Modification of eutectic silicon and β-Al_5FeSi phases in as-cast ADC12 alloys by using samarium addition

The effects of Sm additions （0, 0.5 wt.%, 1.0 wt.%and 1.5 wt.%） on the eutectic Si and β-Al5FeSi phases of ADC12 as-cast alloys were studied by optical microscopy （OM）, scanning electron microscopy （SEM） and differential thermal analysis （DTA）. The experimental results showed that Sm was an effective modifying agent for the eutectic Si of ADC12 alloy, when 1.0 wt.%-1.5 wt.%Sm was added to the alloy, the coarse acicular eutectic Si was modified into fine particle or short rod structure. Moreover, the appropriate addition of Sm （about 1.0 wt.%） had marked effects on shortening the length of needle-likeβ-Al5FeSi phase. Whereas, Sm was less effective on modifying the needle-likeβ-iron to the Chinese script or sphericalα-iron phase. The modification mecha-nism was also discussed.

Modification of primary α-Al, eutectic silicon and β-Al_5FeSi phases in as-cast AlSi10Cu3 alloys with(La+Yb) addition

The study systematically investigated the effects of master alloy addition containing rare earth elements La and Yb on the microstructures characteristic and tensile properties of A1Sil0Cu3 alloy. It was studied by means of optical microscopy, X-ray diffraction, scanning electron microscopy, energy diffraction spectnam and differential thermal analyzer. The results showed that the ad-dition of （La＋Yb） obviously reduced the sizes of the primary a-Al phase and eutectic Si particles as well as 13-A15FeSi phase and im- proved the morphology of the primary a-A1 phase and eutectic Si particles. The optimum addition of（La＋Yb） addition was 0.6 wt.%. Comparing the 0.6 wt.% （La＋Yb） modified A1Sil0Cu3 alloy with the unmodified alloy, it was found that the mean diameter, mean area and SADS of primary a-A1 phase were decreased by 50.80%, 75.74% and 50.83% respectively; the aspect ratio, size （length） and mean area of eutectic Si particles were decreased by 66.30%, 81.78% and 78.99%, respectively, and the average size of the β-AlsFeSi phase was 16.4 pro. In addition, the addition of （La＋Yb） greatly improved the tensile properties ofA1Si 10Cu3 alloy, especially in the ultimate tensile strength and elongation as a result of the significant improvement in microstructure.

Effect of Sr modification on microstructure and thermal conductivity of hypoeutectic Al−Si alloys

Trace amount of Sr(0.05 wt.%)was added into the hypoeutectic Al−Si(3−12 wt.%Si)alloys to modify their microstructure and improve thermal conductivity.The results showed that the thermal conductivity of hypoeutectic Al−Si alloys was improved by Sr modification,and the increment and increasing rate of the thermal conductivity gradually increased with Si content increasing.The improvement of thermal conductivity was primarily related to the morphology variation of eutectic Si phases.In Sr-modified Al−Si alloys,the morphology of eutectic Si phases was a mixed morphology of fiber structure and fine flaky structure,and the proportion of the fine flaky eutectic Si phases gradually decreased with Si content increasing.Under the Si content reaching 9 wt.%,the proportion of fine flaky eutectic Si phases was nearly negligible in Sr-modified alloys.Correspondingly,the increment and increasing rate of thermal conductivity of Sr-modified alloys reached the maximum and tended to be stable.

Effect of the Al–Si eutectic on the microstructure and corrosion behavior of the single-phase Mg alloy Mg–4Li

The influence of alloying with Al–Si eutectic on the microstructure and corrosion resistance of single-phase Mg–4Li was studied in this paper.The microstructure was characterized using an electron microscopy.The corrosion properties were measured by immersion tests and electrochemical impedance spectrum(EIS)measurements.Alloying with Al–Si eutectic caused the formation of Mg_(2)Si and Mg–Al phase.The grain orientation of Mg–4Li was mainly composed of(0001)while the grain orientation of Mg–4Li–6(Al–Si)was mainly consisted of(10–10)and(11–20).Mg–4Li–6(Al–Si)shows worse corrosion resistance than Mg–4Li owing to the galvanic corrosion from the precipitates and texture change.

Influence of surface etching pretreatment on PEO process of eutectic Al–Si alloy

To solve the problems generally encountered during the plasma electrolytic oxidation(PEO) of Al alloys with high Si content, a pretreatment of chemical etching was applied before the process. The influence of such pretreatment was studied by SEM, EDS and XRD. The pretreatment presents a significant effect on positive voltage at the beginning stage of PEO, leading to higher voltage over the whole process. The difference between the positive voltages of non-etched and etched specimens decreases gradually with the increase of processing time. The pretreatment exhibits much less influence on the negative voltage. For the sample with surface pretreatment, the average growth rate of PEO coating is increased from 0.50 to 0.84 μm·min-1and the energy consumption is decreased from 6.30 to 4.36 k W·h·μm-1·m-2. At the same time, both mullite and amorphous Si O2 contents are decreased in the coating.

