Synthesis, Microstructures and Mechanical Behaviour of Cr0.21Fe0.20Al0.41Cu0.18 and Cr0.14Fe0.13Al0.26Cu0.11Si0.25Zn0.11 Nanocrystallite Entropy Alloys Prepared by Mechanical Alloying and Hot-Pressing

Four component Cr0.21Fe0.20Al0.41Cu0.18 medium entropy alloy (Quaternary, 4C-MEA) and six component Cr0.14Fe0.13Al0.26Cu0.11Si0.25Zn0.11 high entropy alloy (sexinary, 6C-HEA) were designed and developed in non-equiatomic ratio to attain improved mechanical properties. These 4C-MEA, and 6C-HEA were synthesized via mechanical alloying (MA), and consolidated by hot pressing (HPing) at 723 K. For comparison, the same atomic ratio of four and six components of coarse grain alloys (4C-CGA and 6C-CGA) were also manufactured by conventional blending method. Nanocrystallite size powders of 27 +/- 5.20 nm and 38 +/- 3.7 nm were achieved for 4C-MEA and 6C-HEA respectively after 20 h MA. The phase evolutions, structural properties, and powder surface morphologies were characterized using X-ray diffraction and several electron microscopes. The 4C-MEA has possessed more quantity of body centred cubic (BCC) and less amount of face centred cubic (FCC) phases due to the more solid dissolution of 4 components. However, 6C-HEA exhibited more quantity of FCC and a small amount of BCC phases due to the incorporation of more FCC components compared to 4C-MEA and less solid dissolution due to more atomic radius difference among the mixing elements (atomic radius of Cr = 166 pm, Fe = 156 pm, Al = 118 pm, Cu = 145 pm, Si = 111 pm and Zn = 142 pm). The HPed samples produced ultra-fine crystallite size of 177 nm and 499 nm for 4C-MEA and 6C-HEA respectively. Further, 4C-MEA and 6C-HEA exhibited the ultimate compressive strength (UCS) of 365 MPa and 456 MPa respectively due to dissolution and lattice distortion of mixing elements. Also, 6C-HEA possessed Vickers hardness strength of around 1.97 GPa which was 2 times higher than 4C-MEA. The theoretical background of various strengthening mechanisms, various physicochemical, thermodynamic parameters, and four core effects behind the improved properties in entropy alloys was discussed and reported. The dislocation strengthening and solid solution strengthening were the major factors in exhibiting more UCS in 4C-MEA and 6C-HEA than 4C-CGA and 6C-CGA.

Automated summary

A medium entropy alloy (4C-MEA) and a high entropy alloy (6C-HEA) were designed and developed through mechanical alloying, achieving nanocrystalline powders and ultra-fine crystallite sizes. The 6C-HEA exhibited higher ultimate compressive strength and Vickers hardness strength compared to the 4C-MEA.