Three dimensional atom probe (3DAP) combined with SEM and TEM was applied to characterize the alloying elements partitioning segregation in cementite in experimental steel tempering at 450℃ for 4h after solution treatment at 1200℃ for 0.5h. The results indicated that, in the as-quenched specimen nearly all the alloying elements such as manganese, silicon and vanadium were dissolved in matrix with homogeneous distribution, and a slight segregation of carbon for the sake of autotempering. A significant segregated zone of carbon found in the 450℃ tempering specimen was detected as a cementite, in which was there an enrichment of manganese, a slight segregation of molybdenum and vanadium and negligible aluminum content, silicon was prone to partitioning at cementite/matrix interface from cementite. A monolayer in thickness enriched with carbon and vanadium was also detected, which may be a V4C3 based G.P. zone acted as an initiator of precipitation of alloyed carbides during an intensity or extensive tempering.
Experimental investigations on rolling texture evolution in plycrystalline aluminum with initial randomly distributed texture and nearly cube texture were carried out via orientation distribution functions and orientation line analysis in this paper. The results show that the rolling textures for the sample with initial randomly distributed texture mainly consists of S、C and B components during rolling deformation while the orientation density values for the three textures only have slight difference, most crystallites aggregate along the α and β orientations lines. The rolling texture for the sample with initial cube texture mainly consists of components C and S while the orientation density value for B component is much lower than those for C and S components, most crystallites accumulate along the β fiber. The volume fraction for S texture component is much higher than those for C and B textures under the two cases.
The rolling texture evolution with initial randomly distributed texture and nearly cube texture was simulated by Taylor-type models. A systematic comparison of the theoretical results obtained by the different type models against the experimental rolling textures is performed by orientation line analysis. The results shows that the orientation distribution calculated by the FC Taylor is closer to experimental results than those by other models at the low rolling deformation degree. During the higher deformation degrees, the results by the lath model agree the measured better for the initial randomly distributed texture while the simulated results obtained by the mixed are closer to the measured than those by the other models for initial cube texture.
Since J.von Neumann derived an exact formula for the growth rate of a cell in a two-dimensional cellular structures in 1952, people has attempted to find an exact extension of this result into three dimensions for a half century. In 2007, an exact three-dimensional von Neumann relation was reported in Nature (Nature, 2007, 446: 1053) by MacPherson and Srolovitz, which was regarded as a great progress of the long-time intense effort. However, the derivation of the exact three-dimensional von Neumann relation was complex and the quantity of “the mean width” of a real grain was difficult to measure. In this paper, based on the capillarity-driven nature of the grain growth, we derived the exact three-dimensional von Neumann-Mullins relation for a convex polyhedron grain in a simple method, which is independent of any additional assumptions concerning any grain size distribution, topology distribution or grain shape. It is shown in this paper that the three-dimensional growth rate of a convex polyhedron grain is related to two one-dimensional quantities: the grain’s mean caliper diameter and the sum of the length of its edges, which agrees with the property pointed by D. Kinderlehrer (Nature, 2007, 446: 995): the rate of change of n-dimensional volume is related to (n-2)-dimensional features of the cell and no others.
Abstract：Considering the solute trapping in non-equilibrium rapid solidification, in this paper, we discuss the effect of solute trapping on the growth process basing on the existent eutectic models. It is found that the growth velocity is increased and lamellar spacing and dendritic tip radius are decreased, and the smaller the equilibrium solute partition coefficient, the more prominent the effect.
Based on Darcy’s law for interdendritic flow, the present study developed a mathematical model for hydrogen porosity formation in Al castings, which specially considers the hydrogen macrosegregation including hydrogen diffusion and transport in macroscopic scale. An upward unidirectional solidification experiment of Al-4.5wt%Cu casting with in-situ melting was carried out. The microstructure analysis showed that the casting included columnar, CET (columnar-to-equiaxed transition), and equiaxed regions in upward direction. It was found that there exist a decreasing distribution of porosity in the columnar region and porosity with high content in casting part next to the bottom chill. The proposed model was applied to the experimental casting. The simulation results were in well agreement with the experiment results. It was indicated that the simulation without the hydrogen macrosegregation can’t well predict the microporosity in the casting, and that the hydrogen macrosegregation had significant influence on the predicted porosities, especially their nucleation and distribution.
