Alloying of pure magnesium with Mg-33.3 wt-%Zr master alloy

To capitalise on the strengthening potential of zirconium as a potent grain refiner for magnesium alloys, the mechanisms of adding zirconium to magnesium and its subsequent grain refining action need to be understood. Using a Mg-33.3Zr master alloy (Zirmax supplied by Magnesium Elektron Ltd) as a zirconium alloying additive, the influence of different alloying conditions on the dissolution of zirconium in magnesium was investigated. It was found that owing to the highly alloyable microstructure of Zirmax, the dissolution of zirconium was generally complete within a few minutes in the temperature range 730 to 780degreesC. Prolonging and/or intensifying stirring were found to have no conspicuous influence on further enhancing the dissolution of zirconium. In all cases studied, the average grain size increased with increasing holding time at temperature while the total zirconium content decreased. The finest grain structure and highest total zirconium content corresponded to sampling immediately after stirring. Pick up of iron by molten magnesium from the mild steel crucibles used for melting and holding, was significantly delayed or avoided in the temperature range 730 to 780degreesC by coating the crucibles with boron nitride. It is therefore feasible to conduct zirconium alloying at 730degreesC without the need of a considerable excess of Zirmax addition using a properly coated or lined steel crucible.

Age hardening of a sintered Al-Cu-Mg-Si-(Sn) alloy

The age hardening response of a sintered Al-3.8 wt% Cu-1.0 wt% Mg-0.70 wt% Si alloy with and without 0.1 wt% Sn was investigated. The sequence of precipitation was characterised using transmission electron microscopy. The ageing response of the sintered Al-Cu-Mg-Si-(Sn) alloy is similar to that of cognate wrought 2xxx series alloys. Peak hardness was associated with a fine, uniform dispersion of lath shaped precipitates, believed to be either the betaor Q phase, oriented along < 010 >. directions and theta' plates lying on {001}(alpha). planes. Natural ageing also resulted in comparable behaviour to that observed in wrought alloys. Porosity in the powder metallurgy alloys did not significantly affect the kinetics of precipitation during artificial ageing. Trace levels of tin, used to aid sintering, slightly reduced the hardening response of the alloy. However, this was compensated for by significant improvements in density and hardness. (c) 2005 Elsevier B.V. All rights reserved.

Stress corrosion cracking and hydrogen embrittlement of an Al-Zn-Mg-Cu alloy

The age hardening, stress corrosion cracking (SCC) and hydrogen embrittlement (HE) of an Al-Zn-Mg-Cu 7175 alloy were investigated experimentally. There were two peak-aged states during ageing. For ageing at 413 K, the strength of the second peak-aged state was slightly higher than that of the first one, whereas the SCC susceptibility was lower, indicating that it is possible to heat treat 7175 to high strength and simultaneously to have high SCC resistance. The SCC susceptibility increased with increasing Mg segregation at the grain boundaries. Hydrogen embrittlement (HE) increased with increased hydrogen charging and decreased with increasing ageing time for the same hydrogen charging conditions. Computer simulations were carried out of (a) the Mg grain boundary segregation using the embedded atom method and (b) the effect of Mg and H segregation on the grain boundary strength using a quasi-chemical approach. The simulations showed that (a) Mg grain boundary segregation in Al-Zn-Mg-Cu alloys is spontaneous, (b) Mg segregation decreases the grain boundary strength, and (c) H embrittles the grain boundary more seriously than does Mg. Therefore, the SCC mechanism of Al-Zn-Mg Cu alloys is attributed to the combination of HE and Mg segregation induced grain boundary embrittlement. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Strength-ductility behaviour of Al-Si-Cu-Mg casting alloys in T6 temper

A comparative study of the mechanical properties of 20 experimental alloys has been carried out. The effect of different contents of Si, Cu, Mg, Fe and Mn, as well as solidification rate, has been assessed using a strength-ductility chart and a quality index-strength chart developed for the alloys. The charts show that the strength generally increases and the ductility decreases with an increasing content of Cu and Mg. Increased Fe (at Fe/Mn ratio 0.5) dramatically lowers the ductility and strength of low Si alloys. Increased Si content generally increases the strength and the ductility. The increase in ductility with increased Si is particularly significant when the Fe content is high. The charts are used to show that the cracking of second phase particles imposes a limit to the maximum achievable strength by limiting the ductility of strong alloys. The (Cu + Mg) content (at.%), which determines the precipitation strengthening and the volume fraction of Cu-rich and Mg-rich intermetallics, can be used to select the alloys for given strength and ductility, provided the Fe content stays below the Si-dependent critical level for the formation of pre-eutectic alpha-phase particles or beta-phase plates.

