Convolutional Multiple Whole Profile Research Articles

Anisotropic and Heterogeneous Development of Microstructures. Combining Laboratory/Synchrotron X-rays and EBSD on a few SPD Metallic Systems

The onset of Severe Plastic Deformation (SPD) regime is quite instructive on the possible origins of the nano-microstructures developed in metals and alloys. It is known that grain fragmentation and dislocation accumulation, among other defects, proceed at different paces depending fundamentally on grain orientations and active deformation mechanisms. There have been many attempts to characterize nano-microstructure anisotropy, leading all of them to sometimes contradictory conclusions. Moreover, the characterizations rely on different measurements techniques and pos-processing approaches, which can be observing different manifestations of the same phenomena.On the current presentation we show a few experimental and computer pos-processing and simulation approaches, applied to some SPD/alloy systems. Williamson-Hall and Convolutional Multiple Whole Profile (CMWP) techniques will be applied to peak broadening analysis on experimental results stemming from laboratory Cu Ka X-rays, and synchrotron radiation from LNLS (Laboratório Nacional de Luz Síncrotron, Campinas, Brazil) and Petra III line (HEMS station, at DESY, Hamburg, Germany).Taking advantage of the EBSD capability of giving information on orientational and topological characteristics of grain boundaries, microstructures, grain sizes, etc., we also performed investigations on dislocation density and Geometrically Necessary Dislocation Boundaries (GNDB) and their correlation with texture components.Orientation dependent nano-microstructures and domain sizes are shown on the scheme of generalized pole figures and discussions provide some hints on nano-microstructure anisotropy.

X-선 회절 패턴 측정과 투과 전자 현미경을 이용한 구리 나노분말의 수소 환원 처리 시 발생하는 미세조직 변화 및 치밀화 시편의 물성 분석

In this study, nano-scale copper powders were reduction treated in a hydrogen atmosphere at the relatively high temperature of <TEX>$350^{\circ}C$</TEX> in order to eliminate surface oxide layers, which are the main obstacles for fabricating a nano/ultrafine grained bulk parts from the nano-scale powders. The changes in composition and microstructure before and after the hydrogen reduction treatment were evaluated by analyzing X-ray diffraction (XRD) line profile patterns using the convolutional multiple whole profile (CMWP) procedure. In order to confirm the result from the XRD line profile analysis, transmitted electron microscope observations were performed on the specimen of the hydrogen reduction treated powders fabricated using a focused ion beam process. A quasi-statically compacted specimen from the nano-scale powders was produced and Vickers micro-hardness was measured to verify the potential of the powders as the basis for a bulk nano/ultrafine grained material. Although the bonding between particles and the growth in size of the particles occurred, crystallites retained their nano-scale size evaluated using the XRD results. The hardness results demonstrate the usefulness of the powders for a nano/ultrafine grained material, once a good consolidation of powders is achieved.

Mechanical and microstructural characterization of dispersion strengthened Al–C system nanocomposites

Al and different amounts of C and C–Cu mixtures were used to produce Al–C and Al–C–Cu powder samples by mechanical milling. Microhardness tests were carried out to evaluate the mechanical properties of the nanocomposites in the as-milled condition. In general, the measured values were considerably higher than pure Al. In order to determine the causes of this hardening, the crystallite size and dislocation density were measured by means of X-ray analyses coupled with a convolutional multiple whole profile (CMWP) fitting program and a comparison with atomic force microscopy (AFM) observations. In Al–C samples, the hardening is mainly due to the decrease of the crystallite size, however for the Al–C–Cu, an additional strengthening mechanism appears and it seems that it is due by a dispersion of graphite nanoparticles in the Al matrix. The strengthening contributions of dislocation density, crystallite size and particle dispersion were modeled by superposing of every single contribution to strengthening (via hardness analyses). We found a direct relationship between the mechanical properties and the nominal amount of C–Cu, where Cu apparently acts as C nanoparticles integration and dispersion agent.

