Papers by Author: Andrej Atrens

Abstract: The structural stability and phase transition of magnesium (Mg) containing different amounts of Al under high pressure was studied by means of first-principles total energy calculations. The cohesive energy calculations showed that the hcp and bcc structures of Mg-4.17 at%Al and Mg-8.33 at%Al were of the strong structural stability. The enthalpy for hcp and bcc structures of Mg was dependent upon the Al content. With increasing Al content from 0 to 8.33 at%, the enthalpy for hcp and bcc structures increased monotonously. Based on the enthalpy differences of the hcp and bcc structures under different pressures, the phase transition pressure under which the hcpbcc structural phase transition may take place for pure Mg, Mg-4.17 at%Al and Mg-8.33 at%Al was 60 GPa, 70 GPa and 85 GPa, respectively, indicating that with the increasing Al content, the phase transition pressure became higher and the hcpbcc transition was more difficult.

Abstract: An Fe content lower than the tolerance limit is critical in controlling corrosion rates of Mg alloys. The possibility of reducing Fe below the tolerance in Mg melts was studied using Zr as a precipitating agent. The experiments were carried out on Mg-X binary alloys with rare-earths, X = Y, Ce, Gd, Nd, and La. The laboratory scale results show that Zr is effective in reducing the Fe content from the Mg melt for Mg-X binary alloys. Purification occurs by the precipitation from the melt of Fe rich precipitates, and the settling of the precipitates to the bottom of the melt. Any desired Fe content down to one wt ppm can in principle be achieved by appropriate melt treatment. The experimental results are discussed with respect to calculated phase diagrams.

Abstract: The corrosion mechanism of Mg alloys in Hank’s solution was elucidated by comparing the corrosion of typical Mg alloys (AZ91, ZE41 and Mg2Zn0.2Mn) and high purity Mg in Hank’s solution at room temperature and in 3% NaCl saturated with Mg(OH)₂. Corrosion was characterised by the evolved hydrogen and the surfaces after the immersion tests. Corrosion in Hank’s solution was weakly influenced by microstructure in contrast to corrosion in the 3% NaCl solution, where second phases cause strong micro-galvanic acceleration. This is attributed to the formation of a more protective surface film in Hank’s solution, causing extra resistance between the alpha-Mg matrix and the second phase. The incubation period in Hank’s solution was alloy dependent.

Abstract: Plug-in specimens enable measurement of reliable Mg polarization curves. Cathodic polarization curves were measured for high purity Mg in 3.5% NaCl saturated with Mg(OH)₂ using (i) mounted specimens and (ii) plug-in specimens. Polarization curves yielded the corrosion current density i_corr and the corresponding corrosion rate P_i, which was compared with corrosion rates evaluated from hydrogen evolution, P_H, and weight loss, P_W. Mounted specimens produce P_i values three times larger than plug-in specimens, due to crevice corrosion in the mounted specimens. Plug-in specimens had no crevice and allow simultaneous measurement of P_H and P_i. P_i was less than P_H and indicated an apparent valence of 1.45 in support of the existence of the uni-positive Mg⁺ ion.

Abstract: The influence of the microstructure, particularly the morphology of the β phase, on the corrosion of Mg alloys has been studied using AZ91 as a model alloy and compared with the corrosion of pure magnesium, used as a standard for comparison. The concentration of the impurity element Fe was below the limit evaluated from theoretical phase diagram construction. Corrosion was measured using hydrogen evolution measurements and some polarization measurements. Corrosion behaviour was characterized for four different microstructures produced by heat treatment of as-cast AZ91: namely (i) as-cast, (ii) homogenization anneal (for 5h and 10h at 380°C), (iii) solid solution and (iv) solution treated and aged. The influence of microstructure can be understood from the interaction of the following three factors: (i) the surface films, (ii) micro-galvanic corrosion acceleration dependant on the amount and distribution of the second phase (the  phase in AZ91) and (iii) the second phase can act as a corrosion barrier and hinder corrosion propagation in the matrix, if the second phase is in the form of a continuous network. It is expected that these factors are important for all multi-phase Mg alloys because all known second phases have corrosion potentials more positive than that of the -phase. The electrochemical measurements did not give good values for the corrosion rate in agreement with the literature.

Abstract: An XPS investigation was carried out on the surface film formed by exposure to high-purity water, on mechanically polished Mg and the two Mg-Al intermetallic compounds: Al3Mg2 and Mg17Al12. The result for mechanically polished pure Mg indicates that a film of MgO covered by a Mg(OH)2 layer, formed by the reaction of MgO with water vapour in the air. On immersion in distilled water, this film was hydrated to a duplex film with an inner MgO layer next to the Mg metal and an external porous layer of hydroxide. For both intermetallics, there was preferential dissolution of magnesium from the mechanically ground surface and also during aqueous immersion. After immersion, there was a 10 nm thick, stable film on the surface; the film composition on Al3Mg2 was whilst that on Mg17Al12 was .