1,721,019 research outputs found
A quantitative interpretation of DSC experiments on quenched and aged SiCp reinforced 8090 alloys
No full textDSC curves of the quenched monolithic 8090 alloy and the 8090 MMC have been used to obtain values for the heats of formation of GPB zones, delta' and S phase. Using these ?H values and a correction for overlap of effects, the DSC curves of the aged alloys have been interpreted in terms of amounts of precipitates present. The presented interpretations are consistent with previous microstructural investigations. The solvus of GPB zones and of S phase in 8090 alloys has been obtained. Significant amounts of GPB zones are formed in the monolithic alloy, whilst the addition of SiC particles greatly reduces the amount of GPB zones formed
Development of reliability theory for the structural integrity assessment of load bearing implant/host tissue constructs
No full textQuantification of particle distribution effects on fatigue in an Al-SiC<sub>p</sub> composite
No full textA detailed experimental investigation has been carried out into the effects of particle distribution and morphology on the smooth specimen fatigue behaviour in a powder-processed 2124-based Al–18.7 vol.% silicon carbide particulate (SiCp) composite with a nominal reinforcement particle size of 6.5 ?m. Experimental results are reported on the development of fatigue damage in the moderate to high cycle regime, highlighting the role of short crack growth behaviour and the limited initiation life in the materials tested. A methodology is presented for quantitatively analysing the effects of reinforcement spatial distribution and morphology on short crack growth using previously reported finite-body tessellation techniques. Tessellation analysis of the composite has demonstrated that direct crack–tip interactions with the reinforcement particles occurred preferentially in small particle/low volume fraction regions, with further analysis suggesting a controlling influence of particle size on crack path behaviour. In addition, multiple linear regressions of the tessellation data identify a combined influence on growth rates in the ‘discontinuous’ regime of reinforcement clustering and reinforcement morphology (orientation and/or aspect ratio). Results are discussed in relation to known short crack growth mechanics and the microstructural and micromechanical characteristics of particle-reinforced systems
DSC sample preparation for Al-based alloys
No full textDifferential Scanning Calorimetry (DSC) is a useful technique for the study of phase transformations and has been widely applied to study precipitation in aluminium alloys (e.g. Ref.1). One reason for its popularity is the perceived ease of sample preparation, which generally involves grinding to a thickness of about 1 mm followed by punching of discs. Alternatively cylindrical rods may be machined and the sample discs can be cut from these rods. It is however well known that punching, grinding, machining and cutting all introduce deformation in Al-based alloys, and this influences precipitation in most heat treatable Al-based alloys by providing sites for heterogeneous nucleation, and by annihilating quenched-in excess vacancies (2). Garcia-Cordovilla and Louis (3) have investigated the influence of specimen preparation on DSC results obtained for a commercial Al-Cu based alloy (AA2011): it was shown that in samples punched and ground after solution heat treatment, the θ’ (Al2Cu) formation effect had shifted to lower temperatures when compared to samples punched and ground prior to heat treatment.In the present work the effect of sample preparation on precipitation during DSC heating of a monolithic 8090 (Al-Cu-Mg-Li-Zr) alloy and an 8090 MMC is investigated. The 8090 alloy system seems especially suited for such a study since the main precipitation reactions which occur in this alloy (GPB-zone, δ’(Al3Li) and S’(Al2CuMg) formation) cover a wide range of different types of precipitation reactions. GPB-zone formation is thought to occur via homogeneous nucleation and is thought to depend critically on the amount of excess vacancies available (4,5). Several possible paths leading to the formation of the L12 ordered δ’ phase from the disordered Al-rich FCC phase have been proposed in the literature (see e.g. Ref. 6,7,8,9 ). Different paths are thought to operate at different temperatures, with homogeneous nucleation and growth occurring in the high temperature range. In each possible path the amount of excess vacancies is expected to influence the rate of δ’ formation by enhancing diffusivity of Li, whilst coarsening of δ’ is thought to be assisted by dislocations (10,11). Additionally heterogeneous nucleation of δ’ on β’ (Al3(Zr,Li)) precipitates occurs (see e.g. Ref.12,13). S’ (Al2CuMg) nucleation occurs mostly heterogeneously and several nucleation sites have been reported in the literature: dislocations (if present) and subgrain boundaries appear to be the most potent sites, followed by dislocation loops resulting from clustering of excess vacancies ( ), although fewer dislocation loops are formed in 8090 alloys as a result of the high Li and Zr vacancy binding energies. Furthermore S’ has been reported to nucleate on δ’ in 8090 alloys (14). An additional complicating factor related to sample preparation in 8090 alloys is lithium loss during solution treatment (15,16,17); Li loss will alter the composition of the surface layer and, unless removed, it will influence thermal reactions associated with precipitation and dissolution of particles
Effect of mechanical surface pretreatment on metal ion release
No full textThe degree of metal ion dissolution from Ti–6Al–4V alloy hip replacement stems subjected to various mechanical and chemical surface pretreatments was analysed in vitro. High-dissolution rates were observed for nitric acid passivated samples that had been mechanically surface treated to increase the implant surface area. Significantly lower ion release levels were observed for mechanically treated samples which had been aged in de-ionised water. The application of an hydroxyapatite coating decreased the metal ion release from the nitric acid passivated samples (compared to the uncoated sample) and increased the metal ion dissolution from the aged samples. The dissolution behaviour of the samples is explained in terms of the diffusion processes occurring at the stem/solution interface and the morphological and chemical characteristics of the surface treated stems
Improvements in quench factor modelling
Full text linkIn this contribution, the validity of a number of key quench factor analysis (QFA) assumptions is discussed. It is shown that the incorporation of a square root dependency of yield strength on precipitate volume fraction provides a sounder physical basis for quench factor modelling. Peak-aged strength/hardness prediction accuracies are not affected, but C-curve positions are. It is also demonstrated that transformation kinetics are described more correctly by a modified Starink–Zahra equation than by a Johnson–Mehl–Avrami–Kolmogorov type equation, yielding better prediction accuracies when a physically realistic Avrami exponent of 1.5 or greater is used. Finally, a regular solution model is introduced to quantify the influence of the solute solubility temperature-dependency on the minimum strength. These improvements are all implemented within the framework of classical QFA
Systematic assessment and validation of compliance-based crack closure measurements in fatigue
No full textDifferent methods of closure measurement and issues related to the identification of closure points are examined. Based on compliance curves obtained from crack mouth clip gauges and near-tip strain gauges, a systematic assessment of closure measurements has been made by a variety of non-subjective methods. Closure results obtained from compliance curves are also compared with direct crack tip observations. For results obtained on a commercial Al alloy, AA2024-T351 plate, it is shown that curve fitting methods based on a combination of linear and quadratic functions provide particularly sensitive and consistent closure measurements. The potential separation of plane stress and plane strain closure effects that arise in real, three-dimensional specimens is demonstrated via a combination of global compliance measurements and the use of side-grooved specimen
The effect of orientation on short crack path and growth rate behaviour in Al-Li alloy AA8090
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