Physicochemical properties of ternary Fe-Si-Nb and Fe-Al-Nb metallic systems

The Russian Federation has a sufficient number of promising deposits of niobium raw materials which can satisfy the niobium and tantalum demands of Russian metallurgical enterprises for many decades. Ferroalloy technologists are faced with the difficult tasks of developing from various types of ore raw materials not only effective processes for its processing but also new acceptable rational compositions of niobium-containing ferroalloys. The chemical composition of niobium ferroalloy should, on the one hand, correspond to the product obtained by benefication (concentrate) and, on the other hand, satisfy the requirements of steelmakers for ferroalloys intended for microalloying niobium steel. To develop rational compositions of new niobium-containing ferroalloys in this work the physicochemical characteristics (which include crystallization temperature and density) of alloys containing 10-50% Nb, 10-40% Si, and 5-30% Al were studied. Two-component Fe-Nb metal alloys have a rational crystallization onset temperature (<1400 °С) only when the niobium content is not more than 10%. To achieve rational crystallization onset temperatures it is necessary to use complex alloys with silicon and aluminum. Studies have shown that a decrease in the crystallization onset temperature of complex niobium alloys occurs when the niobium content decreases with an increase in the concentration of silicon or aluminum. Three-component alloys Fe-Si-Nb and Fe-Al-Nb with a content of 15-20% Nb, 32-40 Si% or 12-30% Al belong to the category of low-melting ferroalloys. To achieve rational density values light metals such as silicon or aluminum must be introduced into a two-component system. The studied three-component alloys with a content of 25-40% Si or 15-30% Al have rational density values both from the point of view of their production and application to the processing of steel melt. The best physicochemical characteristics providing high service properties are possessed by complex niobium (15-20% Nb) FeNbSi alloys with 32-40% Si and FeNbAl with 15-30% Al which are recommended for widespread use in ladle microalloying of steels.

Reordering and Crystallization of Silicon Carbide Amorphized by Neutron Irradiation

ABSTRACTDensification and crystallization kinetics of bulk SiC amorphized by neutron irradiation is studied. The temperature of crystallization onset of this highly pure, fully amorphous bulk SiC was found to be between 875-885°C and crystallization is nearly complete by 950°C. In-situ TEM imaging confirms the onset of crystallization, though thin-film effects apparently alter the kinetics of crystallization above this temperature. It requires >1125°C for complete crystallization of the TEM foil. Annealing at temperatures between the irradiation and crystallization onset temperature is seen to cause significant densification attributed to a relaxation, or reordering, of the as-amorphized structure.

Synthesis of a bulk amorphous alloy by consolidation of the melt-spun amorphous ribbons under high pressure

Based on the experimental observation that high pressure will considerably enhance the crystallization onset temperature of amorphous alloys, an attempt was made to consolidate the melt-spun amorphous ribbons into fully densed three-dimensional bulk amorphous materials under high pressures. An amorphous Ni69Cr7Fe2.5Si8B13.5 (at. %) alloy was used as a model material. Under a pressure of 1.5 GPa, the crystallization onset temperature was found to be increased by about 40 K, resulting in a widened supercooled liquid temperature region (about 68 K) beneath the onset of crystallization. The high pressure consolidation of the amorphous ribbons in this temperature region yielded bulk amorphous compacts with the same density of the melt-spun ribbons. This achievement was attributed to the significant homogeneous viscous flow of materials in the supercooled liquid state that could be maintained at higher temperatures during the high pressure compaction.