Introducing carbon into the novel material class of High entropy alloys (HEAs) often leads to obtaining a high-hardness, chemical-resistant and heat-resistant material called High entropy carbide (HEC). HECs have been the topic of many publications in recent years due to their outstanding thermomechanical properties. Many refractory metal HEA systems have been converted into well-performing HECs, however, one of the most renowned HEA systems has yet to be a subject to any in depth research of adding carbon. The Cantor-like AlCoCrFeNi is well-known for its unique phase composition, displaying high mechanical properties such as fracture toughness, compressive strength or Young’s modulus.
These properties are a result of the alloy’s multiphase composition comprising of fine precipitates homogeneously dispersed within the matrix. The single-phase destabilization is known to be caused by the presence of Al, creating an Al-Ni B2 sublattice system similar to traditional nickel superalloys. While the phase composition and behavior of AlCoCrFeNi has been studied quite thoroughly throughout the years, the stability of these phases under different chemical conditions remains a question unanswered. This work focuses on the addition of carbon to this system, as it is one of the most accessible non-metal enriching elements, which could pave the way for potential large-scope application in the future.
The introduction of this thesis consists of a literary overview of the topic of HEAs, their history and namely the naming and definitions convention. Afterwards, focus is set on the AlCoCrFeNi system specifically, its phases and phase transitions described in detail as well as the effects of different synthesis routes and heat treatments on this material’s microstructure.
Experiments were carried out on a series of samples of (AlCoCrFeNi)100-x+Cx (x = 0; 1,5; 2; 4; 8; 10) consolidated via SPS from atomized pre-alloyed HEA and graphite powders. XRD phase analysis as well as SEM and EDS microstructure and chemical analyses were carried out, tied together with results from Archimedes’ density and Vickers hardness tests. The effect of sintering temperature, dwell time, heating and cooling rates were analyzed.
It was proved that C has an effect on the microhardness of the AlCoCrFeNi system intrinsically linked to further phase destabilization occurring when C > 1,5 at. %. Different C contents cause different phases to stabilize within the microstructure, resulting in non-linear behavior of obtained microhardness. Additionally, C appears to nullify the effect of cooling rates during SPS, possibly stabilizing the phase composition obtained at Tsint. even at lower temperatures. This could also simplify the application process as it was proven that even a large 170g sample retained a near identical microstructure to that of a smaller 4g one.
This thesis offers a large-scale collection of new data for further research. All obtained results are discussed with the limited literature currently available on the topic, expanding the knowledge on the stability of the AlCoCrFeNi system and its interactions with C.