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.

Glass-forming Ability and Corrosion Resistance of Zr—Cu—Al—Co Bulk Metallic Glass

Investigations were carried out to examine the effects of Co addition on the glass-forming ability （GFA） and corrosion resistance of Zr46Cu46AI8 bulk metallic glass in chloride-containing solution. It is found that the GFA of （Zr46Cu46Al8）100-xCox （x = 0, 1, 2, and 4 at.%） alloys reduces with the increase in Co content and correlates well with the parameters, such as the supercooled liquid region width ATx, the reduced glass transition temperature Trg and γ. The corrosion resistance is however found to be enhanced with the in- crease in Co concentration. The addition of Co causes the enrichment of Zr and Al, but depletes Cu in the surface films, which effectively enhances the corrosion potential and lowers the corrosion current density.

Orientation dependence of mechanically induced grain boundary migration in nano-grained copper

Tensile tests were carried out on gradient nanograined copper samples to investigate the grain orientation dependence of mechanically induced grain boundary migration(GBM) process. The relationship between GBM and the orientations of nanograins relative to loading direction was established by using electron backscatter diffraction. GBM is found to be more pronounced in the grains with higher Schmid factors where dislocations are easier to slip. As a result, the fraction of high angle grain boundaries decreases and that of low angle grain boundaries increases after GBM.