MORPHOLOGY AND ORIENTATION CHARACTERISTICS OF DIRECTIONALLY SOLIDIFIED α(A1)-Mg_2Si EUTECTIC

Dependence of the morphology of α(Al)-Mg2Si eutectic on the solidification velocity, and its orientation characteristics were investigated by thermoanalysis, directional solidification and electron diffraction techniques in this study.The eutectic morphology transformed from lamellae to rods with the increasing of the solidification velocity. Simultaneously, the orientation relationship changed from (100)Mg2Si/(100)Al;[110]Mg2si/[110]al to (011)Mg2si/(121)Al;[111]Mg2Si/[111]Al and (010)Mg2Si/(110)Al; [100]Mg2Si/[111]Al. Transformation of the eutectic morphology was related to the change of the preferred orientation of phase Mg2si.

Effect of Li and Ti Additions on Lα(Al)+Mg_2Si Pseudobinary Eutectic Reaction

Effect of Li and Ti additions on Lα(AI)+Mg2Si pseudobinary eutectic reaction in ternary Al-Mg-Si system has been investigated by thermoanalysis, directional solidification and metallographic techniques in this study. It has been found that Li addition causes decreasing of the volume fraction of Mg2Si, while a little amount of Ti causes to increasing, which is of a great importance to the adjustment of phase constitution and alloy properties. Doping components have little influence on the eutectic temperature.

Effects of La-Ce addition on microstructure and mechanical properties of Al-18Si-4Cu-0.5Mg alloy

The microstructural evolution and mechanical properties of Al-18 Si-4 Cu-0.5 Mg alloy modified by the addition of La-Ce rare earth elements through OM,SEM,EPMA and tensile tests were investigated.The results of OM and SEM analyses indicated that primary Si particles were significantly refined from coarse block-like and irregular polygonal shapes into fine flaky shapes,while eutectic Si particles were modified from coarse and needle-like into fine and rod-or coral-like shapes with increase of La-Ce addition.The alloy exhibited the minimum primary Si particle size and the best mechanical properties with the addition of 0.3 wt.%La-Ce.The average particle size decreased from 61 to 28 μm,the ultimate tensile strength increased from 222 to 242 MPa and the elongation increased from 3.2% to 6.3%.In addition,modification mechanisms and fracture modes were explored by the means of SEM and EPMA.

Microstructure and mechanical property of A356 based composite by friction stir processing

Friction stir processing （FSP） was used to incorporate SiC particles into the matrix of A356 Al alloy to form composite material. Constant tool rotation speed of 1800 r/min and travel speed of 127 mm/min were used in this study. The base metal （BM） shows the hypoeutectic Al-Si dendrite structure. The microstructure of the stir zone （SZ） is very different from that of the BM. The eutectic Si and SiC particles are dispersed homogeneously in primary Al solid solution. The thermo-mechanically affected zone （TMAZ）, where the original microstructure is greatly deformed, is characterized by dispersed eutectic Si and SiC particles aligned along the rotational direction of the tool. The hardness of the SZ shows higher value than that of the BM because some defects are remarkably reduced and the eutectic Si and SiC particles are dispersed over the SZ.

Microstructures and corrosion behaviors of Al−6.5Si−0.45Mg−xSc casting alloy

The microstructures and corrosion behaviors of the Al−6.5Si−0.45Mg casting alloys with the addition of Sc were investigated by using scanning electron microscopy,X-ray diffraction,electrochemical measurement techniques and immersion corrosion tests and compared with those of Sr-modified alloy.The results show that Sc has evident refining and modifying effects on the primaryα(Al)and the eutectic Si phase of the alloy,and the effects can be enhanced with the increase of Sc content.When the Sc content is increased to 0.58 wt.%,its modifying effect on the eutectic Si is almost same as that of Sr.Sc can improve the corrosion resistance of the test alloy in NaCl solution when compared with Sr,but the excessively high Sc content cannot further increase the corrosion resistance of the alloy.The corrosion of the alloys mainly occurs in the eutectic region of the alloy,and mostly the eutecticα(Al)is dissolved.This confirms that Si phase is more noble thanα(Al)phase,and the galvanic couplings can be formed between the eutectic Si andα(Al)phases.

