Study On High Temperature Mechanical Properties,Dynamic Behaviors And Deformation Mechanisms Of NiCoFe-based Medium/high Entropy Alloys

The design concept of high entropy alloys（HEAs）breaks the concept of traditional alloys.Unlike traditional alloys with one or two elements as main elements and a small amount of other elements added,HEAs usually consist of five or more principal elements in equal or near equal atomic percentages.MEAs usually consist of three or more principal elements in equal or near equal atomic percentages.The unique atomic structure of MEAs/HEAs results in many unique effects,such as the long range disorder and short range order of atomic arrangement,high entropy effect,severe lattice distortion effect,sluggish diffusion effect and cocktail effect,thus showing excellent properties such as high strength,high ductility,high toughness,high hardness,thermal resistance and impact resistance.Therefore,MEAs/HEAs have a strong industrial application potential,and their unique design concept provides a broad space for directional design and development of MEAs/HEAs with specific properties.The present work focused on the properties and deformation mechanism under high temperature and impact.The Ni₂Co₁Fe₁V_0.5Mo_0.2 MEA and Ni₁₄Co₂₀Fe₂₀Cr₂₆Mn₂₀ HEA were designed and prepared.Firstly,the Ni₂Co₁Fe₁V_0.5Mo_0.2 MEA with high temperature resistance was prepared by introducing large size Mo and V atoms with high melting point.The deformation mechanisms,the strengthening and toughening mechanisms of the Ni₂Co₁Fe₁V_0.5Mo_0.2 MEA at high temperature were studied by combining high temperature quasi-static tension,creep tests and microstructure characterization.Secondly,the stacking fault energy（SFE）of the Ni₁₄Co₂₀Fe₂₀Cr₂₆Mn₂₀ HEA was reduced by optimizing the atomic ratio.The deformation mechanisms,the strengthening and toughening mechanisms of the Ni₁₄Co₂₀Fe₂₀Cr₂₆Mn₂₀ HEA at high strain rate and low temperature were studied in detail by combining the Hopkinson impact at room temperature,Charpy impact at 77 K and microstructure characterization.The main contributions of this paper are as following:（1）The Ni₂Co₁Fe₁V_0.5Mo_0.2 MEA exhibited an excellent combination of strength and ductility and significant work hardening ability in the temperature range of 25～800℃.The ultimate tensile strength（UTS）and uniform elongation at room temperature were 583 MPa and62%,respectively.The excellent properties at room temperature were attributed to the combined effect of strong short-range solute pinning effect and massive dislocation pile-up.The strength and ductility decreased with tensile temperature increase.The UTS and uniform elongation at 800℃were 326 MPa and 44%,respectively.At 800℃,strengthening from solid solution and forest dislocation made the MEA have high work hardening ability in the early stage of deformation.But at the late stage of deformation,the dominant deformation mechanism changed from forest dislocation cutting to dislocation cross-slip,resulting in recovery and annihilation of dislocations.Therefore,the work hardening rate of the alloy decreased rapidly.At the temperature above 900℃,the solution strengthening effect disappeared,and the high temperature accelerated recovery and recrystallization,leading to strain softening of Ni₂Co₁Fe₁V_0.5Mo_0.2 MEA right after yielding and dramatical decrease of uniform elongation.（2）The Ni₂Co₁Fe₁V_0.5Mo_0.2 MEA possessed excellent creep property at 700℃/150 MPa.An increase in creep temperature or an increase in stress could lead to earlier plastic instability.The stress exponent of Ni₂Co₁Fe₁V_0.5Mo_0.2 MEA was 3.1 and the creep activation energy was456.3 k J·mol^-1.The stress exponent indicated that the creep deformation of the Ni₂Co₁Fe₁V_0.5Mo_0.2 MEA was dominated by the viscous slip of dislocation.The creep activation energy was close to the lattice diffusion activation energy of Mo element(480 k J·mol^-1),indicating that the creep rate of the Ni₂Co₁Fe₁V_0.5Mo_0.2 MEA was controlled by the slow diffusion of Mo element.（3）The Ni₁₄Co₂₀Fe₂₀Cr₂₆Mn₂₀ HEA exhibited excellent capability to withstand dynamic loading and shear band in the strain rate of 1000～3000 s^-1.With strain rate increasing,the yield strength increased by 28%from 288 MPa to 360 MPa.The excellent capability to withstand dynamic loading was attributed to high work hardening ability,high strain rate hardening ability and low temperature rise.The work hardening exponent,strain rate sensitivity,activation volume and temperature rise were 0.83～0.95,0.076,11b³ and 64.7 K,respectively.The dominant deformation mechanism changed from dislocation slip at 1000 s^-1 to stacking fault（SF）at 2000 s^-1 and twining at 3000 s^-1.The interactions between dislocations and SFs,the interactions between SFs and SFs and the reactions between SFs and twins were the main reasons for the high work hardening ability of Ni₁₄Co₂₀Fe₂₀Cr₂₆Mn₂₀ HEA.（4）The Ni₁₄Co₂₀Fe₂₀Cr₂₆Mn₂₀ HEA exhibited superior capability to withstand dynamic loading at a temperature range of 77～293 K.With temperature decreasing,the impact toughness increased from 59.5 Jcm^-2 to 66.7 Jcm^-2.At liquid nitrogen temperature（77 K）,twinning and storage capacity of dislocations were enhanced,and additional HCP structuredεphase transformations were introduced.Multiple deformation mechanisms consumed the impact energy more effectively,provided higher work hardening ability,delayed crack propagation,and resulted in better impact toughness of Ni₁₄Co₂₀Fe₂₀Cr₂₆Mn₂₀ HEA at low temperature.