Metastable Phase Evolution And Its Effect On Mechanical Properties Of Heat-resistant High Entropy Alloy

High entropy alloys（HEAs）are considered to have a promising application in high temperature field because of high mixing entropy,strong solid solution strengthening and hysteresis diffusion effect.However,at present,the high entropy alloy with body centered cubic（BCC）matrix has serious brittleness at room temperature.The high entropy alloy with face centered cubic（FCC）matrix has insufficient strength.The high entropy alloy with balanced strength-toughness is lacked.Heat-resistant high entropy alloys are required to serve in intermediate or high temperature environment for a long time,it is very important to study the thermal stability,metastable phase evolution and its influence on high temperature properties of high entropy alloys.In this paper,the heat-resistant high-entropy alloy system is selected for the research.The high-entropy alloy with balanced strengthtoughness and heat-resistant potential is screened through experience criteria of phase formation in high entropy alloy and phase diagram calculation.The addition of Nb,C and Hf forms multiple composite carbides to improve further the high temperature performance.Phase separation behavior,metastable phase evolution and their impact on microstructure and mechanical property during annealing were systematically studied.The main contents and results are as follows:（1）The feasibility of high entropy alloy composition screening by combining the empirical parameters of phase formation in high entropy alloy and phase diagram calculation was verified,and obtained a balanced strength-toughness high entropy alloy,Al₁₆Co₁₆Cr₁₆Fe₁₆Ni₃₀Mo_3.8Nb₁Hf_0.2C₁.The alloy has good casting performance.Its tensile strength and elongation at 600℃ are 805 MPa and 11%.The tensile strength and elongation at 700℃ are 771 MPa and 24.5%,which is more than 90%of the tensile strength at room temperature（858 MPa）.（2）The formation and evolution of ordered phases in Al₁₆Co₁₆Cr₁₆Fe₁₆Ni₃₀Mo_3.8Nb₁Hf_0.2C₁ high entropy alloy during annealing were systematically studied.The results show that the dendritic phase of Al₁₆Co₁₆Cr₁₆Fe₁₆Ni₃₀Mo_3.8Nb₁Hf_0.2C₁ high entropy alloy separated during annealing at 700℃,and phase separation sequence of dendritic phase is FCC solid solution→γ’（Ni,Al rich）→LPSO（Ni,Al rich）→BCC（Ni,Al rich）,LPSO（Mo rich）→BCC（Ni,Al rich）,LPSO（Mo rich）,BCC（Cr,Mo rich）.（3）The effect of order phases on the mechanical properties of Al₁₆Co₁₆Cr₁₆Fe₁₆Ni₃₀Mo_3.8Nb₁Hf_0.2C₁ high entropy alloy was investigated.The results show that the dispersed ordered phase formed in the dendrite after annealed at 700℃ for 48 h can increase the yield strength and tensile strength of the alloy from 498 MPa to 1008 MPa and from 858 MPa to 1242 MPa at room temperature,while the elongation does not obviously decrease.The tensile strength at 600℃ is increased by about 210 MPa,reach 1015 MPa,the yield strength is increased by about 178 MPa,reach 665 MPa.The elongation at 700℃ is increased by about two times,up to 41%.（4）Phase separation and the ordered phase evolution mechanism of Al₁₆Co₁₆Cr₁₆Fe₁₆Ni₃₀Mo_3.8Nb₁Hf_0.2C₁ high entropy alloy during annealing was investigated.The geometrical phase method was used to analyze the strain field of the solid solution before and after annealed,and the lattice distortion was effectively released during the annealing.The high entropy alloy is more complex than the traditional alloy because of the multi-principal element system,and the phase separation occurs quickly in the early stage of annealing,so it is difficult to observe directly in the experiment.The Kinetic Monte Carlo method,which can accurately simulate the microstructure evolution and diffusion behavior of multicomponent alloys,was used to assist the experimental study.The result shows that the new phase in FCC solid solution is easy to form in the place with high vacancy concentration during annealing.Al atoms preferentially occupy vacancies to form A1 clusters,and then combine with Ni atoms to form Ni,Al rich phase.During annealing,vacancies in FCC solid solution will be captured by Ni,Al rich phase,and the captured vacancies further form vacancy groups and promote the growth and transformation of Ni,Al rich phase.The Ni,Al-rich phase will transform from FCC structure to BCC structure finally.（5）The product of strength and elongation and hot working property of Al₁₆Co₁₆Cr₁₆Fe₁₆Ni₃₀Mo_3.8Nb₁Hf_0.2C₁ high entropy alloy are improved greatly by ordered phases.Under equal forging and rolling conditions,the alloy after homogenization at 1200℃ for 6 h cracked seriously at the beganing of processing,while the alloy was found no crack after annealed at 700℃ for 100 h.Moreover,it can be forged from 60～80 mm into 30 mm block,and then hot rolled into 8 mm sheet,its deformation is over 80%and it has a better yield.The microstructure of Al₁₆Co₁₆Cr₁₆Fe₁₆Ni₃₀Mo_3.8Nb₁Hf_0.2C₁ high entropy alloy hot rolled sheet is finer than that of the ingot.NbC is broken in the hot working process.Due to the effect of grain refinement and ordered phase,the elongation of hot rolled sheet increased from 9.5%to 68.5%at 700℃ after annealing at 700℃ for 48 h.The tensile strength decreased slightly,from 682 MPa to 636 MPa,and the product of strength and elongation increased from about 7 GPa%to about 44 GPa%,showing the alloy has excellent comprehensive performance and great application potential.（6）The strengthening and toughening mechanisms of Al₁₆Co₁₆Cr₁₆Fe₁₆Ni₃₀Mo_3.8Nb₁Hf_0.2C₁ high entropy alloy was investigated.The results show that the deformation mechanism of the alloy at high temperature is mainly slip and twin assisted deformation.The stress release of the unannealed alloy during the tensile process is mainly through deformation twin,cross twin,element diffusion and formation of the second phase.Fine and dispersed ordered phases formed in the annealed samples,mainly including（Ni,Al）rich,（Cr,Mo）rich and（Mo,Nb）rich phases,which can effectively hinder dislocation movement and make slip difficult,then promote the formation of deformation twins in FCC solid solution between ordered phases.Due to the small spacing between ordered phases,the size of deformation twins is smaller than that of the as-cast sample.At the same time,the dispersed ordered phase will divide the original FCC solid solution into multiple blocks,resulting in more deformation twins,which can effectively improve the high temperature plasticity and the hot working performance and the product of strength and elongation.