T. Kozieł

The Influence of Cooling Rate During Crystallization on the Effective Partitioning Coefficient in High-Entropy Alloys from Al-Ti-Co-Ni-Fe System

An influence of two different cooling rates on the microstructure and dispersion of the components of high-entropy alloy from Al-Ti-Co-Ni-Fe system has been examined. For investigated alloys, the effective partitioning coefficient has been calculated. This factor indicates the degree of segregation of elements and allows for the specification of the differences between dendrites and interdendritic regions. The obtained results allow for the conclusion that the cooling rate substantially affect the growth of dendrites and the volume fraction of interdendritic regions as well as the partitioning of elements in the alloy. Furthermore, the obtained results made it possible to compare the influence of the cooling rate and the chemical composition on the dispersion of the alloying elements.

Effect of creep deformation at 625°C on microstructure development of VM12 steel

Abstract The microstructure in the new COST martensitic 12% chromium steel VM12 developed for advanced power stations operated around 625 – 650°C has been investigated in the ‘as-received condition’ and after long-term creep exposure at 625°C up for around 29,000 h. Quantitative TEM analyses of VM12 steel microstructure were undertaken to determine the dislocation density within the sub-grain, the width of the martensite laths/sub-grains and the particle parameters (shape, size, distribution). Phase identification was performed using electron diffraction and X-ray spectrometry. The TEM results showed that thermal exposure and creep deformation at 625°C caused some changes of the VM12 steel microstructure. The statistical quantitative analyses of the microstructural parameters, showed that the sub-grain size and the size of M23C6 precipitates slightly increased after exposure up to 29,000 h. In the creep-tested specimens, microstructural changes were much more pronounced than in the thermally-exposed specimens. This paper discusses the influence of the VM12 microstructure changes on long-term rupture strength.

The microstructure of melt-spun alloys with liquid miscibility gap

Effect of the melt ejection temperature on the microstructures of (FexCu1-x)62Si13B9Al8Ni6Y2 (x=0.48, 0.60 and 0.71) alloys was investigated. Rapid cooling of the examined alloys from the temperature range conventional to single-phase amorphous alloys brought about a non-uniform microstructure due to a liquid/liquid phase separation prior to cooling. The systems with a miscibility gap need to be heated over a critical temperature where homogeneous liquid exists. Microstructures of the ribbons melt spun from the homogeneous melt region are characterized by presence of the spherical precipitates distributed in homogeneous matrix. Both, precipitates and matrix, can constitute either the Fe-rich or the Cu-rich phases depending on the alloy composition. The studies confirmed amorphous nature of the Fe-rich phases, both matrix and precipitates whereas the Cu-rich liquid crystallized due to lower glass forming ability.

VM12 Steel for Advanced Power Generation Plants – Metrology of the Precipitates by Electron Microscopy

A new high chromium martensitic steel, VM12, was recently developed for advanced coal-fired ultra-supercritical steam power plants. A very important factor in maintaining high creep resistance during service exposure is its stable microstructure, in particular high dispersion particle strengthening, which slow down the recovery and softening processes of the matrix. Quantitative characterisation of the precipitates in the VM12 steel as received condition and after creep tests up to about 30000 h at 625 °C using TEM was carried out.