Microstructure evolution in adiabatic shear band in α-titanium

The microstructure and microtexture of adiabatic shear bands (ASBs) on the titanium side of a titanium/mild steel explosive cladding interface are investigated by means of optical microscopy (OM), scanning electron microscopy/electron back-scattered diffraction (SEM/EBSD) and transmission electron microscopy (TEM). Highly elongated subgrains and fine equiaxed grains with low dislocation density are observed in the ASBs. Recrystallization microtextures (28°, 54°, 0°), (60°, 90°, 0°) and (28°, 34°, 30°) are formed within ASBs. The grain boundaries within ASBs are geometrical necessary boundaries (GNBs) with high-angles. Based on the relations between temperature and the engineering shear strain, the temperature in the ASBs is estimated to be about 776–1142 K (0.4–0.6 T _m). The rotation dynamic recrystallization (RDR) mechanism is employed to describe the kinetics of the nano-grains’ formation and the recrystallized process within ASBs. The small grains within ASBs are formed during the deformation and do not undergo significant growth by grain boundary migration after deformation.     

Effect of cooling rate on the β-Ti to α-Ti transformations in a Ti-43at%Al intermetallic alloy

Phase transformations of β-Ti to α-Ti was studied in Ti-43at%Al using X-ray diffraction, optical microscopy, scanning electron microscopy, electron microprobe and transmission electron microscopy. Two types of non-equilibrium transformation mechanisms were observed to occur, Widmanstatten plate precipitation and massive transformation. Rapid quench from 1400°C can lead to the precipitation of Widmanstatten plates of α-Ti with the appearance of parallel plates at grain boundaries and basketweave plate colonies within grains. In addition, massive transformation was found to occur at prior β-Ti grain boundaries, resulting in the featureless microstructure and irregular grain boundaries. In all cases, the high temperature α-Ti phase was ordered into α₂ having antiphase boundaries present within massive grains or Widmanstatten plates.     

Structure of grains and grain boundaries in cryo-mechanically processed Ti alloy

A Ti5Ta1.8Nb alloy with the major phase as α (hcp) Ti has been subjected to severe plastic deformation by means of cryo-rolling. Significant grain refinement (from ~5 μm to ~60 nm) has been observed. The mechanism of grain refinement was studied by analysis of lattice strain variations with increase in cold work using XRD technique. Various intermediate stages, such as hardening, alignment of dislocations, cell formation and criticality before new grain formation, were identified. Formation of cells with dislocations alignment at the boundaries and then finally forming an ultra-fine grain structure was confirmed by transmission electron microscopy studies. Detailed grain boundary characterisation has been carried out using high-resolution transmission electron microscopy studies and crystallographic texture analysis. The grain-refined structure was found to possess a large fraction of high angle boundaries identified also as special boundaries by evaluating the misorientation angle/axis sets for a pair of adjacent grain boundaries.     

Columnar grain structure in thick sputtered nickel

The same two types of columnar growth structures were observed in sputtered nickel as have been observed in sputtered chromium. Transmission electron microscopy showed that columnar boundaries oriented perpendicular to the substrate surface were actually grain boundaries originating from rotational mismatch between adjacent grains about a common axis ([110] Ni direction) perpendicular to the substrate surface. No porosity or reduced density was observed in these columnar grain boundaries or in an interface 100–200 Å thick that was produced by an interruption in the sputter deposition experiment.     

Field ion microscope studies of the propagation of substrate grain boundaries into an overgrowth

The roles of substrate grain boundaries and substrate surface topography in the nucleation and growth characteristics of thin and thick overgrowths were evaluated through field ion and transmission electron microscopy techniques. The results of these studies, utilizing both electrodeposited and vapor-deposited overgrowths, indicate that the substrate surface topography is generally continued in the overgrowths at thin coverages. Nucleation and growth characteristics are seen to be influenced more by surface asperities than by the existence of grain boundaries on substrate surfaces. Grain boundaries (or the interaction of grain boundaries with the substrate surface) are not observed to be sites for preferential nucleation and growth of thin films although they can act as a source for recrystallization and grain growth in thicker overgrowths (greater than 20μm).     

