12th Metallic Nanostructure Group Seminar
March 30, 2006, 9:00 am
7th floor seminar room, Sengen
Microstructure and Mechanical Properties of Nanocrystalline Fe-C materials produced by MM-SPS
K. Oishi
Nanocrystalline materials attract our interests because of unusual properties such as ultra-high hardness and strength as compared with coarse-grain ones. Mechanical milling (MM) is one of technique to produce nanocrystalline materials. The pearlite steel powder after MM has shown the formation of a nanocrystalline ferrite structure and the decomposition of cementite. Also, the thermal stability of the powder is very good until 500ºC. Spark plasma sintering (SPS) is comparatively new sintering method. The SPS can achieve a fully dense bulk sample at a rapid heating rate by a pulsed electrical discharge. In this study the SPS has been used to consolidate Fe-C powder subjected to MM. At first, more detailed microstructure analysis was performed on the 600ºC SPS sample which showed high strength and plastic strain in compression test. The microstructure is a bimodal structure consisting of fine and coarse grains. Also, the fine grain region is composed of ferrite- and cementite-phase grains. Next, the SPS was done at different temperatures to produce materials having a high strengthductility combination. The SPS samples consolidated at the sintering temperatures above 610ºC were about 97% in relative density. Results for compression tests show two distinct features; high strength and little ductility for samples sintered below 610ºC and high ductility for ones sintered above 610ºC.

Age hardening and microstructure of Mg – Zn – Cu alloys
J. Buha
Magnesium alloys represent the lightest structural materials and offer a high potential for weight saving in transportations systems and other applications. Many magnesium alloys are heat-treatable and they mechanical properties are derived from the controlled precipitation of strengthening precipitates. Among these, alloys based on Mg-Zn system show the most remarkable age hardening response. Furthermore, addition of Cu to binary Mg-Zn is known to induce significant age hardening response, improve ductility and creep properties. Their high mechanical properties, coupled with their excellent castability and considerably lower cost compared to alloys containing expensive rear earth elements, Mg-Zn-Cu alloys stand out as promising candidates for transportation applications. However, the ageing behavior of these alloys and the effect of Cu on the precipitation and nanostructure are poorly understood.
Ageing response of a number of experimental Mg-Zn-Cu alloys has been investigated by subjecting them to artificial ageing. Artificial ageing at 160ºC was found to induce a significant age hardening characterized by high levels of hardness (up to 109 VHN) and short time-to-peak (less than 30 hours of ageing). The microstructural observations showed that a high density of rod/lath-shaped precipitates forms perpendicular to the basal plane of the Mg matrix. These alloys also showed marked response to room temperature ageing, which suggests that the presence of Cu in sold solution affects the concentration, availability and possibly the distribution of vacancies in the matrix.