Pitfalls in the experimental recording of ultrasonic (backscatter) polar scans for material characterization

The ultrasonic polar scan (UPS), either in transmission, reflection or backscatter mode, is a promising non-destructive testing technique for the characterization of composites, providing information about the mechanical anisotropy, the viscoelastic damping, the surface roughness, and more. At present, the technique is merely being used for qualitative purposes. The limited quantitative exploration and use of the technique can be primarily ascribed to limitations of current theoretical models as well as the difficulty to perform accurate, and more importantly, reproducible UPS experiments. Over the last years, we have identified several potential pitfalls in the experimental implementation of the technique which severely deteriorate the accurateness and reproducibility of a UPS. In this paper, we make an inventory of the most important difficulties, illustrate each of them by a real experiment and present a feasible mediation, either numerical or experimental in nature. Once the experimental set-up is fine-tuned to overcome these pitfalls, it is expected that the recording of high-level UPS experiments, in combination with numerical computations, will facilitate the technique to become a fully quantitative non-destructive characterization method.     

Effect of cooling rate on microstructure of friction-stir welded AA1100 aluminum alloy

In this work, the microstructural changes occurring during cooling of friction-stir welded aluminum alloy AA1100 were evaluated. To this end, friction-stir welding (FSW) was performed in a wide range of cooling rates of 20–62 K/s and the evolved microstructures were studied by using electron backscatter diffraction. Below 0.6 T_m (T_m being the melting point), the stir zone material was found to experience no significant changes during cooling. At higher FSW temperatures, however, notable changes occurred in the welded material, including grain growth, sharpening of texture, reduction of the fraction of high-angle boundaries and material softening.     

Modification of Spalart-Allmaras model with consideration of turbulence energy backscatter using velocity helicity

The correlation between the velocity helicity and the energy backscatter is proved in a DNS case of 256³-grid homogeneous isotropic decaying turbulence. The helicity is then proposed to be employed to improve turbulence models and SGS models. Then Spalart-Allmaras turbulence model (SA) is modified with the helicity to take account of the energy backscatter, which is significant in the region of corner separation in compressors. By comparing the numerical results with experiments, it can be concluded that the modification for SA model with helicity can appropriately represent the energy backscatter, and greatly improves the predictive accuracy for simulating the corner separation flow in compressors.     

Influence of cell packing by centrifugation on 40-MHz ultrasound backscatter

High-frequency ultrasound (HFUS) signals backscattered from RBL-2H3 cell pellets prepared under different centrifugal forces were analyzed to investigate the packing effect of cell aggregates. The measurements were performed in a pulse-echo setup with a 40-MHz transducer. The changes of ultrasound signals from cell pellet in backscattered power, statistical parameter, and pellet thickness were monitored after centrifugation at between 100g and 1600g. Experimental results showed that the HFUS backscattered power from cell pellets was inversely proportional to centrifugal force and increased to a plateau within 1-2 h after centrifugation. The initial thickness of cell pellets decreased with higher centrifugal force, but the changes in thickness and time that took to reach a plateau increased at higher centrifugal force. The envelope statistics of backscattered signals with Nakagami distribution indicates that the centrifugal force and elapsed time after centrifugation affected the backscattering characteristics. The present study suggests that centrifugal force and data acquisition time after cell pellet formation should be considered in in vitro cell packing method with centrifugation to emulate the tissue in vivo.     

Si衬底上生长的铁硅化合物结构和取向分析

采用分子束外延法分别在650-920 ℃的Si（110）和920 ℃的Si（111）衬底表面生长出铁的硅化物纳米结构,并主要分析了920 ℃高温下纳米结构的形貌、组成相及其与Si 衬底的取向关系. 扫描隧道显微镜（STM）研究表明,920 ℃高温下,Si（110）衬底上生长的铁硅化合物完全以纳米线的形式存在,且其尺寸远大于650 ℃低温下外延生长的纳米线尺寸;Si（111）衬底上生长出三维岛和薄膜两种形貌的铁硅化合物,其中三维岛具有金属特性且直径约300 nm、高约155 nm,薄膜厚度约2 nm. 电子背散射衍射研究表明920 ℃高温下Si（110）衬底上生长的纳米线仅以β-FeSi₂的形式存在,且β-FeSi₂相与衬底之间存在唯一的取向关系：β-FeSi₂（101）//Si（111）;β-FeSi₂ [010]//Si[110];Si（111）衬底上生长的三维岛由六方晶系的Fe₂Si 相组成,Fe₂Si 属于164 空间群,晶胞常数为a=0.405 nm,c=0.509 nm;与衬底之间的取向关系为Fe₂Si（001）∥Si（111）和Fe₂Si[120]//Si[112].     

In situ EBSD during tensile test of aluminum AA3003 sheet

Miniature tensile-test specimens of soft-annealed, weakly textured AA3003 aluminum sheet in 0.9 mm thickness were deformed until fracture inside a scanning electron microscope. Tensile strength measured by the miniature tensile test stage agreed well with the tensile strength by regular tensile testing. Strain over the microscope field of view was determined from changes in positions of constituent particles. Slip lines were visible in secondary electron images already at 0.3% strain; activity from secondary slip systems became apparent at 2% strain. Orientation rotation behavior of the tensile load axis with respect to the crystallographic axes agreed well with previously reported trends for other aluminum alloys. Start of the fracture and tensile crack propagation were documented in secondary electron images. The region of fracture nucleation included and was surrounded by many grains that possessed high Schmid factors at zero strain. Crystal lattice rotation angles in the grains surrounding the initial fracture zone were higher than average while rotations inside the initial fracture zone were lower than average for strains from zero to 31%. The orientation rotation behavior of the tensile load axes of the grains around the fracture zone deviated from the average behavior in this material.     