Methanofullerene elongated nanostructure formation for enhanced organic solar cells

Using transmission electron microscopy (TEM) and Z-contrast imaging we have demonstrated elongated nanostructure formation of fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) within an organic host through annealing. The annealing provides an enhanced mobility of the PCBM molecules and, with good initial dispersion, allows for the formation of exaggerated grain growth within the polymer host. We have assembled these nanostructures within the regioregular conjugated polymer poly(3-hexylthiophene) (P3HT). This PCBM elongated nanostructure formation maybe responsible for the very high efficiencies observed, at very low loadings of PCBM (1:0.6, polymer to PCBM), in annealed photovoltaics. Moreover, our high resolution TEM and electron energy loss spectroscopy studies clearly show that the PCBM crystals remain crystalline and are unaffected by the 200-keV electron beam.     

Cobalt intergranular segregation in WC-Co composites

Interfaces between carbide grains in the tungsten carbide-cobalt composite have been considered. Different techniques, such as transmission electron microscopy and energy dispersive X-ray analysis have been used to characterize the orientation relationship, the nature of the planes and the chemical composition of the grain boundaries.The cobalt concentration at WC-WC grain boundaries was determined by X-ray energy selective analysis in the TEM. Cobalt profiles were performed across low-angle grain boundaries, coincidence and general grain boundaries. Cobalt segregation was found whenever dislocations were imaged in the grain-boundary plane of a low-energy grain boundary. The segregation value was compared with the segregation ratio measured in special grain boundaries characterized by a coincidence site lattice.     

Evolution of grain boundary structure in submicrometer-grained Al-Mg alloy

This paper presents high-resolution electron microscopy studies of grain boundary structures in a submicrometer-grained Al-3%Mg solid solution alloy produced by an intense plastic straining technique. The studies include the effect of static annealing on the grain boundary structure. Many grain boundaries are in a high-energy nonequilibrium state in the as-strained sample. The nonequilibrium character is retained on some grain boundaries in samples annealed at temperatures below the onset of significant grain growth. The effect of electron irradiation on the grain boundary structure also is examined.     

Microstructure optimization of austenitic Alloy 800H (Fe-21Cr-32Ni)

The microstructural evolution, specifically of grain boundaries, precipitates, and dislocations in thermomechanically processed (TMP) Alloy 800H samples was characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), electron backscattered diffraction (EBSD), transmission electron microscopy (TEM), and atomic force microscopy (AFM). The TMP not only significantly increased the fraction of low-Σ coincidence site lattice boundaries, but also introduced nanoscale precipitates in the matrix and altered the distribution of dislocations. Statistical analysis indicates that the morphology and distribution of grain boundary precipitates were dependent on grain boundary types. The microstructure optimization played a synergistic effect on the significantly increased strength with comparable ductility and enhanced intergranular corrosion resistance and creep-fatigue life compared to the as-received samples.     

Effect of austenite grain boundary mobility on hardenability of steels containing vanadium

Abstract

A correlation has been established between the rate of grain boundary migration during austenitisation and the hardenability of steels containing 0·2–0·3%C, 1·5–1·7%Mn, up to 0·35% V, and small additions of Al or Ti. Interaction between the austenite grain boundaries and pinning particles was investigated using transmission electron microscopy and segregation to the austenite grain boundaries was examined using Auger electron spectroscopy. It has been concluded that the velocity of grain boundary migration during austenitisation influences the extent of equilibrium segregation to the austenite grain boundaries which, in turn, affects the hardenability. Pinning of the austenite grain boundaries enabled the potential hardenability effect of the alloying elements to be increased. Mechanisms have been discussed for the ways in which segregation, particularly of V, occurs to pinned or immobilised austenite grain boundaries, and the conditions by which most effective grain boundary pinning can be achieved have been considered. Some technological implications have been suggested.

