Papers by Keyword: Synchrotron X-Ray Diffraction

Abstract: Liquid metal structure of Sn-Cu alloys studied by synchrotron high energy X-ray diffraction have been performed to investigate the effect of the trace elements Ni and Al. It has been demonstrated that trace Ni and Al additions are able to alter the liquid Sn-4wt%Cu structure. The refinement of the primary Cu₆Sn₅ phase after trace Al additions is proposed to be caused by the consumption of Cu atoms involved in a AlCu chemical ordering and an associated change in the undercooling for Cu₆Sn₅ nucleation.

Abstract: Manganese-rich austenitic twinning-induced plasticity (TWIP) steels with high strength and superior ductility have received much attention in the past two decades. Tremendous efforts have been made to explore their unusual hardening behaviour which includes contributions from twins, dislocations, grain boundaries and solid solution. Nevertheless, the individual hardening effects of twins, dislocations, grain boundaries and solid solution on the high strength of TWIP steels are still unclear. In the present work, the flow stress of a TWIP steel was experimentally decomposed into the respective contributions of twins, dislocations, grain boundaries and solid solution. For the forest hardening, synchrotron X-ray diffraction experiments with line profile analysis were carried out to measure the dislocation density. It is found that the yield stress of the present TWIP steel is controlled by solid solution and grain boundary hardening, which contribute to 238.3 and 238.5 MPa, respectively. After yielding, the work-hardening rate is dominated by dislocation multiplication which accounts for up to 922 MPa at a true strain of 0.4, equal to about 60% of the flow stress. In comparison, twins contribute to only 118 MPa at the same true strain, equal to about 8% of the flow stress. In other words, twins have minor effect on the flow stress, in contrast to the current understandings in the literature.

Abstract: A novel technique called interdendritic-melt solidified control (IMSC) was developed to manufacture equiaxed superalloy in the efforts of reducing porosity. The effect of process parameter, such as withdrawal rate, of the IMSC on the porosities and mechanical properties of IN792 alloy was investigated, compared with conventional investment casting (CC) technique. The IMSC and CC samples were characterized by optical metallography and scanning electron microscopy. In addition, the minor phases, such as MC carbides and trace amount of η phase, were identified by synchrotron X-ray diffraction. The results indicate that proper withdrawal rate for IMSC can produce castings with much reduced porosity and higher stress rupture properties at elevated temperature compared to conventional investment casting. However, fast withdrawal rate will produce much severe porosity and lower mechanical property.

Abstract: Abstract. Residual stress gradients across the wall of seamless steel tubes influence decisively the mechanical stability and reliability of automotive and industrial constructions. Irreversible bending moments imposed on the tubes induce gradual and asymmetric elasto-plastic deformation across the tube cross-sections which result in very complex residual stress distributions. The aim of this contribution is to present a novel methodology as well as complementary modeling approach to assess the three-dimensional distribution of triaxial residual stresses in bent steel tubes. The stress characterization was performed using high energy X-ray diffraction at the HEMS beamline of PETRAIII synchrotron source in Hamburg as well as using laboratory Drill-hole method. For the complementary modeling of the stress distribution, a FEM software package DEFORM ^HT was used. The results reveal that the stress gradients across the tube wall are primarily influenced by the martensite profile predetermined by the parameters for thermo-mechanical treatment of the tubes. The tube bending causes the formation of continually varying compressive and tensile stresses across the tube circumference whereas the stress magnitude across the wall thickness scales again with the martensite appearance. Finally the results document the importance of the cooling process control and the influence of the applied bending radius on the resulting stress distributions as well as related mechanical parameters like fracture toughness and fatigue behavior.

Abstract: Laser Shock Peening (LSP) is a material enhancement process used to introduce compressive residual stresses in metallic components. This investigation explored the effects of different combinations of LSP parameters, such as irradiance (GW/cm²) and laser pulse density (spots/mm²), on 3.2 mm thick AA6056-T4 samples, for integral airframe applications. The most significant effects that are introduced by LSP without a protective coating include residual stress and surface roughness, since each laser pulse vaporizes the surface layer of the target. Each of these effects was quantified, whereby residual stress analysis was performed using X-ray diffraction with synchrotron radiation. A series of fully reversed bending fatigue tests was conducted, in order to evaluate fatigue performance enhancements with the aim of identifying LSP parameter influence. Improvement in fatigue life was demonstrated, and failure of samples at the boundary of the LSP treatment was attributed to a balancing tensile residual stress.

