Microstructure characterization of 316L deformed at high strain rates using EBSD

Specimens from split Hopkinson pressure bar experiments, at strain rates between ~ 1000–9000 s− 1 at room temperature and 500 °C, have been studied using electron backscatter diffraction. No significant differences in the microstructures were observed at different strain rates, but were observed for different strains and temperatures. Size distribution for subgrains with boundary misorientations > 2° can be described as a bimodal lognormal area distribution. The distributions were found to change due to deformation. Part of the distribution describing the large subgrains decreased while the distribution for the small subgrains increased. This is in accordance with deformation being heterogeneous and successively spreading into the undeformed part of individual grains. The variation of the average size for the small subgrain distribution varies with strain but not with strain rate in the tested interval. The mean free distance for dislocation slip, interpreted here as the average size of the distribution of small subgrains, displays a variation with plastic strain which is in accordance with the different stages in the stress-strain curves. The rate of deformation hardening in the linear hardening range is accurately calculated using the variation of the small subgrain size with strain.

Interface microstructure of alumina mechanically metallized with Ti brazed to Fe–Ni–Co using different fillers

NASCIMENTO,R.M. et al.Interface microstructure of alumina mechanically metallized with Ti brazed to Fe–Ni–Co using different fillers. Materials Science and Engineering A, v.466, n.1/2, p. 195-200, 2007.

Microstructure and tensile properties of carbon-epoxy laminates produced by automated fibre placement: Influence of a caul plate on the effects of gap and overlap embedded defects

Automated fibre placement (AFP) enables the trajectory of unidirectional composite tape to be optimized, but laying down complex shapes with this technology can result in the introduction of defects. The aim of this experimental study is to investigate the influence of gaps and overlaps on the microstructure and tensile properties of carbon-epoxy laminates. First, a comparison between a hand-layup and AFP layup, draped and cured under the same conditions, shows equivalent microstructures and tensile properties. This provides the reference values for the study. Then, gap and overlap embedded defects (more or less severe) are introduced during manufacturing, on two cross-ply layups [(0°/(90°)5/0°] and [(90°/0°)2/90°]. Autoclave cure without a caul plate results in local thickness variation and microstructural changes which depend on the defect type. This has a strong influence on mechanical performance. Use of a caul plate avoids these variations and in this case embedded defects hardly affect tensile properties.

Microstructure, vibrational and visible emission properties of low frequency ultrasound (42 kHz) assisted ZnO nanostructures

Size and shape tuneable ZnO nanostructures were prepared by a low frequency ultrasound (42 kHz) route using various organic solvents as the reaction media. The crystalline nature, lattice parameters and microstructural parameters such as microstrain, stress and energy density of the prepared ZnO nanostructures were revealed through X-ray diffraction (XRD) analysis. The organic solvents influenced the size and morphology of the ZnO nanostructures, and interesting morphological changes involving a spherical to triangular shaped transition were observed. The visible emission properties and lattice vibrational characteristics of the nanostructures were drastically modified by the changes in size and shape. Raman spectral measurements revealed the presence of multiphonon processes in the ZnO nanostructures. The intensity of the visible emission band was found to vary with the size and morphology of the structures. The strongest visible emission band corresponded to the structure with the largest surface/volume ratio and could be attributed to surface oxygen vacancies. The control over the size and morphology of ZnO nanostructures has been presented as a means of determining the intensity of the visible emission band

Additive Manufacturing by LPBF and WAAM of metals: correlation between production process, microstructure and mechanical properties

In the most recent years, Additive Manufacturing (AM) has drawn the attention of both academic research and industry, as it might deeply change and improve several industrial sectors. From the material point of view, AM results in a peculiar microstructure that strictly depends on the conditions of the additive process and directly affects mechanical properties. The present PhD research project aimed at investigating the process-microstructure-properties relationship of additively manufactured metal components. Two technologies belonging to the AM family were considered: Laser-based Powder Bed Fusion (LPBF) and Wire-and-Arc Additive Manufacturing (WAAM). The experimental activity was carried out on different metals of industrial interest: a CoCrMo biomedical alloy and an AlSi7Mg0.6 alloy processed by LPBF, an AlMg4.5Mn alloy and an AISI 304L austenitic stainless steel processed by WAAM. In case of LPBF, great attention was paid to the influence that feedstock material and process parameters exert on hardness, morphological and microstructural features of the produced samples. The analyses, targeted at minimizing microstructural defects, lead to process optimization. For heat-treatable LPBF alloys, innovative post-process heat treatments, tailored on the peculiar hierarchical microstructure induced by LPBF, were developed and deeply investigated. Main mechanical properties of as-built and heat-treated alloys were assessed and they were well-correlated to the specific LPBF microstructure. Results showed that, if properly optimized, samples exhibit a good trade-off between strength and ductility yet in the as-built condition. However, tailored heat treatments succeeded in improving the overall performance of the LPBF alloys. Characterization of WAAM alloys, instead, evidenced the microstructural and mechanical anisotropy typical of AM metals. Experiments revealed also an outstanding anisotropy in the elastic modulus of the austenitic stainless-steel that, along with other mechanical properties, was explained on the basis of microstructural analyses.