Twinning on pyramidal planes in hexagonal close packed crystals determined along with other defects by X-ray line profile analysis

A systematic procedure is developed to evaluate the frequency of {10.1}〈10.\overline 2〉 and {11.2}〈11.\overline 3〉 compressive twins and {10.2}〈10.\overline 1〉 and {11.1}〈\overline 1\overline 1.6〉 tensile twins together with dislocation densities, active slip systems and crystallite size in hexagonal close packed (hcp) metals. The effect of pyramidal twinning on X-ray line broadening in hcp metals is fundamentally different from the effect of twinning on close packed planes in face centred cubic (fcc) crystals. Therefore, the usual theoretical descriptions developed previously for fcc crystals cannot be used for pyramidal twinning in hcp crystals. The profile functions of sub-reflections for this type of twinning are derived to be the sum of a symmetrical and an antisymmetrical Lorentzian function. Sub-profile properties are parameterized and the parameter files are incorporated into the convolutional multiple whole profile (CMWP) procedure. The extended procedure,eCMWP, is applied to determine pyramidal twin frequencies together with dislocation densities, active slip systems and crystallite size in Mg deformed at different temperatures, in commercial purity Ti deformed at high temperature and in high-purity Ti deformed at room temperature.

Microstructure of crystalline materials determined by whole powder pattern X-ray line profile analysis

Powder diffraction patterns consisting of broadened line profiles are used to evaluate microstructure parameters. The microstructure is modelled in terms of crystallite size and microstrain, where microstrain is assumed to be caused by dislocations, triple junctions corresponding to sinter stresses, and to some extent, by planar defects, especially stacking faults and twin boundaries. Crystallite size needs special interpretation, in the case of ceramic powders it is often identical to the particle or grain size, whereas in the case of bulk materials, especially bulk metals or alloys it is equal to the subgrain size. Electron microscopy strongly facilitates the appropriate interpretation of X-ray crystallite size. The diffraction patterns are evaluated by the method of convolutional multiple whole profile (CMWP) fitting procedure [1,2]. In this procedure strain and size is modelled by dislocations and spherical or ellipsoidal crystallites with log-normal size distributions, respectively. The model generated and the measured diffraction patterns are compared by a nonlinear least squares method, in a similar way as in the whole powder pattern modelling (WPPM) method [3]. The effect of stacking faults and twins on X-ray diffraction patterns has been calculated numerically by using the DIFFaX [4] software for the first 15 Bragg reflections in fcc crystals up to 20 % fault densities [5]. It is found that the Bragg reflections consist of 5 types of sub-reflections which are of Lorentzian type. About 15.000 sub-reflections are evaluated and parametrized according to their FWHM and positions relative to the exact Bragg angles. These parameter files are incorporated into the CMWP software for evaluating planar faults together with dislocations and crystallite and/or sub-grain size distributions [5]. In nanocrystalline Cu and Cu-Zn alloys it has been found that twinning becomes a substantial mode of deformation when the grain size is smaller than about 40 and 60 nm, respectively. An unusual behaviour of line broadening in ball milled fluorides has been observed [6]. About twenty diffraction patterns corresponding to different ball milled states of the CaF 2, SrF2, BaF2 and CdF2 fluorides have been analysed in terms of dislocation densities and types, and crystallite size and size distributions by X-ray line profile analysis [6]. The diffraction patterns have been evaluated by the method of convolutional multiple whole profile (CMWP) fitting procedure [2,3]. In about 10 of the 20 patterns the calculated and measured diffraction patterns are in perfect agreement throughout the entire angular range of measurement. However, in the cases of the rest of the patterns the first few measured line profiles appear to be narrower than the calculated profiles based on the model. Nevertheless, also in these latter cases the measured and calculated patterns are in perfect agreement for the higher angular parts of the patterns consisting of 10 to 13 profiles. In the cases where this discrepancy is present, it is interpreted as an X-ray optical interference effect [6] similar to what has been observed by Rafaja et al. [7] in nanocrystalline thin layers. The interference effect has been corrected successfully, either by (i) excluding the affected profiles from the evaluation procedure or by (ii) assuming a diffraction angle dependent apparent bimodal size distribution of crystallites [6].