Production and characterization of al-CoCrFeNi-M (M=Mo, Cu, Mn) high entropy alloys by combustion synthesis method

Abstract

This thesis demonstrates the successful synthesis of high-entropy alloys (HEAs) such as AlxCoCrFeNi, AlxCoCrFeNiCuy, AlxCoCrFeNiMoy, and AlxCoCrFeNiMn using a metallothermic non-centrifugal Self-Propagating High-Temperature Synthesis (SHS) method. This process employed relatively low-cost oxide raw materials like Co3O4, Cr2O3, Fe2O3, NiO, MoO3, and MnO2, with metallic aluminum as a reductant, achieving synthesis within seconds with minimal energy input. The introduction of metallic copper was found effective in promoting exothermic reactions in Cu-containing alloys. Thermochemical simulations using FactSageTM's "Equilib" module were highly effective in predicting SHS outcomes, despite slight deviations due to reactor lid sealing issues and gas release. Experimental results closely matched theoretical predictions, with minor fluctuations attributed to experimental conditions and measurement errors. These simulations were particularly accurate in representing SHS under adiabatic conditions, even though real experiments exhibited some heat loss and scattering. The slag phase separation from alloys was relatively easy, though small amount of slag and inclusions were present within the alloys. The lack of a reliable correlation between adiabatic temperatures or charge mixture compositions and reaction yields was noted due to scattering and gasification. Larger scale experiments are recommended to better assess SHS scalability, as gaseous products and scattering effects may diminish with increased batch sizes. Mn loss in the AlxCoCrFeNiMn system could be mitigated by optimizing the use of Al2O3 as a heat suppressant, reducing MnO2 waste and increasing yield. However, excessive Al2O3 can form MnAl2O4 spinel, lowering manganese reduction efficiency. Optimizing the heat suppressant amount through thermochemical simulations is crucial, with 12 g of Al2O3 found to be optimal for a 150 g charge mixture. Al content influences adiabatic temperature and alloy phase composition. Excess Al absorbs heat without participating in reduction reactions, lowering adiabatic temperature. Reducing Al content below x=0.5 proved difficult, as Al tends to dissolve into the liquid to achieve the desired Cr content. Al's impact extends to altering the crystal structure from FCC to BCC in CoCrFeNi-based HEAs due to atomic radius differences and valence electron concentration. High Al content leads to the formation of BCC phases, including the ordered BCC-B2 phase rich in Al-Ni and the disordered BCC-A2 phase rich in Fe-Cr. This phase transformation, consistent with phase diagram calculations, results in increased alloy hardness, particularly with higher Al content. The addition of Cu to the AlxCoCrFeNi system introduces another FCC phase rich in Cu, with its fraction depending on the Cu content. Low Al and Cu content (e.g., Al0.5CoCrFeNiCu0.5) results in lower secondary FCC phase fractions, while higher Cu and Al content leads to complex duplex FCC+BCC microstructures. The hardness of these alloys varies, with Cu and Al content. Mo addition to AlxCoCrFeNi primarily results in the formation of a brittle sigma phase. As Al content increases, complex microstructures comprising A2, B2, and sigma phases emerge. The hardness of Mo-containing alloys increases with Al content, peaking for the Al1.0Mo1.0 alloy, then decreasing due to the reduced sigma phase fraction at higher Al levels. In the Mn system, SHS alloys typically consist of FCC (A1) and BCC (A2+B2) crystal structures. Increasing Al content leads to a transition from a dual-phase structure to a fully BCC structure, with corresponding increases in hardness. The addition of Mn and Cr enhances hardness in the FCC Al0.5CoCrFeNiMn alloy. Arc-remelted and suction-casted SHS alloys exhibit similar microstructures but are generally porosity-free and contain fewer inclusions. Suction casting results in finer microstructures due to higher cooling rates, with notable differences in dendritic arm spacing observed in Cu-containing alloys. The hardness of these alloys is influenced by the phase fractions present, with BCC phases contributing to higher hardness. FCC-based alloys showed higher cold deformability than BCC alloys, with the Al0.5CoCrFeNi alloy achieving a 131% reduction in area and a 109% increase in hardness after cold rolling. The addition of Cu further enhanced cold deformability in Al0.5CoCrFeNiCu0.5 and Al0.5CoCrFeNiCu alloys, reaching 145% and 170% true reduction in area, respectively, along with significant work hardening. Hot deformability was lower in FCC alloys due to the precipitation of B2 and sigma phases, which increased flow stress through precipitation hardening and dislocation pinning mechanisms. However, increasing the hot deformation temperature could improve hot formability by preventing B2 and sigma precipitation, though temperatures above 1000°C might cause issues like liquefaction of Cu-rich phases. BCC alloys exhibited poor hot deformability, likely due to equipment limitations and high hardness. Annealing the cold-rolled alloys reduced hardness through static recrystallization or recovery, but also resulted in improved hardness and substantial area reduction. Notably, cold rolling followed by annealing produced an equiaxed single-phase FCC structure in Al0.5CoCrFeNiCu0.5, which could be important for corrosion resistance, suggesting further investigation into the corrosion properties of this alloy is warranted. It was found out that introduction of Mo to the AlCoCrFeNi alloy system decreases the high-temperature oxidation resistance (at 800 ºC). Consistent with the CALPHAD simulations, SEM and Raman analyses, the reason for this that presence of low Al and high Mo in the Al1.0Mo01.0 alloy, leads to a thick intermixed oxide layer consisting of spinel and MoOx oxides, before the stabilizaiton of thick protective M2O3 layer (M=Cr, Al). Increasing the Al/Mo ratio, allows the formation of thick protective M2O3 layer and protects and/or slows from further oxidation. Adding Mn to the AlxCoCrFeNi alloy system boosts its saturation magnetization, achieving 83 emu/g (552.4 kA/m) at a density of 6.63 g/cm³. However, this magnetization level is highly influenced by the distribution and ordering of the A2/B2 phases. When the Al content is increased, as in the Al1.5Mn1.0 alloy, the saturation magnetization drops to 147.2 kA/m despite a lower density of 6.40 g/cm³ due to a higher proportion of the ordered B2 phase. The presence of Cr-rich regions near grain boundaries, which are likely paramagnetic due to Cr's antiferromagnetic properties, further diminishes magnetization. To enhance the alloy's soft-magnetic properties, removing Cr could be beneficial. Moreover, the coercivity, which is crucial for minimizing hysteresis loss, rises in alloys with Cr-rich phases and inclusions, with values of 43 Oe (3421 A/m) and 54 Oe (4297 A/m) observed in the Al1.0Mn1.0 and Al1.5Mn1.0 alloys, respectively.