colloidal suspension of submicron spherical particles, approximately hard and uniform, was recently crystallized in space and analyzed for crystal type. The objective was to establish how, and to what structure, hard spheres crystallize without gravity. Computational statistical thermodynamics predicts an equilibrium constant between fcc and hcp of order unity. The microgravity experiments, however, resulted in a random hybrid close-packed structure (rhcp) such that long-range order is two-dimensional. Here we report the mechanism from idealized computer experiments for crystallization of spheres from the metastable fluid. Model systems of up to N=64,000 spheres with infinite spatial periodicity have been crystallized in runs of up to 10 billion collisions. When the fluid, initially in a metastable supercooled state at 58% packing, is allowed to nucleate and freeze, a variety of structures emerges. There are three identifiable stages of structural growth: (i) initial nucleation of fcc, rhcp, and also bcc-like (body-centered cubic) local structures; (ii) rapid growth of all incipient nucleites to random stacked two-dimensional hexagonal (rhcp) grains, plus some fcc, to fill the volume; and (iii) relatively slow dissolution of unstable rhcp faces at grain boundaries. Eventually, stable nucleites emerge comprising hexagonal layers, arranged so as to contain predominantly either fcc arrangements of spheres or rhcp, in roughly 50% proportions.     

The Nature of the Nonsingularity of Inner Interfaces in Hydroxyapatite Ceramics

The morphology of inner interfaces in hydroxyapatite (HA) based calcium phosphate ceramics has been studied by transmission electron microscopy. Grain boundaries in the ceramics have been shown to have a vicinal character, which is related to the mechanism of secondary recrystallization in the material: layer growth of grains via sequential motion of elementary steps on planes corresponding to the \(\{ 1\bar 100\} \) HA prism faces, which grow through transitions of atoms from adjacent grains that are in contact through their planes with large Miller indices. The recrystallization process may be accompanied by a “collision” of vicinal growth surfaces of grains with relatively large misalignment angles and the formation of grain boundaries nanofaceted by prism planes of adjacent grains. The recrystallization process in such a case should be expected to continue in the grain with a smaller nonsingularity of the growth front. Grain boundaries may allow for a match between planes differing in Miller indices, nd_h1k1l1 ≈ md_h2k2l2, and the formation of grain-boundary Pumphrey dislocations, which compensate for the size mismatch between interplanar spacings and/or misalignment of the planes. The observed characteristic grain match configurations are typical of both ceramics produced by sintering HA powders and HA films produced by ion sputtering.     

Deformation induced hcp nano-lamella and its size effect on the strengthening in a CoCrNi medium-entropy alloy

Deformation-induced hcp nano-lamellae with various widths and interspacings were observed in the CoCrNi medium-entropy alloy(MEA)under high strain rate and cryogenic temperature in the present study.Higher hardness was found in the cryogenic-deformed samples compared to the room temperature-deformed samples without hcp phase.Then,size effects of embedded hcp nano-lamellae on the tensile behaviors in the fcc CoCrNi MEA were investigated by molecular dynamics simulations.The overall strengthening was found to have two components:phase strengthening and extra inter-face strengthening,and the interface strengthening was observed to be always stronger than the phase strengthening.Both overall strengthening and interface strengthening were found to increase with increasing width and decreasing interspacing of embedded hcp nano-lamellae.The samples with small spaced hcp nano-lamellae are even stronger than the pure hard hcp phase due to the extra interface strengthening.The samples with larger width of embedded hcp nano-lamellae can provide stronger resistance for dislocation slip and transmission.Nanotwins were observed to be formed in the embedded hcp nano-lamellae.Higher density of phase boundaries and newly formed twin boundaries can provide more barriers for dislocation glide in the other slip systems,resulting in higher strength for samples with smaller interspacing.     

The L12 ↔ DO19 transformation in the intermetallic compound Fe3Ge

From a crystallographic point of view, the transition from the L12 to DO19 phase is an ordered version of the widely investigated fcc to hcp transformation. In the present study, the transformation kinetics of the forward and backward reactions of the L12 DO19transitions in Fe3Ge have been studied and it is shown that a large hysteresis exists between the forward and backward reactions. The detailed microstructural changes that accompany the L12 DO19 transition have been characterized and these explain the observed transformation hysteresis.     

