In Situ HRTEM Observation of Electron-Irradiation-Induced Amorphization and Dissolution of the E(Al18Cr2Mg3) Phase in 7475 Al Alloy

Electron irradiation effects on phase stability of the E(Al18Cr2Mg3) phase have been investigated by highangle annular dark-field scanning transmission electron microscopy and high-resolution transmission electron microscopy(HRTEM). The in situ HRTEM observations show that the Al18Cr2Mg3 particles with different thickness undergo amorphization and dissolution under 300 ke V electron irradiation at 25 °C. The results indicate that the intermetallic compound Al18Cr2Mg3 is unstable under electron irradiation, and structural changes mainly depend on the thickness of particles. Amorphization in the thick particles is caused by a combination of chemical disordering and an increase in point defect concentration. Dissolution after amorphization in the thin particles is attributed to the diffusion of point defect towards the Al matrix.     

Effect of rare earth （RE） on diffusion of aluminum atoms in aluminizing

The RE-aluminized coating and pure aluminized coating on 20 carbons steel were prepared by hot dip aluminizing method at 740 ℃. After diffusion treatment at 850 ℃for 4 h, the distribution of aluminum and lanthanum elements in the coating was analyzed with energy disperse spectroscopy（EDS） and electron probe microanalyses（EPMA）, and the lattice parameter ofa-Fe in the matrix of the coating was measured precisely by X-ray diffractometer（XRD）. The results show that RE permeates into the aluminized coating, leads to lattice disturbance and increases the depth of the aluminized coating. On the basis of the results, the expression of the diffusion coefficient of Al atoms is derived from the diffusion flow, and the effect of the high vacancy concentration and high concentration gradient of vacancies on the diffusion of Al atoms was analyzed by establishing the kinetics model of the vacancy mechanism of diffusion. The results show that the high vacancy concentration and high concentration gradient of vacancies in the RE-aluminized processes are the main reason why the diffusion coefficient of Al atoms in RE-aluminizing is bigger than that in pure aluminizing.     

Microwave permittivity of SiC-Al2O3 composite powder prepared by sol-gel and carbothermal reduction

SiC-Al2O3 composite powder was prepared by sol-gel and carbothermal reduction method. The powder synthesized was characterized by X-ray diffraction（XRD） and scanning electron microscopy（SEM） to confirm the phase formation, and the thermodynamic analysis was performed systematically. Moreover, the variation of its microwave permittivity with different atomic ratio of Al/Si was investigated in the frequency range of 8.2-12.4 GHz. The results show that, the powder obtained consists of spherical particles of 300-400 nm in diameter, which are composed of SiC and Al2O3 microcrystal with the grain size of approximately 45 nm. The results of XRD accord with those of the thermodynamic analysis. It is impossible for Al atoms to dissolve in the lattice of SiC during the carbothermal reduction process. Along with the increase of atomic ratio of Al/Si in the xerogel, the amount of Al2O3 in the powder synthesized increases, which reduces both ε＇, the real part of complex permittivity, and tg δ（ε＂/ε＇）, the dissipation factor, where ε＂ is the imaginary part of complex permittivity.     

Molecular Dynamics Simulations and Experimental Investigations of Atomic Diffusion Behavior at Bonding Interface in an Explosively Welded Al/Mg Alloy Composite Plate

In this study, 6061 aluminum alloy and AZ31 B magnesium alloy composite plate was fabricated through explosive welding. Molecular dynamics(MD) simulations were conducted to investigate atomic diffusion behavior at bonding interface in the AI/Mg composite plate. Corresponding experiments were conducted to validate the simulation results. The results show that diffusion coefficient of Mg atom is larger than that of A1 atom and the difference between these two coefficients becomes smaller with increasing collision velocity. The diffusion coefficient was found to depend on collision velocity and angle. It increases linearly with collision velocity when the collision angle is maintained constant at 10° and decreases linearly with collision angle when the collision velocity is maintained constantly at 440 m/s. Based on our MD simulation results and Fick's second law, a mathematical formula to calculate the thickness of diffusion layer was proposed and its validity was verified by relevant experiments. Transmission electron microscopy and energy-dispersive system were also used to investigate the atomic diffusion behavior at the bonding interface in the explosively welded 6061/AZ31B composite plate. The results show that there were obvious Al and Mg atom diffusion at the bonding interface,and the diffusion of magnesium atoms from magnesium alloy plate to aluminum alloy plate occurs much faster than the diffusion of aluminum atoms to the magnesium alloy plate. These findings from the current study can help to optimize the explosive welding process.     

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.     

