Optical Observations of Disorder in Solid Helium 4

It has been proposed that the supersolidity of helium 4 is associated with disorder in the solid samples. We report optical observations of this disorder in samples grown by different methods. We show that, when grown by the “blocked capillary method” as in most experiments showing anomalous phenomena, solid helium 4 can be polycrystalline with grain sizes down to the micrometer range, much smaller than the 0.1 mm found in some other experiments. We analyze the properties of grain boundaries, and in particular the existence of liquid channels at the contact of grain boundaries with walls. Our analysis includes measurements of the contact angle of the liquid-solid interface with these walls, which exhibits a large hysteresis. Furthermore, we predict that similar channels should exist at the junction between grain boundaries.     

Quantitative characterization of precipitate free zones in Al–Zn–Mg(–Ag) alloys by microchemical analysis and nanoindentation measurement

To correlate quantitatively the mechanical properties of precipitate free zones (PFZ) with the corresponding microstructural and compositional characteristics, TEM observation, EDX analysis and nanoindentation measurement have been performed in the vicinity of grain boundaries in Al–4.9 mass%Zn–1.8 mass%Mg (–0.28 mass%Ag) alloys. The remarkable decreases in hardness and solute concentrations were observed towards grain boundaries even in the regions just outside PFZ. With increasing aging time, it is firstly revealed that the hardness inside PFZ monotonously decreases although the hardness inside grains increases in the earlier stage of aging. Three distinct regions of “PFZ”, “Transition-area” and “Grain-region” were therefore proposed to explain the origins of such age-hardening behavior observed in this work. In the Ag-added alloy, on the other hand, the hardness could be maintained up to closer regions to grain boundaries at the same level as that inside grains.     

Analysis of microstructure evolution and precise solid fraction evaluation of A356 aluminum alloy during partial re-melting by a color etching method

Spheroidization of Al grains is required for the production of semi-solid slurry either by a partial solidification route or partial re-melting route. In this research, A356 aluminum alloy was deformed and partially re-melted to semi-solid state. A segregation sensitive reagent (Weck’s reagent) was used to reveal the inner microstructure of Al grains for the better understanding of the microstructure evolution during partial re-melting. Optical microstructure observation showed that the previously compressed Al dendrites were actually “fractured” during heat treatment and such “fractured” dendrites contributed to the refinement and spheroidization of Al grains. Further study of this phenomenon indicates that the “fractures” are actually migrating high-angle grain boundaries, which was related to the recrystallization that occurred during heat treatment. Moreover, the growth layer of Al grains formed during water quenching is clearly visualized after being etched by Weck’s reagent. Consequently, precise evaluation of solid fractions through image analysis was realized by excluding growth layer when measuring the area of solid phase.     

Orientation splitting and its contribution to grain refinement during equal channel angular extrusion

The early stage mechanisms of grain refinement during ECAE of a single-phase aluminium alloy have been studied using the EBSD technique. It was found that, in addition to the formation of shear-plane cell bands and shear bands by “simple shear”, the development of deformation bands due to orientation splitting contributed significantly to the refinement of microstructure. “Regular” slab-like deformation bands and “irregular” transitional bands were observed after the first pass; both developed boundaries of high misorientations. In the second pass, moderate orientation splitting took place within the deformation bands, although new deformation bands were not detected. With increased strains, fine scale orientation splitting tended to occur in local bands, generating high densities of new high misorientation boundaries. The crystallographic features of the different types of orientation splitting are examined.     

Cryo-Cracking: a technique for creep damage assessment in high temperature components

The characterisation of fracture surfaces of service exposed SA welds created under cryogenic conditions leads to a new fractographic analysis method called “Cryo-Cracking”. The “Cryo-Cracking” method takes advantage of an ability to separate the previous (service) creep damage from the “Cryo- Cracking” induced fracture features. This allows the evaluation of cavities and service damaged grain boundaries and permits a quantitative definition of damage level; aided by the SEM's high resolution and enhanced depth of field. In addition, directed EDS can provide chemical information regarding cavitation and the particulates which nucleate cavities. The “Cryo-Cracking” method does not depend on personal metallographic skills and is thus considered less sensitive to misinterpretation due to specimen preparation techniques.     

