David H. StJohn

The role of solute in grain refinement of magnesium

The effect of separate solute additions of Al, Zr, Sr, Si, and Ca on grain size of Mg has been investigated. Increasing the Al content in hypoeutectic Mg-Al alloys resulted in a continuous reduction in grain size up to 5 wt pct Al, reaching a relatively constant grain size for higher Al contents (above 5 wt pct). The effect of Sr additions was investigated in both low- and high-Al content magnesium alloys, and it was found that Sr had a significant grain refining effect in low-Al containing alloys but a negligible effect on grain size in Mg-9Al. Additions of Zr, Si, and Ca to pure magnesium resulted in efficient grain refinement. The grain refinement is mainly caused by their growth restriction effects, i.e., constitutional undercooling, during solidification, but the effect of nucleant particles, either introduced with the alloying additions or as secondary phases formed as a result of these additions, may enhance the grain refinement. A brief review of grain refinement of magnesium alloys is included in this article to provide an update on research in this field.

The electrochemical corrosion of pure magnesium in 1 N NaCl

An electrochemical investigation was carried out to study the corrosion of pure magnesium in 1 N NaCl at different pH values involving electrochemical polarisation, scanning tunnel microscopy (STM), measurement of hydrogen gas evolution and measurement of the elements dissolved from the magnesium specimen which were determined by inductively coupled plasma atomic emission spectrophotometry (ICPAES). A partially protective surface film was a principal factor controlling corrosion. Film coverage decreased with increasing applied electrode potential. Application of a suitable external cathodic current density was shown to inhibit magnesium dissolution whilst at the same time the hydrogen evolution rate was relatively small. This showed that cathodic protection could be used to significantly reduce magnesium corrosion. A new definition is proposed for the negative difference effect (NDE)

Development of the as-cast microstructure in magnesium–aluminium alloys

Abstract This paper presents an overview of several projects undertaken at CAST to increase our understanding of the solidification characteristics of Mg–Al alloys. With the increased use of magnesium alloys, and with casting dominating as a production route, there is a need for a more comprehensive understanding of the mechanisms of solidification and defect formation to allow further optimisation of alloys and casting processes. The paper starts with considering the formation of the primary magnesium dendrites and the means for grain refinement of magnesium–aluminium alloys. The Mg–Al system is then shown to display a range of eutectic morphologies for increasing aluminium content, ranging from a divorced structure, through several intermediate structures, to a fully lamellar structure at the eutectic composition. The eutectic also influences discontinuous precipitation which occurs in the aluminium-rich regions of the magnesium phase. The paper concludes with a section on porosity formation as a function of aluminium content and an outline of the mechanism responsible for the formation of banded defects in magnesium alloys, particularly in products made in pressure assisted casting processes.

A model of grain refinement incorporating alloy constitution and potency of heterogeneous nucleant particles

A model for the determination of relative grain size is developed based on the assumption that nucleant substrates are activated by constitutional undercooling generated by growth of an adjacent grain. The initial rate of development of constitutional undercooling is shown to be equivalent to the growth restriction factor and is a useful approximation for the effect of solute on grain size when potent nucleant substrates are present. However, when the nucleants are of a poor potency then a calculation of the fraction solid necessary to develop the constitutional undercooling required for nucleation needs to be performed. Using the model, trends in grain size observed experimentally by the addition of titanium to pure aluminium and to a typical casting alloy, AlSi7Mg0.3, can be predicted. II was also found that summing the individual growth restriction factors of each solute element in a multi-component alloy can grossly overestimate the actual value of the growth restriction factor for the alloy

Galvanic corrosion of magnesium alloy AZ91D in contact with an aluminium alloy, steel and zinc

An investigation was carried out into the galvanic corrosion of magnesium alloy AZ91D in contact with zinc, aluminium alloy A380 and 4150 steel. Specially designed test panels were used to measure galvanic currents under salt spray conditions. It was found that the distributions of the galvanic current densities on AZ91D and on the cathodes were different. An insulating spacer between the AZ91D anode and the cathodes could not eliminate galvanic corrosion. Steel was the worst cathode and aluminium the least aggressive to AZ91D. Corrosion products from the anode and cathodes appeared to be able to affect the galvanic corrosion process through an alkalisation, passivation, poisoning effect or shortcut effect