Most of the previous parameters that utilized as a crack driving force were established in modifying the parameter Kop in Elber’s effective SIF range (ΔKeff =Kmax–Kop). This paper focuses on the physical meaning of compliance changes caused by plastic deformation at the crack tip, the test were carried out under constant amplitude loading by using structural steel, and differences of several parameter ΔKeff from literature were analyzed. The effect of actual stress (load) amplitude at the crack tip on fatigue crack growth is investigated based on these test results, and improved two-parameter driving force model ΔKdrive=(Kmax)n(ΔK^)1-n has been proposed. Experimental data for several different types of materials taken from literature were used in the analyses. Presented results indicate that new parameter ΔKdrive was equally effective or better than ΔK(=Kmax-Kmin), ΔKeff(=Kmax-Kop) and ΔK*(=(Kmax)α(ΔK+)1-α) in correlating and predicting the R-ratio effects on fatigue crack growth rate.
The creep behaviors of pure magnesium in different states at the temperatures 75~200℃ under stresses 15~40MPa were studied. The results indicates that the grain size has remarkable effect on the creep behavior. The as-cast magnesium has low creep rate with coarse column crystals. However, the creep rate increases significantly as the grains become fine equiaxed due to dynamic recrystallization after extrusion and deceases when the grains coarsen after anneale-treatments. The stress exponents lie in the range 4.3-4.9 under low stresses,which is consistent with the “Five-Power-Law” suggesting the creep is dislocation climb-controlled. However, the values of over 7 under high stresses indicate power-law breakdown. The apparent activation energies range from 76.0kJ/mol to 89.4kJ/mol. According to the stress exponents and the activation energies as well as the microstructure analysis during creep, the creep are affected by dislocation climb, grain boundary sliding and twinning, among which the former two play dominant role.
The corrosion behavior of Cu in different concentration of NaCl and the inhibition of PASP were studied by photoelectrochemical method. The copper electrode in a borax buffer solution showed p-type photoresponse which came from Cu2O layer on its surface. When a little NaCl(<0.5g/L) was added, the Cu2O layer didn’t change its semiconductor style; When more NaCl(0.5-15g/L) was added, a part of the Cu2O layer changed into n-type because of the doping of the Cl- . When a large amount of NaCl(>15g/L) was added, the Cu2O layer totally changed into n-type. When the concentration of NaCl was 2g/L, PASP played a competitive adsorption with Cl- and prevented Cu2O layer from being doped, so the Cu2O layer showed p-type. When the concentration of NaCl was 30g/L, the competitive adsorption of PASP only prevented Cu2O layer from doping slightly, the Cu2O layer still changed into n-type, but the n-type was weakened..
Wet gas and solution environments containing H2S/CO2 were simulated in a high temperature and high pressure autoclave. Corrosion behaviors of API-X60 pipeline steel were investigated with Scanning Electron Microscopy (SEM) and X-ray Diffraction methodology (XRD). Effects of high H2S partial pressure on corrosion rate and scale were discussed. The results show that corrosion rate increased with H2S partial pressure in wet gas but raised up first and then dropped down in the solution immersion environment. Pitting corrosion was observed at the situations with H2S partial pressure 2.0MPa. The corrosion process was controlled by H2S and mackinawite (Fe1＋XS) was the dominant composition in the corrosion products free of FeCO3. Pitting was occurred in the scale’s defect which was acidified by the HS- and Cl- ions migrating, and the development of the pitting was affected by the electrochemical property of the sulfur-rich phase and FeCl2 admixture. The crystal morphology and structure were more complicated in the scales formed in immersion environment, which were less integrated and bonded than that formed in wet gas. However, hydrogen blistering and hydrogen-induced cracking were more easily occurred for X60 in wet gas than that in immersion environment.
In the present work, silane-based organic-inorganic hybrid films doped with rare earth salt (cerium nitrate) were prepared by sol-gel method using γ-glycidoxypropyltrimethoxysilane (γ-GPTMS) and tetraethoxysilane (TEOS) precursors on 2A12 aluminum alloys. The electrochemical tests (such as polarization curve and electrochemical impedance spectroscopy (EIS)) were preformed to investigate the preparing process of Ce-doped organic-inorganic hybrid films (including the concentration of cerium salt doped in films and curing temperature of coatings) and characterize the performances of corrosion protection of the coatings for 2A12 aluminum alloys. At the same time, the corrosion resistance of Ce-doped coatings was compared with non-doped silane-based hybrid films, chromate conversion coating and rare earth conversion coating. The results of polarization curve tests indicated that the polarization resistance value of Ce-doped silane-based hybrid films increased by more than one order of magnitude compared with that measured for non-doped coating, and much greater than those of chromate and rare earth conversion coatings. EIS measurements gave the results in good agreement with those obtained from polarization curves, suggesting that silane-based hybrid process present promising potentialities as alternative method of chromating by improving the preparing technology of these films.