Non-linear current–voltage characteristics related to native defects in SrTiO3 and ZnO bicrystals

SrTiO₃ and ZnO bicrystals with various types of boundaries were fabricated in order to examine their current—voltage characteristics across single grain boundaries. Their grain boundary structures were also investigated by high-resolution transmission electron microscopy. In Nb-doped SrTiO₃, electron transport behaviors depend on the type of boundaries. Random type boundaries exhibit highly non-linear current—voltage characteristics, while low angle boundaries show a slight non-linearity. On the contrary, undoped ZnO does not exhibit non-linear current—voltage characteristics in any type of boundaries including random ones. It is suggested that the differences observed in current—voltage properties between the two systems are mainly due to the difference in the accumulation behavior of acceptor-like native defects at grain boundaries. A clear non-linearity is obtained by means of Co-doping even for the highly coherent ∑ 1 boundary in a ZnO bicrystal. This is considered to result from the production of acceptor-like native defects by Co-doping.     

MECHANISMS AND FACTORS THAT INFLUENCE POSTWELD INTERGRANULAR UNDERCLAD CRACKING

Abstract— Forged components of ferritic steel can be protected by a welded austenitic stainless steel clad. Intergranular cracking can take place in the ferritic phase close to the ferritic austenitic interface. After developing a technique for fabrication of these cracks, the formation conditions are studied. Auger electron spectroscopy investigations of specimens containing a real crack opened inside the vacuum chamber are used for interpretation. Sulphur segregations embrittle the grain boundaries which are cracked by residual and thermal stresses during the postweld heat treatment.     

Orientation analysis of polycrystalline silicon

An analysis of the crystallographic orientation of Wacker “Silso” polycrystalline silicon is reported. It is observed that many grains are penetrated perpendicular to the sample surface by equally oriented lattice planes. No connection could be found between grain size and grain orientation. 50% of adjacent grains in the center of the plate and 16% of adjacent grains in the edge region of the plate share common lattice planes which traverse the grain boundaries undisturbed.     

Multi-physics analysis of nano-structured ferroelectrics by first-principles simulations

Unusual ferroelectricity emerges in nanoscale Perovskite oxides owing to their characteristic structures, and it strongly interacts with mechanical stress/strain, namely, “multi-physics property”. For systematic understanding, the ferroelectricity and multi-physics property in the nano-components are discussed in terms of their “macroscopic” component shape (e.g., films, wires, tubes and dots) and “inner” inhomogeneous structure (e.g., domain walls and grain boundaries), based on the first-principles density-functional theory calculations. Moreover, this paper also presents a remarkable interplayed effect of the macroscopic shape and the inner structure on the property through a theoretical investigation on polydomain PbTiO₃ ultrathin films.     

Hard magnetic two-phase Co-Cu thin films

The magnetic properties of sputtered films of 25 Co-75 Cu and 50 Co-50 Cu before and after annealing were investigated. In the as-sputtered state the films exhibit the structure of a metastable fcc solid solution. Annealing at 500 to 700°C causes decomposition into two phases, Cu and fcc 89 Co-11 Cu. The decomposition supposedly occurs by heterogeneous nucleation at the grain boundaries, and growth by grain boundary diffusion. The 89 Co-11 Cu phase exists in the form of small particles with magnetic single domain behavior. The films have coercivities up to 280 Oe. Squareness ratios between 0.7 and 0.9 were found. No strain sensitivity of the magnetic properties could be detected. This material is regarded to be suitable for magnetic recording.     

Cracking mechanisms in large size ingots of high nickel content low alloyed steel

This study is focused on determining the possible root causes for cracking after open die forging of large size ingots made of high nickel medium carbon low alloy steels. Optical and scanning electron microscopies as well as Energy Dispersion Spectroscopy (EDS) were used to examine the microstructure of the samples taken out of a cracked forged ingot. The large size of the samples permitted to investigate microstructure at different locations at the surface and in depth. Chemical analysis revealed chromium and oxygen enrichment at the grain boundaries. Grain size measurement indicated clear differences between “clean” surface zones and cracked ones, and between surface and in depth regions. The analyses indicated that fracture phenomena may be due to abnormal grain size which promotes oxide penetration into grain boundaries, resulting in their embrittlement and cracking upon cooling.     

