Influence of Heat Treatment on Cr18Mn18 High Nitrogen Steel Precipitating Behavior and Microstructure

The grain size and precipitate amount which are affected by heat treatment have significant impact on the properties of high nitrogen austenitic stainless steel. In this study, Cr18Mn18 high nitrogen steel sheet is employed to investigate the effects of precipitate on austenitic grain size. It can be seen that the lamella precipitates which are rich in nitrogen and chromium nucleate in the austenite grain boundary and grow inward into grain when aged at 800 ℃ through electron probe micro-analyzer. The transmission electron microscopy results demonstrate that the precipitate is Cr2N and its morphology are detected as ellipsoid-like with major axis of 100-300 nm and minor axis of 50-100 nm roughly. The experiment show that coarsen of the austenite grain is quite critical at 1000-1100 ℃. However, the samples which pre-precipitated at 800 ℃ for 240 min to obtain the most nitride precipitate exhibits much smaller grain size than the as-rolled samples after solid solution treated at 1000, 1050 and 1100 ℃ for 240 min. The results show that the nitride precipitates in the grain boundary can effectively pin the austenite grain boundary and inhibit the grain growth.     

Dynamics in simultaneous electro-generative leaching for sphalerite-MnO2

The principle for the electro-generative leaching was applied to simultaneous leaching of sphalerite-MnO2. A galvanic system for the bio-electro-generative leaching was set up. The effects of grain size and temperature on rate of zinc extraction fi＇om sphalerite under the conditions of presence and absence ofAcidithiobacillusferrooxidans （A.ferrooxidans） were studied, respectively. The results show that with bacteria, the maximum extraction of zinc fi＇om the ores with grain size of 16.6 μm can reach 32.01% after leaching for 12 h, while to obtain the same extraction ratio in the traditional bio-leaching route （i.e. not electro-generative one）10 d is needed to ore granules with same size. The unreacted shrinking core model was used for describing the reaction-relative and diffusion-relative phenomena presented in the process of the electro-generative leaching with and without bacteria, which is considered to be diffusion controlled. The activation energies of the anodic reaction for leaching system in the presence and absence of bacteria are 11.97 and 14.39 kj/mol, respectively, indicating that leaching rate can be decreased by A. ferrooxidans. SEM was used to study the effect ofA. ferrooxidans on the ores in the simultaneous electro-generative leaching, which indicates that the produced sulfur on the surface of the sulfides can be oxidized by A. ferrooxidans after bio-electro-generative leaching for 24 h, and the transferred charge due to the bacterial oxidation is up to 17.86%, which is an important part of the output electric quantity.     

Effects of High Density Electropulsing Treatment on Aging Kinetics of GH4199 Alloy

GH4199 alloy was treated by high density electropulsing for different time, and the microstructure evolution and tensile properties were examined. The results show that the diffusion of solute atoms in GH4199 alloy can be strongly accelerated by electropulsing. The growth activation energy of γ′ phase of 89.86 kJ/mol in the alloy electropulsing-treated, decreases by 64.31 % compared with that normally aged, which accelerates the precipitation and the growth of both the γ′ phase and carbides on the grain boundary of the alloy. The strength of GH4199 alloy increases with increasing of electropulsing treatment time, and the plasticity has no obvious change. The growth of γ′ phase and the precipitation of carbides on the grain boundary is the main reason for obstructing the dislocation motion and improving the plastic deformation resistance of the alloy. The piled-up dislocations can go through the grain boundary across the gap of carbides which is distributed on the grain boundary in chainlike form, and no degradation of the plasticity of GH4199 alloy appears.     

Effect of Zr Addition on Resistance to High Temperature Softening and Resistivity of 6082 Al-Mg-Si Alloys

The influence of zirconium on the resistance to high temperature softening of 6082 Al-Mg-Si alloys has been researched. The softening process of 6082 alloys with Zr and without Zr, isothermally conditioned at 250℃, has been investigated. The results show the inclusion of Zr inhibits the decrease of HB in hardness(HB) compared with the alloys without Zr. This is due to the Zr and Al forming coherent dispersoid-Al3Zr, and Al3Zr particles within an Al matrix, which effectively restricts grain boundary movement. A study is also carried out to investigate the changes in resistivity, which occurs during the ageing of alloys containing Zr, compared to those without Zr. It is found that in both cases there is an initial rapid rise in resistivity followed by a slower rise in resistivity as ageing proceeds. The value of resistivity is lower for 6082 alloys with Zr compared with 6082 alloys without Zr; this is explained in terms of the role of Zr in the 6082 alloys.     

