Showing papers by "University of Science and Technology Beijing published in 2001"

Theoretical study on the phase stability, site preference, and lattice parameters for Gd(Fe, T)12

Abstract: The stability of the intermetallics Gd(Fe, T)12 and the site preferences of the ternary 3d or 4d transition element T are investigated by using a series of interatomic pair potentials, ΦFe-Fe(r), ΦFe-Gd(r), ΦFe-T(r), ΦT-T(r), ΦT-Gd(r), and ΦGd-Gd(r), for the first time. The calculated results show that adding either Cr, Mo, Ti, or V atoms makes the crystal cohesive energy of Gd(Fe, T)12 decrease markedly, proving that these atoms can stabilize Gd(Fe, T)12 with ThMn12 structure even though the GdFe12 crystal structure is itself metastable. The calculated lattice parameters are in good agreement with experiment. The amount of cohesive energy decrease is correlated with the species and occupation site of the ternary atoms. The order of site preference of these stabilizing elements T is 8i, 8j, and 8f, with 8i corresponding to the greatest energy decrease. The calculated results further show that the addition of Co, Cu, Ni, Sc, and Zn does not stabilize the GdFe12 phase in the ThMn12 structure. The calculated results reported correspond well to available experimental data indicating that the ab initio interatomic potentials can be used to describe rare-earth materials.

A preliminary study on the creep behavior of Ti–45Al–10Nb alloy

Abstract: The creep response of a nearly-lamellar Ti–45Al–10Nb at 760°C and 207 MPa suggests that high Nb additions greatly improve the creep resistance of TiAl alloys. The alloy exhibited a minimum creep rate of 6×10−9 s−1 and the time to 0.5% strain of 80 h. These properties are comparable to that of the most creep resistant wrought and cast TiAl-base alloys.

Microstructure and ductile–brittle transition of as-cast Zr-based bulk glass alloys under compressive testing

Abstract: This paper investigates mechanical properties and fracture mechanisms of Zr 52.5 Cu 17.9 Ni 14.6 Al 10 Ti 5 alloys with various volume fractions of quenched-in crystalline. The alloys with various volume fractions of quenched-in crystalline were prepared by controlled oxygen content of alloys and overheating of the pouring. The phase structure, particle size and volume fraction of all samples were identified by X-ray diffraction, differential scanning calorimeter (DSC) curves and scanning electron microscopy (SEM) photographs. The mean sizes of crystalline increased from 0.3 to 1.3 μm with increasing volume fraction of crystalline from 4 to 13%. The compressive mechanical tests show a ductile–brittle transition with significant decrease in the fracture stress and ductility. Detailed observations in the flow deformation and fracture surface illustrate the relationship between the quenching-in crystalline and the mechanical behavior. The full bulk amorphous Zr-based alloy exhibits typical ductile deformation and fracture behavior. The torn shear bands form the typical vein patterns on the fracture surface. The effects of quenching-in crystalline on the flow deformation and fracture behavior depend on the nature, size, volume fraction and distribution. The particle size of the crystalline in the sense of the width of shear bands is critical. When the size is larger than the width of the shear bands the particles induce an obvious inhomogeneity of the flow deformation and more microcracks by the separation of the interfaces. Nano-scale particles, on the other hand, may increase the viscosity of the flow but do not form microcracks, resulting in particle strengthening of the metallic glass. Increasing the volume fraction of large-scale particles is favorable to leaking the microcracks and brittle fracture. With increasing particle size and volume fraction up to two times the width of the shear band and 10% vol., respectively, the ductile fracture of bulk amorphous alloy completely transforms to brittle fracture under compressive testing.

Introduction of a texture component crystal plasticity finite element method for anisotropy simulations

Abstract: The authors present a physically based and time efficient method to include and predict elastic-plastic anisotropy during complex metal forming operations. This new method is based on the direct integration of crystallographic texture components into a nonlinear finite element model, and the approach is designed for performing fast simulations of industry-scale metal forming operations of textiured polycrystalline materials including texture update.

