Modeling of hardening and softening processes in Mg alloys

The tensile deformation behavior of a range of supersaturated Mg-Al solid solutions and an as-cast magnesium alloy AM60 has been studied. The Mg-Al alloys were tested at room temperature while the alloy AM60 was tested in the temperature range 293-573 K. The differences in the deformation behavior of the alloys is discussed in terms of hardening and softening processes. In order to identify which processes were active, the stress dependence of the strain-hardening coefficient was assessed using Lukac and Balik's model of hardening and softening. The analysis indicates that hardening involves solid solution hardening and interaction with forest dislocations and non-dislocation obstacles such as second phase particles. Cross slip is not a significant recovery process in the temperature range 293-423 K. At temperatures between 473 and 523 K the analysis suggests that softening is controlled by cross slip and climb of dislocations. At temperatures above 523 K softening seems to be controlled by dynamic recrystallisation. (C) 2004 Elsevier B.V. All rights reserved.

Corrosion resistance of anodised single-phase Mg alloys

This work studied the effect of the impurity iron and the alloying elements aluminium and zinc in single-phase substrate magnesium alloys on the corrosion resistance of the alloys after anodisation. It was found that increasing zinc content (0-2%) led to increased corrosion resistance of an anodised single-phase Mg-Zn alloy. The addition of Al lowered the corrosion resistance of an anodised commercial purity Mg-Al single-phase alloy, whereas the same addition was found to be beneficial to the corrosion resistance of an anodised high purity Mg-Al single-phase alloy. Heat-treatment made the substrate Mg-Al and Mg-Zn alloys more uniform and hence improved the corrosion resistance of the alloys after anodisation. The detrimental effect of iron impurity on corrosion performance of the unanodised substrate single-phase magnesium alloys was inherited by the anodised alloys. The corrosion resistance of the anodised Mg alloys was found to be closely correlated with the corrosion performance of the unanodised as-cast Mg alloys. (c) 2005 Elsevier B.V. All rights reserved.

Influence of the beta phase on the corrosion performance of anodised coatings on magnesium-aluminium alloys

The effect of the beta phase in Mg-Al alloys on the corrosion performance of an anodised coating was studied. It was found that the corrosion resistance of the anodised coating was closely associated with the corrosion performance of the substrate alloy. In particular, Mg alloys with a dual phase microstructure of alpha + beta with intermediate aluminium contents (namely 5%, 10% and 22% Al) after anodisation had the highest corrosion rate and the worst corrosion resistance provide by the anodised coating. The poor performance of an anodised coating was attributed partly to lower corrosion resistance of the substrate alloy and partly to the higher porosity of the anodised coating. (c) 2004 Elsevier Ltd. All rights reserved.

A critical review of the stress corrosion cracking (SCC) of magnesium alloys

This review aims to provide a foundation for the safe and effective use of magnesium (Mg) alloys, including practical guidelines for the service use of Mg alloys in the atmosphere and/or in contact with aqueous solutions. This is to provide support for the rapidly increasing use of Mg in industrial applications, particularly in the automobile industry. These guidelines should be firmly based on a critical analysis of our knowledge of SCC based on (1) service experience, (2) laboratory testing and (3) understanding of the mechanism of SCC, as well as based on an understanding of the Mg corrosion mechanism.

Formation and mechanical properties of Mg65Cu25Er10 and Mg65Cu15Ag10Er10 bulk amorphous alloys

Mg65Cu25Er10 and Mg65Cu15Ag10Er10 bulk amorphous alloys were produced by a copper mould casting method. The alloys have high glass-forming ability and good thermal stability. The maximum diameter of glass formation (D-c), glass transition temperature (T-g), crystallization onset temperature (T-x), temperature interval of the supercooled region (Delta T-x), melting temperature (T-m), liquidus temperature (T-1) as well as heats of crystallization (Delta H-x) and melting (Delta H-m) are reported for these alloys. Both alloys exhibit high hardness and high strength at room temperature. (c) 2005 Elsevier B.V. All rights reserved.