溅射功率和退火温度对GeSbTe相变薄膜内应力的影响

通过磁控溅射方法制备了GeSbTe薄膜.借助原子力显微镜,X射线衍射仪和应力测试仪等仪器,并结合对薄膜表面形貌和晶体结构的分析,研究了溅射功率和退火温度对薄膜内应力的影响.结果表明:当溅射功率较小时,内应力随着溅射功率的增大而增大,在50W左右时达到最大值,随后又随着溅射功率的增大而减小.退火温度为160℃时,薄膜发生非晶态向fcc晶态结构的相变,由于Te原子析出到晶粒边界,导致薄膜的内应力急剧增大到最大值为100MPa左右,而后随着退火温度的升高而下降,fcc结构向hex结构转变时,内应力变化并不明显.     

Dendritic crystal growth in pure4He

Dendritic crystal growth of pure hcp and fcc⁴He was observed at pressures between 210 and 6500 bar. Dendrite morphology depends on fluid supercooling and crystal phase. At large supercooling, dendrites with side arms are observed, whereas at low supercooling dendrites grow without side arms. The morphology of hcp⁴He dendrites is strongly influenced by crystalline anisotropy. Comparison with present theories of dendrite growth show good agreement with the power law dependencies of velocity, tip radius, and Péclet number on supercooling. Numerically, theory predicts much larger velocities than are observed. The stability parameter is found to be much smaller than theoretically predicted.     

Internal friction and coefficient of linear expansion of zirconium and cobalt in the phase transition region

Results are presented from measurement of the internal friction and coefficient of linear expansion of zirconium and cobalt near the points of firstorder phase transformation. This is the hcp bcc transition beginning at 1135°K for zirconium and the hep fcc transition beginning at 706°K for cobalt.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 50, No. 4, pp. 625–629, April, 1986.     

Crystallographic and morphological aspects of AlN precipitation in high Al,TRIP steels

The dendritic AlN associated with poor ductility on straightening during continuous casting is believed to always crystallise out in the hcp structure. The present paper shows that AlN precipitation can also be present as an fcc phase. Because AlN precipitation is sluggish and needs suitable nucleants like MnS to aid its precipitation, the crystal structure of AlN is dependent on the order in which the austenite and MnS precipitate out during solidification. When austenite forms first, as with the 1%Al transformation induced plasticity (TRIP) steel, MnS nucleants are not available and AlN is forced to precipitate at the austenite grain boundaries as fcc so encouraging intergranular failure. With the 1.5%Al TRIP steel, on solidification, MnS is present in abundance, giving rise to the conventional hcp AlN.     

Secondary grain growth in ammonium nitrate crystallization

Crystallization and ageing of pure ammonium nitrate crystals was studied by scanning electron microscopy. Ageing the crystals beyond 1 h led to the growth of secondary grains along the grain boundaries of primary grains (140–200 m). After 6 h ageing secondary growth along grain boundaries was not observed; instead, distinct clusters with morphologies similar to the secondary grains were formed. The ancillary growth obeys approximately the parabolic relation L=(Kt)^1/n+1 where n=1 justifies grain growth in the pure crystals, and its formation is ascribed to the dissolution of dendrites, edges and corners. The high interfacial activity and interparticle voids can affect the storage and mechanical properties of the material.     

Phase transformations in nanocrystalline alloys synthesized by mechanical alloying

The effect of nanometer grain size and extensive grain boundary regions in nanocrystalline alloy systems was investigated for the chemical order-disorder, structural, precipitation, and spinodal phase transformations. The kinetic paths for approach to the chemically ordered phase from the disordered phase in FeCo-Mo alloys were observed to be the same at different temperatures due to grain boundaries acting as short-circuited diffusion paths for atom movements. The structure of Fe₃Ge was bcc for small crystallite size and the equilibrium fcc phase developed only after a critical grain size was attained. This was understood as a manifestation of the Gibbs Thomson effect. The precipitation phase transformation in Fe-Mo alloys proceeded by a rapid movement and clustering of the Mo atoms to the grain boundaries that was correlated to the size of the nano grains, and subsequent formation of the Mo rich lambda phase directly in the grain boundary regions. The composition fluctuation domains for spinodal decomposition in nanophase Fe-Cr alloys were observed to be linearly correlated to the growth of grains.     