On the role of tin in the infiltration of aluminium by aluminium for rapid prototyping applications

The use of tin as an alloying element in the production of freeformed infiltrated aluminium components is explored. Tin slows the growth of the aluminium nitride skeleton which provides dimensional stability, as well as increasing the rate of infiltration of the aluminium liquid into the aluminium nitride skeleton. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The chemical composition of α cen A: Strong lines and the ABO theory of collisional line broadening

The mean abundances of Mg, Si, Ca, Ti, Cr, and Fe based on both strong and weak lines of alpha CenAare determined by matching the observed line profiles with those synthesised from stellar atmospheric models and comparing these results with a similar analysis for the Sun. There is good agreement between the abundances from strong and weak lines. Strong lines should generally be an excellent indicator of abundance and far easier to measure than the weak lines normally used. Until the development of the Anstee, Barklem, and O'Mara ( ABO) theory for collisional line broadening, the uncertainty in the value of the damping constant prevented strong lines being used for abundance determinations other than in close differential analyses. We found that alpha Cen A has a mean overabundance of 0.12 +/- 0.06 dex compared to solar mean abundances. This result agrees remarkably well with previous studies that did not use strong lines or the ABO theory for collisional line broadening. Our result supports the conclusion that reliable abundances can be derived from strong lines provided this new theory for line broadening is used to calculate the van derWaals damping.

The critical dietary potassium concentration for induction of mineral disorders in non-lactating Welsh Mountain sheep

Diets with more than 30 g K/kg DM have previously been associated with hypomagnesaemia in grazing cattle, and to test whether such diets lead to mineral disorders in sheep, the absorption of Mg and other elements was investigated using experimental diets to which KC I had been added to provide 27, 29, 32 or 34 g K/kg DM. The apparent absorption, balance and apparent retention of Mg, and to a lesser extent Ca, were reduced for sheep offered the diets with 32 or 34 g K/kg DM. The absorption and retention of K, Na, P, Zn, Pb and Cd was not affected by treatment. The blood intracellular Ca concentration was reduced by the diets with 29, 32 or 34 g K/kg DM, compared to the diet with 27 g K/kg DM, but the concentration of other elements was unaffected. Blood plasma Ca concentration was increased at the highest level of K inclusion, providing evidence of mild hyperkalaemia and the involvement of Ca homeostatic mechanisms. It is concluded that Mg absorption by sheep will be impaired if the diet contains more than 30 g K/kg DM, equivalent to an intake of approximately 13 g K/d, but that a high K diet may be beneficial before parturition to accustom the sheep to Ca mobilization before lactation. (c) 2005 Elsevier B.V. All rights reserved.

The corrosion performance of anodised magnesium alloys

The corrosion performance of anodised magnesium and its alloys, such as commercial purity magnesium (CP-Mg) and high-purity magnesium (HP-Mg) ingots, magnesium alloy ingots of MEZ, ZE41, AM60 and AZ91D and diecast AM60 (AM60-DC) and AZ91D (AZ91D-DC) plates, was evaluated by salt spray and salt immersion testing. The corrosion resistance was in the sequential order: AZ91D approximate to AM60 approximate to MEZ >= AZ91D-DC >= AM60-DC > HP-Mg > ZE41 > CP-Mg. It was concluded the corrosion resistance of an anodised magnesium alloy was determined by the corrosion performance of the substrate alloy due to the porous coating formed on the substrate alloy acting as a simple corrosion barrier. (c) 2006 Elsevier Ltd. All rights reserved.

Morphology and crystallography of Mg24Y5 precipitate in Mg-Y alloy

Orientation relationships between Mg24Y5 precipitates and matrix in a Mg-Y alloy were accurately determined using Kikuchi line diffraction. The Burgers relationship with habit planes of {10 (1) over bar0}(H) and {31 (4) over bar0}(H) were observed for all precipitates. Compared with the Mg17Al12 precipitate in AZ91, the precipitation hardening effect in this alloy was significantly increased. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

The effect of iron on liquid film migration and sintering of an Al-Cu-Mg alloy

Analytical transmission electron microscopy indicates that liquid film migration occurs during sintering of an Al-Cu-Mg alloy, that intragranular liquid pools develop from migrating films and that iron segregates to these pools. It is suggested that a high localised iron concentration retards the liquid film migration rate by reducing the coherency strain in the retreating grain, causing a region of the film to detach from the boundary, thus forming an intragranular pool in the advancing grain. Alloys with low iron levels develop few intragranular pools and have high sintered densities. (C) 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Hall-Petch parameters for tension and compression in cast Mg

The flow stress in tensile and compressive deformation has been determined in cast Mg, for a wide range of grain sizes between 36 mum and 1.5 mm. The grain size was varied by alloying Mg with small amounts of Zr. It was found that the 0.2% offset flow stress, sigma(y) (MPa), varies with the grain size, d (m), as sigma(y) = 17.7 + 0.25 d(-1/2) in tension, while sigma(y) = -2.3 + 0.39 d(-1/2) in compression. Possible reasons for the difference between tension and compression are discussed.

