TY - THES T1 - Self formed Cu-W functionally graded material created via powder segregation A1 - Jankovic Ilic,Dragana Y1 - 2008/03/04 N2 - The aim of the present work is to develop a technique to produce continuously formed Cu-W functionally graded materials (FGMs) with adequate mechanical and electrical characteristics for application in extreme environmental conditions. For that purpose a novel process based on size segregation of W bimodal granular media (GM), composed of monomodal W agglomerates A1 (45-60µm) and A2 (200-250µm), has been applied. The study of compaction dynamics and related relaxation behavior of selected granular media have demonstrated that, under weak excitation and under controlled ambient condition, vibrated bimodal W GM segregates in a manner that a gradient in packing is self formed. During sintering, such a graded packing structure turns into a W preform with a gradient in porosity. Studied W GM fulfills both critical demands: i.e. size segregation and effective sintering process. The final Cu-W FGM has been produced by subsequent infiltration of molten Cu into W graded preform. Compaction dynamics of weakly excited W GM (frequency 600 Hz and acceleration 6 g) shows three distinguished stages. The first quasi linear stage is attributed to percolation. This process is controlled by an individual particle relaxation where inelastic collisions between particles dominate and it is effective for a moderate packing density. Through a transient stage, where critical slow down occurs, the system attains a steady state. The steady state stage is driven by collective relaxation of close packed particle clusters, where interparticle forces are of the primary importance. The compaction dynamics of studied W GM can be well predicted by Kohlrausch-Williams-Watts law. Size segregation of polymodal GM is a specific case of vibro-compaction, where collective nature is dominated by excitation level, particle size and friction force. In weakly excited GM, in regimes of geometrical segregation and geometrical segregation aided by convection motion, a gradient in packing can been self formed. Each of these regimes correspond to collective motion of the small agglomerates (A1), whereas the larger agglomerates (A2) play the role of an inert phase. It was shown that a gradient of packing represents the most effective random packing model. Sintering of the constitutive monomodal agglomerates (A1, A2) is strongly inhomogeneous with a statistical nature dependant on initial configuration of selected GM. The low temperature sintering (1400-1700 K) of monomodal W GM is controlled by simultaneous action of rearrangement process and grain boundary diffusion (GBD). A dominance of one of the processes is determined by initial agglomerate size and initial loose packing structure. Agglomerate / particle contact asymmetry and therefore induced transient / residual stresses cause the rearrangement process with the apparent activation energy of Ea~100 kJ/mol and Ea~80 kJ/mol for the agglomerates A1 and A2, respectively. Agglomerate mobility caused by agglomerate structure and polymodal porosity distribution result in viscose flow like regime in the available free volume of the porous W skeleton, similar to rearrangement in liquid phase sintering. Both mechanisms recognize that enhancement of densification can be achieved by improving the packing factor. Rearrangement process continues until a geometrical factor is exhausted, defined by the random close packing density limit, and makes a condition for GBD to be later operating. Sintering kinetics at higher temperature (1700-1950 K) revealed for both monomodal agglomerates important contribution to sintering coming from the action of GBD. Sintering kinetics of bimodal W GM is strongly influenced by a packing structure obtained during vibration. The packing structure evolves through three stages: 1) skeleton of smaller particles (percolation stage); 2) skeleton of larger particles (diffusion stage); and 3) graded structure (steady state). The self formed skeleton and graded structures exhibit different sintering behavior. Sintering kinetics of the skeleton type structure resembles the behavior of skeleton-forming powder. The sintering is controlled by a competition between rearrangement and GBD. Sintering of the W graded structure is similar to the sintering of the homogeneous mixture. Due to a improved packing density, the sintering of the graded structure dominantly controlled by GBD. After sintering the graded structure shows an increase in density of 10 % compared to corresponding percolation structure. The final Cu-W FGMs have been characterized by measurement of electrical resistivity and effective E-Modulus. The content of Cu and W-W contiguity are two features which determine the resistance for current flow and E-modulus of the studied materials. The higher W packing density in graded structure and correspondingly lower Cu content lead to poorer current flow, but higher E-modulus. KW - Funktioneller Gradientenwerkstoff KW - Sinter KW - Pulvermetallurgie KW - Segregation KW - Kupfer KW - Wolfram CY - Saarbrücken PB - Universitäts- und Landesbibliothek SN - 978-3-8322-7070-4 AD - Postfach 151141, 66041 Saarbrücken UR - http://scidok.sulb.uni-saarland.de/volltexte/2008/1461 ER -