Correlation Between Microstructure and Corrosion Behavior of Two 90Cu10Ni Alloy Tubes

Two kinds of 90Cu10 Ni tubes with different service lives(more than 3 years and only 1 year,respectively)under identical working conditions were studied by an immersion test in a 3.5 wt% NaCl solution and the electron backscattered diffraction(EBSD) technique.The morphology after immersion showed severer corrosion attack at the grain boundaries of the tube with shorter service life compared with the tube with longer service life.The grain boundary characterization distributions(GBCDs) of the two tubes obtained by EBSD revealed more Σ3 boundaries and twins,and larger random boundary meshes in the tube with longer service life.A short immersion test in a modified Livingston's solution was conducted to evaluate the tendency to corrosion attack of different types of the grain boundaries.SEM and AFM were used to characterize the corrosion morphologies of the boundaries.A strong correlation between varying depths of corrosion grooves and types of the grain boundaries was obtained.The influence of deviation angle of low Σ boundaries on corrosion resistance of the grain boundaries was also discussed.It is concluded that a special ‘‘grain boundary engineering'(GBE) treatment has been performed on the tube with longer service life.It is proposed that the optimized GBCD is responsible for the better service performance of the tube.     

The Effect of Si Impurity at the Al ∑5 Grain Boundary： a First Principle Computational Tensile Test Study

First principle computational tensile tests （FPCTT） are performed to the Al ∑5 grain boundaries （GBs） with and without substitution or interstitial Si impurity. The obtained stress-strain relationships and atomic configurations demonstrate that the Al ∑5 GBs with and without substitutional or interstitial Si impurity show different fracture modes. The mechanisms of the different fracture modes are analyzed based on the charge density and the density of states. The results show that the charge redistributions of the atoms in the vicinity of GBs and the covalent interactions between Si and its neighboring Al atoms determine the fracture modes.     

Corrosion behavior of NiCr alloys in HCl-containing oxidation atmosphere at 700-800℃

Corrosion behaviors of pure Ni and three NiCr alloys were investigated in an HCl-containing oxidizing at-mosphere at 700℃ and 800℃. All materials suffer from accelerated corrosion at both temperatures. NiCr alloys show an initial mass loss due to the formation of volatile CrCl3 and CrO2Cl2. Some chlorides are detected at the scale/substrate interface and many voids are also found there. NiCr alloys with higher chromium content have better corrosion resistance. However, Ni50Cr is inferior to Ni25Cr due to its two-phase structure, which makes it easy for chlorine to diffuse along grain boundary and to occur inner oxidation. The relevant corrosion mechanism was also discussed.     

Growth of bismuth telluride thin film on Pt by electrochemical atomic layer epitaxy

An automated thin-layer flow cell electrodeposition system was developed for growing Bi2Te3 thin film by ECALE. The dependence of the Bi and Te deposition potentials on Pt electrode was studied. In the first attempt,this reductive Te underpotential deposition (UPD)/reductive Bi UPD cycle was performed to 100 layers. A better linearity of the stripping charge with the number of cycles has been shown and confirmed a layer-by-layer growth mode, which is consistent with an epitaxial growth. The 4 : 3 stoichiometric ratio of Bi to Te suggests that the incomplete charge transfer in HTeO reduction excludes the possibility of Bi2Te3 formation. X-ray photoelectron spectroscopy (XPS) analysis also reveals that the incomplete charge transfer in HTeO2^ occurs in Te direct deposition. The effective way of depositing Bi2Te3 on Pt consists in oxidative Te UPD and reductive Bi UPD. The thin film deposited by this procedure was characterized by X-ray diffraction(XRD), scanning electron microscopy(SEM) and X-ray photoelectron spectroscopy(XPS). A polycrystalline characteristic was confirmed by XRD. The 2 : 3 stoichiometric ratio was confirmed by XPS. The SEM image indicates that the deposit looks like a series of buttons about 0.3 - 0.4/~m in diameter, which is corresponding with calculated thickness of the epitaxial film. This suggests that the particle growth appears to be linear with the number of cycles, as it is consistent with a layer by layer growth mode.     

KINETICS OF TEMPER EMBRITTLEMENT IN 2.25Cr-lMo STEEL USED FOR HOT-WALL HYDROFINING REACTORS

Based on the theory of grain boundary segregation,a kinetics model of temper embrittlement caused by long-term service for hot-wall hydrofining reactors was studied.The kinetics model was applied to phosphorus (P) segregation in 2.25Cr-1Mo steel used for a hot-wall hydrofining reactor,and the kinetics of grain boundary segregation of impurity P in the steel exposed to the process environment of the hydrofining reactor was calculated on the basis of the model.The Auger electron spectroscopy test was performed in order to determine the grain boundary concentration of P.The experimental result is agreement with the theoretical calculated data. The results show that the kinetics equation is reasonable for predicting the levels of grain boundary segregation of impurity P in 2.25Cr-1Mo steel used for hot-wall hydrofining reactors.     

