Soviet Powder Metallurgy and Metal Ceramics

A study was made of the effect of composition on the surface tension of nickel-silicon alloys with up to 50 at.% Si. An inflection was observed on the σ isotherm at 28 at.% Si. The unusual shape of the surface tension versus composition curve is ascribed to a microheterogeneous structure of the melt and the presence in the latter of short-range order.

Nickel powder obtained by reduction from Cuban production waste (PNVK) exhibits good compressibility. Difficult-to-reduce oxide impurities decrease its low-temperature sintering “activity” compared with carbonyl powder and powder reduced from pure nickel oxides. As a result, for sintering PNVK powder higher temperatures (1100–1200°C) are required.

During rapid hardening of 01N17K12M5T steel there are significant changes in its structure and properties. The grain size becomes finer, there is a reduction in the degree of chemical inhomogeneity, and the hardness increases. Foils produced by the “hammer and anvil” method have an extremely dispersed martensitic structure with a martensitic plate width of 0.3–0.4 μm and a hardness of up to 10,000 MPa, which, is an indication of the decisive role of interfaces in strengthening of metal. Rapidly hardened foils may be briquetted at room temperature to a relative density of 85%. Subsequent high-velocity pressing increases the relative density of the briquettes to 95%. The nonuniform temperature distribution and high rate of deformation accompanying high-velocity pressing leads to nonuniformity in the phase transformation and strengthening of chemical inhomogeneity of the material. The high rate of cooling after loading fixes at room temperature the martensitic body-centered cubic phase and austenite of two types differing in chemical composition and accordingly in temperatures of the martensite transformation. The structural and chemical inhomogeneity of 01N17K12M5T steel introduced by a shock wave is so significant that it is not completely eliminated after a 1-h hold at the austenitizing temperature.

The effect of hydrogen heat treatment on impurity contents in aluminum nitride powder of different fineness is studied. Thermodynamic calculations are provided pointing to the possibility of forming gaseous hydrogen compounds with impurity elements. It is established that as a result of treatment the contents of impurity elements, i.e. oxygen, carbon, sulfur, are reduced by a factor of 3-13. The optimum hydrogen heat treatment regime is determined: a temperature of 1323 K and an exposure time of 30 min.

(Ti, W)C–Ni cermets with different contents of Mo2C were produced by the spark plasma sintering (SPS) method. The grain size (GS), composition of ceramic phases, and mechanical properties of the sintered cermets were investigated. The amount of Mo2C had a significant influence on the microstructure and mechanical properties of as-prepared cermets. GS and fracture toughness (KIc) were decreased as a result of increasing the amount of Mo2C. By increasing the amount of Mo2C, the transverse rupture strength (TRS) and hardness (HRA) were enhanced. However, above 10 wt.%, the TRS was reduced. The conventional black cores observed by field-emission scanning electron microscopy (FE–SEM) in backscattered electron imaging (BSE) in (Ti, W)C–Ni cermets will be partially turned into some white cores which contain higher Mo, except for Ti and W elements, when content of Mo2C reaches ~15 wt.%. Batch mechanical tests indicate that cermets with some white cores have refined microstructure and higher hardness, but relatively lower transverse rupture strength (TRS) and fracture toughness (KIc) at room temperature.