Neutron/High-Energy X-Ray Diffraction Studies On The Deformation Behavior Of CoCrNi Medium-and High-Entropy Alloys

High-entropy alloy(HEA)is a new kind of multi-principal component alloy.Its high configurational entropy and excellent comprehensive properties have become the hot spot of metal alloy research.At present,the study on the mechanical properties and deformation mechanism of HEAs is still in the primary stage.The influence of alloying elements and heat treatment process on the micro structure and mechanical properties of HEAs has been systematically studied in previous literature.However,there are few reports on the phase stability as well as the relationship between the phase and the stress partitioning in HEAs.Based on the special atomic structure inherent in HEAs,the factors affecting the mechanical properties can be classified into two categories:(1)elements of different size in the crystal cells may cause large lattice distortion and thus improve the solid solution strengthening effect;(2)The distribution of short-range order(SRO)structure in the alloy(i.e.the element occupying of different sublattice)reduces the stability of phase or twin to improve the plasticity of the alloy.In fact,the HEAs own greater intrinsic lattice strain than conventional alloys since the uncertainty of atomic occupancy in the alloys.The SRO structure elucidates the correlation between the chemical occupation of atoms in random solid solution alloys and the strength on the alloy,which increases strength of HEAs due to the interaction between solute and solvent.There is still lack of research on quantifying peak broadening and its related special phase transformation behavior in HEAs.Based on this,in situ neutron diffraction and synchrotron X-ray diffraction technology has been used to carry out the following researches on CoCrNi HEAs.Firstly,in the paper,a single-phase face-centered cubic(fcc)CoCrNi medium-entropy alloy(MEA)with different boron-doping contents(30?1600 ppm)was prepared by vacuum arc melting.With the addition of element B,the grain size is significantly refined,while the area of the alloy's unrecrystallized area changes and the Cr-rich precipitation segregates at the grain boundary.The uniaxial tensile properties of the alloy change significantly.The tensile yield strength(YS)and ultimate tensile strength(UTS)are 990 MPa and 1178 MPa with the ductility of 26%in 800 ppm boron doping alloy,which were attributed to the strengthening effect of the large unrecrystallized grains.The tensile YS and UTS dramatically increase up to 1456 MPa and 1481 MPa with the ductility of 11%for 1600 ppm boron doping alloy.In situ synchrotron X-ray results show the diffraction peaks of {200} crystal planes in CoCrNi-800 alloy appear asymmetry along the transverse direction(TD)with the increasing strain,and shift to a higher 20 value after deformation.Secondly,this paper used the method of arc vacuum melting to prepare Co19Cr21Ni19Fe22Mn12Al7(at.%)HEA containing fcc and bcc dual phases.The time-of-flight(TOF)neutron diffraction was used to conduct in-situ studies on its uniaxial tensile mechanical behavior at 298 K and 77 K.The results show that the YS and UTS of Co19Cr21Ni19Fe22Mn12Al7(at.%)HEA at room temperature(RT)are 600 MPa and 1187 MPa,respectively,and the elongation is 19.9%.The YS and UTS of the alloy are 750 MPa and 1415 MPa at 77 K,respectively,with the elongation of 14.6%.The striking finding of a colossal elastic strain of 7.2%and 6.0%is reported for the {200}bcc crystal plane,which was achieved at RT and 77 K.Such a colossal lattice strain originates from a new type of stress-induced confined martensitic transformation.Futher transmission electron microscopy(TEM)results found that the disordered bcc phase is randomly distributed in the ordered B2 matrix and keeps coherent with the B2 matrix.The coherent relation is fully reflected in the {100} super lattice lattice strain.In addition,this paper used the high-energy X-ray diffraction technique to study the texture evolution of dual-phase CoCrNiFeMnAl0.5 HEA in a cold-rolled state and after annealing at temperatures ranging from 900? to 1100?.The results show that the main texture component in the fcc phase is {110}<100>,and the main texture components in the bcc phase are {112}<110>and {111}<112>.In fcc phase,{1-1-1}//RD texture decreases and {100}//RD texture increases with the increasing annealing temperature,respectively.In bcc phase,the deformed texture shows the characteristic textures of {110}//RD and<111>//ND.The orientation relationship of the two phases in the alloy is {110}bcc//{111} fcc and<111>bcc//<110>fcc.It is worth noting that the diffraction peaks of {200} and {211} crystal planes show obvious asymmetry in the bcc phase due to the presence of residual stress,while this asymmetry disappears with the decrease of residual stress after annealing.The pole figure shows front and back asymmetry along the transverse axis,which is related to the large shear deformation caused by multiple passes during the rolling process.In summary,this paper mainly used TOF neutron diffraction and high-energy X-ray diffraction techniques to analyze the relationship between lattice strain and micromechanics in HEAs during deformation.The phase stability and the stress distribution of each phase during deformation were determined.The systematic research on texture of dual-phase CoCrNiFeMnAl0.5 HEA was also studied.These results will be helpful to understand the deformation mechanism of HE As and provide a theoretical and experimental basis for strengthening and toughening research.