Jan Vřešťál

Diffusion processes in the FeNi system

Abstract In this paper we present the experimental results of a study on the self-diffusion coefficients of both components in the f.c.c. phase of the FeNi binary system; these results were obtained using the radioisotopes 59 Fe and 63 Ni together with the measurements of the interdiffusion coefficient in this system obtained with the aid of an electron micro-analyser. Diffusion measurements were supplemented by thermodynamic mixing data for the same phase measured by the vapour pressure technique; the two sets of data enabled us to verify Darkens theory of interdiffusion. Experimental results of 185 W impurity diffusion in the f.c.c. phase of the FeNi system are also presented.

Diffusion in FeNiCr alloys with an F.C.C. lattice

The volume self-diffusion coefficients of 51Cr and 59Fe in 16 FeNiCr alloys with an f.c.c. lattice were measured using the modified Gruzin method in the temperature ranges 1298–1548 K for iron and 1286–1536 K for chromium. The self-diffusion coefficients, pre-exponential factors and activation energies for iron and chromium are tabulated. The concentration dependence obtained cannot be expressed by a simple mathematical relation; it is of higher order with a local minimum near the composition Fe-55 wt.%Ni-20 wt.%Cr. A critical analysis of the results obtained as well as previous knowledge made it possible to find the following relation between the self-diffusion coefficients of components which are valid over the whole temperature and concentration ranges examined: DNi∗: DFe∗: DCr∗ = 1:1.2:2 From the thermodynamic activity measurements, values of the thermodynamic diffusion factors in the ternary FeNiCr system and further values of the interdiffusion coefficients in this system were calculated. In this way a complete description of the diffusion processes in the ternary system was obtained. A comparison of the calculated interdiffusion coefficients with the experimental values found by other researchers was made.

Ag-Cu colloid synthesis: bimetallic nanoparticle characterisation and thermal treatment

The Ag-Cu bimetallic colloidal nanoparticles (NPs) were prepared by solvothermal synthesis from metalloorganic precursors in a mixture of organic solvents. The nanoparticles were characterized by dynamic light scattering (DLS) and small angle X-ray scattering (SAXS). The properties of metallic core and organic shell of the nanoparticles were studied by direct inlet probe mass spectrometry (DIP/MS), Knudsen effusion mass spectrometry (KEMS), double-pulse laser-induced breakdown spectroscopy (DPLIBS), and differential scanning calorimetry (DSC). The transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were used for particle characterization before and after thermal analysis. The experiment yielded results that were for AgCu nanoparticles for the first time. The detected liquidus temperature has been compared with the prediction obtained from calculation of the phase diagram of Ag-Cu nanoalloy. The experimental results show that of near-eutectic composition AgCu nanoparticles possess the fcc crystal lattice. Surprisingly, spinodal decomposition was not observed inside the AgCu nanoparticles at temperatures up to 230°C. The depression of the eutectic AgCu melting point was calculated but not observed. The eutectic AgCu microparticles are formed before melting.

Phase changes in superaustenitic steels after long-term annealing

A structural study was performed on the austenitic steels Avesta 254 SMO and Avesta 654 SMO after annealing at 700 C for 500, 3188 and 6170 h. Both Avesta steels initially show an unexpectedly large amount of the Laves phase, followed by a relative slow development of the Sigma phase with equilibrium apparently not yet reached after 3188 h. Thermodynamic calculations confirm that the driving forces for alternative precipitates are very similar thus making it easy to form metastable precipitates that only change very slowly to the equilibrium state. TTT calculations also comnfirm that the Laves phase precipitates earlier than the Sigma phase as the temperature is lowered.

Concentration dependence of the diffusivity of carbon in (γ-Fe)C and in NiC solid solutions

Abstract A semiempirical model expressing the variation in carbon diffusivity with concentration is proposed. It is based on the conclusions resulting from Darkens experiments and on the theory of thermodynamics of irreversible processes. The derived equations are compared with the data of Smith and of Parris and McLellan for carbon diffusion in FeC austenite and in NiC solid solutions. The numerical values of the fitting parameters obtained in these comparisons yield the concentration variation in the correlation factor f ( N C ) for carbon self-diffusion in FeC austenite. The relation of the model to other theoretical treatments is discussed. The semiempirical basis of the model makes it possible to apply the derived equations to polycomponent austenitic systems generally.

Ab initio study of energetics and magnetism of sigma phase in Co-Mo and Fe-Mo systems

We analyse, from first-principles, the energetics and magnetic ordering of sigma phases in Co-Mo and Fe-Mo systems. Total energy differences between the sigma phase and Standard Element Reference (SER) structures are calculated in the whole concentration range at equilibrium volumes by means of the linear muffin-tin orbitals method in the atomic-sphere approximation (LMTO-ASA), the full-potential linearised augmented-plane waves (FLAPW) method and the pseudopotential approach. They are compared with the enthalpy of formation of sigma phase obtained from the phase equilibria calculations at higher temperature based on the semiempirical CALPHAD (CALculation of PHAse Diagram) method. It turns out that the binary sigma phases are more stable than the weighted average of the sigma phase of elemental constituents and that this stability for Fe-Mo is higher than for Co-Mo. On the other hand it was found that the binary sigma phases do not exhibit any stability with respect to the weighted average of the SER structures. The magnetic configurations in all systems are investigated and the stabilizing effect of magnetic order in sigma phase at 0 K is presented. It turns out that the atomic magnetic moment strongly depends on the type of occupied sublattice and total composition of the alloy.

Microstructure of super-austenitic steels after long-term annealing

Abstract Longevity of materials, especially technological parts, is influenced by microstructure development during long-time annealing of such parts during their industrial service. Important relations between the volume fractions of phases and the time of service, especially the precipitation of intermetallics need to be revealed. The present study describes the time dependence of the structure development of two corrosion resistant steels with chromium content of 27 and 32 wt.% and various additions of nickel, molybdenum, manganese, silicon, copper and nitrogen. The microstructure was studied on samples annealed at 700 °C for 500, 3 188 and 6 170 h, revealing the kinetics of the equilibrium process. The homogeneity of samples was checked before heat treatment. The experimental microstructures are compared with the predicted phase composition. The effect of using different databases is also discussed.