Date of Award
8-2024
Document Type
Thesis
Degree Name
Master of Science (MS)
Department
Mechanical Engineering
Committee Chair/Advisor
Dr. Garrett Pataky
Committee Member
Dr. Marian Kennedy
Committee Member
Dr. Rodrigo Martinez-Duarte
Committee Member
Dr. Lonny Thompson
Abstract
Due to their higher wear resistance compared to conventional alloys, high entropy alloys (HEAs) are now being considered as candidates for parts undergoing sliding contact during their lifetimes. While the engineering field has built some knowledge related to the performance of selected high entropy alloys, such as the CoCrFeNi alloy, more research is needed to determine if (how) expansions from four to five principal alloying elements alter the performance compared to the initial alloy. In this study, we started this research effort by investigating the relative performance of CoCrFeNiMn and CoCrFeNiTi with respect to CoCrFeNi. The two primary alloying elements (Mn and Ti) were selected because the structure and tensile performance, along with wear performance of CoCrFeNiMn, which is called Cantor alloy, have been vastly studied, but lacked a comparison of the wear resistance with the CoCrFeNi alloy. There are reports of secondary phase formation in the microstructure as a result of adding Ti to the base CoCrFeNi alloy, which makes the behavior of CoCrFeNiTi composition different from the CoCrFeNiMn. Additionally, there are limited studies on the tribological performance of CoCrFeNiTi alloy.
Contributions included characterization the composition, microstructure, and tribological performance of CoCrFeNi-based alloys, during dry linear reciprocating sliding of a 52100 steel counter-face against each alloy. When needed, homogenization heat treatments were used to break down secondary phases in these alloys. The tribological performance (wear rate and coefficient of friction (COF)) were calculated from wear tests.
The CoCrFeNiTi had the highest wear resistance (4.64 × 10-5mm3/N × m for 1000 cycles test and 8.33 × 10-6mm3/N × m for 1500 cycles test) and the lowest COF (0.58 for 1000 cycles test and 0.59 for 1500 cycles test) due to its higher hardness value (551 HV). This alloy was the only one to have secondary phases (R, Laves, Sigma, and Chi) form during its initial casting, and the higher hardness of CoCrFeNiTi was attributed to the presence of these phases. A reduction in the fraction of secondary phases occurred with homogenization heat treatment (1100 ˚C for 48 h ) and resulted in a decrease in hardness to 200 HV.
The CoCrFeNi alloy had a lower wear rate when the tribo-system underwent 1000 and 1500 cycle tests compared to CoCrFeNiMn alloy. A uniform compact oxide layer was formed on the wear track of CoCrFeNi alloy during the 1000 cycles test and acted as a protective layer, which decreased the volume loss and the wear rate, while CoCrFeNiMn had a negligible amount of oxygen on the wear track. The wear mechanisms of HEA-1 for 1000 cycles test were abrasive and delamination wear with slight oxidation, while HEA-2 experienced abrasive and severe oxidative wear. Wear mechanisms of HEA-3 for 1000 cycles test were abrasive and oxidative wear. By increasing the test duration, a compact uniform oxide layer was formed on the CoCrFeNiMn wear track, decreasing the volume loss and wear rate. Due to the longer test duration, wear debris was generated, and CoCrFeNi wear track surface was not able to form a uniform oxide layer on the wear track, which increased the volume loss. The decrease in volume loss and wear rate of the CoCrFeNiTi with test duration increment was due to the increase in oxygen (oxide) content on the wear track. Wear mechanism of HEA-1 changed to abrasive and oxidative wear with higher oxygen concentration, while wear mechanisms of HEA-2 and HEA-3 remained intact. This formation and localized fracture of the oxide layer could have caused an inverse correlation of wear rate and COF by changing the friction force.
The hardness of HEAs plays a crucial role in dictating the general range for volume loss and specific wear rate values. The active wear mechanisms of HEAs and their changes with modifying test parameters (increasing the test duration in this research) played a significant role in defining the interaction between tribological properties of alloys such as volume loss, wear rate, and COF.
Recommended Citation
Azarmi, Ali, "Understanding The Microstructure and Tribological Performance of CoCrFeNi-based High Entropy Alloys" (2024). All Theses. 4344.
https://open.clemson.edu/all_theses/4344