Synchrotron based reciprocal space mapping and dislocation substructure analysis

During plastic deformation of FCC materials, dislocation density does not evolve uniformly but a dislocation cell/wall structure is formed. X-ray diffraction reciprocal space mapping is used to investigate this substructure within a single grain in a large grained aluminium polycrystal. A simple dislocation cell/wall model capable of computing numerical reciprocal space maps is presented. The model qualitatively captures the experimentally observed features.     

An approach to cyclic plasticity and deformation-induced structure changes of electrodeposited nickel

Bulk nickel samples, produced by electrodeposition, resulting in different initial structure properties, were experimentally studied by X-ray diffraction, as well as by scanning and transmission electron microscopy. Attempts are made to correlate the mechanical behaviour during cyclic plastic deformation with the response of the microstructure. A special effort is made to examine the influence of grain size and internal stresses on the deformation processes.     

The effect of constrained groove pressing on grain size,dislocation density and electrical resistivity of low carbon steel

In this research, constrained groove pressing (CGP) technique is used for imposing severe plastic deformation (SPD) on the low carbon steel sheets. Using transmission electron microscopy (TEM), X-ray diffraction (XRD) and optical microscopy, the microstructural characteristics of produced sheets are investigated. The results show that CGP process can effectively refine the coarse-grained structure to an ultrafine grain range. Dislocation densities of the ultrafine grained low carbon steel sheets are quantitatively calculated and it is found that the CGP can effectively enhance the dislocation density of the sheets. Measurements of their electrical resistivity values show that microstructure refinement and increasing the dislocation density can efficiently increase the electrical resistivity of the CGPed sheets up to ∼100%.     

Microstructures and mechanical properties of Mg–Zn–Y alloy consolidated from gas-atomized powders using high-pressure torsion

In this paper, rapid solidified Mg₉₅Zn_4.3Y_0.7 (at.%) alloy powders produced by an inert gas atomizer were consolidated using a severe plastic deformation technique of high pressure torsion (HPT) at room temperature and 373 K. The behavior of powder consolidation, matrix microstructural evolution, and mechanical properties of the powders and compacts were investigated using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, microhardness, and tensile testing. As the HPT processing temperature increases, the powders are more plastically deformed due to decreased deformation resistance, grain boundaries are more in equilibrium, powder bonding is enhanced due to increased interparticle diffusion, hence, tensile ductility and strength increases. On the other hand, hardness decreases with the increased processing temperature, due to less dislocation density.     

In situ single-grain peak profile measurements on Ti–7Al during tensile deformation

High-energy three-dimensional X-ray diffraction with medium and high reciprocal space resolution was applied to study in situ tensile deformation of Ti–7Al specimens. Samples with planar and random dislocation microstructures were prepared and characterized by electron microscopy. Stress tensors of individual grains were obtained at several loads up to 2% deformation. The stress tensors were found to rotate, and resolved shear stresses were calculated. High-resolution reciprocal space maps of selected grains were recorded. Azimuthal and radial distributions were visualized and discussed in terms of idealized dislocation structures. Heterogeneous grain rotations were observed for the planar microstructure and found to be consistent with activation of the highest stressed basal slip system. Intra-granular strain gradients were detected in excess of the intrinsic radial dislocation peak broadening. The potential of combining the applied techniques with modeling to obtain multiple length-scale information during deformation of bulk specimens is discussed.     

Titanium enrichment and strontium depletion near edge dislocation in strontium titanate [001]/(110) low-angle tilt grain boundary

Dislocations are linear lattice defects in a crystalline solid. Since the unusual atomistic environment of the dislocation may greatly influence various material properties, control of the composition would offer more opportunities to obtain unique one-dimensional structures. In the present study, we have characterized the structure of dislocations in a low-angle tilt grain boundary of strontium titanate (SrTiO₃). High-spatial resolution elemental mapping by electron energy loss spectroscopy combined with scanning transmission electron microscopy has enabled visualization of the enrichment of titanium (Ti) and the depletion of strontium (Sr) near the dislocation cores. The Ti enrichment and the Sr depletion have been observed at all of the dislocations, and the grain boundary is considered to be Ti excess. The extra Ti ions are located on the positions different from the normal perovskite lattice, suggesting that the local structure is largely reconstructed. It has been proposed that tensile strain at the dislocations may be a cause of the Ti enrichment.     

