23rd Magnetic Materials Center Seminar
August 7, 2006, 10:30
7th floor seminar room, Sengen
Effect of Ca additions on the microstructure and mechanical properties of Mg-Zn-Al alloys and Refinement of precipitate microstructures by micro alloying additions
Chamini Mendis
There is an increased pressure on the automotive industry to reduce or eliminate the green house gas emissions. One mode of reducing the green house gas emissions is to reduce the weight of automobiles. Magnesium, being the lightest of the structural metals, is a prime candidate that can be used to light weight an automobile. At present, most creep resistant magnesium alloys, that are commercially available, contain additions of expensive alloying elements such as rare-earth additions, silver or yttrium, making these alloys unsuitable for automotive applications. Previous investigations have shown that creep properties of Mg-Zn-Al alloys were superior to those Mg-Al based alloys[1]. It was also reported that additions of up to 1wt%Ca could further improve the creep resistance of Mg-Zn-Al alloys investigated[1, 2]. However, the microstructure and creep behaviour of the Mg-Zn-Al(-Ca) alloys have not been characterized in detail, and the role of Ca in improving the creep resistance remains to be elucidated. In this work, the age-hardening response of Mg-8Zn-4Al (wt%) alloys, with and without a quaternary addition of 0.5wt% Ca, in the temperature range 150-200°C has been measured using Vickers hardness testing, and precipitate microstructures have been characterized using transmission electron microscopy. The peak-aged samples have been creep tested at selected stress levels in the temperature range 125-175°C. Second part of the presentation considers the effect micro alloying additions and the development of a qualitative thermo-kinetic frame work which assists in the choice of micro alloying additions that provide heterogeneities in the matrix phase. The presence of these sites may enhance the nucleation of precipitates which would refine the distribution of precipitates leading to an increment in the precipitation hardening response observed. The thermo-kinetic criteria are applied to Mg-Sn system which exhibit a relatively small age hardening response and an increment in hardening response and a corresponding increase in the number density of precipitates is observed.

[1] Z. Zhang, Development of Magnesium Based Alloys for Elevated Temperature Applications, in: Department de Mines et Metallurgi, Universite Laval, Laval, Canada, 2000.
[2] M. Vogel, Microstructure and creep behaviour of Mg-Zn-Al-Ca die cast alloys, in: Max-Planck Institut fur Metallforschung und Intitut fur Metallkunde, University of Stuttgart, Stuttgart, 2002, p. 166.

A review of my past research work
R. Shan
In this presentation, I will introduce my research work in last four years.
In my first subject, exchange biasing in CoNi/FeMn and CoCr/FeMn bilayers were investigated, where the CoCr layers are of granular structure and the CoNi layers are in the form of a single phase. In the above two series of bilayers, the different characteristics of the coercivity and magnetization reversal mechanisms which result from the different microstructures were studied.
In my second subject, the magnetic characters of TbFeCo/NiO multilayers were investigated. The out-of-plane coercivity and perpendicular anisotropy of multilayers were enhanced by no loop shift exchange biasing, in comparison with those of TbFeCo single layer films. In my third subject, a series of Co/Pt multilayers with thin Pt layer were investigated. High out-of-plane coercivity of 6.7 kOe was obtained in this kind of Co/Pt multilayers by magnetron sputtering.
At last, I will introduce my some work in Maxtor Company, especially the last unfinished work about simulation of writing process and aftereffect in hard disk based on Arrhenius-Neel model and Monte Carlo method.