Computer simulation of vortex pinning in type II superconductors. I. Two-dimensional simulation

The summation of pinning forces to a volume force exerted on the vortex lattice in type II superconductors allowing it to carry a loss-free current is not fully understood. In order to clarify this question we have started computer simulations of flux pinning. It is shown that for many experimental situations bending of vortices may be neglected since the vortices are too short or pinning is too weak, and thus pinning is two-dimensional. As a first step, two-dimensional pinning simulations will thus be instructive with regard to, say, ribbons of amorphous metals. A general expression for the energy of a vortex-pin system in two and three dimensions is given. The simulation method is presented and illustrated for the isolated pin (with a detailed discussion of the “threshold effect” and of elastic instabilities) and for pin “walls” (grain boundaries) and “nozzles.” Random point pins acting on a perfect or defective vortex lattice are treated in an accompanying paper.     

Development of “Macroscopic Composition Gradient Method” and its application to the phase transformation

A new characterization method, “Macroscopic Composition Gradient (MCG) Method” is proposed to investigate the phase transformations near the phase boundaries. The MCG method is a new technique to investigate the phase transformations in various composition alloys by utilizing a single specimen having the macroscopic solute composition gradient. Since the macroscopic composition gradient in the MCG alloy is so prepared as to cross over the phase boundary, the morphological transition of critical phenomena at the phase boundary can continuously be investigated by means of analytical transmission electron microscopy. By utilizing the MCG method, the various kinds of phase transformation, such as the coherent and incoherent precipitation boundaries, the order/disorder phase transition and the morphological change at the spinodal line have successfully been investigated. Furthermore, to an important thing, the critical size of precipitate-nucleus and the nucleation rate near the solubility limit can be experimentally obtained for respective nucleus. The phase decomposition of supersaturated solid solution progresses by a mechanism of spinodal decomposition even in the N-G region of phase diagram. On the basis of experimental results, the application limit of the conventional nucleation theory is investigated, and hence the failure of Boltzmann–Gibbs free energy becomes obvious in the early stage of phase decomposition.It is noteworthy that the present experiment is systematically conducted for the alloy composition range very close to the solubility limit. Such critical phenomena of phase transformation have been scarcely examined in the past. The MCG method proposed here is considered to open a new way to investigate the critical phenomena in the phase boundary.     

Quantized Grain Boundary States Promote Nanoparticle Alignment During Imperfect Oriented Attachment

Oriented attachment (OA) has become a well‐recognized mechanism for the growth of metal, ceramic, and biomineral crystals. While many computational and experimental studies of OA have shown that particles can attach with some misorientation then rotate to remove adjoining grain boundaries, the underlying atomistic pathways for this “imperfect OA” process remain the subject of debate. In this study, molecular dynamics and in situ transmission electron microscopy (TEM) are used to probe the crystallographic evolution of up to 30 gold nanoparticles during aggregation. It is found that Imperfect OA occurs because 1) grain boundaries become quantized when their size is comparable to the separation between constituent dislocations and 2) kinetic barriers associated with the glide of grain boundary dislocations are small. In support of these findings, TEM experiments show the formation of a single crystal aggregate after annealing nine initially misoriented, agglomerated particles with evidence of dislocation activity and twin formation during particle/grain alignment. These observations motivate future work on assembled nanocrystals with tailored defects and call for a revision of Read–Shockley models for grain boundary energies in nanocrystalline materials.     