GRAIN SHAPE EFFECT IN SUPERPLASTIC DEFORMATION

The method controlling grain shape in TMT processing and the effect of grain shape on char-acteristic parameters in superplastic deformation were discussed.The accommodation velocityof grahl boundary sliding,which is the dominant mechanism in superplastic deformation,andthe contribution of each mechanism to the total strain,as influenced by grain shape,were ana-lyzed.Grain shape has been shown to be an essential structural factor for superplasticity.Thenan analysis was made about the effect of grain shape on the region transition strain rate sothat a new concept,critical aspect for superplasticity,was worked out.These predictions werecompared with the measured results in an Al-Zn-Mg alloy.     

Crystal structure of Mg3Pd from first-principles calculations

Crystal structure of Mg3Pd alloy was studied by first-principles calculations based on the density functional theory. The total energy, formation heat and cohesive energy of the two types of Mg3Pd were calculated to assess the stability and the preferentiality. The results show that Mg3Pd alloy with Cu3P structure is more stable than Na3As structure, and Mg3Pd alloy is preferential to Cu3P structure. The obtained densities of states and charge density distribution for the two types of crystal structure were analyzed and discussed in combination with experimental findings for further discussion of the Mg3Pd structure.     

Electrodeposition of Ni-SiC nanocomposite film

The point of zero charge(PZC) of SiC nanoparticles was determined by means of standard potentiometric titration method, while the influences of the main technological parameters on the microstructure of electrodeposited Ni-SiC composite film were studied and optimized. The results show that high bath pH value favors SiC nanoparticles negatively charged and high bath temperature promotes them positively charged. Under the experimental conditions, sodium dodecyl-glycol is proven to be an effective surface modification anionic surfactant for SiC nanoparticles. The results also show that the optimized Ni-SiC composite film is composed of the nanoparticles with the average grain size in the nanometer range (100 nm), and SiC nanoparticles disperse into the nickel matrix uniformly.     

GRAIN BOUNDARY FEATURE AND CREEP BEHAVIOR OF Fe-15Cr-25Ni ALLOYS

Creep behavior of Fe-15Cr-25Ni alloys with different grain boundary features has been in-vestigated at 850 and 950℃ and in stress range of 14.7～78.5 MPa.The single phase alloyexhibits typical recovery creep characteristics,and for the alloy in which M_(23)C_6 carbidesdensely precipitate at grain boundaries,the creep rate dramatically decreases and creep behav-ior is also significantly different from the single phase alloy.When the carbides precipitate atgrain boundaries,the dislocation density are higher and the size of subgrains near grainboundaries are smaller than those of single phase alloy.The creep mechanisms for two grainboundary features have been discussed.     

Heat transfer and grain refining mechanism during melt treatment by cooling sloping plate

Heat transfer of flow melt and grain refining mechanism during melt treatment by the cooling sloping plate were investigated.The results show that the cooling sloping plate can refine not only grains of alloys but also can obviously refine pure metal.Cooling ability of the plate is the key factor that induces grain refining,the plate material and the flow amount can affect cooling rate of the melt and thus affect refining effectiveness.The cooling rate of the melt on the cooling sloping plate is much faster than that of the conventional casting process,which can reach 1000 K/s and belongs to meta-rapid solidification scope.The thickness of the temperature boundary layer is much larger than that of the velocity boundary layer on the sloping plate,but the temperature gradient is small in the temperature boundary layer.Under strong cooling action by the cooling plate,most parts of the melt on the plate surface can form undercooling,which causes continuous eruptive nucleation,this is the main grain refining mechanism,and the heterogeneous nucleation on the plate surface is a helpful supplement for the nucleation.     

Effect of grain size on corrosion behavior of electrodeposited bulk nanocrystalline Ni

Nanocrystalline (NC) and coarse-grained Ni with different grain sizes (from 16 nm to 2 μm) were fabricated by direct current electrodeposition. Effect of grain size on the electrochemical corrosion behavior of these Ni deposits in different corrosion media was characterized by using potentiodynamic polarization test, electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS) and immersion corrosion test. Results show that in the NaOH or NaCl solution, the NC Ni exhibits improved corrosion resistance with the decrease of grain size. But in H2SO4 solution, the higher grain boundary density accelerates corrosion due to no passive process and the corrosion resistance of NC Ni decreases with refining grain size. The distinct experimental results of NC Ni in corrosion behavior can be reasonably explained by the positive or negative effect of high-density grain boundaries in different corrosion media.     

