Compositionally complex coherent precipitation-strengthened high-entropy alloys:a critical review

The coherent precipitation-strengthened high-entropy alloys(CPS-HEAs)as a new type of structural materials are expected to possess many unique mechanical properties,such as the outstanding strength-ductility com-bination at cryogenic and room temperatures.Apart from this,most of their strengths can even be well retained at elevated temperatures.The compositionally complex matrix and nanoprecipitation phases,as well as the coherent interfaces between them,can potentially bring novel merits of these CPS-HEAs,including sluggish dif-fusion,excellent thermal stability,and controlled magnetic properties.Note that the ductile coherent L1_(2)-nanoparticles can improve the strength of alloys without too much reduction of plasticity,while the coherent B2-nanoparticles strengthened HEAs display completely brittle failure upon tensile test at room temperature.An overview of the alloy design,microstructure evolution,oxidation resistance,mechanical and magnetic properties of CPS-HEAs are briefly discussed here.The advantages of multicomponent coherent precipitation-strengthened HEAs as well as the limitations in this field are also summarized.In addition,this review also points out the future research directions and prospects.

Thermal stability and deformation mechanisms in Ni-Co-Fe-Cr-Al-Ti-Nb-type nanoparticle-strengthened high-entropy alloys

The precipitate morphologies,coarsening kinetics,elemental partitioning behaviors,grain structures,and tensile properties were explored in detail for L1_(2)-strengthened Ni_(39.9)Co_(20)Fe_(15)Cr_(15)Al_(6)Ti_(4-x)Nb_(x)B_(0.1)(x=0 at.%,2 at.%,and 4 at.%)high-entropy alloys(HEAs).By substituting Ti with Nb,the spheroidal-to-cuboidal precipitate morphological transition,increase in the coarsening kinetics,and phase decomposition upon aging at 800°C occurred.The excessive addition of Nb brings about the grain boundary precipitation of an Nb-rich phase along with the phase decomposition from the L1_(2)to lamellar-structured D019 phase upon the long-term aging duration.By partially substituting Ti with Nb,the chemically complex and thermally stable L12 phase with a composition of(Ni_(58.8)Co_(9.8)Fe_(2.7))(Al_(12.7)Ti_(5.8)Nb_(7.5)Cr_(2.3))ensures the stable phase structure and clean grain boundaries,which guarantees the superb high-temperature mechanical properties(791±7 MPa for yielding and 1013±11 MPa for failure)at 700℃.Stacking faults(SFs)were observed to prevail during the plastic deformation,offering a high work-hardening capability at 700°C.An anomalous rise in the yield strength at 800℃was found,which could be ascribed to the multi-layered super-partial dislocations with a cross-slip configuration within the L1_(2)particles.