21st Magnetic Materials Center Seminar
July 3, 2006, 16:00
7th floor seminar room, Sengen
Ageing response in a new Zr-based bulk metallic glass with improved mechanical property
K. Mondal
Recent research thrust in the field of bulk metallic glass is to design glass with better glass forming ability in terms of critical thickness and improved mechanical properties. Another important aspect is how to control the mechanical properties. A new Zr-based composition has been designed. It is a very good glass former with reduced glass forming temperature (Trg) of 0.65 and high thermal stability of 100K (Tx-Tg). It can also be easily cast into 5mm rod. It also shows reasonably good compressive strength of about 2GPa with strain in the range of 6% before failure in the as cast state. The effect of pre-annealing 30K below Tg or above Tg is very effective in improving the compressive ductility of the alloy. Pre-annealing below Tg which is effective in relaxation of quenched-in-strain, improves the strain to failure up to 18%. Pre-annealing 30K above the Tg for different times shows that initially, strain to failure improves gradually and then it falls. The ageing response observed in the alloy will be explained. The effect of casting temperature is also very effective in improving the compressive ductility of this alloy. As the casting temperature is increased, the total fracture strain also increases (up to 16%). The effect of mechanical interlocking during shear band formation will also be discussed with the help of fractography and compression test result.

Plasticity and microstructure of BMGs
G. Kumar
Present focus in bulk metallic glasses (BMGs) is to control the shear bands that are the fundamental elements of plastic deformation in BMGs. Recent studies indicate that the shear bands, at least in some BMGs can be controlled by preparation conditions or appropriate heat treatment. In present work, we report the effect of preparation and heat treatment on the mechanical properties of Zr-based BMGs. The microstructural results of these specimens in the as-cast state and after annealing will be presented along with their mechanical properties. The results suggest that the mechanical properties of BMGs strongly depend on the casting conditions in addition to the alloy composition.

Natural ageing in Mg-Zn-Cu alloys
Joka Buha
Further development and investigation of inexpensive magnesium alloys, as the lightest structural material, with adequate combination of mechanical properties is essential for light-weight and transportation applications. Mg-Zn alloys show the most pronounced age hardening response which is further enhanced by the addition of Cu. Experimental Mg-Zn-Cu alloys have been investigated in this work by subjecting them to artificial and natural ageing. High levels of hardness were achieved by artificial ageing after a relatively short ageing time, due to the formation of a high density of lath-shaped precipitates. However, all alloys investigated showed a significant response to natural ageing, which has not been commonly observed in Mg alloys. Hardness levels comparable to that produced by artificial ageing can achieved after about 8 weeks of natural ageing by the formation of precipitates very different to and much finer than those observed after artificial ageing. Presence of a number of different classes of both elongated and spherical precipitates was revealed by HREM and 3DAP.