We have studied microdefects and interfaces of Al2O3 films on iron and nickel aluminide substrates using variable-energy positron lifetime spectroscopy. Di-vacancies, vacancy clusters, and microvoids were observed in the oxide scales. Their sizes and distributions were determined by the nature of the process used to synthesize the alumina film, and influenced by the composition of the alloy substrates. For oxide–iron aluminide interfaces, positron lifetimes are longer than those for the alumina layer itself, suggesting a greater defect concentration at such sites.
In order to show the suitability of the Positron Lifetime Spectroscopy (POLIS) as a method of Non-Destructive Testing (NDT) several iron alloys / steels were mechanically damaged (tensile stress, fatigue, creeping). The positron annihilation parameters show clear changes during all applied kinds of damage. After tensile stress as well as after creeping a homogeneous distribution of damage in the sample was detected. During the very first elastic cycle of a fatigue experiment, a change in the defect structure occurs in well annealed materials. A modified spectrometer for in the field mapping is presented.
Using positron lifetime spectroscopy (POLIS) the degree of damage in creep-influenced pipe steels was found to decrease with depth, as measured from the physical surface of the samples. Ultrasonic measurements were revealed to mostly note the differences in internal stresses, not in the microstructure. A critical limit of 160 ps average positron lifetime was determined at about 80% of material lifetime consumption in a creep experiment, independent of the material or the creep test parameters. The results suggest the technique of positron lifetime spectroscopy as a method for nondestructive monitoring of creep damage
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