EBSD Study of Microstructural Evolution in a Nickel‐Base Superalloy during Two‐Pass Hot Compressive Deformation

Generally, the high‐temperature deformation characteristics and microstructural evolution of alloys are studied by isothermal compressive experiments at stable strain rates. But, the strain rate is variant during the practice industrial production of components. In this work, isothermal two‐pass hot compression experiments with stepped strain rates are performed to analyze the microstructural evolution of a nickel‐base superalloy with δ phase. Results reveal that the mean grain size decreases, but the percentage of undissolved δ phases increases, as the strain rate of the first pass (SROFP) is increased. However, the mean grain size and the percentage of undissolved δ phases decreases with the increase of inter‐pass time (IPT) or the true strain of the first pass (TSOFP). Meanwhile, the increased deformation temperature easily enlarges the mean grain size, but obviously decreases the percentage of undissolved δ phases. In addition, the evolution of Σ3ⁿ boundaries not only results from the “new twinning mechanism”, but also “Σ3ⁿ regeneration mechanism”. “Σ3ⁿ regeneration mechanism” becomes predominant with decreasing SROFP or increasing IPT/TSOFP. Besides, “new twinning mechanism” plays a major role on Σ3ⁿ boundaries evolution as the temperature is increased from 950 to 980 °C, and then become weaken with further increasing the deformation temperature.

     

The Outstanding Contribution of Basal Slip in Substructure Development during Friction Stir Processing of Magnesium Alloys

This research reveals the critical role of basal slip in the substructure development during friction stir processing of a magnesium alloy. In this respect, the intragranular lattice rotation axes are considered to identify the activity of different slip systems. The applied shear strain during the procedure is stored in the matrix through slip-induced rotations at the grain level. The rotations around distinct Taylor axes produce “slip domains” separated by necessary boundaries from the parent grains, significantly contributing in grain refinement. The basal slip is easily activated in grains holding different stored energy; however, the nonbasal slip has a higher dependency on the amount of local applied strain. Determining the contribution of different slip systems in strain accommodation reveals that the basal slip imposes the highest fraction of low-angle boundaries into the microstructure leading to the development of the ultimate grain boundary structure.     

Mg segregations at and near deformation-distorted grain boundaries in ultrafine-grained Al–Mg alloys

The formation of Mg segregations at and near deformation-distorted grain boundaries (GBs) in ultrafine-grained Al–Mg alloys is theoretically described as a process enhanced by stress fields of extrinsic dislocations existing at such GBs. The equilibrium Mg concentration profiles near low-angle and high-angle GBs containing extrinsic dislocations are calculated. The results of the calculations explain the experimental observations (reported in the scientific literature) of spatially inhomogeneous Mg segregations characterized by high Mg concentrations at and near GBs in ultrafine-grained Al–Mg alloys processed by severe plastic deformation.     

Estimations of grain-boundary surface tension in elemental solids

The empiric equations estimating the temperature dependences of average surface tension of high-angle grain boundaries in elemental solids are presented. The grain boundaries are considered as a homogeneous liquid-like layer in a solid matrix. The expressions present the grain-boundary surface tension as functions of the melting temperature, melting enthalpy, and molar volume of the elemental solid at the melting temperature. The empiric expressions relating grain-boundary surface tension with the surface tensions of elements in the solid and liquid states at melting temperature are obtained on this basis. The relations proposed can be useful to estimate the averaged surface tension of high-angle grain boundaries in elemental solids at high enough homologous temperature.     

Review Stability of nanostructured materials

The different aspects of nanostructured material (NM) stability such as thermal and chemical stability as well as NM behavior under deformation and radiation are characterized and analyzed in detail. Grain growth, phase transitions (including spinodal decomposition), homogenization diffusion processes, relaxation of residual stresses, and behavior of grain boundary and triple junction segregations are discussed in context of the change of nanostructure and properties. Special interest is given to the availability of NMs with ultra-fine grain size and their behavior during annealing as soon as to the possibility of development of nanostructures with high thermal stability. Some unsolved problems are emphasized.     

X-ray diffraction patterns from nanocrystalline binary alloys

X-ray diffraction powder patterns of a random nanocrystalline material containing two atomic species were calculated by evaluation of the Debye function. Atoms of different types were positioned to produce stoichiometric compound, CsCl, and grain boundary segregation in copper-iron alloys. Radial strain fields of the type associated with point or boundary defects were introduced into the structure and the x-ray powder patterns were calculated. Grain boundary defects of different composition than the bulk were introduced and combined with radial strain fields. The diffraction signatures of the “gas-like” and crystalline grain boundaries are calculated and compared.