Title : Microstructure and mechanical properties of high entropy alloys prepared by mechanical alloying
High entropy alloys (HEAs) are recently attracted several researchers, scientists, and academicians due to their extraordinary and outstanding properties that cannot be obtained from conventional alloys. HEAs are multicomponent alloys in which a minimum of five metallic elements are mixed in an equal molar or non-equal molar ratio. The present research work has concentrated on synthesizing four nanocrystalline (NC) alloys of Cr0.26Fe0.24Al0.5 (Ternary, 3E-MEA) Cr0.21Fe0.20Al0.41Cu0.18 (Quaternary, 4E-MEA), Cr0.15Fe0.14Al0.30Cu0.13Si0.28 (Quinary, 5E-HEA) and Cr0.14Fe0.13Al0.26Cu0.11Si0.25Zn0.11 (Sixinary, 6E-HEA) non-equiatomic (equal weight fraction) through mechanical alloying (MA) (250 rpm, BPR of 10:1 and 20h). All synthesized powders were hot-pressed (HPed) at 723k for 30 min subsequently, mechanical properties in terms of compressive stress-strain and hardness were examined. The influence of entropy effect on structural properties and microstructural characterization has been studied for all the as-milled powder alloys and HPed samples. For comparison, the same non-equiatomic ratio and chemical composition for coarse grain alloys, namely Cr0.26Fe0.24Al0.5 (ternary, 3E-CGA) Cr0.21Fe0.20Al0.41Cu0.18 (quaternary, 4E-CGA), Cr0.15Fe0.14Al0.30Cu0.13Si0.28 (quinary, 5E-CGA) and Cr0.14Fe0.13Al0.26Cu0.11Si0.25Zn0.11 (sixinary, 6E-CGA) were manufactured by conventional powder metallurgy (PM) route (blending method) (50 rpm, BPR of 2:1 and 10h). In addition, this thesis reviews various solid-state processes (SSP) to be used in the synthesis of HEA.
Nanocrystallite size powders of 22±4.0 nm, 27±5.20 nm 30±3.5 nm, and 38±3.7 nm were achieved for 3E-MEA, 4E-MEA, 5E-HEA, and 6E-HEA, respectively after 20hr MA. The HPed samples produced ultra-fine crystallite sizes of 144 nm, 177 nm, 277 nm and 499 nm for 3E-MEA, 4E-MEA, 5E-HEA and 6E-HEA, respectively. The phase evolutions, structural properties and powder surface morphologies were characterized using x-ray diffraction (XRD) and advanced electron microscopes. The HPed sample of 3E-MEA of Cr0.26Fe0.24Al0.5 and 4E-MEA of Cr0.21Fe0.20Al0.41Cu0.18 produced more BCC than FCC crystal structures due to more dissolution of Al and Cu atoms in the stronger bonding atoms of Cr-Fe lattice. In contrast, 5E-HEA of Cr0.15Fe0.14Al0.30Cu0.13Si0.28 and 6E-HEA of Cr0.14Fe0.13Al0.26Cu0.11Si0.25Zn0.11 samples has exhibited little more FCC phase than BCC phase due to balance between the dissolution of FCC elements (Al, Cu, Si), BCC elements (Cr, Fe) and HCP (HCP). Further, 3E-MEA, 4E-MEA, 5E-MEA and 6E-HEA have exhibited the ultimate compressive strength (UCS) of 1278 MPa, 1309 MPa, 2060 MPa and 1505 MPa respectively whereas the corresponding conventionally blended alloys much lower UCS are The 268 MPa, 325 MPa, 615 MPa and 662 MPa of 3E-CGA, 4E-CGA, 5E-CGA and 6E-CGA respectively. The Vickers hardness strength (VHS) of 3E-MEA, 4E-MEA, 5E-MEA and 6E-HEA are 3.6, 3.65, 6.06 and 4.56 GPA respectively.
Moreover, the compaction and compressibility behaviors of the powders (green sample) were studied using linear and non-linear compaction equations under different pressures (25-600 MPa for CGAs and 25-1000 MPa for MEAs and HEAs). The results show that the CGAs has more compressibility rate than HEAs. Also, the linear Panelli-Filho equation has the highest regression coefficient among the other linear equations in which it predicts the densifications of both HEAs and CGAs accurately. Finally, 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 MEAs and HEAs than CGAs.
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