Kinetics of reduction and alloy formation in the W-Ni-Fe system

The work described below was undertaken with the aim of studying in detail the simultaneous reduction of mechanical mixtures of oxygen-containing tungsten, nickel~ and iron compounds and of the processes of alloy formation in this system. As starting materials tungstic anhydride, WO 3 (TU 25-68), Fe203 (GOST 4466-70), and NiCO 3 (GOST 4173-66) were employed. In the preparation of mixtures the amounts of the components were chosen so as to obtain alloys of the following compositions (in wt.%): 99.5W-0.5Ni, 99W-1Ni, 90W-7Ni3Fe, 90W-10Ni, and 90W--10Fe. Experimental mixtures were prepared by mechanical mixing in distilled water. The effect of mixture composition on the kinetics of the reduction process was investigated gravimetrically. Reduction was performed in an automatic gravimetric apparatus, using dried hydrogen with a dew point not higher than- 50~ [3]. The quartz spiral had a sensitivity of 13.2 �9 10 -3 g/mm. The weight of each sample was 0.2-0.3 g. To minimize the effect of heating-up to the reduction temperature on the initial stage of the process, the required temperature was established in a vacuum, after which the reactor with the sample was filled with hydrogen. The processes of alloy formation under conditions of reduction and subsequent annealing at temperatures of 700-1450~ in hydrogen were studied by subjecting samples to x-ray phase analyses and electron probe microanalyses. X-ray phase analyses were carried out, using Cu K s radiation, in URS-55 and DRON-0.5 diffractometers. Error in lattice parameter determinations did not exceed 0.003 kX. For electron probe microanalysis a Microscan-5 apparatus, made by the Cambridge Instruments Co. of England, was used. The electron probe was advanced slowly along a polished surface of the specimen. The x radiations of atoms of various elements were simultaneously recorded by two spectrometers on chart paper moving at the same speed as the probe. The intensities of the recorded Ni Ks, Fe K s, and W L~ x radiations were employed for calculating the concentrations of Ni, W, and Fe in the phase constituents of alloy specimens. The calculation program, in which corrections were introduced for atomic numbers, x-radiation absorption, and selective x-ray fluorescence excitation [4], enabled concentrations of elements from boron to uranium to be determined from the K s or L~ x-radiation series. Elements at tungsten grain boundaries were determined in fractures of specimens subjected to transverse rupture tests. In these determinations a Stereoscan $4-10 scanning electron microscope with a Si (Li) x-radiation detector and a Link System (England) analyzer was used. In preliminary stepwise reduction experiments tungsten trioxidewith a specific surface, as measured by the nitrogen thermal desorption method in a gas-chromatograp hic apparatus [5], of 0.1 m2/g was found to be reduced at an appreciable rate under kinetic conditions only at temperatures of 450~ and higher. Below 450~ the rate of reduction of coarse WO 3 was low. In view of this, the effect of additions of the iron-group metals on the reduction process was studied at temperatures both below and above 450~ Nickel and iron were reduced from NiCO 3 and Fe203, respectively, at 300-350~ - a temperature range in which, according to the results of special experiments~ the above compounds are almost completely reduced to Ni and Fe. At 300-350~ tungsten was not reduced from its trioxide.