Utkin S. Phase Diagrams and Alloy Properties in Systems Molybdenum– Iron–Boron and Molybdenum–Nickel–Boron

The scope of this thesis is investigation of phase equilibria in the Mo–Ni–B and Mo–Fe–B ternary systems by a number of physical chemistry analysis techniques. There were studied arc-melted alloys, both as-cast and annealed at subsolidus temperatures (30–50 °С lower than incipient melting temperature). Experimental study resulted in construction of phase diagrams for the Mo−Ni−B and Mo−Fe−B systems for the first time as solidus surface projections and melting diagrams, as well as Scheil's reaction schemes for alloy crystallization in the boron content range up to ~ 50 at. %. It was established for the Mo−Ni−B system that known borides Mo2NiB2, Mo3NiB3 and Mo3Ni10B11 as well as Mo2FeB2 and MoxFe3-xB are stable in the solidus surface and melt incongruently. The homogeneity ranges on metal content were measured thoroughly for the first time, which are equal from 20 to 21,4 at. % Ni for Mo2NiB2 and from 13 to 27 at. % Fe for Mo2FeB2. It was firstly shown that τ1 ternary phases Mo2NiB2 and Mo2FeB2 form via the quasibinary peritectical reactions Lp2 max + α-MoB ↔ τ1 at temperatures not lower than 1793 °С and 1900 °С respectively and are involved in the transitory (peritectic-eutectical) U-type (Übergangsreaktion) phase transformations at lower temperatures. It was shown for the first time that the other transitory (peritectoid-eutectoid) phase transformation Mo2B + MoNi ↔ (Mo) + Mo2NiB2 occurs at 1293 °С in the solid state and the joint solubility of boron and nickel in the (Mo) phase is reached to anomaly great values as 6 and 3,0 at. % respectively. It was proved experimentally that in the constituent binary system Fe−B the Fe3B boride is definitely metastable. At the same time in the Mo−Fe−B ternary system it was shown that in the solidus surface at 1160 °С there is the ternary compound MoxFe3-xB (τ2) containing 1,3−2,0 at. % Mo. This boride is formed via a peritectic reaction and stable up to ~ 1050 °С. It exists as two modifications of crystal structural types Ti3P and Ni3P. As a result of research work within CALPHAD approach for the first time the thermodynamic descriptions of the Fe−B and Mo−Fe−B systems were elaborated, which take into account allotropic transformation α-FeB ↔ β-FeB at 1212 °С and ferromagnetic transformations of α-FeB and Fe2B boride phases as 2-nd order phase transitions. The metastable phase diagram of the Fe−B system including (γ-Fe) + Fe3B eutectic was calculated. It was constructed for the first time The thermodynamic description of the Mo−Fe−B system in the whole concentration and temperature range was firstly elaborated that allows calculating phase diagrams and Scheil's reaction scheme including both 1-st and 2-nd order phase transformations. Due to their high microhardness, abrasive resistance and high-temperature strength, two-phase alloys which consist of ternary boride and binding metal phase based on the metals of iron group are paid attention at development on their basis novel high temperature strength and corrosion-resistant materials, brazing spelters and wear-resistant coatings. It was also revealed regularities in the constitution of the Mo−{Fe, Co, Ni}−B ternary phase diagrams and predicted constitution of the Mo−Co−B system solidus surface.