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Access Type
WSU Access
Date of Award
January 2022
Degree Type
Dissertation
Degree Name
Ph.D.
Department
Mechanical Engineering
First Advisor
Golam Newaz
Abstract
GRCop-84 (Cu-8Cr-4Nb) offers potential for high-heat-flux applications up to approximately 700 °C (1292 °F) and is specifically designed for rocket engine main combustion chamber liners. This alloy gets strength from Cr2Nb dispersoids which strengthen the copper matrix. This alloy approximately gets two-thirds of its strength from the size of the refined grain size, as explained by the Hall-Petch mechanism, and one-third from the dispersion strengthening mechanism. It exhibits excellent elevated temperature strength for high-heat flux applications. In addition, it has enhanced oxidation resistance up to 650 °C because of the formation of Cr-Nb oxide layers above the thick Cu oxide layer. According to the literature, GRCop-84 alloy can only be manufactured using different powder metallurgical techniques and rapid solidification processes due to the presence of the Cr2Nb phase.Since additive manufacturing is a rapid solidification process, in this comprehensive work, the design of experiments was conducted to optimize process parameters, laser power, and travel speed. GRCop-84 was successfully fabricated for the laser metal deposition (LMD) process without any defects. Scanning electron microscopy (SEM) and electron dispersive spectroscopy (EDS) investigation of as-deposited GRCop-84 revealed the hardening Cr2Nb phase forming in a pure copper matrix with particle size between 2.34 ± 0.83 μm. Also, interestingly a very bright Cr-rich phase was found along the cell boundary regions with an average length of 343 ± 113 nm. X-ray diffraction of this alloy confirmed the presence of Cr2Nb precipitates and pure Cr phase in this material. The tension test of the as-deposited samples displayed an average UTS of 410.3 MPa and YS of 232 MPa with 13 % elongation. Which is similar to non-additive processes such as extruded materials In addition, the effect of Cr addition to GRCop-84 was investigated to assess the composition-structure-property relationship. Thermodynamic simulation was used to understand the phase formation during the rapid solidification of this alloy. As per the predictions in GRCop-84 and GRCop-84+Cr 5wt% alloys, they initially start solidification with C15 laves phase, which is Cr2Nb. Also, in both cases, eutectic BCC(Cr) phases are formed in the single-point equilibrium calculation at 800 C. However, in GRCop-84 + Cr 5wt% due to the excess amount of Cr, this BCC phase was predicted to be the primary solidification phase after C15_Laves and before FCC (Cu). Different near-dense samples were successfully deposited using laser metal deposition (LMD) with successive addition of Cr wt.% (0, 5, 10) into the GRCop-84 alloy. The SEM and EDS analysis of the microstructure of all these as-deposited samples revealed the presence of Cr2Nb intermetallic phase, spherical Cr precipitate, and eutectic Cr fiber-like phase with primary Cu matrix. Interestingly, the fiber-like phase was primarily observed along the Cu cell boundaries. The length of these fibers grew from 0.3 to 1.7 μm with the change in Cr present in the alloy composition. This phenomenon was not reported in the literature. Also, there was an increase in the spherical Cr phase particle size and volume with increased Cr content in the alloy. EDS investigation revealed that spherical Cr and nano precipitates are rich in Cr with around 94 At % and 93 At%, respectively. Furthermore, the Cr2Nb phase contains Cr 66.68 At% and Nb 33.8 At%, which is close to the 2:1 ratio. Atom probe tomography revealed that the fiber phase forming along the cell boundary is a Cr-rich phase. The hardness test of these samples reveals that strength increased proportionally from 143 HV to 171 HV with higher Cr content in the alloys. This research successfully created composition-structure-property relation and confirms that additive manufacturing can be a potential route to producing GRCop-84. Also, formation of interesting cell-boundary Cr rich fiber phase morphology and composition was analyzed successfully.
Recommended Citation
Bhagavatam, Ajay Sarma, "Investigation Of Dispersion Strengthened Cu-8cr-4nb (grcop-84) Alloy Using Additive Manufacturing" (2022). Wayne State University Dissertations. 3752.
https://digitalcommons.wayne.edu/oa_dissertations/3752