| Mixtures of single crystalline spherical W particles of 200-250/μm diameter, fine W powder of 10μm size, and fine Ni powder were sintered at 1670℃. Depending on the amount of liquid phase present complete densification is obtained after different sintering times and this is always accompanied by pronounced growth of the large W spheres at the expense of the smaller grains. With limited amounts of liquid phase present, under the influence of the densification force arising from the pores, grain growth occurs preferentially away from the areas where the grains are in close contact. Thus grain shapes accomodation occurs during growth and this enables complete densification. In order to achieve rapid densification during liquid phase sintering, conditions of rapid grain growth are therefore favourable. Zusammenfassung Mischungen aus einkristallinen spha¨rischen W-Teilchen mit 200-250㎛ Burchmesser, feinem W-Pulver von l0㎛ Gro¨Βe und feinem Ni-Pulver wurden bei 1670℃ gesintert. Je nach dem Anteil an flu¨ssiger Phase wird vo¨llige Verdichtung nach verschiedenen Sinterzeiten erreicht. Die Verdichtung ist immer von ausgepra¨gtem Wachstum der groΒen W-Kugeln auf Kosten der kleineren Ko¨rner begleitet. Fu¨r begrenzte Mengen an flu¨ssiger Phase erfolgt unter der Wirkung der Triebkraft fu¨r Verdichtung das Wachstum der Ko¨rner bevorzugt abseits der Stellen, wo die Ko¨rner in nahem Kontakt stehen, Dadurch kommt es wa¨hrend des Kornwachstums zur Gestaltsangleichung der Ko¨rner, welche vo¨llige Verdichtung ermo¨glicht. Um rasche Verdichtung wa¨hrend des Flu¨ssigphasensinterns zu erzielen, sind deshalb Bedingungen fu¨r rasches Kornwachstum besonders gu¨nstig Spherical tungsten particles of 200-250㎛ size were also sintered in liquid nickel at 1640℃. The tungsten spheres grow into their neigh Lours as reported earlier by Huppmann, Riegger, and Petzow. The growing phase etches differently from the original pure tungsten spheres and is a tungsten alloy phase saturated with 0.15wt.-% Ni. It is therefore suggested that the driving force for this process is the chemical potential difference between the equilbrium W-Ni alloy and the pure tungsten phase being dissolved. The growth appears to increase linearly with sintering time and the measured growth rate is 1.1 ㎛/min. The activation energy is (3.7±0.5)×10³J/㏖. The kinetics appears to be controlled mainly by the solution-reprecipitation reaction at the moving boundaries, This chemically driven solution-reprecipitation process is expected to enhance the coalescence and growth of grains during sintering in the presence of liquid or solid elements which form alloys with the mayor component. |
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