Wettability of refractory compounds by intermetallic compounds

Abstract

Hard alloys are widely used in equipment manufacture, but the scarcity of the main components (tungsten and cobalt) makes it necessary to prepare new tungsten-free hard alloys. To ensure a high level of alloy properties, the refractory compound must be wetted well by the liquid binder during sintering.Therefore, we carried out investigations to determine the main wetting characteristics, which are especially important in the preparation of modern alloys using intermetallic compounds as the binder.Data on the wettability of refractory compounds by intermetallic compounds are limited, and the general characteristics have not been developed.We investigated the wettability of titanium carbide TiC and titanium niobium carbonitride (Ti0.TvNb0.23C0.422 N0.48) by liquids of various intermetallic compositions.The re ~, fractory compounds used in the investigation were selected because of their wide use as the matrix of modern tungsten-free hard alloys.The composition of the intermetallic compounds was selected so that they differed in the average number of unpaired electrons per atom (i.e., in the s + d electron concentration for transition metals and the s + p electron concentration of nontransition metals), the average atomic weight, and the average specific volume (see Table I).Samples of refractory compounds with diameter 15 mm and height 5 mm were prepared from homogeneized powders after vibratory milling in an ethanol medium using balls made of hard alloy VK8 for 60 h with powder:ethanol:ball ratio 1:1:7. After semidry pressing, tablets were sintered for 1 h under vacuum at 1900~ and in a nitrogen atmosphere at 1800~The sintered samples were subjected to diamond polishing over the wetting surface. The pomosities of titanium carbide and carbonitride samples were 4-5 and 20-30%, respectively. The wetting samples were made from starting metallic powders with weight ratios corresponding to the chemical formula of the intermetallic compound.From the obtained batch, we pressed samples with diameter 3 n~n and height 5 mm, which were pre-sintered at 900~ under vacuum and then used for a study of the wetting process.Wetting was carried out in a vacuum chamber under pressure 10-I-10 -2 mm. The contact angle was determined by the sessile-drop method.The investigation was carried out in the range 920-1620~ It was determined that the minimum contact angle of titanium carbide and titanium niobium carbonitride changed in a complex relation to the electron concentration of the wetting intermetallic compounds (Fig. I). Intermetallic-compound alloys having an electron concentration of about 4.5 and 6.5 electrons per atom had poor wetting properties (large contact angles).It was expected that poor wetting should be observed for intermetallic compounds with an electron concentration of the order of 6-6.5 electrons per atom because they have the highest energy of bonding between metal atoms with the greatest number of s + d unpaired electrons.This assumption was confirmed, i.e., an increase of the bond energy in the intermetallic compou...