F.A. Garcia-Pastor

Microstructure and properties of hot isostatically pressed powder and extruded Ti25V15Cr2Al0??? 2C

Abstract The influence of process route on the microstructure and tensile behaviour of specimens prepared from hot isostatically pressed powders and extruded ingot of the burn resistant alloy, Ti–25V–15Cr–2Al–0·2C (wt-%), has been investigated. Samples based on gas atomised (GA) and plasma rotating electrode process (PREP) powders have been studied. Microstructural examination shows that many PREP powder particles are single crystals, whereas GA particles are polycrystalline. The mechanical properties of hot isostatically pressed specimens have been assessed using tensile testing monitored by acoustic emission, while microstructures have been characterised by synchrotron X-ray microtomography and optical and analytical scanning electron microscopy. Tomographic examination revealed a small fraction (<0·002 vol.-%) of pores in samples made from hot isostatically pressed GA powders, but no porosity was detected in samples made from hot isostatically pressed PREP powder. In view of their similar tensile behaviour, it is concluded therefore that the porosity does not contribute to the scatter and poor ductility in these hot isostatically pressed samples. These pores increased in size and volume fraction after heat treatment above the hot isostatic press temperature. The large scatter in tensile properties of both hot isostatically pressed GA and PREP samples was correlated with the presence of large (100–400 μm) circular crack initiation sites on the fracture surfaces, but the origin of these initiation sites has not been identified.

In Situ Synchrotron Studies of Reversible and Irreversible Non-elastic Strain in a Two-Phase TiAl Alloy

This paper contrasts the cyclic tensile loading of two-phase titanium aluminide Ti-44Al-8Nb-1B microstructures, namely fully lamellar (FL) and duplex (DP). The former, in contrast to the latter, shows premature yielding and hysteresis loops on cycling. These phenomena were studied by in situ cyclic loading of these specimens using high energy synchrotron X-ray diffraction. The results show significant differences in the micromechanics of deformation. Load transfer between the γ and α2 phases has been identified at stresses below the macroscopic yield point in the FL microstructure, while the DP microstructure showed no signs of such load transfer before its well-defined macroscopic yield point. A partially reversible pseudo-plastic deformation mechanism seems to be operating at relatively low stresses in the FL specimen. This mechanism is believed to be twinning/partial reversible twinning of the γ phase. The presence of twins at relatively low applied stresses has been confirmed for the FL microstructure by electron channeling contrast imaging. Further support of twinning/partial reversible twinning in FL but not DP microstructures below the macroscopic yield point was obtained by following the evolution of the integrated intensity of particular diffraction peaks measured during the in situ synchrotron X-ray experiments.

Effect of Immersion Routes on the Quenching Distortion of a Long Steel Component Using a Finite Element Model

Distortion while quenching steel is a commonly found problem in industrial practice. This issue arises from the combined effect of thermal contraction and martensite transformation stresses. This problem may be exacerbated in components with long geometries. This work presents the results of a numerical and experimental investigation that assessed the effect of both the austenite grain size (AGS) of three sizes 8, 9 and 10 ASTM, and the immersion route on the distortion of a long component of SAE 5160 steel during oil quenching. The transformation kinetics were calculated using JMatPro and were validated by quench dilatometry. A three-dimensional finite element model was developed in Deform 3D. This model was then validated using thermocouple data. Results showed that the distortion was minimized when the long component was dipped sideways in the quenching medium instead of immersing the component from the distal ends when the AGS was 8 ASTM.

Preparation and characterization of sol–gel silica based neutral optical density coatings by the addition of graphite particles

Abstract Silica based coatings with various thicknesses (600–3000 nm) were prepared from the gellation of aqueous suspensions composed of colloidal silica particles, small graphite particles and inorganic binder materials, the latter were included to improve the coating structure. The graphite particles added in amounts of 0.8–3.6 wt.%, in the dried coating, absorb visible light with the same efficiency, which results in coatings with neutral optical density in this range of the spectrum. The optical density of the coatings can be changed from basically 0, in coatings without graphite, to more than 2 in thick coatings with 3.6 wt.% of graphite. The structural characterization of the coatings was carried out using X-ray diffraction, Raman scattering and atomic force microscopy.