High Temperature High Strain Rate Behavior of Superalloy MA 760
Mardoukhi, Ahmad (2013)
2013
Master's Degree Programme in Materials Science
Teknisten tieteiden tiedekunta - Faculty of Engineering Sciences
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Hyväksymispäivämäärä
2013-10-09
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi:tty-201310211355
https://urn.fi/URN:NBN:fi:tty-201310211355
Tiivistelmä
The objective of this work was to investigate the high strain rate and high temperature behavior of mechanically alloyed and oxide dispersion strengthened nickel based superalloy MA 760. These types of alloys are used in many high temperature applications, such as turbine blades, where also impact type loadings can occur. Therefore, understanding the behavior of the alloy at its operating temperatures can help designing better and safer components in the cases of high rate impacts and collisions.
The high strain rate high temperature tests were carried out using the Split Hopkinson Pressure Bar device at different strain rates and temperatures. The tests were carried out at strain rates between 1050 s-1 and 3800 s-1 and at temperatures ranging from room temperature up to 900 ⁰C. The obtained data was analyzed based on the principles of the Split Hopkinson Pressure Bar, focusing on the yield strength, strain rate, and fracture strain.
Based on the test results, the effects of strain rate and temperature on the mechanical behavior of the MA 760 was described. Yield strength increases as a function of temperature until temperatures close to 700⁰C, after which the yield strength decreases. However, even after this decrease the material is still very strong, which makes this material suitable for high temperature applications. The reason for this observed behavior is the anomalous yielding behavior of the γ’ phase. The flow stress increases with increasing temperature until the maximum. At higher temperatures (above 700 ⁰C), the deformation starts in the γ matrix, which causes the reduction in the yield strength of the material. Around 900 ⁰C, the initial cuboidal microstructure changes its morphology, which leads to the further reduction of the strength of the material.
During the work of this thesis, a high temperature apparatus for the Taylor impact test was designed and built. The apparatus consists of the sample holder made of Teflon. The sample is placed in the sample holder with a ceramic wool ring. Two thermocouples are attached to each end of the specimen to monitor the temperature of the specimen. A stopper filled with ceramic wool was built to catch the specimen and the projectile. An induction heater was used to heat up the specimen to the test temperature. The impact process was recorded with a high speed camera to measure the speed of the projectile. The device was successfully tested and the results obtained from the tests were comparable with literature and the result obtained from the Split Hopkinson Pressure Bar.
The high strain rate high temperature tests were carried out using the Split Hopkinson Pressure Bar device at different strain rates and temperatures. The tests were carried out at strain rates between 1050 s-1 and 3800 s-1 and at temperatures ranging from room temperature up to 900 ⁰C. The obtained data was analyzed based on the principles of the Split Hopkinson Pressure Bar, focusing on the yield strength, strain rate, and fracture strain.
Based on the test results, the effects of strain rate and temperature on the mechanical behavior of the MA 760 was described. Yield strength increases as a function of temperature until temperatures close to 700⁰C, after which the yield strength decreases. However, even after this decrease the material is still very strong, which makes this material suitable for high temperature applications. The reason for this observed behavior is the anomalous yielding behavior of the γ’ phase. The flow stress increases with increasing temperature until the maximum. At higher temperatures (above 700 ⁰C), the deformation starts in the γ matrix, which causes the reduction in the yield strength of the material. Around 900 ⁰C, the initial cuboidal microstructure changes its morphology, which leads to the further reduction of the strength of the material.
During the work of this thesis, a high temperature apparatus for the Taylor impact test was designed and built. The apparatus consists of the sample holder made of Teflon. The sample is placed in the sample holder with a ceramic wool ring. Two thermocouples are attached to each end of the specimen to monitor the temperature of the specimen. A stopper filled with ceramic wool was built to catch the specimen and the projectile. An induction heater was used to heat up the specimen to the test temperature. The impact process was recorded with a high speed camera to measure the speed of the projectile. The device was successfully tested and the results obtained from the tests were comparable with literature and the result obtained from the Split Hopkinson Pressure Bar.