Hall-Petch parameters in tension and compression in cast Mg-2Zn alloys

The flow stress in tension and compression has been measured as a function of plastic strain in Mg-2Zn castings with grain sizes ranging from 55 to 340 mum. Hall-Petch parameters have been calculated and are compared to those previously reported. In contrast to the behaviour of pure Mg grain refined with Zr and of hot-worked and recrystallised pure Mg and Mg-Zn alloys, the cast material shows little tension/compression asymmetry of the flow stress. The possible effects of texture and of twinning are noted. (C) 2004 Elsevier B.V. All rights reserved.

delta O-18, Sr/Ca and Mg/Ca records of Porites lutea corals from Leizhou Peninsula, northern South China Sea, and their applicability as paleoclimatic indicators

Combined seasonal to monthly resolution coral skeletal delta(18)O, Sr/Ca, and Mg/Ca records are reported for one modem and two late Holocene Porites lutea corals from a fringing reef at Leizhou Peninsula, the northern coast of the South China Sea (SCS). All the profiles for the period 1989-2000 reveal annual cycles well correlated with instrumental sea surface temperatures (SST), and display broad peaks in summer and narrow troughs in winter, reflecting seasonal growth rate variations. Calibration against instrumental SST yields the following equations: delta(18)O=-0.174(+/- 0.010)xSST(degrees C)-1.02(+/- 0.27) (MSWD=5.8), Sr/Ca-(mmol/mol)=-0.0424(+/- 0.0031)xSST(degrees C)+9.836(+/- 0.082) (MSWD=8.6), and Mg/Ca-(mmol/mol)=0.110(+/- 0.009)XSST(degrees C)+ 1.32(+/- 0.23) (MSWD=55). The scatter in the Mg/Ca-SST relationship is much larger than analytical uncertainties can account for, suggesting the presence of SST-unrelated components in the Mg/Ca variation. Calculated Sr/Ca-SST values for two later Holocene Porites lutea samples (U-series ages similar to 541 BC and similar to 487 AD, respectively) from the same reef suggest that SST in the SCS at similar to 541 BC was nearly as warm as in the 1990s (the warmest decade of the last century), but at similar to 487 AD, it was significantly cooler. This observation is consistent with climatic data reported in Chinese historic documents, confirming that the Sr/Ca-SST relationship is a reliable thermometer. Removing the SST component in the delta(18)O variation based on calculated Sr/Ca-SST values, the residual delta(18)O reflects the deviation of the Holocene seawater delta(18)O from the modem value, which is also a measure of the Holocene sea surface salinity (SSS) or the summer monsoon moisture level in mainland China. Such residual delta(18)O was close to zero at similar to 541 BC and -0.3 parts per thousand at similar to 487 AD, suggesting that it was as wet as in the 1990s at similar to 541 BC but significantly drier at similar to 487 AD in mainland China, which are also consistent with independent historic records. Calculated Mg/Ca-SST values for the two late Holocene corals are significantly lower than the Sr/Ca-SST values and are also in conflict with Chinese historic records, suggesting that coral Mg/Ca is not reliable proxy for SST. At comparable Sr/Ca ranges, fossil corals always display negative Mg/Ca offsets if compared with the modem coral of the same site. We interpret this observation as due to preferential loss of Mg during meteoric dissolution of cryptic Mg-calcite-bearing microbialites in the exposed fossil corals. Microbialites (MgO up to 17%, Sr only 100-300 ppm) are ubiquitous during reef-building processes and their presence in only a trace amount will have a significant impact on coral Mg/Ca ratios without detectable influence on coral Sr/Ca ratios. (c) 2004 Elsevier B.V. All rights reserved.

Precipitation induced densification in a sintered Al-Zn-Mg-Cu alloy

It is demonstrated that slow cooling to 200 degrees C from a high sintering temperature (620 degrees C) reduces porosity in an Al-8Zn-2.5Mg-1Cu powder compact when compared to isothermal sintering at the higher temperature for a longer time. The reduction in porosity is attributed to shrinkage associated with removal of solute from the aluminium solid solution and heterogeneous precipitation of the eta phase (MgZn2), particularly onto pore surfaces. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.