ndcrostructural changes and dissolution of carbides and Ti carbonitride in the Ni matrix. W(" was the most fawmtrrable for refinement of microstructure and gives the lowest rate of dissolution. Dissolution rates with HfC and TaC were 1.6 and 1.9 times greater. Core/rim structures were observed.
Mechanical properties of a hybrid cemented carbide composite B.R.Patterson eta[. (University of Birmingham, Birmingham, UK.) Int J. Refac. Metal/Hard Mater., Vol 19, No 4-6, 2ooi, 547-552. A (WC-Co)-Co hybrid cemented carbide composite was investigated. The material consists of granules of WC-Co in a Co matrix. Hardness and high stress abrasive wear resistance increase with decrease in %Co and decreased matrix %Co. Fracture toughness increased with increase m %Co in both matrix and granules. Increase in WCCo granule size increases wear resistance and fracture toughness. The material is shown to be superior to conventional WC-Co.
Microstructure and mechanical properties of ultra-fine grained hardmetals T.Sailer et al. (University of ErlangenNCirnberg, Erlangen, Germany.) Int J. Refac. Metal/Hard Mater., Vo119, No 4-6, 2ooi, 553-559. Studies to correlate mechanical properties and microstructure of ultra-fine grained, tip to 0.3mm, hardmetals are described. Different WC powders and binder systems, Co, Co-Fc, Fc-Ni and Co[-Ni-Fe, were investigated under cyclic bending loads and uniformly increasing loads. Behaviour under cyclic loading was sensitive to binder type. Samples with Co binder had high bend strength but high fatigue sensitivity Bend strength was lower for samples with the complex binders. It is implied that different damagmg mechanisms exist for Co and the complex binders.
Submicron and ultrafine grain hardmetals for microdriUs and metal cutting inserts (3.Gille et al. (H.C.Starck GmbH, Goslar, Germany.) Int J. Refac. Metal/Hard Mater., Vol 20, No i, 2002, 3-22. It is noted that, rccentl); demand for microdrills has greatly increased. The latest duvclopmctlts are reviewed and it is shown that proper doping and optimised processmg ha\e cxtendcd service lives. Details of a
new ultrafine WC powder grade, with a 0.lmm intercept in sintered WC-10%Co, a sintered Vicker's hardness of 2050 for lwt°/,,VC-Cr2C3 dopmg were presented.
transformation may be an intermediate stage in the growth of large WC crystals.
Production and characterisation of ultrafine tungsten carbide powders
S.Put et al. (Katholieke University Leuven, Heverlee, Belgium.) PM Science/Technol. Briefs, Vol 3, No 3, 2ooi, 14-17.
B.Zeiler, A.Bock. (Wolfram Bergbau- und HLitten GmbH., St.Martin, Austria.) Int J. Refac. Metal/Hard Mater., Vol 20, No 1, 2002, 23-30. The conventioal calcination-reduction-carbursation route for manufacture of WC powder is shown to be able to make powders with grain sizes below 0.5ram. Strict process control has given good powder uniformity and lot-to-lot reliabilit): Laboratory scale powders have been produced down to 0.15 to 0.2ram Characteristics of ultrafine grades are discussed. A wide range of techniques was used in characterisation of the WC powders.
Computational study of grain growth in cemented carbides J,Nishino et al. (Mitsubishi Materials Corporation, Ibaraki, Japan.) Int J. Refac. Metal/Hard Mater., Vol 20, No i, 2002, 31-40. Continuous and discontinuous grain growth in micrograined cemented carbides was investigated by Monte Carlo computer simulation. Ostwald ripening and grain boundary migration are assumed to occur. Effects of liquid phase fraction, grain boundary energy and an implanted coarse grain are considered. It is considered that these factors are important in consideration of grain growth. Good agreement with experiment is reported.
Formation of large tungsten carbide crystals during sintering of ultrafine tungsten carbide-cobalt W.D.Schubert et al. (Technicalo University of Vienna, Vienna, Austria.) Int J. Refac. Metal/Hard Mater., Vol 20, No i, 2002, 41-5 o. It is shown that during sintering of very fine grain WC powders, particle sizes of 100 to 200nm, large plate like WC crystals, above 20mm, may be formed. This growth may start in the solid state sintering stage and may reach 7ram before liquation. This early growth was noted preferentially in the surface zones where there is contact with the sintering atmosphere. The presence of twins in the crystals suggests that the growth of WC is defect assisted. It is also suggested that h carbides and their
Electrophoretic shaping of graded hardmetals
Electrophoretic deposition of compositionally graded hardmetals is described and discussed. It is possible to deposit WC-Co but not Co alone. Deposition of a graded hardmetal, with 6%Co on the 'hard' side and 17%Co on the soft, is reported. High density can be attained by solid or liquid state sintering but only solid state sintering, up to 1290°C, retained the graded structure.
New oxide-based sintering aids for microwave sintering of silicon nitride T.Ueno et al. (Osaka University, Ibaraki, Japan.) ].Japan Soc. Powder/Powder Metall.. Vol 48, No 6, 2ooi, 551-557. In Japanese. Si3N4 materials were microwave-sintered with additions of 5wt%Yb203-3%A1203 or 5wt%Y20.3-3%AI203. The material with Yb addition sintered to full density at 1600°C in microwaves but required 1850°C in conventional heating. The difference was less with Y additions, 1700°C and 1750°C respectively. It is deduced that the effectiveness of the sintering aid depends on the ratio of Yb:AI. 1600°C is close to the liquidus for Yb203-AleO3. The lower temperature for microwave sintering resulted in finer microstructures.
Densification of silicon nitride with alumina and yttria or ytterbia sintering aids T.Ueno et al. (Osaka University, Ibaraki, Japan.) J.Japan Soc. Powder/Powder Metall.. Vol 48, No 6, 2ooi, 558-564. In English. Densification and phase transformations in Si3N4, microwave sintered at 28 and 30GHz, with additions of 5wt%Yb203-3%A1203, 5wt%Y203-3%A1203 or 2.5wt%Yb2031.5%A1203 were investigated at heating rates of 5°C/min-I to 900°C/min I. The activation energy apparently decreased with increase in microwave power. Densification rate depended on the a to b transformation in the initial sintering stage. Microwaves did not affect the transformation rate. With the sraall additions sintering was not enhanced by increase in microwave power.