| 3D transition metal high-entropy alloys have good strength-plasticity matching,excellent low temperature performance and irradiation resistance,and have broad application prospects in aerospace,ultra-low temperature containers and nuclear energy and other fields.Compared with conventional alloys,more serious defects in casting billet and easier cracking in processing are the main challenges faced by the large-scale production of this kind of high-entropy alloys.In order to break through this bottleneck,in this paper,four kinds of CrxMnFeNi(x=0.8～1.5)transition metal high-entropy alloys were designed,and systematic research on solidification characteristics,hot deformation behavior and service performance of the alloys were carried out.The solidification mechanism and hot deformation mechanism of single-and dual-phase high-entropy alloys were clarified,and the effect of phase composition on the room temperature mechanical properties and high-temperature oxidation resistance of CrxMnFeNi high entropy alloys was revealed.The main findings are as follows:(1)The composition design and preparation of CrxMnFeNi high-entropy alloys.The effects of Cr,Mn,Fe and Ni elements on the phase composition of highentropy alloys were studied through theoretical analysis and phase diagram calculation,and it was clear that Cr was an effective element for the formation of BCC phase.Four high-entropy alloys of Cr0.8MnFeNi,CrMnFeNi,Cr1.2MnFeNi and Cr1.5MnFeNi with increasing BCC phase were designed by adjusting Cr content.The verification results of induction melting shown that the hardness increases with the increase of BCC phase content.(2)Study on solidification characteristics of CrxMnFeNi high-entropy alloys.The high-temperature solidification modes and microstructure evolution during solidification of single-phase Cr0.8MnFeNi and dual-phase Cr1.5MnFeNi high entropy alloys were investigated by Thermo-Calc theoretical calculation,differential scanning calorimeter(DSC)and directional solidification experiment.The results shown that the solidification paths of single-and dual-phase alloys were obviously different,which were L→L+FCC→ FCC(single-phase)and L→L+BCC→L+BCC+FCC→BCC+FCC(dual-phase),respectively.Under the experimental conditions of directional solidification,both single-and dual-phase high-entropy alloys grew as dendrites,and the dendrites of dual-phase alloys were more developed.During the solidification process,Cr and Fe preferentially aggregated,nucleated and grew continuously,followed by the nucleation and growth of Mn and Ni segregated in the liquid phase.The primary dendrite spacing(λ1)and secondary dendrite spacing(λ2)had an exponential relationship with the drawing speed(V):λ1=250.1V-0.27875 and λ2=250.1V-0.32735 in single-phase alloy,λ1=983.4V-0.27625 andλ2=199.4V-0.4358 in dual-phase alloy.It shown that increasing cooling rate can effectively refine the microstructure of high-entropy alloy.It was revealed that the Cr content in the high-entropy alloy affected the temperature gap of solidus and liquidus,liquidus slope and solidification mode,which leaded to the complex change of dendrite spacing.(3)Study on hot deformation behavior of CrxMnFeNi high-entropy alloys.The hot compression experiments of single-phase Cr0.8MnFeNi and dual-phase Cr1.5MnFeNi high-entropy alloys were carried out by thermomechanical simulator.The results shown that discontinuous dynamic recrystallization(DDRX)was dominant in single-phase alloys.However,the continuous dynamic recrystallization(CDRX)of BCC phase was dominant in the dual-phase alloy,and the increase of temperature and the decrease of strain rate promoted the dynamic recovery of FCC phase.The hyperbolic sine Arrhenius constitutive models were established for single-and dual-phase alloys,respectively.The average thermal activation energy of single-and dual-phase alloys were 404.1245 kJ/mol and 349.4594 kJ/mol,respectively,and the relative error was less than 4%.In addition,the hot processing maps were established and the optimum processing intervals were determined:the strain rate was less than 0.2 s-1 and the temperature was higher than 1000 ℃ for single-phase alloys;the strain rate was less than 0.15 s-1 and the temperature was higher than 975m4 for dual-phase alloys.(4)Study on mechanical properties of CrxMnFeNi high-entropy alloy.The yield strength and tensile strength of Cr0.8MnFeNi single-phase alloy were 366 MPa and 654 MPa,respectively.The introduction of BCC phase enhanced the strength of dual-phase alloy,and the yield strength and tensile strength of Cr1.5MnFeNi dualphase alloy were 596 MPa and 915 MPa,respectively.The tensile strength of the four high-entropy alloys had a linear relationship with the BCC phase content.During the tensile process of CrxMnFeNi high-entropy alloy at room temperature,the FCC phase was dominated by plane slip,and the BCC phase was dominated by cross slip.Furthermore,with the increase of deformation,FCC phase gradually formed<111>//DD and<100>//DD fiber texture,BCC phase gradually formed<110>//DD and<100>DD fiber texture,and no phase transition occurred.(5)Study on high-temperature oxidation resistance of CrxMnFeNi highentropy alloys.The oxidation process of CrxMnFeNi alloy in air at 800℃ was mainly controlled by the diffusion of Mn and Cr elements in the oxide layer.The oxidation products were composed of Mn2O3 in the outer layer and(Mn,Cr)3O4+Cr2O3 in the inner layer.The high-temperature oxidation resistance of the four alloys increased first and then decreased with the increase of Cr content.The oxidation rate constants(kp)were 0.0267 mg2cm-4h-1,0.0167 mg2cm-4h-1,0.0193 mg2cm-4h-1 and 0.0247 mg2cm-4h-1,respectively.This was due to the fact that although the increase of Cr content can enhance the oxidation resistance of the alloy,it also increased the FCC/BCC interface,promoted the diffusion and oxidation of Mn elements,and formed Mn oxide nodules,pores and internal oxidation,which greatly reduced the oxidation resistance. |
PDF Full Text Request