Effect of a homogeneous recrystallized microstructure and a bimodal microstructure on mechanical properties in Mg-5Zn-0.6Zr alloys

For typical Mg-Zn-Zr alloys,exhilaratingly high strength of a yield strength(YS)higher than 300 MPa can hardly be attained by traditional rolling.In this paper,we compare the mechanical properties and strengthening mecha-nisms of the Mg-5Zn-0.6Zr alloys having a homogeneous dynamical recrystallized microstructure and a bimodal microstructure with high-density nano substructures.The Mg-5Zn-0.6Zr alloy with the bimodal microstructure(rolled at 150℃ with a thickness reduction of 60%)exhibits a YS of 332 MPa,an ultimate tensile strength(UTS)of 360 MPa,and an elongation of 5%.The high strength is attributed to the microstructure with high-density nano substructures,high-density nano(Mg,Zr)Zn_(2) precipitates,ultrafine recrystallized grains,and strong basal texture.In comparison,the Mg-5Zn-0.6Zr alloy with homogeneous microstructure(rolled at 200℃ with a thick-ness reduction of 70%)exhibits a YS of 209 MPa,an UTS of 317 MPa,and an elongation of 17%,which contains coarser recrystallized grains,coarser precipitates,weaker texture,and lower density of dislocations,further re-sulting in low strength.The difference between the strengthening mechanism in two kinds of microstructure is discussed in detail.The results facilitate the preparation of wrought Magnesium alloy with high strength by reasonable microstructure construction.

The controlling factors on metallurgical properties of advanced high strength steel sheets

Recent progress in production process for advanced high strength steel sheets（AHSS） is described focussing on Bainitic ferrite steel,transformation induced plasticity（TRIP） steel and precipitation hardened steel by NANO sized precipitates are reviewed. In higher tensile strength（TS） grade,elongation（El）,hole expansion ratio（λ） and their balance are eagerly requested to improve by customers.Bainitic ferrite steel which has shear deformed lath with various distribution of iron carbides and retained austenite should be distinguished to produce different balance of El/λ. Chemical composition and heat cycle are reviewed to produce various kinds of metallurgical phase and mechanical property.5%Mn -2%Si TRIP steel is introduced in which a plenty of retained austenite at about 30%and larger absorbed energy（AE） more than 3 000 kgf/mm;TS·%El（gauge length:50 mm） can be obtained. That steel also showed excellent upper shelf energy in Charpy test of 140 J/cm;,as well as excellent property of 20%El and 1 400 MPa TS after forging. A new type of precipitation hardened steel having NANO sized（Ti,Mo） C in ferrite matrix is introduced.It satisfies the both of excellent El andλat the same time. Bending property and holes expansion property obey the same kind of deforming criteria which depends on the forming capability of work hardened parts.Homogeneity of structure is their controlling factor and a new parameterαwhich is obtained by gauge length dependency of total elongation is proposed.

ZrNiSn-based compounds with high thermoelectric performance and ultralow lattice thermal conductivity via introduction of multiscale scattering centers

The high lattice thermal conductivity of half-Heuslers(HHs)restricts the further enhancement of their thermoelectric figure-of-merit(ZT).In this study,multiscale scattering centers,such as point defects,dislocations,and nanoprecipitates,are synchronously introduced in a n-type ZrNiSn-based HH matrix through Nb doping and Hf substitution.The lattice thermal conductivity is substantially decreased from 4.55(for the pristine ZrNiSn)to 1.8 W·m^(−1)·K^(−1) at 1123 K via phonon scattering over a broad wavelength range through the adjustment of multiscale defects.This value is close to the theoretically estimated lowest thermal conductivity.The power factor(PF)is enhanced from 3.25(for the pristine ZrNiSn)to 5.01 mW·m^(−1)·K^(−2) for Zr_(0.66)Hf_(0.30)Nb_(0.04)NiSn at 1123 K owing to the donor doping and band regulation via Nb doping and Hf substitution.This can be ascribed to the synergistic interaction between the lowering of the lattice thermal conductivity and retention of the high PF.Consequently,a ZT value of as high as 1.06 is achieved for Zr_(0.66)Hf_(0.30)Nb_(0.04)NiSn at 1123 K.This work demonstrates that these actions are effective in jointly manipulating the transport of electrons and phonons,thereby improving the thermoelectric performance through defect engineering.

Effect of Isothermal Annealing on Nano Ti-Carbides Precipitation

Transmission electron microscopy was used to investigate the effect of isothermal holding temperature and time on the nano Ti-precipitates.A holding temperature was varied systematically from 400℃ to 1200℃.The isothermal holding was continued for 30s,300s and 900s,respectively.Nano carbides of (Ti,Nb)C were precipitated significantly at 900℃.The size of carbides was approximately 10nm at 30s holding and increased to 20～30nm at 900s holding.Isothermal holding at 1000℃ showed the increased amount of carbides larger than 100nm.At 800℃,nano (Ti,Nb)C was not observed at 30s and it was examined at 300s.The size of nano (Ti,Nb)C was smaller than that of 900℃.As the isothermal temperature decreased to 700 ℃,the nano (Ti,Nb)C was only seen at 900s holding and the size of carbides was smaller than 10nm.Nano (Ti,Nb)C was disappeared at isothermal holding below 600℃.The kinetics of nano (Ti,Nb)C precipitation were studied as a function of isothermal holding temperature and time,respectively,using the precipitate growth equations.

Altered age-hardening behavior in the ultrafine-grained surface layer of Mg-Zn-Y-Ce-Zr alloy processed by sliding friction treatment

To gain insight into the ageing behavior of ultrafine grain(UFG)structure,the precipitation phenomena and microstructural evolutions of Mg-6 Zn-1 Y-0.4 Ce-0.5 Zr(wt.%)alloy processed by sliding friction treatment(SFT)were systematically studied using hardness texting,transmission electron microscopy(TEM)equipped with high-angle annular dark-field scanning(HADDF-STEM),X-ray diffraction(XRD)and XRD line broadening analysis.The microhardness of the SFT-processed(SFTed)sample initially decreases from 109.6 HV to 104.8 HV at ageing for 8 h,and then increases to the peak-ageing point of 115.4 HV at16 h.Subsequently,it enters the over-aged period.The un-SFTed sample,as the counterpart,follows a regular ageing behavior that increases from 89.9 HV to 99.6 HV when ageing for 12 h,and then drops.A multi-mechanistic model is established to describe the strengthening due to grain refinement,dislocation accumulation,precipitation etc.The analysis reveals that the temperature sensitive UFG structure has an obvious grain coarsening effect,which arouses the soft phenomenon in the early ageing stage.But precipitation hardening provides an excellent hardness enhancement for overcoming the negative influence and helping to reach the peak-aged point.In our microstructural observations,a lot of equilibrium ultrafine Mg Zn2 precipitates precipitate along dislocations because defects can provide the favorable conditions for the migration and segregation of solute atoms.