In this experimental investigation, the tensile creep behavior of commercial-purity molybdenum sheet at temperatures between 1300°C and 1700°C is critically evaluated, based upon experimental creep testing and microstructural characterizations. The high-temperature properties of molybdenum are of interest because there are many applications in which molybdenum and molybdenum alloys are used at elevated temperatures. Understanding of the creep mechanisms and the constitutive relations between stress and strain at elevated temperatures is needed in order to determine if molybdenum is an appropriate choice for a given high-temperature design application and to accurately predict its creep life. The creep behavior of two commercially-available grades of molybdenum was determined using short-term creep tests (1/2 to 14 hours) at slow to moderate true-strain rates of 10⁻⁶ to 10⁻⁴ s⁻¹ and temperatures between 1300°C and 1700°C. High-temperature, uniaxial tensile testing was used to produce data defining the relationship between tensile creep strain-rate and steady-state flow stress at four temperatures: 1340°C, 1440°C, 1540°C, 1640°C. Microstructural changes that occurred during creep testing were evaluated and compared to changes resulting from elevated temperature exposure alone. Mechanisms for dynamic abnormal grain growth that occurred during creep testing and the causes of the microstructural changes that occurred as a function of temperature are discussed. / text

The thesis deals with the study of surface-active properties, zeta potential, particle size and adhesion of surfactants (Arkopal N110, Igepal CO-520, Igepal CO-890, Dehyton PL, sodium dodecyl sulfate and dodecyltrimethylammonium bromide) selected for full notch creep test method. To determine the surface phenomena occuring in a solution due to their presence at 25, 50 and 80 °C, the Du Noüy ring method of surface tension measurement was used. The importance of factors such as concentration, ionic character and the molecular structure of the surfactants, temperature and dispersion media (pH, purity) were analysed. Dynamic light scattering measurements were performed in order to evaluate the particle size and the electrophoresis method was used to determine the zeta potential. The results were compared to the contact angle measurements, i. e. adhesion tendency between surfactant solutions and the surface of selected types of high density polyethylene. The optical analysis was used to evaluate the contact angle. The negative impact of water purity was negated. The temperature dependecy was specific for each of the examined surfactant species, however the relationship between temperature and the length of hydrophilic chain was discovered. Alkaline pH was less convenient for amphoteric surfactant than acid or neutral environment, but this might have been caused by components contained in the buffer. The character of surfactant solutions was mostly monodisperse. The only exceptions were 1mM Igepal CO-520 and 3mM Dehyton PL. Further investigation led to conclusion that the molecules tend to form agglomerates. Adhesion was in accordance with surfactant effectiveness of surface reduction and adsorption. However, this theory did not match the data obtained from testing acid and neutral Dehyton PL solutions' adhering to polyethylene species containing carbon black.