Materials Science Forum Vols. 654-656

Abstract: X-ray phase-contrast imaging and tomography add an additional dimension to conventional x-ray methods by exploiting the x-ray refraction effects in addition to x-ray absorption in forming an image. This greatly enhances the visibility of edges, voids and boundaries within a sample. It also makes it possible to characterise weakly x-ray absorbing samples which would produce little or no contrast in conventional x-ray imaging. Here we described the application of a laboratory-based x-ray phase-contrast microscope to x-ray micro-tomography of a self-healing polymer system and the quantitative analysis of the resulting three dimensional (3D) datasets to better understand the healing and ageing processes.

Abstract: . A low carbon steel is austenitized and isothermally held at 680°C to form allotriomorphic ferrite and followed by a holding at lower temperature to form bainite. The morphology of allotriomorphic ferrite/bainite interfaces is studied using optical microscope. Three kinds of combination are observed: Type I: interface on one side is clear while on the other side, unclear; Type II: unclear on both sides; Type III: clear on both sides. Clear interface indicates a large difference in the orientation between the bainite and the ferrite, and unclear interface, a very small difference. The statistical counting shows that the ratio of Type I is about 80-82%, and that of Type II, 7-8%, and Type III, 9-11%. It is observed that this ratio does not change with the austenite grain size and bainite forming temperature. And the clear and unclear side of allotriomorphic ferrite may have different influence on the nucleation rate of bainite at allotriomorhic ferrite/prior austenite interface.

Abstract: The precipitation behavior in the AL6XN austenitic stainless steel after creep deformation at temperatures 500~750 °C up to 3600 hours has been studied by electron microscope. The results showed that precipitates were hardly observed for the steel crept at 500~550 °C, and that the precipitates of carbides were mainly found at grain boundaries in samples crept at 600 °C. When the creep temperature was increased to 650~750 °C, a high density precipitates was found both at grain boundaries and within grains. The electron diffraction pattern and energy-dispersive X-ray spectroscopy analyses confirmed that these precipitates are  and Laves phases.

Abstract: 3-D FIB-EBSD tomography was used to analyze the structure and crystallography of nuclei, and nucleation process in 3-D space during annealing a cold rolled IF steel. It is revealed that the structure of nucleus at its very initial formation stage can be divided into two parts: (1) nucleation core, having a dislocation-contained subgrain structure, mainly bounded by low angle grain boundary with surrounding deformation subgrain, (2) newly-grown region, having a dislocation free structure, formed from high store energy deformation structures surrounding nucleus, and mainly bounded by high angle grain boundary.

Abstract: Rapid cooling of TiAl-based alloy from α phase (disordered hexagonal, A3) generates  phase (ordered tetragonal, L1o) grains through massive transformation nucleating mostly over the α/α grain boundaries. This current work deals with the identification and the validation of different nucleation mechanisms during  massive transformation in TiAl-based alloys. Special attention has been given to the variant selection criteria for the nucleation of the massive structures along different types of α/α grain boundaries. The  massive domains formed along the grain boundaries were analysed using high resolution electron backscattered diffraction (EBSD). Statistical studies were made on different nucleation sites and different mechanisms are proposed. Two–dimensional studies of the nucleation mechanism suggest that the minimization of the interfacial energy could be the predominant criteria during the grain boundary nucleation. In order to verify this nucleation criterion in three-dimensions, serial sections were made and EBSD maps were taken and analysed in each section. The variant selection observed during the nucleation and the growth of the  massive grains is further discussed after getting a broader view under three-dimensional investigations.

Abstract: The precipitation behavior of nano–scale magnetic particles formed in a Cu–Fe–Ni alloy on isothermal annealing at 873K and 1073K have been investigated by means of transmission electron microscopy (TEM). Nano–scale magnetic particles were formed randomly in the Cu–rich matrix after receiving a short annealing due to phase decomposition in the alloy. With increasing the isothermal annealing time, however, the striking features that two or more nano–scale particles with a cubic shape and precipitates with a needle shape were aligned linearly along <100> directions were observed on isothermal annealing at 873K and 1073K, respectively. To investigate the relationship between microstructures and magnetic properties of precipitates in a Cu–Fe–Ni alloy were also carried out the superconducting quantum interference device (SQUID) magnetometer. The present study revealed that several significant influences to magnetic properties were induced during the precipitation process in this alloy.

Abstract: A study of the coarsening process of the decomposed phases was carried out in the Cu-34wt.%Ni-4wt.%Cr and Cu-45wt.%Ni-10wt.%Cr alloys using transmission electron microscopy. As aging progressed, the morphology of the coherent decomposed Ni-rich phase changed from cuboids to platelets aligned in the <100> Cu-rich matrix directions. Prolonged aging caused the loss of coherency between the decomposed phases and the morphology of the Ni-rich phase changed to ellipsoidal. The variation of mean radius of the coherent decomposed phases with aging time followed the modified LSW theory for thermally activated growth in ternary alloy systems. The coarsening rate was faster in the symmetrical Cu-45wt.%Ni-10wt.%Cr alloy due to its higher volume fraction of precipitates. The activation energy for thermally activated growth was determined to be about 182 and 102 kJ mol-1 in the Cu-34wt.%Ni-4wt.%Cr and Cu-45wt.%Ni-10wt.%Cr alloys, respectively. The size distributions of precipitates in the Cu-Ni-Cr alloys were broader and more symmetric than that predicted by the modified LSW theory for ternary alloys.

Abstract: Literature data on grain boundary properties show surprisingly in some cases reversed courses of the structural dependences than expected, or directly prove their qualitative changes with changing temperature. This “reversed anisotropy” is demonstrated for example of grain boundary segregation. It is shown that this reversed anisotropy of grain boundary properties can be explained by the enthalpy–entropy compensation effect. Serious consequences of altered structural dependence of grain boundary properties for Grain Boundary Engineering are discussed.

Abstract: The texture distribution and development of grain oriented silicon steel with different production stages have been investigated by electron backscatter diffraction technique (EBSD). The results indicated that below the surface the intensity of {001}<110> of α-fiber was strongest with a amount of {110}<001> texture and γ-fiber. After first cold rolling and normalizing, in the corresponding position the {110}<001> texture and γ-fiber appeared again. The {110}<001> grains and γ-fiber after decarb-annealing was helpful to accelerate the abnormal growth of second grain nucleation.

Abstract: Synchrotron X-ray microtomography (SPring-8, Japan) has been used for the microstructure characterization in a closed cell Al-Zn-Mg alloy foam. Some sophisticated microstructure features, such as micropores and intermetallic particles inside the cell wall, were visualized and quantified three dimensionally(3D) by the high-resolution phase contrast imaging technique. By microtomographies tuned to energies above and below the Zn K-absorption edge, the 3D quantitation of Zn distribution was obtained using subtraction imaging technique. It has been clarified that the Zn distribution was inhomogeneous in the cell wall of the foam. And the agglomeration of Zn-bearing particles was confirmed to induce the brittle fracture of cell wall. The distributions of Ti and Ca in the foam were also visualized by subtraction method. The current tomographic techniques provide novel solutions for the 3D microstructure analysis in the highly inhomogeneous foam materials.