Influence of Temperature on in Nanocrystalline Materials： the Inverse Hall-Petch Effect Phase Field Crystal Simulation

The influence of temperature on the inverse Hall-Petch effect in nanocrystalline(NC) materials is investigated using phase field crystal simulation method.Simulated results indicate that the inverse Hall-Petch effect in NC materials becomes weakened at low temperature.The results also show that the change in microscopic deformation mechanism with temperature variation is the main reason for the weakening of the inverse Hall-Petch effect.At elevated temperature,grain rotation and grain boundary(GB) migration seriously reduce the yield stress so that the NC materials exhibit the inverse Hall-Petch effect.However,at low temperature,both grain rotation and GB migration occur with great difficulty,instead,the dislocations nucleated from the cusp of serrated GBs become active.The lack of grain rotation and GB migration during deformation is mainly responsible for the weakening of the inverse Hall-Petch effect.Furthermore,it is found that since small grain size is favorable for GB migration,the degree of weakening decreases with decreasing average grain size at low temperature.     

Different grain refinement mechanisms of minor zirconium and cerium in magnesium alloys

Zirconium and rare earth element cerium were added in magnesium and magnesium alloys to study their different grain refinement mechanisms. The results show that zirconium has an obvious refinement effect on the cast grain of magnesium and its alloys without the alloy element Al because the crystal structure of zirconium is the same as magnesium matrix, and the lattice parameters are close to magnesium. Zirconium can decrease the grain size of magnesium from 150 to 20 pm. The rare earth cerium also has a grain refinement effect on Mg and Mg-Al alloy. The cerium atoms tend to remain in the liquid rather than solidify with the solvent atoms magnesium at the solid-liquid interface. The liquid constitutional undercooling can provide a heterogeneous crystal nucleation. The grain is refined from 200 μm to 40-80 μm. These two elements have different grain refinement mechalfism on Mg alloy. The mechanism of zirconium is that it acts as the nuclei of α-Mg. But the mechanism of cerium is that it increases the liquid constitutional undercooling that can provide a heterogeneous crystal nucleation for the alloy.     

Microstructure and magnetic properties of Nd-Fe-B sintered magnets with P addition

In this paper,the effects of P doping on magnetic properties and microstructure were studied in Nd-Fe-B sintered magnets.With P doping,the grain size gets refined and the distribution of the main phase is optimized due to the reduction of the liquidus temperature.The liquidus temperature for the 0.05 wt% P-doped magnets is 1022 K,while that for the P-free magnets is 1038 K.As P content increases,the liquidus temperature significantly decreases.Clear and continuous grain boundary phases are formed in the P-containing magnets with smaller grain size.The optimized microstructure with average grain size of 8.43 μm is obtained in the 0.05 wt% P-doped magnets,which is approximately 0.69 μm smaller than that of P-free sintered magnets(9.12μm).Though P is usually thought to be an impurity element,it might be beneficial in Nd-Fe-B sintered magnets with proper addition.The coercivity of the0.05 wt% P-doped magnets could be increased to1283 kA·m~(-1),with slight changes of the remanence and the maximum magnetic energy product.NdP04 phases in the grain boundary are of hexagonal structure,while those at the triple junctions have monoclinic structure.Activated sintering is achieved by doping proper P element in the Nd-FeB sintered magnets.     

Microstructures of an AlSi7Mg alloy by near-liquidus casting

Semi-solid ingots of an AlSi7Mg alloy were obtained using the method of near liquidus casting. Their micro- structures exhibit the characteristics of fine, equiaxed, and non-dendrite, which are required for semi-solid forming. The in- fluences of casting temperature, heat preservation time, and cooling rate on the microstructure were also investigated. The results show that in the temperature region near liquidus the grain size becomes small with a decrease in casting temperature. Prolonging the heat preservation time makes grain crassitude at the same temperature. And increasing the cooling rate makes grain fine. The microstructure of the alloy cast with iron mould is finer than that with graphite mould.     

Grain refinement and mechanism of CAl_（0.5）W_（0.5） compound in Mg-Al alloys

The effect of CAl0.5W0.5(CAW) compound on the grain refinement of Mg-Al based alloys was investigated.The results show that CAW compound is an effective and active grain refiner.The grain size of binary Mg-Al alloys is more than 500 μm,and it is changed to about 110 μm with a 1 wt.% CAW addition.The hardness increased with the decease of grain size monotonously.The mechanical properties are improved by the addition.The fine grain size is mainly ascribed to the dispersed Al2CO particles,which are very potent nucleating substrates for Mg-Al alloys.The nucleation cores formed by chemical reaction directly are well-distributed in the matrix.