Influence of plastic deformation on occurrence of discontinuous precipitation

We have studied the effect of plastic deformation by compression on the occurrence of discontinuous precipitation in Al-30% Zn alloy after ageing at two different temperatures (348 and 423 K) has been studied. Optical microscope, scanning electron microscope, X-ray diffraction and differential scanning calorimetry were used for characterization.During ageing of undeformed alloy, the grain boundary of supersaturated solid solution represents the favorite site of precipitation by discontinuous mechanism. We found that the occurrence of discontinuous precipitation depends mainly on the degree of plastic deformation before ageing. The grain boundary act as reaction front and migrates in case of low degree of prior deformation.     

Large elastic strains and ductile necking of W nanowires embedded in TiNi matrix

The deformation behaviors of W nanowires embedded in a TiNi matrix were investigated by means of in-situ synchrotron high energy X-ray diffraction(HEXRD) and in-situ transmission electron microscopy(TEM) analysis during tensile deformation.The HEXRD measurement indicated that the W nanowires exhibited an average lattice strain of about 1.50 %,whereas the TEM examination revealed a local elastic strain of about 4.59 % in areas adjacent to the TiNi matrix where stress-induced martensitic transfo rmation occurred.This strain corresponds to a stress of ~15 GPa for the W nanowires.In addition,in areas adjacent to the TiNi matrix where plastic deformation and cracking were generated,the W nanowire showed significant ductile necking with ~80 % reduction in cross-section area.The ductile necking of W nanowire is attributed to the lack of protection from the stress-induced martensitic transformation of the TiNi matrix.     

Recent developments in modelling of microhardness saturation during SPD processing of metals and alloys

Ultrafine-grained and even nanostructured materials can be fabricated using severe plastic deformation to ultra-high strains in equal-channel angular pressing (ECAP), high-pressure torsion (HPT), machining and their combinations, such as machining of ECAP specimens, HPT of ECAP billets and HPT of machining chips. This report presents recent results of investigations of the microstructures and microtextures of pure copper, nickel and aluminium subjected to different deformation processes to ultimately high imposed strains. A comparison of the microstructure, dislocation density and microhardness developed during combinations of different strain paths is performed. All characteristics were analysed by X-ray, transmission and scanning electron microscopy, and electron backscatter diffraction (EBSD). The influence of different processing routes is discussed in terms of the accumulated strain and microstructure refinement. The saturation in grain refinement is examined with reference to the recovery taking place during ultra-high strain deformation. A phenomenological model based on the Voce equation is applied for fitting parameters based on the experimental data and this is suggested for a prediction of microhardness evolution for pure metals (Ag, Au) and Cu-based (Zn, Al) alloys.     

Nanocrystallization of zirconium subjected to surface mechanical attrition treatment

A nanostructured surface layer with thickness of about 20?μm was formed on commercially pure zirconium using surface mechanical attrition treatment (SMAT). The microstructural features of the surface layer were systematically investigated using optical microscopy (OM), x-ray diffraction (XRD), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM), respectively. Based on the results obtained, a grain refinement mechanism induced by plastic deformation during SMAT of Zr is proposed. At?the initial stage of SMAT, twinning dominates the plastic deformation of Zr and divides the coarse grains of Zr into finer twin plates. With increasing strain, intersection of twins occurs, and dislocation slips are activated, becoming the predominant deformation mode instead of twinning. As a result of the dislocation slips, high-density dislocation arrays are formed, which further subdivide the twin plates into subgrains of size about 200-400?nm. With a further increase of strain, the dislocations accumulate and rearrange to minimize the energy state of the high-strain-energy subgrains, the dense dislocation walls convert to grain boundaries, and the submicronic grains are subdivided, leading to the formation of nanosized grains at the top of the treated surface.     

New opportunities for 3D materials science of polycrystalline materials at the micrometre lengthscale by combined use of X-ray diffraction and X-ray imaging

Non-destructive, three-dimensional (3D) characterization of the grain structure in mono-phase polycrystalline materials is an open challenge in material science. Recent advances in synchrotron based X-ray imaging and diffraction techniques offer interesting possibilities for mapping 3D grain shapes and crystallographic orientations for certain categories of polycrystalline materials. Direct visualisation of the three-dimensional grain boundary network or of two-phase (duplex) grain structures by means of absorption and/or phase contrast techniques may be possible, but is restricted to specific material systems. A recent extension of this methodology, termed X-ray diffraction contrast tomography (DCT), combines the principles of X-ray diffraction imaging, three-dimensional X-ray diffraction microscopy (3DXRD) and image reconstruction from projections. DCT provides simultaneous access to 3D grain shape, crystallographic orientation and local attenuation coefficient distribution. The technique applies to the larger range of plastically undeformed, polycrystalline mono-phase materials, provided some conditions on grain size and texture are fulfilled. The straightforward combination with high-resolution microtomography opens interesting new possibilities for the observation of microstructure related damage and deformation mechanisms in these materials.     