Effect of compression ratio on microstructure evolution of Mg−10%Al−1%Zn−1%Si alloy prepared by SIMA process

A new Mg−10%Al−1%Zn−1%Si alloy with non-dendritic microstructure was prepared by strain induced melt activation(SIMA)process.The effect of compression ratio on the evolution of semisolid microstructure of the experimental alloy was investigated.The results indicate that the average size ofα-Mg grains decreases and spheroidizing tendency becomes more obvious with the compression ratios increasing from 0 to 40%.In addition,the eutectic Mg2Si phase in the Mg−10%Al−1%Zn−1%Si alloy transforms completely from the initial fishbone shape to globular shape by SIMA process.With the increasing of compression ratio,the morphology and average size of Mg2Si phases do not change obviously.The morphology modification mechanism of Mg2Si phase in Mg−10%Al−1%Zn−1%Si alloy by SIMA process was also studied.

Effects of Cu addition on microstructure and mechanical properties of Er-modified Al-10Mg_(2)Si cast alloys

The solidification microstructure and mechanical properties of hypoeutectic Al-10Mg_(2)Si cast alloys with different Cu contents and 0.45 wt.% Er were investigated using an optical microscope, scanning electron microscope, and an electronic universal testing machine. Results showed that Cu and Er could significantly reduce the grain size of the eutectic Mg;Si phase from 15.4 μm for the base alloy to 5.8 μm for the alloy with 1.5 wt.% Cu and 0.45 wt.% Er.Meanwhile, the needle-like β-Al5Fe Si phase was modified to fine sized irregular shaped Cu, Er and Fe-rich particles with the additions of Cu and Er. The strength and ductility of the Al-10Mg_(2)Si as-cast alloys were simultaneously improved by the additions of Cu and Er, and the ultimate tensile strength, yield strength and elongation increase from 223 MPa,136 MPa and 2.1% to 337 MPa, 169 MPa and 4.7%, respectively. The heterogeneous nucleation of Mg_(2)Si on Al P was avoided by forming Cu, Er and P-containing phases, due to the additions of Cu and Er. Moreover, the Cu and Er atomic clusters and their intermetallic segregated on the surface of the eutectic Mg_(2)Si and inhibited the growth of the eutectic Mg_(2)Si, which were responsible for the modification of the eutectic Mg_(2)Si.

Influence of samarium content on microstructure and mechanical properties of recycled die-cast YL112 aluminum alloys

The influence of Sm （Samarium） content on microstructure and mechanical properties of recycled die-cast YLl12 aluminum alloys was investigated. The results show that many small Sm-rich particles form in the recycled die-cast YLl12 alloys with Sm addition. At the same time, the secondary dendrite arm spacing in the YLl12 alloys modified with Sm is smaller than that of the unmodified alloy. The eutectic Si of recycled die- cast YL112-xSm alloys transforms from coarse acicular morphology to fine fibres. Mechanical properties of the investigated recycled die-cast YLl12 aluminum alloys are enhanced with Sm addition, and a maximal ultimate tensile strength value （276 MPa） and elongation （3.76%） are achieved at a Sm content of 0.6wt.%. Due to the modification of eutectic Si by Sm, numerous tearing ridges and tiny dimples on the fractures of tensile samples are observed.

Microstructure evolution of a hypereutectic Nb–Ti–Si–Cr–Al–Hf alloy processed by directional solidification

In this work, the Nb-14Si 24Ti-10Cr-2Al-2Hf-0.1Y alloy （at.%） was processed by the liquid-metal-cooled directional solidification （DS） at 1750 ℃ with withdrawal rates of 1.2, 6, 18 mm/min and post heat treatment （HT） at 1450 ℃ for 10 h. The microstructures of the direction- ally solidified and heat treated samples were investigated. The results show that the microstructure of directionally solidified alloy mainly consists of petaloid Nbss ＋ Nb5Si3 eutectics and Ti-rich Nbss ＋ Nb5Si3 ＋ Cr2Nb eutectics. With the increase of withdrawal rate, the primary NbsSi3 is eliminated, Nbss ＋ Nb5Si3 eutectic cells turn round and connected with the microstructure refine- ment and Nbss ＋ Nb5Si3 ＋ CrzNb eutectics turn to a river-like morphology. After heat treatment, Nbss ＋ Nb5Si3 ＋ Cr2Nb eutectics disappeared and petaloid Nbss ＋ Nb5Si3 eutectics turn to a spe- cific fiber-mesh structure gradually, which is promoted by higher withdrawal rates. Furthermore, both the volume fraction of Cr2Nb and the content of Cr in Nbss of Nbss ＋ Nb5Si3 eutectics change regularly with the increase of withdrawal rate and heat treatment at 1450 ℃ for 10 h.