MST/804     

Microstructural characterisation of friction stir processed aluminium

Abstract

Friction stir processing was carried out on commercially pure aluminium, and a detailed microstructural characterisation was performed by electron backscattered diffraction and transmission electron microscopy. Friction stir processing resulted in significant grain refinement with narrow grain size distribution. The microstructure showed fine and equiaxed grains, with some ultrafine grains being also observed. Electron backscattered diffraction studies showed majority of the boundaries to be high angle, confirming the occurrence of dynamic recrystallisation (DRX). Transmission electron microscopy observations revealed dislocation arrangement into subgrain boundaries, grains having different dislocation densities and in different stages/degrees of recovery. Electron backscattered diffraction analysis also revealed a progressive transformation of sub-grain boundaries into high angle grain boundaries. A multimechanism of dynamic recovery, continuous DRX and discontinuous DRX seems to be operating during the process. The microstructure is not affected by changing the rotation speed from 640 to 800 rev min^?1, except that the grain size was marginally larger for higher rotational speed.     

Structural dependence of gold deposition by nanoplating in polycrystalline copper

In the present work, the gold-nanoplating technique is used to monitor differences in the electrochemical activity of different types of grain boundaries in high-purity copper. Gold-nanoplating is based on the electrochemical displacement of gold, which is deposited as particles from an aqueous solution on the polycrystalline copper surface. The complementary use of electron backscatter diffraction for revealing microstructural features, field emission scanning electron microscopy for imaging, and energy-dispersive X-ray analysis for quantification of the deposited gold makes it possible to detect differences in the grain boundary activity for different types of grain boundaries. In this way, it becomes possible to distinguish special from random boundaries in an efficient way. Also quantitative experimental results on grain boundary activity are produced, which correlate strongly with literature predictions on grain boundary energy.     

Atomic structure of a CeO2 grain boundary: the role of oxygen vacancies

Determining both cation and oxygen sublattices of grain boundaries is essential to understand the properties of oxides. Here, with scanning transmission electron microscopy, electron energy-loss spectroscopy, and first-principles calculations, both the Ce and oxygen sublattices of a (210)Σ5 CeO(2) grain boundary were determined. Oxygen vacancies are shown to play a crucial role in the stable grain boundary structure. This finding paves the way for a comprehensive understanding of grain boundaries through the atomic scale determination of atom and defect locations.     

Exudation of Ag and Cu in internally oxidized Ag-Sn-In-(Cu) alloys

Growth and structure of pure Ag layers are investigated on the surface and at grain boundaries of Ag alloys that have been internally oxidized. Two Ag-Sn-In and an Ag-Sn-In-Cu alloy were studied by ion polishing, field emission scanning electron microscopy and optical microscopy. As already known from previous studies Ag atoms diffuse towards the surface. In the present study it is shown that grain boundaries can act as a sink for diffusing Ag atoms in competition with the surface. In the quaternary alloy Ag and Cu diffuse towards the surface simultaneously. The Cu diffusion is confined by the nucleation of CuO particles. The progress of the internal oxidation front is discussed in conjunction with the nucleation of pure Ag crystallites and CuO particles at grain boundaries.     

Effect of intermetallic particles and grain boundaries on short fatigue crack growth behaviour in a cast Al–4Cu–3Ni–0.7Si piston alloy

The short fatigue crack growth behaviour in a model cast aluminium piston alloy has been investigated. This has been achieved using a combination of fatigue crack replication methods at various intervals during fatigue testing and post‐mortem analysis of crack profiles. Crack–microstructure interactions have been clearly delineated using a combination of optical microscopy, scanning electron microscopy and electron backscatter diffraction. Results show that intermetallic particles play a significant role in determining the crack path and growth rate of short fatigue cracks. It is observed that the growth of short cracks is often retarded or even arrested at intermetallic particles and grain boundaries. Crack deflection at intermetallics and grain boundaries is also frequently observed. These results have been compared with the long crack growth behaviour of the alloy.     

