11th Metallic Nanostructure Group Seminar
March 9, 2006, 9:00 am
7th floor seminar room, Sengen
Microstructural characterization of mechanically milled martensite steel powder
S. Ohsaki
The decomposition of cementite and the formation of nanocrystalline structure from pearlite steel are commonly observed in heavily drawn pearlite wire, weared surface of rail steel and mechanically milled pearlite. TEM and 3DAP observation of mechanically milled pearlite steel powder showed that formation of nanocrystalline ferrite and segregation of carbon at grain boundaries were observed in the powder after complete decomposition of cementite by mechanical milling. On the other hand, martensite steel is also important industrial steel which is used as many bearing products. Appearance of a white structure under rolling contact fatigue is established in the bearing products as well as the pearlite steel products. However, the microstructural change is much less understood when compared with that of the pearlite steel because of small number of investigations due to difficulty of the sample preparation. In this talk, I will present the results of the microstructural observation and the carbon distribution in the martensite steel powder by TEM and 3DAP. Additionally, the TEM and 3DAP sample preparation techniques by Focused ion beam (FIB) will be also introduced in the presentation.

Microstructure and mechanical properties of Ti-Zr-Cu-Ni-Be metallic glass
D. Nagahama
Most of Bulk Metallic Glasses (BMGs) reported so far exhibit a plastic strain in the range of 0-3% when deformed under compression mode. This limited plastic strain in monolithic BMGs is usually correlated to their failure along a single shear band. The plastic strain of BMGs can be significantly improved by forming a composite structure composed of nanocrystallites embedded in amorphous matrix. However, recent reports show that the compressive strain of monolithic amorphous Ti-based and Pt-based BMGs can reach up to ~7% and ~ 25%, respectively. Such a high plastic strain is explained by the formation of multiple shear bands. However, the correlation between the microstructure and the multiple shear band formation in the Ti-based and Pt-based BMGs has remained unclear. The purpose of this investigation is to study the correlation between the microstructure and the mechanical properties of Ti-based BMG in the as-cast state and upon annealing.

Revisiting glass forming ability criteria by considering size of nuclei and activation energy for nucleation
K. Mondal
Better glass forming ability (GFA) can be decided by lower critical cooling rate (Rc) and higher critical thickness (Zc). There are many existing semi-predictive models for the determination of GFA, which are termed as GFA criteria. All of them consider better linear correlation with Rc and Zc by taking three material parameters, melting temperature (Tm), crystallization temperature (Tx) and glass forming temperature (Tg). Though, there is very good linear correlation of GFA criteria with Rc, correlation of GFA with Zc is poor. In principle, a glass having low Rc should have higher Zc. This is not observed in a number of cases.
Present model speaks of GFA criteria in terms of Zc, which is developed from the basic theory of homogeneous nucleation theory. It considers two aspects, Gibbs free energy change for the crystallization (ΔGV) at 0.8Tm for glassy alloys and solid-liquid interfacial energy (σ) calculated from homogeneous nucleation theory. ΔGV and σ take considerable part in deciding the size of nucleating phase and activation energy for the nucleation. It is shown that this model is a much better approach for predicting an alloy with better glass formation in terms of Zc in a particular alloy system. It tries to cite possible reason for poor correlation between Zc and existing GFA criteria.