Grain boundary and microstructure engineering of Inconel 690 cladding on stainless-steel 316L using electron-beam powder bed fusion additive manufacturing

This research explores the prospect of fabricating a face-centered cubic(fcc) Ni-base alloy cladding(Inconel 690) on an fcc Fe-base alloy(316 L stainless-steel) having improved mechanical properties and reduced sensitivity to corrosion through grain boundary and microstructure engineering concepts enabled by additive manufacturing(AM) utilizing electron-beam powder bed fusion(EPBF). The unique solidification and associated constitutional supercooling phenomena characteristic of EPBF promotes[100] textured and extended columnar grains having lower energy grain boundaries as opposed to random, high-angle grain boundaries, but no coherent {111} twin boundaries characteristic of conventional thermo-mechanically processed fcc metals and alloys, including Inconel 690 and 316 L stainless-steel.In addition to [100] textured grains, columnar grains were produced by EPBF fabrication of Inconel 690 claddings on 316 L stainless-steel substrates. Also, irregular 2–3 μm diameter, low energy subgrains were formed along with dislocation densities varying from 108 to 109 cm~2, and a homogeneous distribution of Cr_(23)C_6 precipitates. Precipitates were formed within the grains(with ~3 μm interparticle spacing),but not in the subgrain or columnar grain boundaries. These inclusive, hierarchical microstructures produced a tensile yield strength of 0.527 GPa, elongation of 21%, and Vickers microindentation hardness of 2.33 GPa for the Inconel 690 cladding in contrast to a tensile yield strength of 0.327 GPa, elongation of 53%, and Vickers microindentation hardness of 1.78 GPa, respectively for the wrought 316 L stainlesssteel substrate. Aging of both the Inconel 690 cladding and the 316 L stainless-steel substrate at 685?C for50 h precipitated Cr_(23)C_6 carbides in the Inconel 690 columnar grain boundaries, but not in the low-angle(and low energy) subgrain boundaries. In contrast, Cr_(23)C_6 carbides precipitated in the 316 L stainless-steel grain boundaries, but not in the low energy coherent {111} twin boundaries. Consequently, the Inconel690 subgrain boundaries essentially serve as surrogates for coherent twin boundaries with regard to avoiding carbide precipitation and corrosion sensitization.     

Revealing the Role of Electrocatalyst Crystal Structure on Oxygen Evolution Reaction with Nickel as an Example

Establishing a correlation between the crystal structure and electrocatalytic activity is crucial to the rational design of high performance electrocatalysts. In this work, taking the widely investigated nickel (Ni) based nonprecious oxygen evolution reaction (OER) catalyst as an example, for the first time, it is reported that the crystal structure plays a critical role in determining the OER performance. Similar‐sized nickel nanoparticles but in different hexagonal close‐packed phase and face‐centered cubic phase coated with N‐doped carbon shells, noted as hcp‐Ni@NC and fcc‐Ni@NC, are successfully prepared, respectively, in which the N‐coated carbon shell structures were also similar. Surprisingly, a dramatically enhanced OER performance of hcp‐Ni@NC in comparison with fcc‐Ni@NC is observed. The hcp‐Ni@NC only requires 305 mV overpotential to achieve the current density of 10 mA cm^?2, which is 55 mV lower than that of fcc‐Ni@NC, which can be ascribed to the influence of nickel crystal phase on the electron structure of N‐doped carbon shell. This finding will bring new thinking toward the rational design of high performance non‐noble metal electrocatalysts.     

The structure and properties of a hard alloy coating deposited by high-velocity pulsed plasma jet onto a copper substrate

Using a plasmatron operating in specially calculated regimes, tungsten carbide (WC) based coatings were deposited onto a copper crystallizer plate. It was found that a local hardness of the WC-Co coating may reach up to 1.3×10₄ N/mm² and the coating adhesion to substrate may be as high as 270 MPa. The elemental and phase compositions of coatings were studied by Rutherford backscattering spectroscopy, X-ray diffraction, and transmission electron microscopy with electron diffraction. The surface morphology and depth-composition profiles of the coatings were studied by optical and scanning electron microscopy. The coating is composed of WC crystal grains with hexagonal close packed (hcp) lattice, α-and β-Co grains, and cubic WC grains. The average size of the hcp WC grains is 0.15 μm and that of the cobalt particles is about 25 nm. In addition, the grain boundaries contain W₃Co₃C particles with an average size of 15 nm.     

