Giovanni Bruno

Simultaneous determination of high-temperature crystal structure and texture of synthetic porous cordierite

The evolution of the crystal structure and crystallographic texture of porous synthetic cordierite was studied by in situ high-temperature neutron diffraction up to 1373u2005K, providing the first in situ high-temperature texture measurement of this technologically important material. It was observed that the crystal texture slightly weakens with increasing temperature, concurrently with subtle changes in the crystal structure. These changes are in agreement with previous work, leading the authors to the conclusion that high-temperature neutron diffraction allows reliable crystallographic characterization of materials with moderate texture. It was also observed that structural changes occur at about the glass transition temperature of the cordierite glass (between 973 and 1073u2005K). Crystal structure refinements were conducted with and without quantitative texture analysis being part of the Rietveld refinement, and a critical comparison of the results is presented, contributing to the sparse body of literature on combined texture and crystal structure refinements.

Young’s modulus and Poisson’s ratio changes due to machining in porous microcracked cordierite

Microstructural changes in porous cordierite caused by machining were characterized using microtensile testing, X-ray computed tomography, and scanning electron microscopy. Young’s moduli and Poisson’s ratios were determined onxa0~215- to 380-μm-thick machined samples by combining digital image correlation and microtensile loading. The results provide evidence for an increase in microcrack density and decrease of Young’s modulus due to machining of the thin samples extracted from diesel particulate filter honeycombs. This result is in contrast to the known effect of machining on the strength distribution of bulk, monolithic ceramics.

Stress-induced damage evolution in cast AlSi12CuMgNi alloy with one- and two ceramic reinforcements

Two composites, consisting of an as-cast AlSi12CuMgNi alloy reinforced with 15xa0vol% Al2O3 short fibres and with 7xa0vol% Al2O3 short fibresxa0+xa015xa0vol% SiC particles, were studied. Synchrotron computed tomography disclosed distribution, orientation, and volume fraction of the different phases. In-situ compression tests during neutron diffraction in direction parallel to the fibres plane revealed the load partition between phases. Internal damage (fragmentation) of the Si phase and Al2O3 fibres was directly observed in CT reconstructions. Significant debonding between Al matrix and SiC particles was also found. Finally, based on the Maxwell scheme, a micromechanical model was utilized for the new composite with two-ceramic reinforcements; it rationalizes the experimental data and predicts the evolution of all internal stress components in each phase.

Primary and secondary creep in aluminum alloys as a solid state transformation

Despite the massive literature and the efforts devoted to understand the creep behavior of aluminum alloys, a full description of this phenomenon on the basis of microstructural parameters and experimental conditions is, at present, still missing. The analysis of creep is typically carried out in terms of the so-called steady or secondary creep regime. The present work offers an alternative view of the creep behavior based on the Orowan dislocation dynamics. Our approach considers primary and secondary creep together as solid state isothermal transformations, similar to recrystallization or precipitation phenomena. In this frame, it is shown that the Johnson-Mehl-Avrami-Kolmogorov equation, typically used to analyze these transformations, can also be employed to explain creep deformation. The description is fully compatible with present (empirical) models of steady state creep. We used creep curves of commercially pure Al and ingot AA6061 alloy at different temperatures and stresses to validate the propos...

Effect of Plastic Deformation on the Microscopic Residual Stresses in 6061Al-15vol%SiCw Composites

Projects MAT 97-1059-C02-01 from CICYT and 07N-0066-98 from CAM, Spain, and support from NFL (Studsvik) under contract no N01 HPRI-CT-1999-00061 in the frame of ARI Program. Help from Mihail Butman, NFL, Studsvik, who performed the compression tests is gratefully acknowledge.

Improving the visibility of phase gratings for Talbot-Lau X-ray imaging

Abstract Talbot-Lau interferometry provides X-ray imaging techniques with significant enhancement of the radiographic contrast of weakly absorbing objects. The grating based technique allows separation of absorption, refraction and small angle scattering effects. The different efficiency of rectangular and triangular shaped phase gratings at varying detector distances is investigated. The interference patterns (Talbot carpets) are modeled for parallel monochromatic radiation and measured by synchrotron radiation. In comparison to rectangular shapes of phase gratings much higher visibility is obtained for triangular shapes which yield enhanced contrast of a glass capillary test specimen.

