Heterogeneous lamella design to tune the mechanical behaviour of a new cost-effective compositionally complicated alloy

A heterogeneous lamella(HL)design strategy was applied to manipulate mechanical properties of a new cost-effective Fe_(35)Ni_(35)Cr_(25)Mo_(5)compositionally complicated alloy(CCA).The HL structure was produced by single-step heat treatment(800℃for 1 h)after cold rolling.This HL structure consists of alternative lamellae regions of coarse-grained FCC matrix(5-20μm),and regions containing ultra-fine grains or subgrains(200-500 nm)together with nanoprecipitates(20-500 nm)and annealing twins.As compared with other cost-effective CCAs,the 800℃annealed sample with HL structure demonstrated a comparable tensile property,with yield strength over 1.0 GPa and total elongation of~13%.Formation of the annealing twins and nanoprecipitates decorated HL structure was a result of the concurrent partial recrystallization and precipitation ofσphase at the shear bands with a high density of lattice defects(e.g.high-density dislocation walls and deformation twins).The latter restricted the growth of recrystallized grains,leading to the formation of ultrafine subgrains within the HL structure.The high yield strength resulted from the multistage hetero-deformation induced(HDI)strengthening and precipitation strengthening associated with heterogeneous lamella structures containing nanoprecipitates.The ductility was originated from the coexistence of multiple deformation mechanisms,which started with dislocation slip and formation of stacking faults at the initial stage,followed by nano-twinning at the higher strain level.This HL design strategy,comprising composition and thermomechanical process designs,and the resultant microstructure tuning,open a broader window for the development of cost-effective CCAs with enhanced performance.

A new strategy for fabrication of unique heterostructured titanium laminates and visually tracking their synchronous evolution of strain partitions versus microstructure

Heterostructured(HS)material with extraordinary mechanical properties has been regarded as one of the most promising structural materials.Here,we reported a new strategy for preparing heterostructured pure titanium laminates that possess a good combination of strength and ductility by combining gradient structure(GS)and heterogeneous lamella structure(HLS).The deformation characteristic versus microstructure evolution of GS/HLS titanium laminates,namely the strain partitions between different-sized grains(480–25μm)was visualized using a scanning electron microscope(SEM)equipped with electron backscattered diffraction(EBSD)mode combined with the digital image correlation(SEM-DIC)with an ultrahigh spatial resolution for the first time.As a result,the hetero-deformation of unique GS/HLS structure by the characteristic of strain partitions could be accurately captured.While the hetero-deformation could result in the hetero-deformation induced(HDI)stress strengthening and HDI hardening,which were regarded as the key reason that the resulting GS/HLS Ti laminates showed a superior combination of strength and ductility.This could promote a more in-depth understanding of the strengtheningtoughening mechanism of heterostructured material.

Strengthening and toughening bulk Ni_(2)CoFeV_(0.5) medium-entropy alloy via thermo-mechanical treatment

Single-phase face-centered cubic(fcc)medium-and high-entropy alloys(MEAs/HEAs)have high ductility but low yield strength.In this work,the microstructures of single-phase fcc Ni_(2)CoFeV_(0.5) MEAs were tailored by cold-rolling and subsequent annealing and typical heterogeneous lamella(HL)structures composed of recrystallized micro-grain lamellae(with an averaged grain size of∼4μm)and nonrecrystallized nano-/ultrafine-grain lamellae were obtained.Tensile tests revealed that most HL samples exhibited excellent strength and ductility synergy.The HL sample with∼23 vol%recrystallized grains annealed at 590℃ for 1 h had a high yield strength of 1120 MPa and a good fracture elongation of 12.3%,which increased by 5%and 46%,respectively compared with those of as-rolled sample.Annealing-induced yield strength increase is attributed to high-density annealing twin boundaries(TBs)in the recrystallized grains,the annihilation of mobile dislocations inside the non-recrystallized grains,and extra heterodeformation-induced strengthening produced by the HL structure.Hall-Petch relationship of Ni_(2)CoFeV_(0.5) MEA can be reasonably described by counting both TBs and grain boundaries,with lattice friction stress of 87.3 MPa and coefficient of 722.8 MPaμm1/2.Our work provides optional and controllable solutions for preparing MEAs/HEAs with excellent mechanical properties by low-cost and high-efficiency thermomechanical treatments.