Comparative determination of the α/β phase fraction in α+β-titanium alloys using X-ray diffraction and electron microscopy

A comparison is made between the measured α/β phase fractions in Ti-6246 using X-ray diffraction (XRD) and electron microscopy. Image analysis of SEM and TEM images was compared to the phase fraction estimate obtained using electron backscattered diffraction, lab and high-energy synchrotron XRD. There was a good agreement between the electron microscopic and diffraction techniques, provided that the microstructural parameters of grain size and texture are estimated correctly when using quantitative Rietveld refinement.     

中锰Q&P钢残余奥氏体的稳定性及其TRIP效应

通过单向拉伸及平面应变实验研究了Mn含量为7%的中锰淬火-配分(QP)钢残余奥氏体的机械稳定性,利用X射线衍射仪(XRD)测定试验钢残余奥氏体的含量,通过观察试验钢的拉伸曲线及扫描电镜(SEM)、透射电镜(TEM)照片,分析变形前后的微观组织,研究中锰QP钢的变形机制。结果表明:应力状态对残余奥氏体稳定性有较大的影响,平面应变更有利于相变诱导塑性(TRIP)效应的发挥;中锰QP钢的拉伸变形特征是由超细晶硬化机制和TRIP效应相互作用产生的,通过微观组织观察发现中锰QP钢的塑性变形主要是残余奥氏体的TRIP效应,其中薄膜状的残余奥氏体的稳定性最高。     

The nanocrystalline structure formation in germanium subjected to severe plastic deformation

The peculiarities of nanocrystalline (NC) structure formation in germanium subjected to severe plastic deformation (SPD) are examined in this paper. Transmission electron microscopy (TEM), X-ray analysis and differential scanning calorimetry were employed in the structural study of germanium specimens. The crystal-to-amorphous transition induced by SPD in germanium is observed. The NC structure formation is the result of annealing at 850°C. Crystallites in the NC state have non-equilibrium grain boundaries (GB) and a particular “spread” diffraction contrast, observed by TEM, testifies to this.     

Experimental Revelation of Surface and Bulk Lattices in Faceted Cu2O Crystals

Semiconductor crystals have generally shown facet-dependent electrical, photocatalytic, and optical properties. These phenomena have been proposed to result from the presence of a surface layer with bond-level deviations. To provide experimental evidence of this structural feature, synchrotron X-ray sources are used to obtain X-ray diffraction (XRD) patterns of polyhedral cuprous oxide crystals. Cu₂O rhombic dodecahedra display two distinct cell constants from peak splitting. Peak disappearance during slow Cu₂O reduction to Cu with ammonia borane differentiates bulk and surface layer lattices. Cubes and octahedra also show two peak components, while diffraction peaks of cuboctahedra are comprised of three components. Temperature-varying lattice changes in the bulk and surface regions also show shape dependence. From transmission electron microscopy (TEM) images, slight plane spacing deviations in surface and inner crystal regions are measured. Image processing provides visualization of the surface layer with depths of about 1.5–4 nm giving dashed lattice points instead of dots from atomic position deviations. Close TEM examination reveals considerable variation in lattice spot size and shape for different particle morphologies, explaining why facet-dependent properties are emerged. Raman spectrum reflects the large bulk and surface lattice difference in rhombic dodecahedra. Surface lattice difference can change the particle bandgap.     

Ti-WC nanocrystalline coating formed by surface mechanical attrition treatment process on 316L stainless steel

Nanocrystalline coatings were performed on the surface of 316L stainless steel plates mechanically with a mixture of Ti and WC powders under vacuum conditions. The targets were replaced in the end of the high energy milling rig, while Ti-WC mixture was milled as usual. It is shown that the coatings are nanocrystalline in nature with narrow distribution of average size of nanocrystallites. X-ray diffraction and scanning electron microscopy (with energy-dispersive spectrometer) revealed that the top layer of the coatings is uniform. Microhardness, roughness and primary corrosion tests (tafel tests) proved enhancement of coated samples with respect to raw materials. Transmission electron microscope image of deformed surface confirmed surrounding of nanoparticles by dislocation loops after plastic deformation.     

