Band 52, Heft 6

Due to their high melting point and good creep resistance, Mo-Si-B alloys have the potential to be used as structural materials at extremely high temperatures in the future. Apart from the relatively ductile Mo solid solution, three-phase molybdenum-based alloys including the intermetallic phases Mo 3 Si und Mo 5 SiB 2 , which enable low fracture toughness, especially at room temperature. A promising approach for fracture toughness improvement consists in the production of a microstructure in which the intermetallic phases are embedded in the ductile Mo solid solution matrix. This paper looks at different powder- and ingot-metallurgical production processes and the resulting microstructures. Particular attention will be paid to the impact of structural component size and phase distribution on the fracture toughness at room temperature.

Microscale machining processes modifying component surfaces are applied in various fields: from drive technology through plant engineering to medical technology. The impact of the fine structures formed in the material on its behavior and mechanical properties need to be investigated in more detail. Besides the examination of the cyclic and quasi-static behavior of micromachined samples, analysing the near-surface microstructure and the hardness is of great importance. The nanoindentation allows for the determination of local mechanical properties in small analysis volumes. Its applicability to determine the hardness in micromachined near-surface areas including the metallographic preparation for commercially pure titanium is presented in this work.

The powder metallurgically processed carbon-free alloy Fe-25Co-15Mo (in m.-%) can be hardened via the precipitation of an intermetallic μ phase (Fe, Co) 7 Mo 6 by solution annealing, quenching and subsequent aging. Solution annealing is carried out in the austenite region which leads to a supersaturation of the matrix with molybdenum and the precipitation of nm sized μ phase particles during subsequent aging. “Primary” μ phase particles that are 1–2 μm in diameter formed during prior hot isostatic pressing are the source of molybdenum for aging. However, during solution annealing, these “primary” μ phase particles also impede grain growth. Therefore, the solution annealing process should be optimized to avoid extensive grain growth while still obtaining sufficient molybdenum contents in the matrix. In order to characterize the solution annealing process as a function of time and temperature, dilatometer experiments were performed to monitor the dissolution kinetics of the μ phase. The remaining μ phase fraction was determined in a complementary way by applying two characterization methods: Quantitative analysis of scanning electron microscope images and X-ray diffraction.

Burners are key components of turbomachinery with internal combustion. They allow for a particularly lean combustion in heavy-duty stationary gas turbine engines for power stations and thus contribute significantly to economical operation. Depending on their design, burners are often subject to thermomechanical stress. This failure analysis is concerned with thermal cycling cracks at the burner outlet. Circumferential cracking, parallel to the component surface, is untypical for this damage mechanism. Thermal cycling and thermal-mechanical fatigue cracks usually spread in a radial direction. Possible causes could be the special kind of centrifugal casting structure or the high thermal tension, which might have built up during burner operation.