Fracture Morphology and Quenched-in Precipitates Induced Embrittlement in a Zr-base Bulk Glass

Abstract: The fracture morphology and quenched-in embrittlement in Zr 52.5 Ni 14,6 Al 10 Cu 17.9 Ti 5 bulk glass were investigated by tensile and compressive tests at room temperature at the same strain rates of 4 × 10 -4 s -1 and scanning electron microscopy (SEM) observation. SEM analysis based on the deformation and fracture features indicates that the normal stress and shear stress on the fracture surface play a different role in the shearing-off of the specimens in tension and compression. The shear stress is the main controlling factor for the fracture in compression. and the fracture surface is along the maximum shear stress plane. In tension, however, both the shear and normal stresses govern the fracture process together, and the fracture surface is along the plane with an angle of 56 deg away from the axial direction. The fracture flrstly starts from a random region on the surface where there is a stress concentration due to the voids or shear bands. The shearing-off leads to a dilatation and softening of the local glass. The softening then promotes the shearing-off and leads to final catastrophic fracture. The crystalline precipitates significantly influence the tensile and compressive properties. With increases in the volumetric fractions and the sizes of the precipitates, both the tensile and compressive strength and fracture strains decrease. The fracture mode changes from ductile to brittle. Vein patterns, shear-bands and local melting can still be observed when the volume fractions of quenched-in precipitates are less than 3 ∼ 5%. When the precipitates exceed 5% in volume fraction, fracture surface becomes rock strata-like feature, and samples lose almost their strength. The precipitates with larger sizes and no-spherical shapes play a role in rising stress concentration, resulting in decreasing the fracture strength.

Moisture induced environmental embrittlement of intermetallics

Abstract: This paper reviews recent investigations of the kinetic aspects of moisture induced environmental embrittlement of intermetallic alloys. The main kinetic steps of the embrittlement include surface adsorption, oxidation to form atomic hydrogen, diffusion of dissolved hydrogen [H] from surface to bulk, and hydrogen induced embrittlement. The first part of the paper reviews the chemical aspects of surface adsorption and oxidation reactions using thermodynamic approaches and kinetic equations. The second part of the paper reviews the aspects of hydrogen diffusion in fcc and bcc related ordered structures and the hydrogen concentration profile at room temperature. Then, three possible cases (diffusion rate limited, adsorption reaction rate limited, and mixed rate limited) and their related kinetic equations are discussed. The influence of certain important kinetic factors on the hydrogen concentration profile is also considered. The beneficial effect of B additions on environmental embrittlement is ill...

Prediction and Analysis on Formation of Internal Cracks in Continuously Cast Slabs by Mathematical Models

Abstract: The formation of internal cracks in continuously cast slabs is mainly attributed to the strain status and microsegregation near the solidifying front of the slabs. Based on this understanding, the effects of the strain status at solidifying front and the chemical composition of liquid steel on the internal cracks were studied using a strain analysis model and a microsegregation model developed in the present study. The tensile strains at the solidifying front caused by bulging, unbending, and misalignment of supporting rolls in a four-point-unbending bow caster were calculated. The roll gap in the caster was measured for the calculation of the strains caused by the misalignment of the supporting rolls. The calculated strain status near the solidifying front was used to predict the internal cracks. Critical strains based on some experimental data were adopted as the crack criteria. Sulfur prints of the slab transverse sections were used to verify the model predictions. The enrichment of chemical compositions in the interdendritic liquid and its effect on the freezing temperature of the liquid were studied with the microsegregation model, in which the transition of ferritic/austenitic solidification and the precipitation of MnS were taken into account. S and P were revealed to strongly accumulate at the columnar grain boundaries, and the segregation of P increases significantly when C content increases from 0.1 % to 0.2 %. With the accumulation of P and S in the interdendritic liquid, the freezing temperature of the liquid decreases obviously, thus the internal crack tendency is greatly increased.

Modelling coal gasification in an entrained flow gasifier

Abstract: Coal gasification processes in a slurry-feed-type entrained-flow gasifier are studied. Novel simulation methods as well as numerical results are presented. We use the vorticity-stream function method to study the characteristics of gas flow and a scalar potential function is introduced to model the mass source terms. The random trajectory model is employed to describe the behaviour of slurry-coal droplets. Very detailed results regarding the impact of the O2/coal ratio on the distribution of velocity, temperature and concentration are obtained. Simulation results show that the methods are feasible and can be used to study a two-phase reacting flow efficiently.

Stress corrosion cracking relation with dezincification layer-induced stress

Abstract: Brass foil with a protective layer formed on one side was deflected during corrosion in an ammonia solution under various applied potentials, and then corrosion-induced stress generated at brass/dezincification layer under different potentials could be measured. At the same time, susceptibility to stress corrosion cracking (SCC) of brass in the ammonia solution under various applied potentials was measured by using a single-edge notched specimen. At open-circuit potential, both corrosion-induced tensile stress and susceptibility to SCC (Iσ) had a maximum value. Both tensile stress σp and susceptibility Iσ decreased slightly with decreasing potential under anodic polarization, but reduced steeply with a decrease in potential under cathodic polarization. At the cathodic potential of − 500 mVSCE, corrosioninduced stress became compressive because of the copper-plating layer; correspondingly, susceptibility to SCC was zero. Therefore, the variation of SCC susceptibility with potential is consistent with that of the corrosion-induced additive stress.