Effect of Cerium Doping on Microstructure and Dielectric Properties of BaTiO3 Ceramics

A solid state reaction method was used to synthesize barium titanate (BT) and barium cerium titanate (BCT) ceramics at sintering temperature of 1473 K for 4 h. The effect of cerium (Ce) on the structure, microstructure and dielectric properties of BCT was investigated. The scanning electron microscopy (SEM) investigations revealed that the grain size increases with increasing Ce content. The X-ray diffraction (XRD) patterns showed mostly the BT phase, where the lattice parameter decreased with the addition of Ce. The temperature dependence of dielectric constant showed decrease in the phase transition temperature with higher Ce content. The dielectric constant decreased slightly with increasing frequency. The direct current (dc) density-voltage characteristics of the ceramics showed ohmic behavior for both the BT and BCT. As the temperature increased, the dc resistivity of the ceramics decreased. The activation energy increased with increasing Ce content.     

Measurement of the evolution of internal strain and load partitioning in magnesium hybrid composites under compression load using in-situ synchrotron X-ray diffraction analysis

Energy dispersive synchrotron X-ray diffraction analysis has been applied to evaluate the evolution of average internal elastic lattice strains under compression load within the phases of magnesium hybrid composites, reinforced by silicon-carbide particles and Saffil® alumina short fibres. This allows for the calculation of phase stresses and thus the load partitioning. The mean elastic misfit stresses were calculated using an Eshelby type modelling. Considering the external load, prediction of the phase-specific stresses for elastic composite deformation was performed and the results were compared to the experimental data obtained.Matrix elastic lattice strains reveal high plastic anisotropy due to the activation of different deformation modes in the form of crystallographic slip and mechanical twinning. The formation of twins, verified by diffraction intensity shifts due to crystallographic reorientation, was found to affect the sharing of load between the participating phases. Consequently different regimes of composite deformation were specified. This comprises elastic regions characterized by linear strain and stress growth for all phases as well as plastic regions showing nonlinear distributions.     

Microscale characterization of granular deformation near a crack tip

This paper presents a study of microscale plastic deformation at the crack tip and the effect of microstructure feature on the local deformation of aluminum specimen during fracture test. Three-point bending test of aluminum specimen was conducted inside a scanning electron microscopy (SEM) imaging system. The crack tip deformation was measured in situ utilizing SEM imaging capabilities and the digital image correlation (DIC) full-field deformation measurement technique. The microstructure feature at the crack tip was examined to understand its effect on the local deformation fields. Microscale pattern that was suitable for the DIC technique was generated on the specimen surface using sputter coating through a copper mesh before the fracture test. A series of SEM images of the specimen surface were acquired using in situ backscattered electronic imaging (BEI) mode during the test. The DIC technique was then applied to these SEM images to calculate the full-field deformation around the crack tip. The grain orientation map at the same location was obtained from electron backscattered diffraction (EBSD), which was superimposed on a DIC strain map to study the relationship between the microstructure feature and the evolution of plastic deformation at the crack tip. This approach enables to track the initiation and evolution of plastic deformation in grains adjacent to the crack tip. Furthermore, bifurcation of the crack due to intragranular and intergranular crack growth was observed. There was also localization of strain along a grain boundary ahead of and parallel to the crack after the maximum load was reached, which was a characteristic of Dugdale–Barenblatt strip-yield zone. Thus, it appears that there is a mixture of effects in the fracture process zone at the crack tip where the weaker aspects of the grain boundary controls the growth of the crack and the more ductile aspects of the grains themselves dissipate the energy and the corresponding strain level available for these processes through plastic work.     

Characterization of r.f.-sputtered ZnO thin films by X-ray diffraction and scanning electron microscopy

Thin films of ZnO, ranging in thickness from 0.08 to 6 μm, have been prepared by r.f. sputtering on substrates of either quartz or glass under various deposition conditions and subsequently characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Results from XRD indicate that the grain size increases from 0.01 to 0.5 μm as the film thickness is increased from 0.08 to 6 μm for deposition at 65 °C and increases from 0.1 to 0.3 μm as the deposition temperature is increased from 65 to 480 °C for a constant film thickness of 2 μm, whereas the lattice strain and dislocation density decrease slightly under similar conditions. Results from SEM indicate that the particle size parallel to the plane of the film is approximately equal to the mean grain size perpendicular to the plane of the film, suggesting that growth proceeds by the nucleation of new grains rather than by the elongation of columnar grains in the growth direction. General observations indicate that microstructural parameters, such as grain size, grain shape and lattice strain, depend sensitively on the exact nature of the deposition conditions.