Analysis of Valence Electron Structure in Compound Layer of Steel 42CrMo Plasma Nitrided with Rare Earth Addition

The valence electron structure (VES) in compound layer of steel plasma-nitrided at 560℃ with rare earth (RE) addition was calculated based on the empirical electron theory (EET) of solids and molecules and BLD method. The results show that the presence of RE atoms diffused into surface layer leads to an increase of phase structure factor, which explains the catalyzing and micro-alloying effects of RE.     

Effect of mischmetal and yttrium on microstructures and mechanical properties of Mg-Al alloy

The effect of yttrium and mischmetal(MMs)on the as-cast and solid solution treated structures of Mg-Al alloys with different Al-contents was investigated.The results show that the MMs in Mg-Al alloy existed in rod Al4(Ce,La)compound while Y in Mg-Al alloy in polygonal Al2Y compound.The amount of Mg17Al12 in Mg-Al alloy is decreased with increasing Y or MMs addition,and Mg17 Al12 intermetallic compound is changed from continuous network to discontinuous one.The Al4(Ce,La)and Al2Y compounds are not dissolved into Mg-Al alloy matrix during solid solution treatment so that their high heat stability can be exhibited.The experiment of mechanical properties indicate that elongation and impact toughness of the Mg-Al-Y alloy with polygonal Al2Y compound are higher than those of Mg-Al-MMs alloy with rod A4(Ce,La) compound.     

Evolution of undissolved phases in high-zinc content super-high strength aluminum alloy during ageing

The evolution of undissolved phases in the high-zinc content super-high strength aluminum alloy during ageing was investigated by means of SEM and EDS. The results show that undissolved phases of Cu-rich M(AlZnMgCu) exist in the silver-free alloy at solid-solution state. With increasing the ageing time, the precipitation of agehardening precipitates MgZn2 stimulates Zn atoms within the undissolved phases to diffuse into the matrix, and thus the Cu content in the M(AlZnMgCu) phase increases relatively. For the silver-bearing alloy, small addition of Ag promotes the formation of Ag-rich M(AlZnMgCuAg) undissolved phases and deteriorates mechanical properties of the alloy. At the early stage of ageing, Ag content within the M(AlZnMgCuAg) phases greatly decreases due to rapid diffusing of Ag atoms into the matrix and the co-clustering of Ag and Mg atoms. As the ageing time prolonging, the precipitation of MgZn2 results in the decrease of Zn content in the undissolved phases, and the relative increase of Ag and Mg contents.     

Study on the Formation Mechanism of the Solid Siliconized Layer on Ti-48Al Alloy

The microstructures of the siliconied specimens of Ti-48Al alloy were analyzed by SEM equipped with XEDS.The specimens were pack siliconized with the two different cementations,15%Si 85% Al2O3 and 15%Si 85%ZrO2.The results show that a composite siliconized layer is formed on the surface of the Tial alloy.The outer layer is the continuous Al2O3 where a lot of Si particles adhered;the inner layer is most of Ti5Si3 with amount of Al2O3 particles dispersed in.It was deduced that the Al2O3 in the cementation layer is formed by the Al atoms in the TiAl substrate react with the residual O in the fumace and in the TiAl substrate.     

Structural stability of intermetallic compounds of Mg-Al-Ca alloy

A first-principles plane-wave pseudopotential method based on the density functional theory was used to investigate the energetic and electronic structures of intermetallic compounds of Mg-Al-Ca alloy, such as Al2Ca, Al4Ca and Mg2Ca. The negative formation heat, the cohesive energies and Gibbs energies of these compounds were estimated from the electronic structure calculations, and their structural stability was also analyzed. The results show that Al2Ca phase has the strongest alloying ability as well as the highest structural stability, next Al4Ca, finally Mg2Ca. After comparing the density of states of Al2Ca, Al4Ca and Mg2Ca phases, it is found that the highest structural stability of Al2Ca is attributed to an increase in the bonding electron numbers in lower energy range below Fermi level, which mainly originates from the contribution of valence electron numbers of Ca（s） and Ca（p） orbits, while the lowest structural stability of Mg2Ca is resulted from the least bonding electron numbers near Fermi level.     

Theoretical study and numerical simulation of the stress fields of the Al2O3 joints brazed with composite filler materials

Non-linear finite element code MSC. Marc was utilized to analysis the field of stress of the Al2O3 joints brazed with composite filler materials. The properties of the filler materials were defined by using the mixing law, method of Mori-Tanaka and theory of Eshelby to ensure the accuracy and reliability of results of finite element method （FEM）. The results show stress in brazed beam is higher than that in base material. The maximal stress can be found in the interface of joint. And the experimental results show that the shear strength of joints increases from 93.75 MPa （ Al2O3p Ovol. % ） to 135.32 MPa （ Al2O3p 15vol. % ） when composition of titanium is 3wt% in the filler metal.     

