A study of defects in epitaxial films of ZnTe by transmission electron microscopy

The electron diffraction patterns of sphalerite structure ZnTe films evaporated in vacuum onto NaCl, BaF₂ and CaF₂ substrates contained numerous satellite spots. The diffraction patterns from (100) ZnTe films grown on cleavage faces of NaCl always contained satellite spots that arose from grains of wurtzite structure material with the (0001) plane parallel to the plane of the film and doubly positioned by a 30 rotation about the normal to the film. Other satellite spots occurred due to the presence of cubic microtwins and other types of included grains of wurtzite. Films grown under cleaner conditions had simpler structures.The most prominent satellite spots in diffraction patterns from (111) films of ZnTe grown on NaCl, BaF₂ and CaF₂ were due to twinned sphalerite double positioning. Additional spots arose from microtwins.Evidence of double diffraction was found in both the (100) and (111) diffraction patterns.     

Characterisation of residual stresses in heat treated,high strength aluminium alloy extrusions

Residual stresses were measured in rectilinear aluminium bars quenched using an aqueous polyoxyethylene glycol (PAG) solution or cold-water. Residual stresses were measured with neutron diffraction and a superposition-based method using mechanical strain release measurements. Three orthogonal stress components were measured along two transverse lines using neutron diffraction. The longitudinal residual stresses were mapped over a transverse cross-section using the contour technique. A primary slice removal technique mapped three orthogonal residual stresses over a transverse cross-section in the PAG extrusion. Residual stresses were found to vary from biaxial compressive in the part boundaries to triaxial tensile in the interiors. There was close correlation between the neutron diffraction and mechanical strain release techniques. PAG quenching demonstrated lower residual stresses.

This paper is part of a Themed Issue on Measurement, modelling and mitigation of residual stress.     

A transmission electron microscope study of electrodeposited nickel films on (001) copper substrates

Nickel was electrodeposited on to (001) single-crystal copper films. Deposit thickness were in the range 10 Å to 500 Å. Bicrystals and stripped deposits were examined by transmission electron microscopy and high-resolution electron diffraction. Misfit, as a function of thickness, was determined from moiré fringe spacings, splitting of diffraction spots and the spacing of misfit dislocations. The misfit was found to be less than that of vapour-deposited films at corresponding thicknesses. The misfit as calculated from the spacing of misfit dislocations was not in agreement with the misfit as determined from moiré fringes or electron diffraction. These discrepancies might be due to foreign material incorporated in the deposit.     

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.     

Microscopic analysis of Fe–Cr alloy produced by single roll strip casting

Abstract

In the present investigation, the microstructures and mechanical properties of Fe–Cr alloy prepared by single roll strip casting were studied. Optical microstructure showed subgrain boundaries inside large grains. Cracks were observed along the grain boundary. Scanning electron microscopy, X-ray diffraction study confirms the formation of chromium carbide at the grain boundary in the case of as cast alloy. Electron backscatter diffraction showed preferred orientation of grains in the as cast alloy. Carbides and undesired phases were not observed in heat treated alloy. Pores present in as cast samples expand after heat treatment process. Mechanical properties, like tensile strength, yield strength, elongation and hardness, of Fe–Cr single roll strip casting alloys were improved after heat treatment.     

The Effect of Electron Beam Irradiation on Electron Diffraction Patterns of Bi-Sr-Ca-Cu-O High- Tc Superconductors

The effect of electron irradiation having the energy of 75, 100, and 200 keV on structural modifications of Bi-2212 superconducting samples has been studied. For the last energy, the irradiation time from zero to 150 min was used. At a constant energy of the electrons, the observed phenomena consist in the disappearance of the incommensurate unidimensional modulation, in the decreasing of spots' intensity and their elongation along the equivalent crystallographic axis a, and even spot splitting with the occurrence of double extra spots, with the increase of the irradiation time.After electron irradiation with energy of 75 and 100 keV, the structural modifications lead to some spot patterns consisting of some planar lattices (in some cases a pseudo-tetragonal one) that are twisted on each other at different angles (8°, 13.6°, 19°, etc.) around the axis of the incident electron beam. For the irradiation at increased doses of thin microcrystals having reduced lateral dimensions, the electron diffraction spots were arranged in discrete or partial continuous Debye rings or continuous concentric Debye rings characteristic for the polycrystalline state.After electron irradiation with energy of 200 keV, the effects of electron irradiation on Bi-2212 samples depend strongly on irradiation fluence rate and time and consisted in the following: disordering defects in the diffraction patterns (disappearance of some spots, spot intensity modification, streaks occurrence, spot elongation); alteration and disappearance of incommensurate structural modification; conversion of single crystal particle areas into polycrystalline material; and quasi-amorphization.A simple approach based on the evaluation of the displacement yield of in-plane oxygen atoms vs. irradiation time for the different incident energy and electron fluence rates could explain the general trend of irradiation damage in HTS materials.     

