相场法模拟流动下非等温多晶粒枝晶生长形貌研究

基于相场方法对包含多晶粒的非等温合金凝固过程进行了二维数值模拟。以Ni-Cu合金为例,展示了多核晶粒形貌的生长过程,研究了流速、各向异性和过冷度、晶核个数等参数对多枝晶生长形貌的影响。结果表明:随着模拟时间的增加,各晶核由独立生长演化为竞争生长;引入强制流动后,枝晶上游臂的生长得到促进,下游臂的生长受到抑制,垂直臂逐渐向迎流方向倾斜生长;随着流速的增大,多枝晶的非对称性增强,垂直臂在迎流一侧逐渐长出二次枝晶,而逆流侧保持光滑。随着各向异性值和过冷度的增大,各枝晶形貌大小均增大。

等温淬火球铁齿轮的试制

介绍了等温淬火球铁齿轮的主要生产工艺 ,以及合金与非合金等温淬火球铁齿轮的试制应用。其结果表明 ,用Cu Mo合金化的等温淬火球铁 (370℃× 1.5h)可代替 2 0CrMnTi钢制造拖拉机末端传动齿轮 ;用含Mn达 0 .7%左右的球铁棒材 ,经 2 80～ 30 0℃× 1.5h等温淬火处理得到贝氏体马氏体球铁 ,可用来代替 2 0Cr MnTi钢正时齿轮 ;采用地方生铁 (0 .6 %～ 0 .8%Mn) ,通过增加硅量、强化孕育处理、降低奥氏体化温度以及等温淬火后回火处理等措施 ,可生产出接近于合金等温淬火球铁性能水平的齿轮。以上

Evolution of mechanical properties,localized corrosion resistance and microstructure of a high purity Al-Zn-Mg-Cu alloy during non-isothermal aging

The evolution of mechanical properties,localized corrosion resistance of a high purity Al-Zn-Mg-Cu alloy during non-isothermal aging(NIA)was investigated by hardness test,electrical conductivity test,tensile test,intergranular corrosion test,exfoliation corrosion test,slow strain rate tensile test and electrochemical test,and the mechanism has been discussed based on microstructure examination by optical microscopy,electron back scattered diffraction,scanning electron microscopy and scanning transmission electron microscopy.The NIA treatment includes a heating stage from 40℃to 180℃with a rate of 20℃/h and a cooling stage from 180℃to 40℃with a rate of 10℃/h.The results show that the hardness and strength increase rapidly during the heating stage of NIA since the increasing temperature favors the nucleation and the growth of strengthening precipitates and promotes the transformation of Guinier-Preston(GPI)zones toη'phase.During the cooling stage,the sizes ofη'phase increase with a little change in the number density,leading to a further slight increase of the hardness and strength.As NIA proceeds,the corroded morphology in the alloy changes from a layering feature to a wavy feature,the maximum corrosion depth decreases,and the reason has been analyzed based on the microstructural and microchemical feature of precipitates at grain boundaries and subgrain boundaries.

Non-isothermal oxidation and ignition prediction of Ti-Cr alloys

The non-isothermal oxidation behavior and oxide scale microstructure of Ti-Cr alloy （0≤w（Cr）≤25%） were studied from room temperature to 1723 K by thermogravimetric analysis （TGA）, X-ray diffraction （XRD） and scanning electron microscopy （SEM）. The influencing mechanism of chromium on the oxidation resistance of Ti-Cr alloys was discussed. The results show that the oxidation resistance of the alloys decreases with Cr below a critical chromium content wC and increases above wC; above 1000 K, the oxidation kinetics obeys parabolic rule and titanium dominates the oxidation process; after oxidation, the oxygen-diffusing layer is present in the alloy matrix, the oxide scale is mainly composed of rutile whose internal layer is rich in chromium, and chromium oxides separated out from TiO2 near the alloy-oxide interface improve the oxidation resistance. Ignition of metals and alloys is a fast non-isothermal oxidation process and the oxidation mechanism of Ti-Cr alloys during ignition is predicted.

Phase field simulation for non-isothermal solidification of multicomponent alloys coupled with thermodynamics database

In order to quantitively model the real solidification process of industrial multicomponent alloys, a non-isothermal phase field model was studied for multicomponent alloy fully coupled with thermodynamic and diffusion mobility database, which can accurately predict the phase equilibrium, solute diffusion coefficients, specific heat capacity and latent heat release in the whole system. The results show that these parameters are not constants and their values depend on local concentration and temperature. Quantitative simulation of solidification in multicomponent alloys is almost impossible without such parameters available. In this model, the interfacial region is assumed to be a mixture of solid and liquid with the same chemical potentials, but with different composition. The anti-trapping current is also considered in the model. And this model was successfully applied to industrial A1-Cu-Mg alloy for the free equiaxed dendrite solidification process.

