Spark Plasma Sintering of Cobalt Powders in Conjunction with High Energy Mechanical Treatment and Nanomodification

Spark plasma sintering (SPS) investigations were carried out on three sets of Co specimens: untreated, high energy mechanically (HEMT) pre-treated, and nanomodified powders. The microstructure, density, and mechanical properties of sintered pellets were investigated as a function of various pre-treatments and sintering temperatures (700–1000 °C). Fine-grained sinters were obtained for pre-treated Co powders; nano-additives tended to inhibit grain growth by reinforcing particles at grain boundaries and limiting grain-boundary movement. High degree of compaction was also achieved with relative densities of sintered Co pellets ranging between 95.2% and 99.6%. A direct co-relation was observed between the mechanical properties and densities of sintered Co pellets. For a comparable sinter quality, sintering temperatures for pre-treated powders were lower by 100 °C as compared to untreated powders. Highest values of bending strength (1997 MPa), microhardness (305 MPa), and relative density (99.6%) were observed for nanomodified HEMT and SPS processed Co pellets, sintered at 700 °C.

The Structure, Role and Flexibility of Grain Boundaries

ABSTRACTThe structure and role of grain boundaries is investigated using an atomic analysis of the grain boundary movement during Molecular Dynamics displacement cascade simulations of bcc Fe. The results show the grain boundary to be a flexible entity. Local restructuring of the GB accommodates the incoming self interstitial atoms with local kinks, or small movements of a few atomic spacings occurring when the grain boundary is engulfed in the displacement cascade. The damage created is investigated using two potentials: the Ackland (non-magnetic) and the Dudarev- Derlet (magnetic) to study the role and influence of magnetism on the results obtained.

Theory of Grain Growth in the Presence of Atoms Drag Effects

The statistical model of grain growth is able to predict the effect of Zener drag on the grain size distribution evolution and on grain growth kinetics [1, 2]. This paper, in the same framework, will treat the case of atoms drag on grain boundary movement. The mechanism by which atoms drag operates is significantly different by that of Zener. The corresponding peculiar features will result in a specific grain size distribution evolution with considerable change of grain growth kinetics and distribution shape from that of normal grain growth case as a function of the intensity of the pinning conditions.

Influence of Lattice Anisotropy on Models Formulated by Cellular Automata in Presence of Grain Boundary Movement: A Case Study

This paper continues in the previous research focussed to two simple questions. The first one reads: ”What is the influence of anisotropy of computational lattice on simulations of boundary movement?” where grain boundary movement typically appears in simulations of grain boundary migration and static/dynamic recrystallization. The second question reads: ”How is the computational anisotropy related to natural anisotropy of the material lattice itself?” This study is focussed on the influence of change of the computational algorithm and/or lattice on the grain boundary movement. Two algorithms, the majority rule and the simple modification of the Monte Carlo method for two different lattices – namely square and hexagonal one – are used.