The effect of zirconium grain refinement on the corrosion behaviour of magnesium-rare earth alloy MEZ

Corrosion performance of sand cast magnesium alloy MEZ was investigated for unrefined (MEZU) and Zr-grain-refined (MEZR) microstructures in 5% NaCl solution using salt spray, immersion, in situ examination of the corrosion morphology, ESEM, electron probe microanalysis, hydrogen evolution and polarisation curves. MEZU demonstrated higher rates of anodic dissolution and cathodic hydrogen evolution than MEZR. The central zirconium-rich areas within the grains of the MEZR microstructure was more corrosion resistant than the outer zirconium-depleted areas of the grains, whereas MEZU showed little difference in corrosion between the centre and the edge of the grains. Based on the analyses of the corrosion process, polarisation behaviour and the microstructure of these two alloys, it is postulated that zirconium acts in a number of ways to improve corrosion resistance. Zirconium stabilises the solid solution and makes it inactive in anodic dissolution, significantly passivates the precipitated particles reducing cathodic hydrogen evolution and increases the barrier effect of the grain boundary phase through a finer grain size and thus a more continuous layer of the grain boundary phase.

Rheological behaviour of the mushy zone and its effect on the formation of casting defects during solidification

A new conceptual framework has been developed which explains the formation of shear-related casting defects such as porosity, segregation and tears. The theory relates defect formation to the mechanical behaviour of the partially solidified microstructure when shear stresses are developed during the filling of a casting and by the subsequent feeding processes during solidification. Two transition points, the dendrite coherency point and the maximum packing solid fraction, divide the mushy zone into three regions of different mechanical and feeding behaviours. The response of the mush to shear is related to the presence of these zones during solidification of a casting. The resulting defects are rationalized by considering the governing local shear stress and shear rate, local strength and time available for fluid flow. The design of the casting, the casting process used and the alloy composition all influence the relative importance of shearing on defect formation.

The effect of grain refinement and silicon content on grain formation in hypoeutectic Al-Si alloys

The effect of increasing the amount of added grain refiner on grain size and morphology has been investigated for a range of hypoeutectic Al-Si alloys. The results show a transition in grain size at a silicon concentration of about 3 wt% in unrefined alloys; the grain size decreasing with silicon content before the transition, and increasing beyond the transition point. A change in morphology also occurs with increased silicon content. The addition of grain refiner leads to greater refinement for silicon contents below the transition point than for those contents above the transition point, while the transition point seems to remain unchanged. The slope of the grain size versus silicon content curve after the transition seems to be unaffected by the degree of grain refinement. The results are related to the competitive processes of nucleation and constitutional effects during growth and their impact on nucleation kinetics

Effect of a short solution treatment time on microstructure and mechanical properties of modified Al–7wt.%Si–0.3wt.%Mg alloy

Abstract Microstructural change caused by a short solution treatment and the corresponding change in tensile properties and impact energy of a strontium modified Al–7wt.%Si–0.3%Mg cast alloy were studied. It was found that a solution treatment of 10 min at 540 or 550 °C is sufficient for the α-aluminium phase to homogenise and achieve the maximum level of magnesium and silicon as predicted by the solubility and alloy composition limits. A solution treatment of 30 min causes spheroidisation, coarsening and an increase in inter-particle spacing of the eutectic silicon particles leading to a significant improvement in ductility and impact resistance. Compared with a standard 6 h solution treatment, solution treatment of 30 min at 540 or 550 °C is sufficient to achieve more than 90% of the maximum yield strength and more than 95% of the maximum UTS and the maximum average elongation to fracture. However, only 80% of the maximum impact energy can be attained by the short solution treatment. The values of the ductility and impact energy pass through a minimum between 1.5 and 10 min of solution treatment time indicating that solution treatments of less than 10 min should be avoided.

Characteristic zirconium-rich coring structures in Mg-Zr alloys

The most characteristic feature of the microstructure of a magnesium alloy that contains more than a few tenths per cent soluble zirconium is the zirconium-rich cores that exist in most grains. The morphology, distribution and composition of cores observed in a Mg-0.56%Zr alloy and the small particles present in them were investigated