Hot compression deformation of AZ91 magnesium alloy has been performed on Gleeble-1500 under conditions of strain rates of 0.001～1s-1 and deformation temperatures of 250～400℃，the flow stresses in different deformation conditions are investigated. The results show that the flow stress is significantly affected by both deformation temperature and strain rate，the flow stress decreases with the deformation temperature increasing and strain rate decreasing. The relations of the deformation activation energy and stress exponent with strain are obtained using the hyperbolic-sine mathematics model，and the hot deformation constitutive relationship being established. Comparing with experimental results，it is proved that the model reflects the real feature of the deformation of AZ91 alloy.
An Al85Ni5Y6Fe2Co2 metallic glass was prepared by melt spinning. Two endothermic steps were observed on the normal differential scanning calorimetry, which is mostly ascribed to two glass transition processes caused by amorphous phase separation. Temperature modulated differential scanning calorimetry (TMDSC) reveals that the two endothermic steps are not attributed to two glass transition processes. High-angel annular detector dark-field scanning transmission electron microscopy (HAADF-STEM) and EDS formerly gave evidence of amorphous phase separation for the sample heated to 300oC. Only one glass transition was observed on the reversible heat flow of the TMDSC curves for this sample.
Al85Ni5Y10-xCox (x = 0, 2) and Cu46Zr54-xAlx (x = 0, 7) metallic glasses (MGs) have been examined using fluctuation electron microscopy (FEM). Strong medium-range order (MRO) was revealed in both MG systems, by peaks in intensity variance curves. Minor changes in composition result in differences in these peaks, while diffraction patterns remain identical. Specifically, minor alloyings of Co into the Al85Ni5Y10 and Al into the Cu46Zr54, forming Al85Ni5Y8Co2 and Cu46Zr47Al7 MGs, render the medium-range structure more homogeneous. This is in a good agreement with the increase of glass-forming ability in the two systems. This result sheds light on FEM as a useful tool to totally sense the structural changes in the medium range, helpful to study the effect of sample composition on properties of metallic glasses.
TiO2 composite nanoparticles were synthesized by adding iron salt and surfactant Cetyl trimethyl ammonium bromide (CTAB) into the sol-gel process of Tetrabutyl titanate (TBOT). The composite particles, whose absorption maximum was appeared in about 530nm, showed the photocatalytical activity toward Rhodamine B. The effect of Fe3+ mass on the structure and properties of composite materials was systematically studied. Some means such as TEM, XRD, TG-DTA and UV-Vis were used to character these particles. The results showed, that the Fe3+ iron was doped in the crystal lattice of TiO2 and the new bandwidth (Eg=1.37eV) was appeared. Because of that, the spectral response property of TiO2 composite materials was changed into visible light range.
The microstructure of the room temperature ferromagnetic semiconductor Zn1-xCoxO1-δ was investigated by analytical electron microscopy. The experimental results indicated that the oxygen content decided the microstructure and the magnetic property of Zn1-xCoxO1-δ. The films deposited under poor oxygen were consistent of 5nm nanocrystal wurtzite Zn1-xCoxO1-δ and amorphous Co. They were all ferromagnetic phases. The films deposited under rich oxygen were consistent of 10~20nm nanocrystal Zn1-xCoxO1-δ and antiferromagnetic phase CoO. The concentration of the oxygen vacancies was greatly reduced, and the room temperature ferromagnetism was greatly weakened and even disappeared.
Microstructure of weld nugget zone and mechanical properties of the whole and slice in the thick plate joints obtained by friction stir welding (FSW) were studied. The results show that the tensile strength σb, yield strength σ0.2 and elongation δ decrease with increasing weld speed when the rotary speed is constant. The maximum σb, σ0.2 and δ lie in the top weld and reach 186.7MPa, 100.3MPa and 14.1% respectively, while the minimum σb, σ0.2 and δ in the bottom joint only attain 157.5MPa, 80.2MPa and 10.1% separately. The fracture morphologies show that there are lots of meshy dimple and the deepest locates in the top joint. The secondary intercrystalline crack and quasi-cleavage face lie in the root of weld. The microhardness at the top is higher than that at the bottom and presents asymmetrically through the weld centerline. The fine and equiaxial grain size is bigger at the top than that at the bottom and clings together at the root of weld. Compared with the bottom, second-phase particle at the top redissolves into the matrix abundantly and the strengthening effect boosts up.
