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 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.

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.

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.

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.

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.

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.

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.

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.