| Multi-elemental high-entropy alloy (HEA) has drawn extensive attentions in the past decade. As a new type of alloy, HEA is composed of at least five principle elements with the content of each component less than35%, which has gone beyond the designing concept of traditional alloys that basing on only one or two major elemental compositions. The high entropy effect in HEA induced by the increased species of major components results in the easily formation of simple body centered (BCC) or face centered (FCC) structures, and probably accompanied by the formation of inter-crystalline compounds or nano-crystals. Therefore, three strengthening effects can be achieved:solid solution strengthening, precipitation strengthening and dispersion strengthening. Through optimizing the composition, HEAs are capable of possessing higher hardness, higher strength, and better high temperature oxidation and corrosion resistance than traditional alloys.In the present study, AlxCoCrCuFeNi (x=0.5,1.0,1.5)(denoted as Alx in the following section) high entropy alloys were prepared using the medium frequency electromagnetic induction method. With the aim of improving the overall performance of HEA, TiC particle-reinforced Al0.5CoCrCuFeNi alloy based composites with different volume fractions of TiC, Al0.5CoCrCuFeNi-y vol.%TiC (y=5,10, and15, denoted by Al0.5-TiCy), were synthesized by in situ reaction. Furthermore, aging procedure was performed on the above materials at different temperatures. The microstructure and mechanical properties of these self-produced alloys were investigated using high resolution transmission electron microscope, scanning electron microscope, material testing system and hardness tester. In addition, the high temperature oxidation behavior of Alx alloys and Al0.5-TiCy composites were also examined as HEAs are potential in high-temperature applications. The findings and conclutions have been drawn as follows.Al0.5CoCrCuFeNi alloy with dentrite crystal structures formed with lower cooling rate is composed of simple FCC solid solution. However, small amount of BCC structured phases were generated in the equiaxed polygrain structured Al0.5alloy prepared by induction melting and casting (rapid silidification). Intriguingly, small amount of phases with BCC structure can be also produced in the dentrite structured Al0.5alloy after subjected to annealing at600℃for24h. Moreover, large amount of Cu-rich nano-precipitations existed in the interdendritic regions after annealing treatment, and the compressive yield strength was increased from487MPa to600MPa. In contrast, no obvious variation of phase composition was found in the Al0.5alloy with equiaxed structure under annealing, and its compressive yield strength was enhanced by11.9%compared to the alloy with dendrite structure in as-cast state.Microstructural investigations of the dentrite structured Alx (x=0.5,1.0and1.5) alloys revealed that Co, Cr, Fe and Ni are majorly distributed in grain matrices while Cu is riched in grain boundary regions. It indicates that the molar fraction of Al has insignificant effects on the element distribution of Alx series of HEAs. However, lattice distortion occurred in the crystal structure of solid solutions with increasing Al content due to the obvious atomic size difference. As a result, the main FCC phase transformed into BCC accompanied by spinodal decomposition of the latter. It is worth noting that large amount of strip and spherical Cu-rich nano-phases were precipitated with x increasing from0.5to1.0, leading to significant improvement in the overall mechanical properties of the alloy. The ultimate strength of Al1.0alloy reached1739.3MPa and the extension rate was more than12%. The hardness and strength of the alloy can still be enhanced via further increasing Al content. Nevertheless, remarkable degradation of plasticity occurred, and the Alx alloys exhibited variations in the failure modes from ductile fracture to cleavage fracture. The results indicated that solid solution strengthening and precipitation strengthening are the prominent strengthening mechanism of Alx series of HEAs.The matrices of in situ synthesized Al0.5-TiCy (y=5,10and15) composites exhibited dentrite structured with TiC particles distributed homogeneously. With the increase of the TiC volume fraction, the average size of TiC ceramic particles increases from200nm of Alo.5-TiC5to3um of Al0.5-TiClO. The compressive yield strength of Al0.5-TiCy (y=5,10and15) composites reached740MPa,709MPa and680MPa, respectively. The mechanical properties of TiC particle-reinforced composites have been improved remarkably compared with those of Al0.5HEA. Take Al0.5-TiC5for an example, the compressive yield strength is almost50%higher than that of Al0.5alloy matrix.The high temperature oxidation behavior of Alx alloys and Al0.5-TiCy (y=5,10, and15) composites were examined between850and1050℃in still atmosphere. Results revealed that the oxidation dynamics for Alx alloy exposed at850and950℃are similar, abiding by parabolic law. However, the oxidation kinetic curve becomes almost linear when oxidated at1050℃, with serious cracking and spalling of oxide layer and severe internal oxidation. However, oxidation dynamics for Al1.0and Al1.5alloys with higher Al content deviated from the parabolic raw with much less oxidization. The formation of dense Al2O3film as Al content increases prevented the inner part of the sample from further oxidation. Compared to Al0.5CoCrCuFeNi alloy, the oxidation rate was decreased dramatically in the composites. In particular, the oxidation weight is only0.5mg/cm2for Alo.5-TiC5after oxidized at950℃for100h. |
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