The influence of high temperature ageing on the Structure of Alloy 800

The report presents investigations of the microstructure of commercial Alloy 800 after isothermal ageing at 900°C, carried out by optical microscopy and by transmission electron microscopy using replica and thin foil techniques. The high temperature ageing was characterized predominantly by precipitation of carbides on grain and twin boundaries of the austenitic matrix, as well as on dislocations within the grains. The carbides were identified, using electron microscope diffraction and X-ray phase analysis of electrolytic extractions, as (Cr, Fe)₂₃C₆, Ti (C, N) and TiC.     

Fatigue crack nuclei in austempered ductile cast iron

ABSTRACT Short fatigue crack nuclei in austempered ductile cast iron have been studied using optical microscopy, scanning electron microscopy, atomic force microscopy and X‐ray microtomography and by electron backscatter diffraction analysis. Fatigue cracks nucleate at graphite nodules and shrinkage microporosity. The crack nuclei are arrested and retarded by barriers in the microstructure, by either blocking of slip at boundaries or owing to the requirement for tilt and twist of the stage I crystallographic crack at grain boundaries. These observations indicate that both the size of the defects, such as graphite nodules and microporosity, and the size of the prior austenite grains control the largest crack nucleus that can develop, and hence determine the component fatigue limit.     

Effect of the structural state on the cold brittleness and fracture toughness of low-alloyed molybdenum

We investigate the effects of the chemical and phase compositions of grain boundaries and the state of an excess phase on the fracture toughness, cold brittleness, and strength of low-alloyed molybdenum by the methods of scanning electron microscopy, transmission electron microscopy, and Auger electron spectroscopy. The structural state of the material was changed without changing grain sizes by annealing in various modes at different temperatures. It was shown that the level of segregation of interstitials on grain boundaries is determined by both the annealing temperature and distinctive features of the evolution of excess phases. Embrittlement of grain boundaries results in a decrease in the strength and fracture toughness of the material and in an increase in its temperature of cold brittleness. In the case of transcrystalline fracture, the material exhibits better mechanical properties correlated with distinctive features of the evolution of excess phases inside the grains.Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 30, No. 5, pp. 34–44, September–October, 1994.     

Effect of processing parameters on quench sensitivity of an AA7050 sheet

Secondary precipitation takes place in alloy 7050 during slow quenching after solution and results in a significantly decreased content of Mg, Zn elements. Optical microscopy, transmission electron microscopy (TEM), and electron backscatter diffraction (EBSD) study showed the distribution of Al₃Zr particles plays a vital role in quenching process. The equilibrium η phases mainly precipitate at Al₃Zr particles within the recrystallized grains and other high energy areas (primarily grain boundaries). The influence of the processing (homogenization, hot rolling and solution) parameter on the quench sensitivity is also investigated by mechanical property examination (T6 temper). A ramping heat homogenization, controlled hot rolling (67% rolling reduction and 3-5 s⁻¹ deformation rate) and two-stage solution treatment result in lesser recrystallization and fewer high angle grain boundaries, and lower the boundary angles within sub-structures. The decreasing number of heterogeneous precipitation sites endows the study alloy with good quenching sensitivity and better mechanical properties.     

Nanoscale investigation of charge transport at the grain boundaries and wrinkles in graphene film

The influence of grain boundaries and mechanical deformations in graphene film on the electric charge transport is investigated at nanoscale with conductive atomic force microscopy. Large area monolayer graphene samples were prepared by the chemical vapor deposition technique. Field emission scanning electron microscopy confirmed the formation of grain boundaries and the presence of wrinkles. The presence of the D-band in the Raman spectrum also indicated the existence of sharp defects such as grain boundaries. Extremely low conductivity was found at the grain boundaries and the wrinkled surface was also more resistive in comparison to the plain graphene surface. Many samples were experimented with to justify our findings by selecting different areas on the graphene surface. Uniform conductivity was found on grain boundary and wrinkle free graphene surfaces. We made channels of varied lengths by local anodic oxidation to confine the charge carrier to the smallest dimensions to better confirm the alteration in current due to grain boundaries and wrinkles. The experimental findings are discussed with reference to the implementation of graphene as transparent conductive electrode.