Journal of the Japan Society of Powder and Powder Metallurgy

Based on the oxidation mechanism of the alloy, auther considered previously that Ni-Cr alloy is formed in binder phase and it may be responsible for a good corrosion-resistance and oxidation-resistance.
In the present work, process forming the alloy binder during the sintering was investigated by X-ray diffraction and thermomagnetic analysis. During the sintering below eutectic temperature, Cr₃C₂ decomposed into Cr₇C₃, Cr and carbon and the resulting Cr₇C₃ or Cr diffused rapidly in Ni phase compared with Cr₃C₂. As a result of such reaction the Ni-Cr alloy binder is formed, at the same time free carbon is formed in binder phase.
In the specimens sintered above eutectic temperature, it was also found on the basis of the lattice constant and Curie temperature of Ni solid solution containing Cr₃C₂ that the alloy phase with the properties as a nichrome is formed in the binder. Such binder phase was heterogeneous.
Formation of the alloy binder phase is desirable for the corrosion-resisting and the oxidation-resisting properties of the alloy but formation of graphite result a poor mechanical strength. Therefore elimination of graphite is important practically.

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The crystals of ammonium paratungstate were thermally decomposed in both air and vacuum, and the process of thermal decomposition of this salt and the formation of tungsten trioxide particles were studied by means of following measurements, (i) weight (ii) determination of the evolved ammonia gas, (iii) the differential thermal analysis, (iv) the X-ray analysis, (v) the surface area measurement and also (vi) the observation by electron microscope.
The result obtained are as follows;
(1) The dehydration of the crystal takes place completely below 250°C. The evolution of ammonia occurs in several stages extending to 420°C.
(2) The a-tungsten trioxide are obtained at the temperatures above 420°C by the decomposipion in air. The particle of a-tungsten trioxide consists of small unit particles which are linked with each other, forming an aggregate whose shape are resemble that of the mother salt.
(3) When the salts are decomposed in vacuum, 7-tungsten oxide are obtained at the temperature ranging from 400-500°C, and 8-tungsten oxide at the temperature higher than 500°C. The shape of the particle thus obtained was somewhat different from that of a-tungsten obtained in air.
Experimental results, and the considerations on the mechanism of decomospition and also the formation of tungsten trioxide are described in detail.

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