Two-stage dynamic recrystallization and texture evolution in Al-7Mg alloy during hot torsion

Hot torsion tests were performed on the Al-7Mg alloy at the temperature ranging from 300 to 500℃ and strain rates between 0.05 and 5 s^(-1) to explore the progressive dynamic recrystallization(DRX)and texture behaviors.The DRX behavior of the alloy manifested two distinct stages:Stage 1 at strain of≤2 and Stage 2 at strains of≥2.In Stage 1,there was a slight increase in the DRXed grain fraction(X_(DRX))with predominance of discontinuous DRX(DDRX),followed by a modest change in X_(DRX) until the transition to Stage 2.Stage 2 was marked by an accelerated rate of DRX,culminating in a substantial final X_(DRX) of~0.9.Electron backscattered diffraction(EBSD)analysis on a sample in Stage 2 revealed that continuous DRX(CDRX)predominantly occurred within the(121)[001]grains,whereas the(111)[110]grains underwent a geometric DRX(GDRX)evolution without a noticeable sub-grain structure.Furthermore,a modified Avrami’s DRX kinetics model was utilized to predict the microstructural refinement in the Al-7Mg alloy during the DRX evolution.Although this kinetics model did not accurately capture the DDRX behavior in Stage 1,it effectively simulated the DRX rate in Stage 2.The texture index was employed to assess the evolution of the texture isotropy during hot-torsion test,demonstrating significant improvement(>75%)in texture randomness before the commencement of Stage 2.This initial texture evolution is attributed to the rotation of parent grains and the substructure evolution,rather than to an increase in X_(DRX).

Effect of Pre-Ageing on the Precipitation Behaviors and Mechanical Properties of Al-7Si-Mg Alloys

For alloys that are quenched to 25℃ room temperature, there are Mg-Mg, Si-Si and Mg-Si clusters shown by exothermic peaks in a A357 alloy. However, there are no clear Mg-Mg, Si-Si and Mg-Si cluster exothermic peaks in a A356 alloy. Hence, the low Mg content in a A356 alloy has less impact with natural ageing. The natural ageing impact on the mechanical properties of a A357 alloy is higher than a A356 alloy due to the precipitation of Mg-Si clusters for which the nucleation size does not reach the critical size. The 90℃ pre-ageing process could promote Mg-Si clusters to a critical size to become the nucleation site for of β''. This then increases the artificial ageing strength and mitigates the impact of natural ageing.

Effects of Mn and Sn on microstructure of Al-7Si-Mg alloy modified by Sr and Al-5Ti-B

The effects of Mn and Sn on the microstructure of Al?7Si?Mg alloy modified by Sr and Al?5Ti?B were studied. The results show that the columnar dendrites structure is observed with high content of Sr, indicating a poisoning effect of the Al?5Ti?B grain refinement. In addition, Sr intermetallic compounds distribute on the TiB2 particles, which agglomerate inside the eutectic Si. The mechanism responsible for such poisoning was discussed. The addition of Mn changes the morphology of iron intermetallic compounds fromβ-Al5FeSi toα-Al(Mn,Fe)Si. Increasing the amount of Mn changes the morphology ofα-Al(Mn,Fe)Si from branched shape to rod-like shape with branched distribution, and finally convertsα-Al(Mn,Fe)Si to Chinese script shape. The microstructure observed by transmission electron microscopy (TEM) shows that Mg is more likely to interact with Sn in contrast with Si under the effect of Sn. Mg2Sn compound preferentially precipitates between the Si/Si interfaces and Al/Si interfaces.

MICROSTRUCTURE AND MACROSEGREGATION OF Al-7%Si ALLOY SOLIDIFIED UNDER COMPLEX EFFECTS OF ELECTROMAGNETIC AND CENTRIFUGAL FORCES

Macrosegregations and microstructures of Al-7%Si alloy solidified under complex of fects of magnetic field and centrifugal forces are studied by means of a set of selfdesigned electromagnetic centrifugal casting (EMCC) device. It is shown that electromagnetic field (EMF) has an important effect on the macrosegregation of centrifugal casting specimen of Al-7%Si alloy in two respects: one is that there exists always a kind of convection in the liquid in front of the S/L interface caused by effect ofthe electromagnetic force; the other is that different atomic clusters of solidparticles with different physical characteristics are subjected to quite different electromagnetic (Lorentz) force. Therefore, their movements get changed. In addition, the formation process of a complex band structure consisting of primary α-Al dendrites and (α-Al+β-Si) eutectics in hypoeutectic Al-Si alloys during EMCC and the effect of EMF are discussed.

