Ordering and the micromechanics of Ti-7Al

The evolution of intergranular lattice strain in the α titanium alloy Ti–7Al wt% was characterised using in situ time-of-flight (TOF) neutron diffraction during room temperature tensile loading. Samples were aged to promote ordering and the formation of nanometre-scale α2 (Ti3Al). On ageing, at 550°C and 625°C, dislocations were observed to travel in pairs, and in planar arrays, which has been attributed to the presence of ordering. A slight change in c/a was observed, from 1.6949 to 1.6945, and a slight increase in the macroscopic modulus. However, no changes were observed in the residual lattice strains, which are the grain-orientation average elastic strains produced by plasticity. Therefore it is inferred that the changes in deformation mechanisms caused by ordering that result in an enhanced vulnerability to dwell fatigue affect primarily the extent of slip localisation. The overall strain distributions between grains in different orientations are not changed

Protocol for the perfusion and angiography imaging sub-study of the Third International Stroke Trial (IST-3) of alteplase treatment within six-hours of acute ischemic stroke

RATIONALE: Intravenous thrombolysis with recombinant tissue Plasminogen Activator improves outcomes in patients treated early after stroke but at the risk of causing intracranial hemorrhage. Restricting recombinant tissue Plasminogen Activator use to patients with evidence of still salvageable tissue, or with definite arterial occlusion, might help reduce risk, increase benefit and identify patients for treatment at late time windows. AIMS: To determine if perfusion or angiographic imaging with computed tomography or magnetic resonance help identify patients who are more likely to benefit from recombinant tissue Plasminogen Activator in the context of a large multicenter randomized trial of recombinant tissue Plasminogen Activator given within six-hours of onset of acute ischemic stroke, the Third International Stroke Trial. DESIGN: Third International Stroke Trial is a prospective multicenter randomized controlled trial testing recombinant tissue Plasminogen Activator (0·9 mg/kg, maximum dose 90 mg) started up to six-hours after onset of acute ischemic stroke, in patients with no clear indication for or contraindication to recombinant tissue Plasminogen Activator. Brain imaging (computed tomography or magnetic resonance) was mandatory pre-randomization to exclude hemorrhage. Scans were read centrally, blinded to treatment and clinical information. In centers where perfusion and/or angiography imaging were used routinely in stroke, these images were also collected centrally, processed and assessed using validated visual scores and computational measures. STUDY OUTCOMES: The primary outcome in Third International Stroke Trial is alive and independent (Oxford Handicap Score 0-2) at 6 months; secondary outcomes are symptomatic and fatal intracranial hemorrhage, early and late death. The perfusion and angiography study additionally will examine interactions between recombinant tissue Plasminogen Activator and clinical outcomes, infarct growth and recanalization in the presence or absence of perfusion lesions and/or arterial occlusion at presentation. The study is registered ISRCTN25765518

Dislocation interactions in near-alpha Titanium alloy Ti6242Si under LCF

Dislocation interactions were investigated in near-alpha titanium alloy Ti6242Si after low cycle fatigue. Samples from the gauge section and the crack initiation site on the fracture surface were studied. Grain pairs with different crystallographic orientations were analysed from the gauge section to understand the dislocation interactions near the boundary. Deformation was primarily caused by planar slip, localized into slip bands in primary alpha (αp) grains. Direct slip transfer was observed within grains having similar orientations. In contrast, slip transfer resulted in a different kind of dislocation nucleation in the neighboring hard grain where there was misorientation between the grains, with the observation of cross-slip. Strain transfer was observed between highly misoriented grain pairs. Crack nucleation occurred on an αp grain by basal plane splitting, due to the large tensile stress developed by a double ended pile-up. This kind of pile-up is suggested to result from the incomplete reversibility of dislocation motion during load reversals. The observation of superjogs on the basal dislocations in the crack nucleated αp grain provides a rationale for why cracks nucleate near, rather than on, the basal plane

Embrittlement of alloy 625 and effect of remedial treatments

This study investigated the susceptibility of alloy 625 to embrittlement in the temperature range 600–800 ℃ using simulated thermal treatments, specimens from ex-service petroleum industry components and subsequent remedial heat treatment. Embrittlement was quantified by conducting impact testing using the Charpy V-notch method and the results discussed in terms of precipitate formation and ageing. The results of annealing the exposed samples at high temperature are presented, demonstrating that embrittlement is most detrimental at intermediate temperatures (i.e. 650 ℃). The study confirms that a 24-h remedial heat treatment of embrittled alloy 625 leads to significant recovery of impact energy on ex-service samples. </jats:p

Thermal imaging and stress analysis for predicting the behaviour and long-term performance of flare tips

A combination of computational and imaging tools was developed to characterise the performance and assess the lifetime of flare tips, the top portion of stacks used to burn off excess gas in the gas and petroleum industry. Integrating the use of (1) an infrared imaging study to obtain temperature profiles of flare tips in operation and (2) finite element modelling of stress distributions based on temperature profiles allowed lifetime prediction for creep and fatigue failure. The techniques were successfully applied to a complex flare tip design that included a venturi shaped windshield. It was concluded that in this instance, the flare tip lifetime would be limited by a combination of creep and fatigue of the support brackets and by plastic deformation at the top of the windshield. Design methodologies for producing flare tips with improved lifetimes are suggested, including the use of computational fluid dynamics modelling to assess the combined effects of gas flow rates and wind conditions on combustion behaviour. Thus, the prediction prior to installation, of both the performance of a new design and the long-term behaviour, can be a valuable assessment process for a proposed flare tip assembly. </jats:p