| High entropy alloys (HEAs) were composed of five or more than five elements. The content of each element should be more than 5 at.% and less than 35 at.%. It breaks the design concept of the conventional alloys which are based on one or two principle elements. Because of its high mixing entropy, the microstructure and properties of the high entropy alloys are different from the conventional alloys.In this thesis, FeCoNiAlxCr(x=0.3、0.5、0.8、1.0、1.5) and FexCoNiCuCr(x=0.2、0.5、 0.8、1.0、2.0) HEAs were prepared by the arc melting method. Meanwhile, FeCoNiAlxCr and FexCoNiCuCr HEAs were heated at different temperature (400℃、600℃、800℃、1000℃、 1200℃) for 5h, then be quenched in water(WQ) and cooled down to room temperature in the furnace(FC). The microstructure of the as-cast and annealed FeCoMAlxCr and FexCoNiCuCr HEAs were investigated using XRD、OM、SEM、DSC and TEM. The hardness and compression properties were tested and analyzed.It has been found that with the phases existing in the FeCoNiAlxCr HEAs have the transition of fcc'fcc+bcc'bcc structure with x increasing, and larger x enhances the formation of the bcc phase. With x increase, the microstructure changed form pure fcc solid solution to dendrite structure, then to equiaxed grain with dendrite. The ordered bcc nano-scale particles have been found in the alloys with x of 0.5、0.8、1.0. The compressive strength and hardness of the FeCoNiAlxCr HEAs increases with x increasing.After heat treatments, the phases in the FeCoNiAl0.3Cr alloy remain fcc structure. The FeCoNiAl0.3Cr alloys heated at 400 and 600℃ have similar microstructure as the as-cast alloys. The microstructures with rich Al and Ni are formed in the alloys heated at 800 and 1000℃. However, the alloy heated at 1200℃ still has typical single fcc phase. All the alloys have good plasticity, and the heat treatment improves the strength and hardness of the alloys.The FeCoNiAl0.5 Cr alloys heated at different temperature remains its typical dendrite microstructure with bcc and fcc phases. However, the content of the fcc phases increases with the rising temperature. The heat treatment at 400 and 600℃ has no obvious effect on the microstructure, and improves the strength and hardness of the alloys. When the alloys are heated at 800℃, a large amount of needle-shape phases (NSP) were found in the dendrite region, which hardened the specimen. When the alloys are heated at 1000℃, the spinodal microstructure in the inter-dendrite region grows up, and the strength and hardness of the alloy become smaller. The alloy heated at 1200℃ has simple dendrite microstructure, and nano-scale ball-like phases were found in the inter-dendrite region of the alloys cooled in the furnace.The FeCoNiAl0.8Cr alloys heated at different temperature consist of bcc and fcc phases. The amount of the fcc phase increases with the increasing temperature. The heat treatment at 400 and 600℃ has no obvious influence on the microstructure and properties of the alloy. The alloys heated at 800 and 1000℃ have larger size of fiber microstructure, and lower strength and hardness value. When the alloys are heated at 1200℃, the size of the fiber was 10 times larger than before, and the properties decreased. Furthermore, spinodal microstructure was found in the region between the fibers of FC specimen.Similarly, the heat treatment has no obvious effect on the microstructure, strength and hardness of the FeCoNiAl1.0Cr alloys. However, the fcc phase has been found in the alloys heated at 800 and 1000℃, and amount of fcc phase increases with the rising temperature. Lots of needle-shaped precipitations have been found around the grain boundary, the size of spinodal microstructure increased, and its strength and hardness decreased. When the temperature reached 1200℃, net-shaped phases were found around the grain boundary. TEM observation shows that the plate-like precipitation is order bcc structure, the short precipitation is fcc phase, and the matrix has bcc structure.The FeCoNiAl1.5Cr alloys after heat treatment has developed dendrite microstructure and bcc phase, and has high hardness and poor plasticity. The heat treatment below 800℃ has no obvious effect on the microstructure of the alloy, and slightly improves the strength and hardness of the alloy. The alloys heated at 1000℃ has nano-scale precipitates around the grain boundary, and lower strength and hardness. The alloys heated at 1200℃ have net-shape precipitates, and lots of nano-scale precipitates were found around the grain boundary in the FC specimen, but no nano-scale precipitates in the WQ specimen.The FexCoNiCuCr HEAs after heat treatment compose of two fcc solid solutions. The heat treatment below 800℃ has no obvious effect on the microstructure of the alloy. However, the nano-scale ball-shape precipitates were found in the dendrite regions of the alloys heated at 1000℃. The alloys heated at 1200℃ has many micro-scale ball-shape phase in the dendrite region, whose amount decrease with the increasing Fe content. The hardness decreases with the increasing temperature, and has smallest value at the heat treatment temperature of 1000℃.In a summary, the heat treatment at 400 and 600℃ has no obvious effect on the microstructure of both the FeCoNiAJxCr and the FexCoNiCuCr HEAs, slightly improves the strength and hardness of the alloys, indicating that the alloys have higher thermal stability. |
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