The Study Of Phase Component And Mechanical Performance In Solid Solutions High Entropy Alloys

High entropy alloys (HEAs) are usually composed of more than five metallic elements in equal or near-equal atomic ratio, and interestingly they tend to form simple solid solution structure rather than the intermetallic compounds in solidification because of the effect of high entropy. The intrinsic cystal structure makes alloys have high thermodynamic stability, and the alloys have great development potential in terms of high-temperature strength, corrosion resistance, wear resistance and oxidation resistance, etc. So the solid solution is the microstructure with high expectations at room temperature for HEAs. Then the phase component and mechanical performance is the main problem needed to study for HEAs, so this paper studies are the following aspects:Firstly, according to the crystal structure of constituent element at high temperature and the principle of structural similarity for solid-solution, the constituent elements for high-entropy stabilized solid-solution alloys were divided into two groups:BCC phase forming elements and FCC phase forming elements. The stability of FCC and/or BCC phase is mainly depended on the competition of mixing free energy between BCC phase and FCC phase from the thermodynamics point of view. The phase with lower mixing free energy will be formed preferentially. Furthermore, the thermodynamic parameter η was defined as the ratio of mixing free energy of BCC (a) phase and total mixing free energy of BCC and FCC {a+β) phases, that is,η= △Gamix/△Ga+βmix. It is shown that FCC single-phase solid-solutions are stable at η≤0.50, and instead of BCC single-phase solid-solutions are stable at η≥ 0.64, and their mixtures are stable between the two boundaries. The η-defined criterion to predict the phase stability is appropriate for most HEAs with solid solution structure, which is vitally important for alloy design and for controlling the mechanical behavior in HEAs.Secondly, when the elements Al and Ni exist simultaneously, HEAs generally form the B2 structure. In order to clarify the formation mechanism of B2 ordered-phase, Ni was substituted by Co in HEAs is to explore the causes of B2 structure and changes in microstructure and properties of alloys. The results show that the microstructures of them are the simple solid solution structures that composed of the BCC phase or/and FCC phase, the BCC phase lies in dendrite and the FCC phase locates at interdendrite. With the increasing of Al content, the phase structure of as-cast alloys change the crystal structure from FCC and BCC mixed phases to BCC single-phase, which also illustrates that Al is BCC phase-formed elements. Worthy of note is that both the HEAs all form the B2 super structure, the main reason for this phenomenon is that the mixing enthalpy between Al and Ni or Co is relatively large. HEAs tend to form the AINi or AICo super structure in solidification, that is, the BCC phase modulated structure is made up of B2 ordered phase and B1 disordered-phase in HEAs. And the strength of alloys is greatly enhanced.Thirdly, based on Cr-Ni binary phase diagram, Cr is selected as the basic element of BCC phase and Ni is selected as the basic element of FCC phase. Further, the CoCrFeNiAlo.86 HEAs was designed by the model of Cr0.56Ni0.44, and CoCrFeNiAl HEAs was prepared as a comparative study. The results show that the microstructure of CoCrFeNiAl0.86 is isometric crystal structure, which is composed of BCC and FCC mixed-phase solid solution. However, the distribution of phase component is inhomogeneous in microstructure for CoCrFeNiAlo.86 HEAs, the eutectic microstructures composed of FCC phase and BCC phase distribute in some cells and the phase structures of other cells are the BCC amplitude modulation structures constituted by B2 ordered-phase and B1 disordered-phase. The strength of CoCrFeNiAl0.86 is significantly lower than CoCrFeNiAl HEAs because of the existence of FCC phase and the microstructural nonuniformity. However, the plasticity of CoCrFeNiAl HEAs with BCC phase modulated structure is not greatly reduced because of the phase components are distributed homogeneously.