كليدواژه :
High entropy alloys , Mechanical alloying , AlCoCuMnNi , Al content , Sintering temperature
چكيده فارسي :
High entropy alloys (HEAs) represent a class of materials composed of at least four components in near equiatomic ratios. Introduced in 2004, HEAs have intrigued researchers due to their inherent complexity arising from multiple constituent elements. The HEAs properties are influenced by various factors, including the alloy composition, crystallographic structure and the chosen production route, leading to diverse structural/mechanical characteristics. Ongoing research continually introduces new and promising HEAs driven by theoretical calculations and simulation predictions together with practical investigations. This study focuses on examining the microstructure and hardness of AlCoCuMnNi (S1) and Al0.25CoCuMnNi (S2), two novel high entropy alloys produced by utilizing powder metallurgy and Spark Plasma Sintering (SPS) to create fine-grained alloys with superior properties. Mechanical alloying using a planetary ball mill was employed for milling of commercially pure Al, Co, Cu, Mn and Ni powders with particle sizes smaller than 63 µm. Following previous investigations, 45h milling resulted in a single phase FCC for both compositions. The milled powders were subsequently sintered using SPS at temperatures of 850°C and 1000°C. Sample preparation involved grinding, polishing and etching with a marble solution for 90 seconds enabling examination through Optical Microscopy (OM) and Scanning Electron Microscopy (SEM). Finally, Vickers hardness testing was conducted by applying a 0.5 kg load with a dwell time of 10 seconds. These characterization techniques as well as X-ray Diffraction (XRD) were employed to assess the impact of sintering temperature and aluminum content on the microstructure and hardness of the produced HEAs. XRD analysis revealed a duplex BCC+FCC microstructure for both S1 samples sintered either at 850°C (S1-850) or at 1000°C (S1-1000). Moreover, the phase ratio analysis indicated a higher BCC phase content in S1-1000 compared to S1-850, possibly due to the accelerated FCC to BCC diffusional transformation at 1000°C sintering temperature. Comparative OM and SEM analysis of S1-850 and S1-1000 samples revealed distinct morphologies. The S1-1000 sample displayed a bimodal microstructure with nano and micro grains, while S1-850 exhibited a unique structure with two regions: one resembling S1-1000 and another with evident elemental segregations. The XRD patterns of both S2-850 and S2-1000 samples exhibited a single-phase FCC structure. Nevertheless, the SEM images of S2-850 suggested the presence of second-phase precipitates with varying BSE contrasts. However, detailed analysis using elemental mapping and EDX inputs indicated similar elemental distributions in both light gray and dark gray regions, implying that minor variations in some elements caused the observed BSE contrast for S2-850. This suggests that during the low-temperature sintering process at 850°C for S2, there was insufficient diffusional energy to redistribute the elements in the bulk alloy resulting in localized elemental segregations. Hardness measurements indicated values of 390 HV for S1-850, 450 HV for S1-1000, 290 HV for S2-850, and 300 HV for S2-1000. The higher hardness values of S1 alloys as compared to those of their S2 counterparts is attributed to their higher aluminum content resulting in the formation of a second BCC phase. The elevated sintering temperature enhanced hardness in both S1 and S2 alloys due to both creation of higher BCC phase content and imposing more uniform elemental distributions.