Cavitation and fracture of mechanically alloyed aluminium at high homologous temperatures

The tensile behaviour of mechanically alloyed (dispersion strengthened) IN90211 was characterized at strain rates between 0.0001 and 340 sec^–1 at temperatures between 425 and 475 ° C, At strain rates above 0.1 sec^–1, superplastic elongations were obtained (maximum elongation 525% at 475 ° C, 2.5sec^–1. Large elongations were possible due to the lack of cavitation, even though the strain-rate sensitivity was lower (m 0.25) than usually found in superplasticity. Cavitation was precluded by the morphology of the platelet-shaped grains in which low-angle subgrain boundaries were predominantly perpendicular to the tensile axis. Grain-boundary sliding was observed along high-angle grain boundaries which were generally parallel to the tensile axis. At the high homologous testing temperatures (0.76 to 0.81), concurrent grain-boundary sliding and lattice slip was made possible by the rapid lattice diffusivity and easy climb of lattice dislocations over dispersions in the matrix and grain boundaries.     

Lateral grain growth during reactive isothermal solidification in the Ni-Sn system

In this research lateral growth of Ni₃Sn₄ grains in the Ni-Sn system during isothermal solidification was investigated. Experiments involved multi-layered samples heat-treated for different durations at temperatures in the range 235–600C. During solidification Ni₃Sn₄ was the only intermediate phase found to grow as a layer at the solid/liquid interface. SEM and optical microscopy cross-sectional views of samples heat-treated at 500C and at 600C reveal that lateral grain growth occurs, the mean lateral grain size being proportional to t(t-the duration of the heat treatment). This process was found to be connected with solution reprecipitation process between adjacent grains in order to equate the local curvature of the solid/liquid interface at groove zone. Rotation of the grain boundary groove due to this mass transfer induces a curvature of the grain boundary, which is the controlling mechanism for its migration. The process was numerically formulated, which provides quantitative fitting to the observed kinetics. The formulation includes a contribution due to the good wetting of the grain boundaries by the liquid phase supporting the observed enhancement in growth rate.     

On the Nature of Disorder in Solid 4He

We apply a modified Debye approach to calculate the Gibbs free energy for different structural phases and crystallite sizes in ⁴He. Atoms are assumed to interact via the Aziz potential. We have found that some intermediate (between hcp and bcc) phase predicted previously is more favorable than hcp at low temperatures and for small sizes. We show that it can exist in a wide pressure range up to 60 bar in ⁴He for crystallite sizes about 3,000 atoms. For larger sizes (10,000 atoms or more) this phase becomes unfavorable. In multidomain structures the intermediate phase competes with hcp and metastable fcc that can be a reason for disorder in solid ⁴He.     

Microstructure and lattice bending in polycrystalline laser-crystallized silicon thin films for photovoltaic applications

Grain size, grain boundary population, orientation distribution and lattice defects of polycrystalline silicon thin films are investigated by electron backscatter diffraction (EBSD). The silicon thin films are produced by a combination of diode laser melt-mediated crystallization of an amorphous silicon seed layer and epitaxial thickening of the seed layer by solid phase epitaxy (SPE). The combined laser-SPE process delivers grains exceeding several 10 μm of width and far larger than 100 μm in length. Strong lattice rotations between 10 and 50° from one side of the grain to the other are observed within the larger grains of the film. The misorientation axes are well aligned with the direction of movement of the laser. The intragranular misorientation is associated both with geometrically necessary dislocations and low angle boundaries, which can serve as recombination centres for electron-hole pairs. Since the lateral grain size is up to two orders of magnitude larger than the film thickness, the high dislocation density could become an important factor reducing the solar cell performance.     

Microstructural dependence of giant-magnetoresistance in electrodeposited Cu-Co alloys

The relationship between the microstructure and the magnetic properties of heterogeneous Cu-Co [Cu92.5-Co7.5] (at.%) thin films prepared by electrodeposition was studied. Electron spectroscopic imaging (ESI) studies clearly revealed the evolution of the cobalt microstructure as a function of thermal treatments. The as-deposited film is composed of more than one phase; metastable Cu-Co, copper and cobalt. During annealing the metastable phase decomposes into two fcc phases; Cu and Co. Grain growth occurs with increasing annealing duration, such that the cobalt grains are more homogeneously distributed in the copper matrix. A maximum GMR effect was found after annealing at 450°C for 1.5 h, which corresponds to an average cobalt grain size of 5.5 nm according to magnetization characterization. A significant fraction of the cobalt in the Cu-Co film did not contribute to the GMR effect, due to interactions between the different magnetic grains and large ferromagnetic (FM) grains. The percolation threshold of cobalt in metastable Cu-Co alloys formed by electrodeposition is lower (less than 7.5 at.%) than that prepared by physical deposition methods (35 at.%).