Effect of trabecular bone material properties on ultrasonic backscattering signals

In this study, ultrasonic backscattering signals in cancellous bones were obtained by finite difference time domain （FDTD） simulations, and the effect of trabecular material properties on these signals was analyzed. The backscatter coefficient （BSC） and integrated backscatter coefficient （IBC） were numerically investigated for varying trabecular bone material properties, including density, Lame coefficients, viscosities, and resistance coefficients. The results show that the BSC is a complex function of trabecular bone density, and the IBC increases as density increases. The BSC and IBC increase with the first and second Lame coefficients. While not very sensitive to the second viscosity of the trabeculae, the BSC and IBC decrease as the first viscosity and resistance coefficients increase. The results demonstrate that, in addition to bone mineral density （BMD） and microarchitecture, trabecular material properties significantly influence ultrasonic backseattering signals in cancellous bones. This research furthers the understanding of ultrasonic backscattering in cancellous bones and the characterization of cancellous bone status.     

EBSD-XPS法分析磷石膏中杂质物相

磷石膏是湿法磷酸过程形成的固体副产物。磷石膏中含有磷、氟、硅等有害杂质组分,极大影响磷石膏制品的质量和性能,巨量磷石膏堆存严重威胁生态环境和生命安全。确定磷石膏中杂质物相的赋存状态,为磷石膏除杂净化和综合利用提供理论指导,非常重要。以低温干燥后的磷石膏为研究对象,利用X射线荧光光谱（XRF）分析确定磷石膏中杂质元素的组成,结果表明,磷石膏中的杂质元素含量较高的有P,Si,F和Al,含量较低的有Ba,Fe和Mg等。因二水硫酸钙物相强峰对杂质物相峰有较强遮蔽作用,X射线衍射光谱（XRD）分析不能确定磷石膏杂质的物相。利用扫描电子显微镜对磷石膏进行电子背散射衍射（EBSD）分析,根据被检样品衬度的区别探明磷石膏的杂质物相,利用X射线能谱分析（EDS）成分确定杂质物相组成;利用X射线光电子能谱（XPS）对硫酸钙晶体表面以及混合杂质物相作进一步分析。EBSD分析结果表明,磷石膏中杂质物相主要包括二氧化硅、氟硅酸钠、氟硅酸钾、氟磷酸钙、氟化钙、硫酸钡、硫化铁、三氧化二铝等,此外还有硅、铝、磷、氟等杂质混合组成的复盐物相,其中二氧化硅、硫酸钡、硫化铁、氟磷酸钙和三氧化二铝为独立赋存物相,氟硅酸钠和氟硅酸钾的物相则混合分布在硫酸钙晶体之间,氟化钙杂质与硅、铝、磷、氟杂质复盐物相结合赋存。XPS分析结果表明,磷石膏中还存在硅酸钙、氟化铝、氟化镁、硫酸铝、磷酸铝、磷酸钙、磷酸氢钙和磷酸二氢钙等物相,其中磷酸钙、磷酸氢钙、磷酸二氢钙和氟磷酸钙四种物相的特征峰位分布极为接近。采用EBSD-XPS组合分析方法,不仅确定了磷石膏中杂质的物相,还阐明了杂质物相与硫酸钙晶体之间的构效关系。该研究为磷石膏杂质物相分析提供新途径,为磷石膏除杂净化及其综合利用提供坚实的理论依据。     

Relative angular precision in electron backscatter diffraction: A comparison between cross correlation and Hough transform based analysis

In the present study the relative angular resolution of an electron backscatter diffraction system based on Hough transform analysis has been determined with a silicon single crystal wafer. The resolution is found to be better than 0.1° and can be easily improved by repetition of measurements. A test measurement on a BaFe₂As₂ thin film, where disorientations of 0.1° and less are present, was performed using the cross correlation electron backscatter diffraction technique. The same measurement is evaluated with the Hough transform technique. Comparing both techniques give evidence of a relative resolution of better than 0.1°. However, in specimen areas with strain inhomogeneities a deviation along one rotation axis can be observed.     

Improved growth of solution‐deposited thin films on polycrystalline Cu(In,Ga)Se2

CdS and Zn(O,S) grown by chemical bath deposition (CBD) are well established buffer materials for Cu(In,Ga)Se₂ (CIGS) solar cells. As recently reported, a non‐contiguous coverage of CBD buffers on CIGS grains with {112} surfaces can be detected, which was explained in terms of low surface energies of the {112} facets, leading to deteriorated wetting of the chemical solution on the CIGS surface. In the present contribution, we report on the effect of air annealing of CIGS thin films prior to the CBD of CdS and Zn(O,S) layers. In contrast to the growth on the as‐grown CIGS layers, these buffer lay‐ ers grow densely on the annealed CIGS layer, even on grains with {112} surfaces. We explain the different growth behavior by increased surface energies of CIGS grains due to the annealing step, i.e., due to oxidation of the CIGS surface. Reference solar cells were processed and completed by i‐ZnO/ZnO:Al layers for CdS and by (Zn,Mg)O/ZnO:Al for Zn(O,S) buffers. For solar cells with both, CdS and Zn(O,S) buffers, air‐annealed CIGS films with improved buffer coverage resulted in higher power‐conversion efficiencies, as compared with the devices containing as‐grown CIGS layers. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)