Simonov

pp.30-44

This work presents results of thermodynamic studies and searching for a rational chemical activation dopant for deposition of tantalum from the gaseous phase onto surfaces of parts of gas-turbine engines made of heatproof nickel alloys in order to increase the diffusion stability of heatproof aluminide coatings. Calculations showed a possibility of using nickel dichloride for tantalum and tantalum-chromium coating of nickel alloys. During this investigation several restrictions on the number of additional alloying elements in the saturating mixture were discovered. An effective amount of chromium for the combined process of tantalum-chromium coating was also determined.

This work presents results of thermodynamic studies and searching for a rational chemical activation dopant for deposition of hafnium from the gaseous phase onto the surface of gas-turbine engine’s parts made of heatproof nickel alloys in order to increase diffusion stability of heatproof aluminide coatings. Calculations showed impossibility of using a traditional activation dopant, that is nickel dichloride, for hafnium coating because liberation of gaseous hafnium compounds occurs at temperatures which are beyond reach for modern equipment for thermochemical treatment. During this investigation it was established that ammonium chloride could be used as an activation dopant. If hafnium consists one fifth of ammonium chloride in the saturating mixture, amount of gaseous hafnium chloride reaches its maximum.

In this work the authors calculated phase transformation kinetics in non-equilibrium solid solutions of binary alloys such as Fe-2 Cr (at.%), Fe-4 Cr (at.%), Fe-24Cr (at.%). In the literature [1] there is information on the impact of pre-heat treatment (PHT) on the size of iron nitrides after nitriding. Using the “Termodin” program [2, 3], they calculated cluster sizes, diffusion coefficients at temperatures of isothermal ageing, time of nucleation and time of the growth of the clusters, nucleation rate and growth rate of clusters which were formed at initial stages of decomposition of the supersaturated solid solution. Formation kinetics of a given volume fraction of the σ phase was also calculated. The obtained results allowed to control the structure of alloys, depending on heat treatment

The authors propose an energy method for calculating degradation rate of a non-equilibrium structure while in opeartion. With the use of differential scanning calorimetry it is possible to determine the presence of phase transformations in case of alloy aging, decay of supersaturated solid solutions, secondary crystallization and also assess heat content of the system during the transient process from the non-equilibrium state into the thermodynamic equilibrium state with extra energy release. Differentiating the current value of extra energy with respect to time, it’s also possible to determine the value of the chemical activity coefficient during phase transformations in alloys at working temperatures. Estimated dependence of the rate of energy dissipation χ on the initial value of a superfluous chemical potential was derived for the first time.

 The authors calculated the effect of temperature and time parameters of thermal pre-treatment of structural steels at the initial stages of formation of a fine structure before nitriding. With the program "Termodin" developed at the department "Materials science" in Bauman MSTU the authors calculated the critical radius of the nuclei of iron nitride, diffusion parameters, kinetic parameters of nucleation and growth phase Fe4N, time of formation of a given volume fraction of γ^/ phase during isothermal holding in the high holiday for binary alloys Fe-5, 6% N by mass. Kinetic curves of decomposition of the supersaturated solid solution with formation of a given volume fraction of γ^/ phase were built. The authors also calculated prediction modes of thermal pre-treatment on the size of hardening phases.

This article deals with the results of a thermodynamic analysis of nitrides phase state, their quantity in the alloy HA30-VI when the quantity of nitrogen varies in the initial furnace-charge at different melting stages. Thermodynamic calculations allow to estimate the phase composition of liquid melt and crystal nickel alloy HA30-VI. It was investigated whether titanium nitride, hafnium and carbides in equilibrium conditions were part of that alloy. Minimal permissible quantity of nitrogen required to eliminate the formation of nitrides was calculated.

Nitrogen behavior under different thermo-time modes of the melting alloy HA30-VI in the induction furnace was investigated; optimal temperature of melted nitrogen depuration was determined. The negative influence of nitrogen on the formation of parasitic grains with random orientation near the surface of mono-crystal casting was shown. Non-uniform distribution of nitrogen with the height of cast mono-crystals was determined.

By the method of differential scanning calorimetry the authors studied phase transformations in the binary alloy Ni-Al (7%Al) formed at the disintegration of a supersaturated solid solution. It was shown that formation process of phase Ni3Al is connected with hidden crystallization heat release.

Computation of the phase change sequence during the incubation interval of oversaturated solid solution of Ni-Al alloy breakage      was carried out (93% Ni, 7% Al by weight). Quantity of conversion stages in the process of oversaturated solid solution breakage, atom’s superfluous chemical potential with every metastable phase equilibrium, mass content of precipitated phases, newly formed phases’ chemistry, gradient of Al atoms’ chemical potential between solution and nucleating phases could be determined with formed digram of non-equilibrium phase states.