Continuous cooling precipitation diagram of cast aluminium alloy Al-7Si-0.3Mg

The precipitation behaviour during quenching of cast Al-7Si-0.3Mg aluminium alloy was investigated by DSC in the cooling rate range of 0.01 K/s to 3 K/s and by quenching dilatometry for higher rates. Two main precipitation reactions were observed during cooling, a high temperature reaction starting almost directly with quenching from 540℃ and a low temperature reaction starting at about 400℃. Quenching with 3 K/s already significantly suppresses precipitation during quenching. Hardness after T6 ageing increases with increasing quenching rate, due to the increasing content of supersaturated solid solution. By dilatometry and hardness results the critical cooling rate can be estimated as about 60 K/s. Quenched Al-7Si-0.3Mg microstructures have been investigated by light microscopy. The microstructures consist of an aluminium-silicon eutectic structure, aluminium solid solution dendrites and precipitates inside the aluminium dendrites, depending on quenching rate.

Effect of La addition on microstructure and mechanical properties of hypoeutectic Al-7Si aluminum alloy

The effect of La addition(0,0.1,0.2,0.4,wt.%)on the microstructure,tensile properties and fracture behavior of Al-7Si alloy was investigated systematically.It is found that the La appears in the Al-7Si alloy in the form of Al4La and Al2Si2La phases.La addition can refine the secondary dendrite arm spacing(SDAS)and eutectic Si particles,which are decreased by 7.9%and 7%,respectively,with the optimal La content of 0.1wt.%.Because when 0.1wt.%La is added,a relatively higher nucleation undercooling of 37.47℃ is observed.Higher undercooling degree suggests that nucleation is accelerated and subsequent growth is restrained.After T6 heat treatment,compared with the without La,the ultimate tensile strength of the alloy with 0.1wt.%La is enhanced by 5.2%from 333 MPa to 350.2 MPa and the elongation increases by 73%from 7.37%to 12.75%,correspondingly.The fracture mode evolves from the ductile-brittle mixed fracture to ductile fracture mode.However,when La element content reaches a certain value of 0.4wt.%,serious segregation takes place during the solidification process.The formed brittle phases deteriorate the tensile properties of the alloy and the fracture mode of Al-7Si-0.2/0.4 La changes to mixed ductile-brittle fracture mode.

Optimization of processing parameters for preparation of Al-3Ti-1B grain refiner

Al-3Ti-1B master alloys were prepared at different processing parameters by the reaction of halide salts,and the grain refining response of Al-7Si alloy was investigated with Al-3Ti-B master alloy.The microstructure of master alloy and its grain refining effect on Al-7Si alloy were investigated by means of OM,XRD and SEM.Experimental results show that,the size of Al3Ti particles presented in Al-3Ti-1B master alloys increases with the increase of reaction temperature and decreases with the increase of cooling rate.The grain refining efficiency of Al-3Ti-1B master alloy on Al-7Si alloy is mainly attributed to heterogeneous nucleation of Al3Ti particles,and the morphology ofα(Al)changes from coarse dendritic to fine equiaxed.As a result,Al-3Ti-1B master alloy is prepared by permanent mold,and holding at 800 ℃for 30min,which has better grain refining performance on Al-7Si alloy.

Electrochemical anodization of cast titanium alloys in oxalic acid for biomedical applications

Titanium and its alloys have numerous biomedical applications thanks to the composition and morphology of their oxide film.In this study,the colorful oxide films were formed by anodizing cast Ti-6Al-4V and Ti-6Al-7Nb alloys in a 10%oxalic acid solution for 30 s at different voltages(20–80 V)of a direct current power supply.Atomic force microscopy was used as an accurate tool to measure the surface roughness of thin films on the nanometer scale.Scanning electron microscopy and X-ray diffraction were performed to analyze surface morphology and phase structure.According to the results,the produced titanium oxide layer showed high surface roughness,which increased with increasing anodizing voltage.The impact of anodizing voltages on the color and roughness of anodized layers was surveyed.The corrosion resistance of the anodized samples was studied in simulated body fluid at pH 7.4 and a temperature of 37℃ utilizing electrochemical impedance spectroscopy and the potentiodynamic polarization method.The anodized samples for both alloys at 40 V were at the optimal voltage,leading to a TiO_(2) layer formation with the best compromise between oxide thickness and corrosion resistance.Also,findings showed that TiO_(2) films produced on Ti-6Al-7Nb alloys had superior surface roughness properties compared to those of Ti-6Al-4V alloys,making them more appropriate for orthopedic applications.From the obtained data and the fruitful discussion,it was found that the utilized procedure is simple,low-cost,and repeatable.Therefore,anodization in 10%oxalic acid proved a viable alternative for the surface finishing of titanium alloys for biomedical applications.

