Václav Sklenička

The creep behavior of the cast nickel-based superalloy EEQ111 was investigated over a range of temperatures and applied stresses relevant to high-temperature turbine applications. Constant-load creep tests were performed to evaluate the evolution of creep strain, minimum creep rate and time to fracture. The dependence of creep resistance on stress and temperature was analyzed to identify the dominant deformation mechanisms governing the creep response. The experimental results reveal a pronounced sensitivity of the minimum creep rate to both applied stress and temperature. The stress exponent and apparent activation energy for creep were determined and discussed in relation to the underlying microstructural features of the alloy. The observed trends indicate that creep deformation in EEQ111 is controlled by thermally activated dislocation processes, strongly influenced by the presence and stability of γ′ precipitates and grain-boundary carbides. At higher temperatures and lower stresses, a transition in the creep mechanism is suggested by changes in the stress exponent and damage evolution behavior. The results contribute to a more detailed understanding of the creep mechanisms operating in cast nickel-based superalloys and provide insight into the role of microstructural stability in controlling long-term creep performance.

The creep behavior of the cast nickel-based superalloy EEQ111 was investigated over a range of temperatures and applied stresses relevant to high-temperature turbine applications. Constant-load creep tests were performed to evaluate the evolution of creep strain, minimum creep rate and time to fracture. The dependence of creep resistance on stress and temperature was analyzed to identify the dominant deformation mechanisms governing the creep response.The experimental results reveal a pronounced sensitivity of the minimum creep rate to both applied stress and temperature. The stress exponent and apparent activation energy for creep were determined and discussed in relation to the underlying microstructural features of the alloy. The observed trends indicate that creep deformation in EEQ111 is controlled by thermally activated dislocation processes, strongly influenced by the presence and stability of γ′ precipitates and grain-boundary carbides. At higher temperatures and lower stresses, a transition in the creep mechanism is suggested by changes in the stress exponent and damage evolution behavior.The results contribute to a more detailed understanding of the creep mechanisms operating in cast nickel-based superalloys and provide insight into the role of microstructural stability in controlling long-term creep performance.

The origin of the thermal creep of zirconium cladding tubes in all light water reactors is still the subject of deep confusion and intricate controversies. The reason for this inconvenient situation is obviously that microstructural processes in thermal creep remain poorly understood and this is due to the relatively small number of studies that have been carried out. In this study uniaxial creep tests in tension of zirconium alloy cladding tubes in the as-received and pre-hydrided states are followed by metallographic analysis of the as-received and crept specimens by light microscopy and SEM to explain the observed high-temperature creep behavior of the tubes.

This work investigates the creep behavior of severely deformed commercial aluminum. The commercial aluminum was processed by helical rolling (HR) and equal-channel angular pressing (ECAP) at room temperature. During these processes, the equivalent strain up to about 4 was imposed into the as-received material. The creep testing at 200 °C revealed that HR and ECAP significantly increased the time to fracture compared to the as-received material. The stress dependences showed that the value of stress exponent n decreased with the value of the imposed strain. The stress-change tests showed that as-received and severely deformed states exhibited different recovery rates after unloading. The microstructure analysis showed that creep behavior was influenced by the microstructure formed during severe plastic deformation. The relationships between creep behavior and microstructure in the investigated states are discussed.

Damage and fracture processes in high temperature creep of an investment cast B1914 Ni-based superalloy with the increased amount of boron to 0.08wt.% for high temperature applications were analysed. Constant load creep tests in tension were conducted at temperatures from 800 to under applied stress ranging from 150 to 700 MPa. The microstructure of fractured specimens was investigated by scanning electron microscope Tescan equipped with an electron-back scatter diffraction. Microstructure investigation showed that the microstructure of the B1941 superalloy consists of a gamma (γ) phase with a dendritic structure and gamma prime (γ ́) phase with a cuboidal shape. Precipitates of γ ́and a lamellar eutectic, composed of γ/(Mo,Cr,Ni)₃B_2, were identified in the interdendritic region. Creep damage and fracture are closely connected with decohesion of the interface between M₃B₂ boride and matrix.

This work deals with the influence of the processing technology of ultrafine-grained titanium by the application of severe plastic deformation (SPD) on its degradation processes in creep at elevated temperatures. Commercial grade titanium heats with different content of carbon were processed by multi-stage rolling. Creep tests under constant tensile stress were performed in a protective atmosphere of argon at a temperature of 673 K with application of different stress levels. The microstructure of the fractures creep specimens were analysed by metallographic and fractographic analyses using scanning and transmission electron microscopy (SEM, TEM). Under the same creep loading conditions the heat with higher content of carbon exhibited better creep resistance due to stronger effect of precipitation hardening by titanium carbides.

The commercial purity of VT1-0 titanium was processed by the rolling process and executed at elevated, room, and cryo-temperatures. These processings led to the formation of an ultrafine-grained microstructure, with the mean grain size at a nanometer level. Some of these materials were statically annealed at a temperature of 823 K for 1 h, which led to significant subgrains and grain coarsening. The constant load creep tests in tension were carried out in argon on all states of materials, at temperatures of 648–723 K and different ranges of applied stresses. From the value of the steady-state creep rate, the control creep mechanisms were determined. The microstructure analyses were carried out via SEM and TEM. It was found that titanium prepared at elevated and room temperatures have a higher creep strength than titanium prepared at cryo-temperatures. Furthermore, the post-SPD —annealing led to a significant decrease in the creep properties. The influence of the preparation temperature on the difference of the creep behavior were discussed and explained using the microstructure analyses of the tests’ samples.