Abstract: Carbon-carbon composites are deemed as candidate materials for application in very high temperature reactors. In a very high temperature reactor, carbon-carbon composite materials would experience severe environmental impacts from high temperatures. As a result, we applied non-destructive ex-situ diffraction experiments to investigate the microstructure changes of the carbon-carbon composite materials experiencing different temperatures. In this study, the samples were prepared in a format of a three-dimensional pitch-based carbon-carbon composite. The samples were heated to 500 (°C), 700 (°C), and 900 (°C) for 2 minutes, respectively. In order to understand the temperature effect on carbon-carbon composite, we facilitated the high penetration of the synchrotron X-ray diffraction at National Synchrotron Radiation Research Center to examine the evolution of microstructures subjected to heat treatment. The results show that the lattice parameters of a-axis and c-axis evolve upon heating. The molecular dynamics simulation results suggest that the early-stage rearrangement is originated from the release of the defects.

Abstract: Bi-metallic piping welds are frequently used in light water nuclear reactors to connect ferritic steel pressure vessel nozzles to austenitic stainless steel primary cooling piping systems. An important aspect for the integrity of such welds is the presence of residual stresses. Measurement of these residual stresses presents a considerable challenge because of the component size and because of the material heterogeneity in the weld regions. The specimen investigated here was a thin slice cut from a full-scale bi-metallic piping weld mock-up. A similar mock-up had previously been investigated by neutron diffraction within a European research project called ADIMEW. However, at that time, due to the wall thickness of the pipe, stress and spatial resolution of the measurements were severely restricted. One aim of the present investigations by high energy synchrotron radiation and neutrons used on this thin slice was to determine whether such measurements would render a valid representation of the axial strains and stresses in the uncut large-scale structure. The advantage of the small specimen was, apart from the easier manipulation, the fact that measurement times facilitated a high density of measurements across large parts of the test piece in a reasonable time. Furthermore, the recording of complete diffraction patterns within the accessible diffraction angle range by synchrotron X-ray diffraction permitted mapping the texture variations. The strain and stress results obtained are presented and compared for the neutron and synchrotron X-ray diffraction measurements. A strong variation of the texture pole orientations is observed in the weld regions which could be attributed to individual weld torch passes. The effect of specimen rocking on the scatter of the diffraction data in the butt weld region is assessed during the neutron diffraction measurements.

Abstract: We have performed in-situ magnetization and high-energy X-ray diffraction measurements on two aluminum-based TRIP steels from room temperature down to 100 K in order to evaluate amount and stability of the retained austenite for different heat treatment conditions. We have found that the bainitic holding temperature affects the initial fraction of retained austenite at room temperature but does not to influence significantly the rate of transformation upon cooling.

Abstract: Laser shock peening offers potential advantages over conventional peen technologies in terms of the depth of the residual stresses that can be induced, and improvements in surface roughness. In this study the application of laser peening to thin aluminium plates such as are used in aerospace applications is investigated. Peening of thin plates presents challenges in balancing the peen intensity to prevent overpeening that will actually lower the stress field. Strain profiles for different laser peening parameters were obtained using synchrotron X-ray diffraction at the ESRF, France. Results are presented and discussed of the residual strain profiles in terms of the laser power density and the number of peen passes. When the power density and number of passes are increased the compressive strain magnitudes are also increased, as has been observed in previous studies. However, the strain components longitudinal and transverse to the peen line are not identical to each other, with the transverse component being much less compressive.

Abstract: Residual strain profiles were measured by synchrotron X-ray radiation in Al2O3/Y-stabilized ZrO2 (YSZ) ceramic laminates. Different stacking sequences were employed, including alternating layers containing 5 and 40 vol.% YSZ. Residual strains were found to be fairly constant within each layer; although they change at the interface between layers with different compositions. Different behaviour is observed for the strains along the in-plane and normal directions.