A mechanism of ferrite softening in a duplex stainless steel deformed in hot torsion

A mechanism of dynamic softening of ferrite was studied in a 21Cr-10Ni-3Mo austenite/ferrite duplex stainless steel subjected to torsion at a strain rate of 0.7 s⁻¹ at 1200°C. Transmission electron microscopy together with convergent beam electron diffraction were used with major emphasis on the study of misorientations across ferrite/ferrite boundaries. No evidence of discontinuous dynamic recrystallisation involving nucleation and growth of new grains was found within ferrite contrary to some suggestions made in the literature for similar experimental conditions. The softening mechanism has been classified as extended dynamic recovery characterised by a gradual increase in misorientations between neighbouring subgrains that were created by dynamic recovery processes at the earlier stages of deformation. The resulting dislocation substructure was a complex network of subgrain boundaries composed of a mix of higher- and lower-angle walls characterised by misorientation angles not exceeding 20° at a maximum obtained strain of 1.3.     

Microstructural development in a low carbon Ti-microalloyed steel during deformation within the ferrite region

Microstructural evolution of ferrite in a low carbon Ti-microalloyed steel, during deformation within the ferrite region, was investigated by using torsion testing. Warm deformation characteristics of ferrite were studied by analyzing of its flow curves, optical microstructures and electron back-scattered diffraction maps. The results show that an unstable sub-boundaries network forms and then these transform into high angle boundaries, with further straining. It was found that the occurrence of continuous dynamic recrystallization is responsible for the development of very fine ferrite grains with high angle boundaries.     

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.     

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.     

Incorporation of paclitaxel solid dispersions with poloxamer188 or polyethylene glycol to tune drug release from poly(ϵ-caprolactone) films

Here we report the application of solid dispersion (SD) technique to improve paclitaxel (PTX) release from poly(?-caprolactone) (PCL)-based film. Paclitaxel solid dispersions (SDs) with either poloxamer188 (PXM) or polyethylene glycol (PEG) were successfully prepared by a melting method and then incorporated into PCL films, which were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC) and In vitro drug release/dissolution studies. It was found that PTX was faster released from the SDs than the corresponding physical mixtures (PMs) or PTX alone. For the PCL films with almost the same PTX loading, drug release from films containing SDs was remarkably faster than that from the film directly incorporated with PTX particles, and the films containing SDs with PXM exhibited a faster drug release than those with PEG. An increase In the content of PXM had no significant influence on PTX release from the films containing SDs. Incorporation of a higher content of SDs led to slower drug release from PCL films, indicating that PTX loading had a dominating effect on drug release. Through this study, we demonstrated the feasibility of the application of SD technique on the improvement of PTX release from PCL films and offered some beneficial information on modulating drug release behavior by changing the compositions and contents of the SDs-loaded PCL films.     

Development of microstructure and mechanical properties of as cast Al–Mg–Mn alloy during high temperature ECAP

Abstract

A large scale billet with diameter of 58·5 mm of an as cast Al–Mg–Mn alloy was processed by equal channel angular pressing (ECAP) at 350°C up to six passes. A significant refinement of the grains was observed after six pressings to ～2 μm. And the selected area electron diffraction (SAED) pattern showed that almost all of the grains were separated by boundaries with high angles of misorientation. A banded substructure was not observed during the hot ECAP, and a reasonably equiaxed structure was obtained just after one single pressing. Both the strength and the elongation increased abruptly in a single passage through the die, but thereafter, the increase was more gradual and exhibited a saturation effect after the fourth pressing. The good combination of strength and ductility of the Al–Mg–Mn alloy attained by the hot ECAP appeared to be attractive properties for industrial applications. Moreover, hot ECAP could possibly be used as an alternative step to hot extrusion or hot rolling in industrial processing, to break down an initial coarse as cast structure in a quite large scale billet.     

Microstructural evolution analysis of grains and tensile properties of tin bronze in hot extrusion at different temperatures

In this paper, Electron Back-Scattered Diffraction is adopted to analyze the tin bronze microstructural changes of grains and grain boundaries in hot extrusion from 620 to 720°C. The result shows that the size of grains grows as the temperature rises with a competitive status between recrystallisation and subgrains and deformed grains, and shows a random distribution with no obvious preferred orientation in structure. High angle grain boundaries (>30°) are most common with a large quantity of Σ3 boundaries at 60°. Twin grains firstly appear in recrystallisation and grow continuously at subgrains. In the tensile test, re-crystallised grains had certain influence on the plasticity of tin bronze. Along with the elongation of tin bronze at temperatures above 700°C, its plasticity also declined.     