INTERGRANULAR CORROSION AND PHOSPHOR SEGREGATION AT GRAIN BOUNDAR1FS OF NON-SENSITIZED SUPERPURE AUSTENITIC STAINLESS STEEL 00Cr25Ni22Mo2N

The corrosion behaviour and mechanism of superpure austenitic stainless steel00Cr25Ni22Mo2N in urea processing environment was studied using metallography,SEM,TEM,SIMS and AES techniques.The results show that the gas extraction tube made ofnon-sensitized 00Cr25Ni22Mo2N steel suffered intergranular corrosion.Corrosive mediapenetrated not only into the tube wall through grain boundaries but also expanded from theboundary towards the interior of the grain.Neither depletion of Cr nor precipitates were foundat the grain boundaries.However,P(and Si)was segregated at the grain boundaries to a greatextent.Semiquantitative calculation indicates that the P content at the grain boundaries isabout 25 wt-%,three orders of magnitude higher than the content within the grain.Sugges-tion is made that the potential difference between the grain and its boundary due to the segre-gation results in the observed intergranular corrosion.     

INTERFACIAL ATOMIC-REACTION MODEL FOR DIFFUSION BONDING OF METALS

According to probability theory and atomic activation on bonding interface of metals,a mathematical model was developed for the atomic interfacial reaction during diffusionbonding.The effect of parameters of bonding processing and material on the bondingstrength was then gained.It was suggested that the activation centre of atomic interfacialreaction of bonding may be,in situ,the boundary dislocation and its elastic stress field.A substantial agreement about the quantitative prediction of the model was made withthe results of diffusion bonding experiments for 7075Al alloy     

Decomposition of Nb3Si and mechanical-property improvement by adding appropriate amount of MgO in Nb-16Si-20Ti alloy

Five Nb-16Si-20Ti-xMgO alloys(x=0,0.1,0.5,1.0 and 3.0) were prepared via arc melting in this study,and the effect of MgO addition on their phase composition,microstructure evolution,and mechanical properties was examined.The results demonstrated that MgO reacted with the Nb-Si-Ti alloy while Mg atoms replaced Nb atoms in the Nbss phase.The hypoeutectic alloy was transformed into a hypereutectic alloy upon the addition of 3.0 at% MgO,and the Nb₃Si phases decomposed into a fine Nbss/α-N...     

Effect of La and Nd on microstructures and mechanical properties of AZ61 wrought magnesium alloy

Effects of La and Nd addition on the microstructure and mechanical properties of the AZ61 alloy have been investigated. The results show that when La and Nd are added into the AZ61 alloy respectively, the β(Mg17-Al12) phase is refined and granulated, and new phases are formed in the form of small rod-like shape, which are verified as La3 Al12 and Nd3Al11 phase by X-ray diffraction and TEM observation. Microstructure observations show that the effective efficiency of La addition is higher than that of Nd addition, thus the sizes of β(Mg17 Al12) and La3Al11 phase are relatively smaller than those of β(Mg17 Al12 ) and Nd3 Al11 phases in both AZ61 alloy and Nd-containing alloy. The increase of the tensile strength and elongation of AZ61 alloy refers to the existence of small rod-like La3Al11 and Nd3Al11 phases, and fine granulated β(Mg17 Al12 ) phase.     

Molecular simulation of interfacial reaction between TiAl alloy melts and different coatings

The effect of coatings (Y2O3, ZrO2 and Al2O3) on the interfacial reaction of TiAl alloys was studied with molecular dynamics. The binding energy of coatings and the diffusion process of oxygen in the melt were simulated, and then the simulation results were compared with the experimental results. The simulation results indicate that for each of the three simulated coatings, inordinate interfacial reactions have occurred between the coating and the melt. The binding energy results show that Y2O3 has the best stability and is the most difficult to break down. ZrO2 has the greatest decomposition energy and is the easiest to break down in the melt. Besides,the molecular dynamics indicate that the diffusion coefficient of the oxygen atom in Al2O3 is larger than that in the other two coatings, indicating that oxygen diffusion in Al2O3 is the fastest at a given temperature. The experimental results show that the oxygen concentration of the melt with Al2O3 coating is the highest, and the oxygen diffusion is of similar magnitude to the simulation values, from which the conclusion can be obtained that the oxygen concentration is significantly influenced by the coating materials.     

