2021 Virtual Undergraduate Research Symposium
2021 Virtual Undergraduate Research Symposium
Understanding Influence of Heat-Treatment on Serrated Yielding in a Ni Superalloy
Understanding Influence of Heat-Treatment on Serrated Yielding in a Ni Superalloy
PROJECT NUMBER: 66 | AUTHOR: Nathan Brown, Metallurgical and Materials Engineering
MENTOR: Kester Clarke, Metallurgical and Materials Engineering and Amy Clarke, Metallurgical and Materials Engineering
ABSTRACT
Nickel-based superalloys have been reported to experience serrated yielding in specific temperature regimes which overlap with their operating temperature range. This phenomenon results in increased strain with a nominal change in stress, causing possible undesired dimensional changes. Understanding the origins of localized deformation that accompanies serrated yielding would allow for improved mechanical properties, leading to better performance and more efficient turbine engines. Since serrated yielding is sensitive to small changes in temperature, thermal gradients during elevated temperature tensile testing must be reduced. Testing of sample geometry modification and equipment optimization has been conducted to limit the thermal gradient present in the sample. It was determined that by increasing the free span of the sample geometry and by raising grip chiller temperatures, thermal gradients can be reduced.
PRESENTATION
AUTHOR BIOGRAPHY
Nathan Brown is a junior majoring in Metallurgical and Materials Engineering (MME) at Colorado School of Mines. He is currently involved in The Center for Advanced Non-Ferrous Structural Alloys (CANFSA) where he is studying the influence of serrated yielding on nickel-based superalloys. Before this position, Nathan aided in projects of aluminum roll bonding for superplasticity and general sample preparation for other members of CANFSA. In the future, Nathan will be pursuing a graduate degree in MME with a focus on microstructural evolution.
When tensile testing at a specific strain rate, load control or strain control is usually used? Which will you utilize to control the strain during your tests?
Hi Caroline,
Load control is often used in tensile testing as it does not require external equipment such as an extensometer. Additionally, strain control presents some issues if testing to failure once the sample fractures. As such, load control provides sufficient resolution for my tensile tests.
Hi Nathan, what is the minimum thermal gradient you could achieve with the Gleeble with optimal sample design and equipment modification?
Hi Joe,
With optimal conditions, which would include a large free span, specialty grips, and optimized chiller temperatures, one may expect to see temperature differences of less than 5 degrees Celcius.
Excellent, Nathan!
Nathan, how does the geometry of your specimen affect the thermal gradient (flat plate dog bone versus cylinder)?
Hi Nikki,
Cylindrical sample geometries are ideal because they have radial (one dimension) thermal gradients whereas plate sample geometries have more complex gradients. The reason sample geometries were shifted was because of stock material size constraints. Looking at the stock for Alloy 10, I would be able to get 1 cylindrical specimen from each wedge while I am able to machine 3 dogbone specimens.