Precipitation Characteristics And Regulatory Mechanism Of Second Phase In CrFeCoNi-based High Entropy Alloy

In 2004,a creatively alloy design concept by Yeh,namely,high-entropy alloys(HEAs)concept,was proposed and the basic idea is to simultaneously alloy multiple principal elements in equimolar or near equimolar ratios.The CrFeCoNi HEAs with a face-centered cubic(FCC)structure has excellent plasticity(toughness)but insufficient strength.In response to this problem,a large number of studies have shown that precipitation strengthening has achieved good results in this series of HEAs.For example,adding a certain amount of Al to the alloy precipitates a plate of B2 phase,adding a certain amount of Ti to precipitate a spherical L1₂ phase,and compound addition can further adjust the characteristics of L1₂.It can be seen that Ti and Al elements have different roles in the formation of the second phase.In order to explore the effect of the interaction of these two elements on the precipitates,CrFeCoNi was selected as the matrix in this paper,and the effects of different Al and Ti ratios on the characteristics of the second phase were studied.First,the role of Al in CrFeCoNi matrix was studied.The(CrFeCoNi)₉₄Al₆ alloy(Al6alloy)was aged at 750?and 800?for 17 h,72 h,100 h and 500 h.It was found that the incoherent B2 phase precipitated after aging.After aging at high temperature for a long time,the precipitates grow from square to needle.The average size of precipitates apparently increases with increasing aging temperatures and aging time.The coarsening rate constant of B2 phase at 750?is about 1.65?10^-25m³/s which is higher than the??phase in many Ni-based superalloys,indicating that the precipitated B2 phase has poor thermal stability.After adding Ti alone,the precipitated L1₂phase is easy to coarsen,and a brittle phase will also be precipitated when the Ti content is high.In summary,there are certain limitations when adding Al or Ti alone.The effect of Ti on the second phase was further studied,and part of the Al element was replaced.In this paper,the typical composition is selected,which is to replace 2at.%Al(Ti2Al4 alloy).After aging at 750?/500h,the diameter of the spherical L1₂ phase is about48nm.The B2 phase precipitated after aging at 750?/500h after adding Al alone is needle-shaped,with an average diameter of about 0.7mm;the diameter of the L1₂ phase precipitated after adding Ti alone at 800?/48h reaches about 0.4mm.This shows that after Ti is substituted for Al,the precipitation of L1₂ phase is promoted,and the thermal stability is greatly improved.The precipitate coarsening rate in Ti2Al4 alloy is lower than that in many traditional Ni-based alloys,which mainly due to the small elastic energy between the matrix and the interface or/and the low diffusion coefficient.In this paper,the effects of Ti and Al alloying on the second phase characteristics of CrFeCoNi are studied.It was found that a needle B2 phase precipitated when Al was added alone,and a spherical L1₂ phase precipitated when Ti was added alone.After substituting2at.%Ti for Al,the precipitate is a spherical L1₂ phase with nano-scale uniform distribution.And because Ti and Al work together to reduce the diffusion rate,the thermal stability is better than when Ti is added alone.It provides a certain reference for controlling the precipitation of the second phase of HEAs.