Investigation on the modification behavior of A356 alloy inoculated with a Sr-Y composite modifier

In the present paper, differential scanning calorimetry （DSC）, scanning electron microscopy （SEM）, and X-ray diffraction （XRD） were used to examine the effects of a Sr-Y composite modifier on the microstructure of A356 alloy. After adding Y to A356, YAl 3 compounds formed, and the size of the α （Al） crystal nucleus increased. The degree of supercooling caused by Sr-Y composite modifier was higher than Sr modification by 2.7 °C, leading to an increased nucleation rate. This increase in supercooling temperature was favorable to the refinement of eutectic structure of the alloy and its eutectic reaction was delayed to the maximum extent. The Si phase in the as-cast Sr-Y composite-modified A356 alloy was either granular or flaky. No large flakes of eutectic Si were found, and the modification effects were completely comparable with those obtained using a lone Sr modifier. After T6 heat treatment, most of the eutectic Si showed a grain-like shape with smaller grains. No eutectic Si with long-strip shapes, significant enhancements in the particle roundness and evenness of the Si crystals, and increased globosity were observed. Both the roundness and evenness of the grained Si crystals were enhanced, and the amount of globular eutectic Si available increased, these findings showed that excellent modification effects were achieved.

Influences of different Zr/Sc ratios on microstructure and mechanical properties of Al-2Si alloy

The influences of different Zr/Sc ratios on the microstructural evolution and mechanical performances of Al-2 Si alloy were investigated in this paper.The as-cast samples were characterized and analyzed by the scanning electron microscope(SEM)with an energy dispersive spectrometer(EDS)to indicate the refining effect of the different Zr/Sc ratios on theα-Al and Si phases in Al-2 Si alloy.The results indicate that theα-Al phases are refined from coarse dendritic structure to relatively fine equiaxed grains when the Zr/Sc ratio is equal to 1:1.Meanwhile,the eutectic Si phase in Al-2 Si-0.2 Zr-0.2 Sc alloy is modified from the coarse plate and needle to fine fiber and partial granular structure,and the average roundness of eutectic Si is decreased by 75%from 16 to 4.With the refinement ofα-Al phases and the modification of eutectic Si structure,the ultimate tensile strength(UTS)is improved by 53%and the elongation(EL)is increased by 64%when the Zr/Sc ratio is equal to 1:1.In addition,the UTS decreases and EL is enhanced with the increase of tensile temperature.Moreover,the fracture model of Al-2 Si-0.2 Zr-0.2 Sc alloy in250℃changes into the ductile fracture.

Solidification behavior,microstructure and silicon twinning of Al-10Si alloys with ytterbium addition

The solidification behavior, micro structure and silicon twinning of Al-10 Si alloys with Yb addition were investigated by thermal analysis, optical microscopy, X-ray diffraction. scanning electron microscopy, and transmission electron microscopy. The results indicate that the nucleation temperature, minimum temperature, and growth temperature of Al-lOSi alloys decrease with increasing Yb content. The cooling curves of the Yb-modified alloys exhibit marked recalescence. The recalescence of the modified alloy peaks at 2.3 ℃ when the Yb content is 0.7 wt%. The 3 D morphologies of eutectic Si in Yb-modified alloys change from a coarse plate-like structure to a honeycomb structure with many fine fibrous structures.The Al-Si-Yb intermetallic compound is observed in the 1.0 wt% Yb-modified alloy. Meanwhile, XRD analysis and TEM results indicate that the average twin spacing in the Yb-modified alloys is 18-46.2 nm.The average twin spacing of eutectic Si decreases with increasing Yb content. When the Yb content in the modified alloy is increased to 0.7 wt%, the average twin spacing value of eutectic Si reaches to 18 nm,which promotes the formation of twins and refinement of eutectic Si.

Microstructure and mechanical properties of AlSi10Cu3 alloy with (La＋Yb) addition processed by heat treatment

The optical microscopy, scanning electron microscopy （SEM） and energy-dispersive spectrometry （EDS） were used to as-sess the influence of micro-addition of （La＋Yb） on the microstructure and mechanical performance of the AlSi10Cu3 alloy in heat treatment conditions. It was shown that the appropriate （La＋Yb）addition （0.3 wt.% or 0.6 wt.%） transformed the needle-likeβ-Al5FeSi phase into Chinese script or sphericalα-Al8Fe2Si phase. Eutectic silicon refined the long needle-like particles into granular or round particles at 0.6 wt.% （La＋Yb） content. Moreover, the La3Al11 and YbAl3 phases acted as strengthening phases during the heat treatment processing in the alloy with the addition of （La＋Yb）. Consequently, the alloy with 0.6 wt.% （La＋Yb） exhibited an en-hanced mechanical properties response with ultimate tensile strength, elongation, and hardness at 69.35%, 113.26% and 23.61% higher than those of the unmodified alloy, respectively. Further addition （0.9 wt.%） of （La＋Yb） resulted in the increasing of the black acicular RE-rich intermetallics during heat treatment, which could aggravate the situation of stress concentration leading to deteriora-tion of the mechanical properties of alloy.

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.