Non-isothermal retrogression kinetics for grain boundary precipitate of 7A55 aluminum alloy

The retrogression kinetics for grain boundary precipitate （GBP） of 7A55 aluminum alloy was investigated by transmission electron microscopy （TEM） observation. The results reveal that the coarsening behavior of GBP obeys “LSW” theory, namely, the cube of GBP average size has a linear dependence relation to retrogression time, and the coarsening rate accelerates at the elevated retrogression temperature. The GBP coarsening activation energy Qo of （115.2±1.3） kJ/mol is obtained subsequently. Taking the retrogression treatment schedule of 190℃, 45 min derived from AA7055 thin plate as reference, the non-isothermal retrogression model for GBP coarsening behavior is established based on “LSW”theory and “iso-kinetics” solution, which includes an Arrhenius form equation. After that, the average size of GBP r（t） is predicted successfully at any non-isothermal process T（t） when the initial size of GBP r0 is given. Finally, the universal characterization method for the microstructure homogeneity along the thickness direction of TA55 aluminum alloy thick plate is also set up.

Comparative analysis of isothermal and non-isothermal solidification of binary alloys using phase-field model

Based on the entropy function, a two-dimensional phase field model of binary alloys was established. Meanwhile, an explicit difference method with uniform grid was adopted to solve the phase field and solute field controlled equations. And the alternating direction implicit(ADI) algorithm for solving temperature field controlled equation was also employed to avoid the restriction of time step. Some characteristics of the Ni-Cu alloy were captured in the process of non-isothermal solidification, and the comparative analysis of the isothermal and the non-isothermal solidification was investigated. The simulation results indicate that the non-isothermal model is favorable to simulate the real solidification process of binary alloys, and when the thermal diffusivity decreases, the non-isothermal phase-field model is gradually consistent with the isothermal phase-field model.

Simulation of faceted dendrite growth of non-isothermal alloy in forced flow by phase field method

Numerical simulation based on a new regularized phase field model was presented to simulate the dendritic shape of a non-isothermal alloy with strong anisotropy in a forced flow. The simulation results show that a crystal nucleus grows into a symmetric dendrite in a free flow and into an asymmetry dendrite in a forced flow. As the forced flow velocity is increased, both of the promoting effect on the upstream arm and the inhibiting effects on the downstream and perpendicular arms are intensified, and the perpendicular arm tilts to the upstream direction. With increasing the anisotropy value to 0.14, all of the dendrite arms tip velocities are gradually stabilized and finally reach their relative saturation values. In addition, the effects of an undercooling parameter and a forced compound flow on the faceted dendrite growth were also investigated.

Effect of non-isothermal aging on microstructure and properties of Al−5.87Zn−2.07Mg−2.42Cu alloys

The evolution of microstructure and properties of Al−5.87Zn−2.07Mg−2.42Cu alloys during non-isothermal aging was studied.The mechanical properties of the alloy were tested by stretching at room temperature.The results show that in the non-isothermal aging process,when the alloy is cooled to 140℃,the ultimate tensile strength of the alloy reaches a maximum value of 582 MPa and the elongation is 11.9%.The microstructure was tested through a transmission electron microscope,and the experimental results show that the GP zones andη'phases are the main strengthening precipitates.At the cooling stage,when the temperature dropped to 180℃,the GP zones were precipitated again.Besides,the experimental results show that the main strengthening phase during non-isothermal aging isη'phases.

Free dendritic growth model considering both interfacial nonisothermal nature and effect of convection for binary alloy

Considering both the effect of the nonisothermal nature of the interface as well as the effect of forced convection,an extended free dendritic growth model for binary alloys was proposed.Comparative analysis indicates that the effect of convection on solute diffusion is more remarkable compared with the ignorable effect of convection on thermal diffusion at low bath undercooling,due to the fact that solute diffusion coefficient is usually three orders of magnitude less than thermal diffusion coefficient.At high bath undercooling,the effect of convection on the dendritic growth is very slight.Furthermore,a satisfying agreement between the model predictions with the available experiment data for the Cu70Ni30 alloy was obtained,especially at low bath undercoolings,profiting from the higher values of interfacial migration velocity predicted by the present model with nonideal fluid case than that predicted by the one ignoring the effect of convection.