Interactions between iron, manganese, and the Al-Si eutectic in hypoeutectic Al-Si alloys

Sand-cast plates were used to determine the effect of iron and manganese concentrations on porosity levels in Al-9 pet Si-0.5 pet Mg alloys. Iron increased porosity levels. Manganese additions increased porosity levels in alloys with 0.1 pet Fe, but reduced porosity in alloys with 0.6 and I pet Fe. Thermal analysis and quenching were undertaken to determine the effect of iron and manganese on the solidification of the Al-Si eutectic. At high iron levels, the presence of large beta-Al5FeSi was found to reduce the number of eutectic nucleation events and increase the eutectic grain size. The preferential formation of alpha-Al15Mn3Si2 upon addition of manganese reversed these effects. It is proposed that this interaction is due to beta-Al5FeSi and the Al-Si eutectic having common nuclei. Porosity levels are proposed to be controlled by the eutectic grain size and the size of the iron-bearing intermetallic particles rather than the specific intermetallic phase that forms.

Light alloy castings for automotive applications: The case of Al vs Mg

The economical and environmental effects of mass reduction through Al and Mg primary alloys substitutions for cast iron and steel in automotive components are discussed using MF. Ashby's penalty functions method The viability of Mg alloy substitutions for existing Al alloy cast components is also considered. The cost analysis shows that direct, equal-volume, Al alloy substitutions for cast iron and steel are the most feasible in terms of the CAFE liability, followed by substitutions involving flat panels of prescribed stiffness. When the creation of CO2 associated to the production of Al and Mg is considered, the potential gasoline savings over the lifespan of the car compensate for the intrinsic environmental burden of Al in all applications, while electrolytic Mg substitutions for cast iron and steel are feasible for equal volume and panels only. Magnesium produced by the Pidgeon thermal process appears to be too primary energy intensive to be competitive in structural applications. Magnesium substitutions for existing Al alloy beams and panels are generally unviable. The current higher recycling efficiency of Al casting alloys confers Al a significant advantage over Mg alloys.

Service performance of engineering materials

Corrosion research by Atrens and co-workers has made significant contributions to the understanding of the service performance of engineering materials. This includes: (1) elucidated corrosion mechanisms of Mg alloys, stainless steels and Cu alloys, (2) developed an improved understanding of passivity in stainless steels and binary alloys such as Fe-Cr, Ni-Cr, Co-Cr, Fe-Ti, and Fe-Si, (3) developed an improved understanding of the melt spinning of Cu alloys, and (4) elucidated mechanisms of environment assisted fracture (EAF) of steels and Zr alloys. This paper summarises contributions in the following: (1) intergranular stress corrosion cracking of pipeline steels, (2) atmospheric corrosion and patination of Cu, (3) corrosion of Mg alloys, and (4) transgranular stress corrosion cracking of rock bolts.

An evaluation of steels subjected to rock bolt SCC conditions

In order to understand the metallurgical influences on Rock Bolt SCC, an evaluation has been carried out on carbon, carbon + manganese, alloy and microalloyed steels subjected to the conditions previously identified as producing laboratory SCC similar to that observed for rock bolts in service. The approach has been to use the LIST test (Linearly increasing stress test) for samples exposed to a dilute pH 2.1-sulphate solution, as per our prior studies. SCC was evaluated from the decrease in tensile strength, ductility and fractography as revealed by SEM observation. A range of SCC susceptibilities was observed. Ten of these steels showed SCC, however there was no SCC for one carbon, two carbon + manganese and two alloy steels.

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.

Effect of iron on grain refinement of high-purity MG-Al alloys

The effect of iron on the grain refinement of high-purity Mg-3%Al and Mg-91%Al alloys has been investigated using anhydrous FeCl3 as an iron additive at 750degreesC in carbon-free aluminium titanite crucibles. It was shown that grain refinement was readily achievable for both alloys. Fe- and Al-rich intermetallic particles were observed in many magnesium grains. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

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.