The influence of microstructure on the sintering process in crystalline metal powders investigated by positron lifetime spectroscopy: II. Tungsten powders with different powder-particle sizes

• Published in: Journal of physics. Condensed matter Vol. 11; no. 7; pp. 1787 - 1806
• Main Authors: Staab, T E M, Krause-Rehberg, R, Vetter, B, Kieback, B, Lange, G, Klimanek, P
• Format: Journal Article
• Language: English
• Published: Bristol IOP Publishing 22.02.1999; Institute of Physics
• Subjects: Applied sciences; Cross-disciplinary physics: materials science; rheology; Exact sciences and technology; Materials science; Materials synthesis; materials processing; Metal powders; Metals. Metallurgy; Physics; Powder metallurgy. Composite materials; Powder processing: powder metallurgy, compaction, sintering, mechanical alloying, and granulation; Production techniques; Particle size; Metal powder; Metallography; Positron annihilation; Plastic deformation; Crystal defect; Tungsten; Powder compaction; Defect structure; Grain size; Annealing; Physical radiation effects; Recovery(properties); Electron beam; Transition metal; Vacancy cluster; Shrinkage; Experimental study; X ray diffraction; Lifetime; Sintering; Recrystallization; Models; SEM; Microstructure
• ISSN: 0953-8984; 1361-648X
• DOI: 10.1088/0953-8984/11/7/010

Compacts of tungsten powder with five different powder-particle sizes (from 0.4 mu m to 75 mu m) are subjected to pressureless sintering. We investigate the change in microstructure during the sintering process by positron lifetime spectroscopy. So as to be able to distinguish between defects having the same positron lifetime, we investigate their kinetics when the sample is annealed. In particular, we consider the annealing out of vacancy clusters after low-temperature electron irradiation, as well as recovery and recrystallization of a tungsten sheet, in as-manufactured form. Making measurements on uncompacted powder, we find an increasing fraction of positrons annihilating in surface states with decreasing powder-particle size. The powder-panicle and grain sizes (influencing the x-ray domain size) are monitored additionally by means of metallography and x-ray diffraction. We find that all of the methods give results in agreement with each other. The small grain sizes at lower temperature, about one fifth of the powder-particle size, cause positrons to annihilate at grain boundaries, leading to vacancy-cluster-like signals. At the intensive-shrinkage stage, there are certainly contributions from different shrinkage mechanisms. The observed shrinkage rates can be explained by Coble creep. It is possible that dislocations also play a role as vacancy sources and sinks, since the intensive-shrinkage stage occurs in a temperature region wherein recrystallization takes place.