Microstructure and mechanical properties of Al-7Si-0.7Mg alloy formed with an addition of(Pr+Ce)

Effects of （Pr＋Ce） addition on the Al-7Si-0.7Mg alloy were investigated by optical microscope （OM）, energy diffraction spectrum （EDS）, X-ray diffraction （XRD） and tensile tests. The results showed that the Al-7Si-0.7Mg alloy was modified with （Pr＋Ce） addition. The needle-like eutectic silicon phase developed into rose form and the crystalline grains decreased in size and showed a high degree of spheroidization. When the amount of the （Pr＋Ce） addition reached 0.6 wt.%, the mean diameter was 31.8μm （refined by 50%）. The aspect ratio decreased to 1.35, and the tensile strength and ductility reached 192.4 MPa and 2.18%, respectively At higher levels of addition, over-modification occurred, as indicated by increased grain size and reduced mechanical properties. The poisoning effect of the （Pr＋Ce） addition on eutectic silicon and the constitutional supercooling caused by the （Pr＋Ce） addition were the major causes of alloy modification, grain refinement, and the improvement of mechanical properties.

Hot deformation behavior and process parameters optimization of Ti-6Al-7Nb alloy using constitutive modeling and 3D processing map

The isothermal compression test for Ti-6Al-7Nb alloy was conducted by using Gleeble-3800 thermal simulator.The hot deformation behavior of Ti-6Al-7Nb alloy was investigated in the deformation temperature ranges of 940-1030℃and the strain rate ranges of 0.001-10 s^(-1).Meanwhile,the activation energy of thermal deformation was computed.The results show that the flow stress of Ti-6Al-7Nb alloy increases with increasing the strain rate and decreasing the deformation temperature.The activation energy of thermal deformation for Ti-6Al-7Nb alloy is much greater than that for self-diffusion ofα-Ti andβ-Ti.Considering the influence of strain on flow stress,the strain-compensated Arrhenius constitutive model of Ti-6Al-7Nb alloy was established.The error analysis shows that the model has higher accuracy,and the correlation coefficient r and average absolute relative error are 0.9879 and 4.11%,respectively.The processing map(PM)of Ti-6Al-7Nb alloy was constructed by the dynamic materials model and Prasad instability criterion.According to PM and microstructural observation,it is found that the main form of instability zone is local flow,and the deformation mechanisms of the stable zone are mainly superplasticity and dynamic recrystallization.The optimal processing parameters of Ti-6Al-7Nb alloy are determined as follows:960-995℃/0.01-0.18 s^(-1)and 1000-1030℃/0.001-0.01 s^(-1).

Modification Mechanism and Uniaxial Fatigue Performances of A356.2 Alloy Treated by Al-Sr-La Composite Refinement-Modification Agent

Modification mechanism and uniaxial fatigue properties of A356.2 alloy treated by Al-6Sr-7La and traditional Al-5Ti-1B/Al-10Sr(hereinafter refers to traditional treated alloy) were investigated by constant stress amplitude method. Microstructure, dislocation and Si twinning of the alloys were studied by thermal analysis, scanning electron microscopy(SEM) and transmission electron microscopy(TEM). The results showed that Al-6Sr-7La possesses better refining and modification effect than Al-5Ti-1B/Al-10Sr. Meanwhile, fatigue properties of the alloy treated by Al-6Sr-7La are higher than traditional treated alloy, and this is mainly owing to that Al-6Sr-7La treated alloy has more twins in eutectic Si and lower twin spacing. In addition, higher density of nanophases formed on twin faces and La-rich clusters appear at multiple twin intersections. Stacking faults and entrapped nanophases appeared on growing Si twin faces. Impurity induced twinning(IIT) mechanism and twin plane re-entrant edge(TPRE) mechanism are valid for eutectic Si which are important for mechanical optimization of A356.2 alloy.