Effect of Deep Cryogenic Treatment on Microstructure and Properties of AE42 Magnesium Alloy

Abstract

There has been a great interest of using magnesium (Mg) alloys in automotive industries owing to its high specific strength, high thermal conductivity, good damping capacity and good machinability. The use of Mg alloys in automobile increases fuel efficiency and reduce emissions by reducing the vehicle weight. The AE42 alloy is a promising creep resistant Mg alloy and it is suitable for application up to 175°C.

Cold treating is a very old process and is widely used for high precision parts to enhance their properties. Cold treatment can be done in 2 ways. When treatment is done at temperature down to -80°C it is called ‘Cryogenic Treatment (CT)’. On the other hand, when treatment is done at liquid nitrogen temperature (i.e., -196°C) it is called ‘Deep Cryogenic Treatment (DCT)’. Deep cryogenic treatment generally improves certain proper ties beyond the improvement obtained by normal cryogenic treatment. When CT was carried for steel, it was observed that there was complete transformation of retained austenite into marten site. Also, there was improvement of mechanical properties which can be explained by the precipitation of sub microscopic carbides as a result of the CT. When deep cryogenic treatment was carried out on steel, it showed significant increase in wear resistance. It was theorized that the increase in wear resistance was a direct result of the reduction in the amount of retained austenite and change in the carbide morphologies. Therefore, it is concluded that DCT has pronounced effect to alter microstructure and thereby the mechanical properties. The number of literature that report DCT of different kinds of steels, ZnO nanowires and pure Zr is plenty. Unfortunately, the number of literature on CT/DCT for Mg alloy is rare. Recently, CT/DCT on AZ91 and AZ31 Mg alloys resulted homogeneity in microstructure that exhibited superior wear and tensile properties. However, the precise effect of CT/DCT on properties like hardness, creep, corrosion and so on is still not clear. Therefore, in the present study, mechanical properties including wear as well as corrosion behavior of the AE42 (Mg-4Al-2RE) Mg alloy subjected to DCT is planned for investigation. For comparison the same tests will be carried out on AE42 alloy without DCT. DCT was carried out on specimens for 0, 4, 8 and 16 h. A detailed microstructural characterization of the untreated and treated AE42 alloy has been done. The specimens were prepared according to required dimensions for the above-mentioned tests. The phase analysis using XRD as well as optical and SEM micrographs were taken on the untreated and treated AE42 alloy. It was observed that with increase in DCT time the volume fraction of secondary Al4RE phase was reducing and that of primary α-Mg was increasing. Tensile, hardness, wear and corrosion tests were carried out on all specimen and its results were compared. The ductility of tensile specimens was increasing with increase in DCT time. The hardness of the specimens decreased with increase in DCT time. The wear rate increased with increase in DCT time. Corrosion specimens showed increase in corrosion rate with increase in DCT time.