Grain size refinement of magnesium composite alloys by addition of B2O3

The high performance magnesium alloy was investigated by adding B2O3 in magnesium and magnesium alloys. Experiments include adding B2O3 in Mg, Mg-AI and Mg-RE alloys, respectively, studying the effects of B2O3 on the microstructure, were studied measuring the change of grain size and microhardness of the materials, discussing the change of grain size, morphology and distribution. The results show that adding 3% or 6%（mass fraction） B2O3 in Mg can bring twinning in Mg, adding B2O3 in Mg-Al alloys and Mg-RE alloys can refine the alloy grain size. Adding 3?O3 in Mg-6Al alloys can refine the average grain size by about 5 pro, with the average hardness increased by 13.3% （53.3-60.4 HV0.O3）; adding 6?O3 in Mg-6Al alloys can refine the average grain size by about 13 pro, with the average hardness increased by 15.8% （53.3-61.73 HV0.O3）; adding 3% and 6?O3 into Mg-6RE alloys can refine the grain size by about 5 and 9/am, respectively, with the average hardness decreased to HV0.03 64.66 and HV0.O3 57.86, respectively from HV0.03 88.57. In the Mg-6Al alloy the content of aluminum is increased, while in the Mg-6RE alloy the content of oxygen is decreased. It can be concluded that it is beneficial to develop Mg-Al-B-O particle reinforce composite alloys, and it is feasible to develop nanometer crystallization of block material by Mg-B-O-RE.     

Microstructure of as-extruded 7136 aluminum alloy and its evolution during solution treatment

With the aid of scanning electron microscopy(SEM), energy-dispersive spectroscopy(EDS), X-ray diffraction(XRD), differential scanning calorimetry(DSC)analysis and electron backscatter diffraction(EBSD), the microstructure of the alloy in as-extruded state and various solution-treated states was investigated. The results indicate that second phase of the as-extruded 7136 aluminum alloy mainly consists of Mg(Zn, Cu, Al)_2and Fe-rich phases. The Mg(Zn, Cu, Al)_2phase directly dissolves into the matrix during solution treatment with various solution temperatures. After solution treated at 475 °C for 1 h,Mg(Zn, Cu, Al)_2phases are dissolved into the matrix,while Fe-rich phases still exist. Fe-rich phases could not dissolve into the matrix by prolonging solution time. The mechanical property test and EBSD observation show that two-stage solution treatment makes no significant improvement in mechanical properties and recrystallization of the alloy. The optimized solution treatment parameter is chosen as 475 °C/1 h.     

Protective behavior of an SO2/CO2 gas mixture for molten AZ91D alloy

The protective behavior for a molten AZ91D alloy in an open melting furnace was investigated under a protective gas mixture containing 3% SO2 and 97% CO2, and the protection mechanism was discussed. Experimental results show that the gas mixture provides effective protection for AZ91D melt in the temperature range from 680 oC to 730 oC. The microstructure, chemical composition and phase composition of the surface film formed on the molten AZ91D alloy were analyzed using scanning electron microscopy (SEM) with energy dispersive spectrometer (EDS) and X-ray diffraction (XRD). The SEM results demonstrate that the surface films with an average thickness between 0.5 μm and 2 μm are dense and coherent in the protected temperature range. The EDS results reveal that the surface film mainly contains elements S, C, O, Al and Mg. The XRD results show that the surface film consists of MgO, MgS and a small amount of C phase.     

Influence of crystal structure and formation energies of impurities （Mg, Zn and Ca） in zinc blende GaN

First-principles calculations on neutral metal impurities （Mg, Zn and Ca） in zinc blende GaN were studied. Formation energies were calculated for substitution on the gallium site, the nitrogen site and incorporation in the octahedral interstitial site and the tetrahedral interstitial sites. The calculated results show that the major defects studied have a high formation energy in excess of 5 eV, and the gallium substitutional site is favorable for incorporation. MgGa has particularly low formation energy 1.19 eV and can be expected to incorporate readily into GaN. The local crystal structural changes around the impurity in the lattice were studied after metal atoms occupying the gallium substitutional site. It shows that the lattice constant becomes bigger and the tetrahedral angle between impurities and its nearest N atom becomes smaller mainly due to the extended M-N bond length and big size of impurities atoms, which results in a local lattice distortion. The Zn-N （2.04 A） bond strength is the smallest among the three impurities which raises the formation energy. CaGa is unfavorable due to a large size mismatch in spite of a large bond strength （2.25 A）. The calculated results identify the two key factors determining impurities incorporation in zinc blende GaN： the atomic size of impurities comparing to that of host atoms and the bond strength between the impurities and its neighbors. The results are in well agreement with other calculated and experimental results.