19th Magnetic Materials Center Seminar
June 19, 2006, 16:00
7th floor seminar room, Sengen
Mechanical Properties of Nanostructured Materials
B. S. Rao
Nanocrystalline materials are the polycrystalline materials with grain sizes below ~100nm. These materials have attractive properties such as increased strength/hardness, improved toughness, enhanced diffusivity, high specific heat, enhanced thermal expansion coefficient and superior soft magnetic properties in comparison with conventional polycrystalline materials. These properties led to widespread research in this area. Although the strength/hardness of these materials are very high but they lack in ductility, in the present seminar I will discuss some of the current trends in improving the ductility of these materials.

Improved hardening behavior of Mg-Sn-Zn alloy
Taisuke SASAKI
The recent demand for the magnesium alloy is to develop the wrought alloy with excellent heat resistance. Precipitation hardening is well known as one of the very effective strengthening mechanism from the ambient temperature to the elevated temperature. Mg-Sn alloy is the typical precipitation hardnenable alloy system. Sn has several characteristics suitable as an alloying element for elevated temperature application; low diffusivity in Mg, low solid solubility in Mg and high liquid solubility in molten Mg. furthermore, the Mg2Sn phase has very high melting temperature of about 770℃. Up to now, the micro-addition of Zn is found to be very effective to enhance the hardening behavior in Mg-Sn alloy. For more strengthening, further improvement of hardening responce is required. This improvement can be achieved by the refinement of the precipitates and the by the precipitates on non-basal plane. To develop ideal microstructure, the isothermal aging treatment at 160℃ and the two-step aging is conducted. . To compare to the Mg-Sn-Zn alloy aged at 200℃, the peak hardness increased by the two-step aging and isothermal aging at 160℃. The increase in the hardness can be explained by the change in the mictrostructure. tep aging, the change in the morphology of the precipitates was observed. In addition, the drastic refinement of the precipitates was achieved by the aging treatment at 160℃. Together with these topics, the mocrostructure of wrought magnesium-tin alloy will also be presented.

Microstructure Characterization of two step aged Mg-Zn alloys by TEM and 3DAP
K. Oishi
Mg-Zn alloys are most widely used wrought magnesium materials. It shows age hardening due to the precipitation of supersaturated Zn. The precipitation sequence is well documented starting from the G.P. zone formation, followed by the formation of β1’ and β2’, precipitate, before reaching to the formation of equilibrium β phase. To enhance the age hardening response, two-step aging was reported to be effective [G.Mima & Y.Tanaka, Trans. Japan Inst. Met., vol. 12 (1971)]. In this study, the variations of age hardening response and corresponding microstructures in Mg-Zn alloys were investigated by using hardness test, transmission electron microscopy (TEM) and 3-dimendional atom probe (3DAP). Billets of Mg-6Zn-1Mn and Mg-6Zn-3Al-1Mn (wt.%) alloys were prepared by hot-extrusion at 350 oC. These alloys were designated as ZM61 and ZAM631, respectively. The hardness of the two-step aged samples was higher than that of the single-aged samples for both alloys. TEM observations showed that the peak aged samples have two kinds of precipitates: one was a rod along to the c-axis of matrix phase, another was a plate lying on the basal plane. In both alloys, rod-like precipitates were dominant. The two-step aged samples exhibited finer microstructure than the single-aged ones. GIF elemental mapping showed the rod-like and the cuboidal precipitates had a high concentration of Zn, but the distribution of Al was almost homogeneous. Atom probe analyses of ZAM631 pre-aged at 70 oC suggested the formation of Zn-rich zones.