Microstructure and mechanical properties of variable-plane-rolled Mg–3Al–1Zn alloy

A flawless bulk AZ31 magnesium alloy with extensive mechanical twins was produced by variable-plane rolling, in which the sample was rotated 90° around its longitudinal axis between passes. The unique orientation relationship between the parent grains and the twin grains favours twinning during variable-plane rolling, which leads to the formation of extensive twins. Tensile testing revealed an excellent balance of mechanical properties, with a yield strength of 280 MPa and 15.5% elongation to failure. The significant strengthening originates from the effective blockage of glide dislocations by numerous conventional grain boundaries and twin boundaries. A weak double-peak (slightly off-basal) texture is formed during variable-plane rolling, which helps in achieving the desired level of ductility.     

Dynamic recrystallization of Ni-base alloys—Experimental results and comparisons with simulations

The dynamic recrystallization of the Ni-base alloy Böhler L306 VMR (Alloy 80A) in a transient state was investigated both by light microscopy and electron backscatter diffraction (EBSD), and the experimental results were compared with those from simulations. Subgrain structures of the size of the recrystallized grains were observed close to the grain boundaries of the original grains. With increasing strain a texture developed in the deformed fraction. Strong twinning was found in the recrystallized fraction, with area fractions of the twinned grains of around 80% for higher strains. Thus the measured grain sizes strongly depend on the handling of the twins. A pronounced increase in the average grain size of the recrystallized fraction with increasing strain (time) was only observed after twin removal. There was generally good agreement between the measured and the simulated results.     

镁合金AZ31高温形变机制的织构分析

利用X射线衍射和背散射电子衍射方法测定了镁合金AZ3l高温动态再结晶和超塑形变时的宏观和微观织构,分析了晶粒内部的形变机制.结果表明,在动态再结晶和超塑形变过程中,晶粒内部的滑移机制仍起重要作用,表现为再结晶晶粒出现择优取向以及一些晶粒可充分均匀形变成长条状.宏观织构的测定表明,具有不同初始织构的两类样品高温动态再结晶时,新晶粒有不同的取向择优过程,形成相似的织构;长条形变晶粒内部开动的滑移系也有一定的差异.分析了不同温度下相同的织构对应的不同塑变机理取向成像分析表明,基面织构取向的晶粒间总伴随着较高比例的小角晶界和30°(0001)的取向关系,这是六方结构的六次对称性限制了动态再结晶时(亚)晶粒间取向差的有效增大的缘故.     

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.     

Microstructure and mechanical properties of Mg-Al-Zn alloy sheet fabricated by cold extrusion

Cold extrusion of AZ31 magnesium alloy sheets was studied in this paper. Microstructure and texture distributions of the as-extruded sheet were investigated by electron backscattered diffraction (EBSD) method. The grains were significantly refined and the average grain size was 1.6 μm. Dynamic recrystallization has taken place during the extrusion process, which resulted in the high frequency of high angle grain boundaries in the sheet. After the cold extrusion, a weak double-peak type basal texture was formed. The formation of the texture was ascribed to the non-basal <c + a> slips. Tensile tests revealed that mechanical properties were enhanced due to grain size refinement, but mechanical anisotropy was obvious. It is believed that mechanical anisotropy was related to the splitting of basal texture.     

热处理对铸造Ti15-3合金显微组织和力学性能的影响

借助光学显微分析、ＴＥＭ和ＳＥＭ分析手段研究了不同热处理工艺对Ｔｉ１５－３合金显微组织和力学性能的影响。结果表明：合金在铸态时的组织特征为粗大的β相,由于合金中没有析出相的弥散强化作用,因而合金的强度低,在不同温度时效处理后,在晶内和晶界析出α相,随着时效温度和时效时间的增加,析出相不断粗化,与铸态相比,合金时效后强度大幅度提高而处伸率大幅度下降,在变形过程中,合金中的位错在析出相周围形成缠结,合     

Relation between microstructures of martensite and prior austenite in 12 wt% Cr ferritic steel

Tempered martensitic 9–12 wt% Cr ferritic steels are used as heat resistant materials in power plant, where service under conditions of high temperature and pressure for several decades is required, and an adequate resistance to creep is one of the key requirements. In this type of steels, failure has been found to occur preferentially at prior austenite grain boundaries if the prior austenite grains are coarse. It appears that the prior austenite grain boundaries can act as a site of especial weakness in the tempered martensitic microstructure. It would therefore be useful to investigate whether the properties of prior austenite grain boundaries could be modified by some appropriate thermomechanical processing method. One approach to this is to attempt to increase the fraction of annealing twins in the austenite phase and to investigate whether this has an effect on the properties of the martensite after transformation and tempering. In this study, thermomechanical treatments involving hot-rolling have been applied and the fraction of austenite twins produced determined using electron backscatter diffraction analysis. The treatment giving the highest fraction of austenite twins was identified and the effect of the increase in twin fraction on the characteristics of the martensite was investigated. It was found that the fraction of coincidence site lattice boundaries in martensite along prior austenite grain boundaries increased with increasing fraction of prior austenite twin boundaries.