Formation And Mechanical Properties Of Fe-Co-Cr-Ni-Al-M High Entropy Alloys Fabricated Via Sub-rapid Solidification

High entropy alloys（HEAs）have attracted extensive attentions due to their unique phase formation and excellent mechanical properties in the field of metallic materials.Recent investigation shows that FeCoCrNiAl_0.6 HEAs with coarse FCC dendrite+BCC inter-dendrite structure could be formed under arc-melting and further cooling with relatively low cooling rates,while the composite-like BCC matrix+B2 nano-crystalline structure was obtained under laser surface melting strategy with ultrahigh cooling rates,and B2 phase is precipitated from the BCC matrix.Accordingly,it could be deduced that the FeCoCrNiAl_0.6HEAs can form supersaturated solid solution under rapid-solidified condition,and such solid solution is metastable.Therefore,it is possible to have phase transformation in this alloy system under sub-rapid solidification.If such metastability in solid solution of HEAs could be managed by proper cooling rates,it may be possible to produce desired ultrafine structure and achieve a substantial improvement in comprehensive mechanical properties.However,there is still lack of investigation on this issue.Therefore,in this work,copper mold casting was utilized to fabricate FeCoCrNiAl_0.6,FeCoCrNiAl_0.6Nb_x（x=0.06～0.24）and FeCoCrNiAl_0.6Mo_x（x=0.1～0.3）HEAs with different cooling rates.Particular attention was focused on the relations among the processing and composition parameters（i.e.,cooling rates and Nb and Mo microalloying）,microstructure,and mechanical properties.The main findings of this work are summarized as follows:1.Sub-rapid solidified FeCoCrNiAl_0.6 HEA with cooling rates（R_c）of～250 K/s,～111.1 K/s,～40 K/s and～20.4 K/s were produced by remelting and injecting the alloy ingots into copper mold with different casting diameters（d）of 2 mm,3 mm,5mm,and 7 mm,respectively.Then,the effects of R_c on the phase formation mechanism,microstructure,and mechanical properties of this HEA were systematically investigated and detailedly discussed.The results show that complex and refined sideplate structure with FCC+BCC dual-phase could be formed in this HEA under rapid-solidification.Further analysis shows that the orientation relationship（OR）between BCC and FCC phases is close to K-S and N-W ORs.Accordingly,it is deduced that such structure may have close correlation to BCC→FCC solid-state phase transition during sub-rapid solidification.The formation of FCC phase is closely related to R_c.The higher the R_c,the less content and the finer the structure of FCC phase in this HEA.R_c could significantly improve the mechanical properties of this HEA with yield strength and hardness.2.Based on the above studies,the sub-rapid solidified FeCoCrNiAl_0.6Nb_0.1 HEA is designed by Nb microalloying in FeCoCrNiAl_0.6 and fabricated under R_c of～111.1K/s,and the effects of minor Nb on the phase formation mechanism,microstructure and mechanical properties of this HEA were investigated.It is found that the FeCoCrNiAl_0.6Nb_0.1 HEA could form unique microstructure and outstanding mechanical properties comparable to FeCoCrNiAl_0.6HEA produced under the same condition.The FCC+BCC structure is composed of fine dendrites,while the submicron-scale Nb-rich regions are located at the boundary of these two kinds of phases.Further,the complex ultrafine/nanocrystalline structures could be observed via TEM.It implys that Nb plays two roles in this HEA.On one hand,it changes the phase transition mechanism,and on the other hand,it induces the formation of a fine-ultrafine-nanocrystalline hierarchical structure,which could induce great improvement mechanical properties compared with FeCoCrNiAl_0.6.Then,we studied the phase formation mechanism of FeCoCrNiAl_0.6Nb_0.1 HEA and its microstructure and mechanical properties fabricated under different R_c.It is found that the microstructure is refined and the mechanical properties are improved with the increase in R_c.When R_c changes from～40 K/s to～250 K/s,the yield strength increases from～950 MPa to～1500 MPa,Vickers hardness enhances from～515 HV to～600 HV,and the plasticity increases from～33%to～38%.Finally,the effects of Nb concentration on the microstructure and properties of sub-rapidly solidified FeCoCrNiAl_0.6Nb_x HEAs were investigated.The results show that the microstructure and mechanical properties are significantly varied by changing Nb contents.With the increase in Nb,the microstructure is more refined and strength is improved,while the plasticity deteriorates in FeCoCrNiAl_0.6Nb_x HEAs.Moreover,the Nb-rich phase and the proportion of BCC phase increases with higher Nb content.When x increases from 0.06 to 0.24,the yield strength enhances from～1000 MPa to～2000 MPa,and Vickers hardness increases from～480 HV to～660 HV.This work exhibits that,Nb not only promotes the refinement of the microstructure in the BCC phase,but also leads to the formation of the Nb-rich phase and induce the formation of the FCC phase during the solidification,leading to the change in the phase transformation.3.Based on the results mentioned above,the sub-rapid solidified FeCoCrNiAl_0.6Mo_0.2 HEA was designed by Mo microalloying in FeCoCrNiAl_0.6 and fabricated under R_cof～111.1 K/s.and its microstructure and mechanical properties have been systematically investigated.It is found that,Mo could refine the microstructure and improve the mechanical properties.The OR between FCC and BCC phases is close to the K-S OR.By further studying the effects of R_c on the microstructure and properties of FeCoCrNiAl_0.6Mo_0.2HEA,it is found that when R_cincreased from～40 K/s to～250 K/s,the yield strength and Vickers hardness enhanced greatly from～1000 MPa to～1700 MPa and from～530 HV to～620 HV,respectively.Furthmore,the microstructure and mechanical properties of sub-rapidly solidified FeCoCrNiAl_0.6Mo_x（x=0.1 and 0.3）with different Mo contents are developed,which possess excellent mechanical properties and refined microstructure.The results show that BCC phase increases and the OR between the FCC and BCC phases tends to the K-S OR with inceasing Mo addition.The yield strength and Vickers hardness has positive correlation with Mo content,while the plasticity decreases significantly with the incease in Mo concentration.