Search

Additive manufacturing processes supplement traditional material processing routes with unique capabilities which can have profound impacts on component production. Physical prototyping is accelerated and the fabrication of complex components, difficult or impossible to produce conventionally, is realized. Metals research is often focused on identifying process windows to avoid defects which thereby yield desirable properties. In electron beam melting fusion processes, however, precise spatial control of the heat source allows for detailed microstructure manipulation. Design is therefore extended to the microstructure scale offering greater overall flexibility towards engineering high performance components. In this work the role of geometry and beam path sequencing in a powder bed electron beam melting process is investigated. It is observed that by carefully engineering the melting sequence the morphology and texture at the mesoscale can be controlled. Solidification in the build direction, which usually prefers [001] directions, is tilted by control of the heat flux vector which yields large columnar crystals with a strong [011] build direction preference. This newly developed conduction control strategy is demonstrated for producing alternating mesoscale structures in bulk samples. Furthermore, a new scanning strategy is demonstrated which may be suitable for promoting a randomized crystallographic texture during the additive manufacturing process.

This investigation reports an unusual effect of hold time (up to 10seconds) on retardation in the growth of creep-fatigue small cracks at 550 degrees C in Grade 91 steel. The observed phenomenon was interpreted by elucidating multiple processes that are active in the plastic zone at the crack tip. To this effect, microstructural and mechanical responses of the crack tip plastic zone were compared with the mechanical and microstructural responses during low cycle fatigue/creep-fatigue. It is proposed that the stress relaxation that occurs during the hold time can reduce the stress intensity in the plastic zone of crack tip thus, contributing to retardation in the small crack growth rate. Aside from stress relaxation, stress intensity in the crack tip can be further reduced by the phenomenon of cyclic softening that occurs because of plasticity-induced microstructural coarsening. Separately, the contribution of oxidation-induced crack tip shielding is also considered to explain the observed effects of hold time on crack growth rates. Taken together, a combination of stress relaxation, enhanced rate of cyclic softening, and higher degree of oxidation with the introduction of a hold time is demonstrated to be responsible for reduction in the crack growth under creep-fatigue conditions.

International audience; Cyclic oxidation testing was conducted on alloy MA956 and two different batches of alloy PM2000 at 1,100 and 1,200 °C in different atmospheres rich in O2, H2O and CO2. Compared to 1 h cycles in dry O2, exposure in air + 10 vol.% H2O resulted in an increase of the oxidation rate and a decrease of the time to breakaway for all alloys at 1,200 °C, and a faster consumption of Al in the MA956 alloy. One hour cyclic testing in 49.25 % CO2 + 50 % H2O + 0.75 % O2 had a smaller effect on the oxidation rate but led to increased formation of voids in alloy MA956, which had an impact on the alloy creep resistance. At 1,100 °C, exposure in 50 % CO2 + 50 % H2O resulted in significant oxide spallation compared with oxidation in air, but this was not the case when 0.75 % O2 was added to the CO2/H2O mixture as a buffer. The control of impurity levels drastically improved the oxidation resistance of PM2000.

International audience; Increasing efficiency is a continuing goal for all forms of power generation from conventional fossil fuels to new renewable sources. However, increasing the process temperature to increase efficiency leads to faster degradation rates and more components with corrosion-limited lifetimes. At the highest temperatures, oxidation-resistant alumina-forming alloys and coatings are needed for maximum lifetimes. However, lifetime models accurate over the extended application durations are not currently available for a wide range of candidates and conditions. Increased mechanistic understanding and relevant long-term data sets will assist in model development and validation. Current progress is outlined for applying a reservoir-type model to Fe-base alloys and coatings. However, more work is needed to understand environmental effects, such as the presence of H2O, and to extend the current model to NiCrAl and NiCr alloys. As the critical performance factors are better understood, it will be easier to evaluate new materials in laboratory screening experiments.

Creep tests were performed on thin wall specimens made of MC2 single crystal superalloy at 1150 °C and under controlled atmosphere. The results highlight the deleterious oxidation effect on creep properties. The assumption that oxidation leads to a non-load-bearing affected zone is insufficient to explain the difference in creep rate that was noticed between tests performed under synthetic air and under hydrogenated argon, and cannot explain the decrease of the strain rate during the tests that were carried out with a change of atmosphere from synthetic air to hydrogenated argon. On the other hand, these experimental results are consistent with vacancy injection due to partial cationic oxidation, which accelerates the creep rate by promoting creep mechanisms controlled by diffusion. The anionic protective alumina scale formed under hydrogenated argon prevents this vacancy flux. The integration of this dynamic and long range oxidation effect in creep strain models is discussed.