X-ray refraction distinguishes unprocessed powder from empty pores in selective laser melting Ti-6Al-4V

ABSTRACT For the first time, X-ray refraction techniques are proven for the identification of void formation in Ti-6Al-4V parts produced by selective laser melting. The topology and volume fraction of pores are measured in samples produced with different laser energy density. Unique X-ray refraction methods identify different kinds of defects, characteristic to the regions below and above the optimum laser energy density, namely unprocessed powder (unmolten powder particles, balling effect, and fusion defects) from empty keyhole pores. Furthermore, it is possible to detect small inhomogeneities (voids or cracks) with sizes below the spatial resolution of optical microscopy and X-ray computed tomography. IMPACT STATEMENT For the first time, we show that, unparalleled by high-resolution X-ray-computed tomography or electron microscopy, X-ray refraction can distinguish unprocessed powder from empty pores in additive manufactured materials. GRAPHICAL ABSTRACT

The Connection between Micro-Residual Stress and Thermo-Mechanical Treatments in 6061Al-15vol%SiCw Composites

In this work it is shown how the microscopic residual stress state in 6061Al-15vol%SiCw composites can be modified by plastic strain and by isothermal annealing (the latter, affecting also the precipitation state). The composites were obtained by a powder metallurgical route, which involves extrusion of Al-SiCw powder blends. Neutron diffraction measurements were carried out to determine the RS state. Several degrees of plastic strain, from 1% up to 15% in compression, have been applied ex-situ to the same specimen on several composite materials. Furthermore, the m-RS was also investigated in one of the composites at several precipitation states as resulting from isothermal annealing at 300°C during different times. Investigations were carried out from a fully hardened state, T6 up to an overannealed condition, OV (obtained after annealing during 100 h). The results show that quenching from solid solution treatment temperature prior to annealing at 146°C to achieve the T6 condition provokes a RS state, which does not relax during this treatment. No decrease of the deviatoric component and partial relaxation of the hydrostatic term of the micro- RS developed in the composite occur when treating from the T6 to the OV condition. Instead, only 1% plastic strain is sufficient to produce total relaxation of the hydrostatic term of the m-RS. Even inversion of the radial component occurs on higher straining. Again, no relaxation of the deviatoric component occurs. This may be explained bearing in mind that the deviatoric component is linked to the geometry of the reinforcing particles (whiskers degree of orientation). The hydrostatic term does not relax totally after heat treatment because is partially re-generated on cooling, while the plastic deformation is ‘locked’ in the microstructure via dislocation activity.

The influence of the support structure on residual stress and distortion in SLM Inconel 718 parts

The effect of support structure and of removal from the base plate on the residual stress state in selective laser melted IN718 parts was studied by means of synchrotron X-ray diffraction. The residual stresses in subsurface region of two elongated prisms in as-built condition and after removal from the base plate were determined. One sample was directly built on a base plate and another one on a support structure. Also, the distortion on the top surface due to stress release was measured by contact profilometry. High tensile residual stress values were found, with pronounced stress gradient along the hatching direction. In the sample on support, stress redistribution took place after removal from the base plate, as opposed to simple stress relaxation for the sample without support. The sample on support structure showed larger distortion compared to sample without support. We conclude that the use of a support decreases stress values but stress-relieving heat treatments are still needed.

Residual Stresses in Selective Laser Melted Samples of a Nickel Based Superalloy

Additive Manufacturing (AM) through the Selective Laser Melting (SLM) route offers ample scope for producing geometrically complex parts compared to the conventional subtractive manufacturing strategies. Nevertheless, the residual stresses which develop during the fabrication can limit application of the SLM components by reducing the load bearing capacity and by inducing unwanted distortion, depending on the boundary conditions specified during manufacturing. The present study aims at characterizing the residual stress states in the SLM parts using different diffraction methods. The material used is the nickel based superalloy Inconel 718. Microstructure as well as the surface and bulk residual stresses were characterized. For the residual stress analysis, X-ray, synchrotron and neutron diffraction methods were used. The measurements were performed at BAM, at the EDDI beamline of -BESSY II synchrotronand the E3 line -BER II neutron reactor- of the Helmholtz-Zentrum fur Materialien und Energie (HZB) Berlin. The results reveal significant differences in the residual stress states for the different characterization techniques employed, which indicates the dependence of the residual state on the penetration depth in the sample. For the surface residual stresses, longitudinal and transverse stress components from X-ray and synchrotron agree well and the obtained values were around the yield strength of the material. Furthermore, synchrotron mapping disclosed gradients along the width and length of the sample for the longitudinal and transverse stress components. On the other hand, lower residual stresses were found in the bulk of the material measured using neutron diffraction. The longitudinal component was tensile and decreased towards the boundary of the sample. In contrast, the normal component was nearly constant and compressive in nature. The transversal component was almost negligible. The results indicate that a stress re-distribution takes place during the deposition of the consecutive layers. Further investigations are planned to study the phenomenon in detail.