Annealing hardening effect aroused by solute segregation in gradient ultrafine-grained Mg-Gd-Zr alloy

In this study,a remarkable annealing hardening effect was detected in gradient ultrafine-grained(UFG)Mg-0.32Gd-0.11Zr(at.%)alloy sheet fabricated by sliding friction treatment(SFT).Under the precipitation-free condition,the annealed UFG structure shows an obvious hardness increment from 1.40 GPa to 1.89 GPa after 200℃ heating for 12 h,which exhibits a much higher hardening response than the annealed coarse-grained(CG)structure.The high-angle annular dark-field scanning transmission elec-tron microscopy(HAADF-STEM)and elemental mapping reveal prominent segregation of solute Gd atoms along grain boundaries,which endows the UFG structure with excellent grain boundary stability.More-over,Gd segregation is also found around the extrinsic stacking fault(E-SFs)and the low-angle grain boundaries composed of edge dislocations.The large-scale solute partitioning provides a significant seg-regation hardening effect,which completely resists the softening effect aroused by the grain coarsening and dislocation annihilation.This work realizes a good combination of surface mechanical processing for fabricating UFGs and subsequent heat treatment,which earns desirable segregation hardening effects.

Unveiling anneal hardening in dilute Al-doped Al_(x)CoCrFeMnNi(x = 0,0.1) high-entropy alloys

Anneal hardening has been one of the approaches to improve mechanical properties of solid solution alloys with the face-centered cubic(FCC) structure,whereby a considerable strengthening can be attained by annealing of cold-worked alloys below the recrystallization temperature(T_(rx)).Microscopically,this hardening effect has been ascribed to several mechanisms,i.e.solute segregation to defects(dislocation and stacking fault) and short-range chemical ordering,etc.However,none of these mechanisms can well explain the anneal hardening recently observed in phase-pure and coarse-grained FCC-structured high-entropy alloys(HEAs).Here we report the observations,using high-resolution electron channeling contrast imaging and transmission electron microscopy,of profuse and stable dislocation substructures in a cold-rolled CoCrFeMnNi HEA subject to an annealing below T_(rx).The dislocation substructures are observed to be thermally stable up to T_(rx),which could arise from the chemical complexity of the high-entropy system where certain elemental diffusion retardation occurs.The microstructure feature is markedly different from that of conventional dilute solid solution alloys,in which dislocation substructures gradually vanish by recovery during annealing,leading to a strength drop.Furthermore,dilute addition of 2 at.% Al leads to a reduction in both microhardness and yield strength of the cold-rolled and subsequently annealed(≤500℃) HEA.This Al induced softening effect,could be associated with the anisotropic formation of dislocation substructure,resulting from enhanced dislocation planar slip due to glide plane softening effect.These findings suggest that the strength of HEAs can be tailored through the anneal hardening effect from dislocation substructure strengthening.