Al Dendrites Research Articles

Formation and Prevention of Snow-shaped Speckles on the Worked-face of Al-Si Alloys

The mechanism forming of snow-shaped speckles on the worked-face of the castings of Al-Si Alloys was investigated by analyzing ZL104 and ZL108 alloys etc. The snow-shaped speckles usually appear on the worked-face of the castings of Al-Si alloys whose macro-grain sizes are coarse. Microcosmically, the formation of snow-shaped speckles is related to large secondary dendrite arm spacings, which cause the uneven distribution of hard eutectic structure and soft a -Al dendrite. In terms of the factors that cause the snow-shaped speckles, it is practicable to preclude them by accelerating cooling rate or adding 2wt% Al5Ti1B to ZL104.

Structure and stability of the platinum/aluminium interface: alloying and substrate vacancy formation on Pt{111}/Al

The structure and stability of Al overgrowths on platinum have been studied by LEED, STM, Auger spectroscopy and photoemission. Annealing dendritic Al islands on Pt{111} to 500 K promotes intralayer Al adatom diffusion across the substrate terraces. Evaporation from smaller Al dendrites results in island densification and coalescence, accompanied by Al/Pt intermixing at ascending steps. Simultaneously, triangular vacancy islands appear that decorate the step edges of the platinum surface. This process plays an integral part in the intermixing of Pt and Al at step sites. Metastable compact islands of the Al overgrowth persist up to 800 K, above which temperature interlayer transport commences, resulting in the formation of an ordered (2×2) Pt 3Al surface alloy. Thus, a low-temperature quasi-one-dimensional alloying mechanism is succeeded by a place exchange mechanism at higher temperatures. The (2×2) alloy, formed for Al coverages above 0.5 ML, remains stable up to ∼1000 K, at which point, complete Al dissolution occurs, though some Pt vacancy islands persist at the surface.

The effect of vanadium and grain refiner additions on the nucleation of secondary phases in 1xxx Al alloys

High purity Al–0.3 wt% Fe–0.1 wt% Si alloys with different Si, V and grain refiner contents were melt spun to produce microstructures of submicron secondary phases entrained in a higher melting point Al matrix. On reheating, a dispersion of eutectic liquid droplets forms that represents an exaggerated version of the liquid puddles that solidify pinched-off between Al dendrite arms during conventional casting. The subsequent resolidification of the droplets, analysed using differential scanning calorimetry (DSC), allows the nucleation-controlled aspects of secondary phase selection to be studied. The droplets solidify as the metastable FeAl m phase in ribbons containing ≃500 ppm V or ≃100 ppm V plus Al–Ti–B, Al–Ti–C or Al–B grain refiner. This phase contributes to the “fir-tree” surface defect in commercial sheet products. This work suggests that the combination of V and Al–Ti–B promotes FeAl m in commercial ingots, and confirms that solidification rate and bulk Si content also influence phase content.

Microstructural characterization and solidification behavior of atomized Al–Fe powders

The effect of solidification history on the resultant microstructure in atomized Al–2.56 wt% Fe and Al–6.0 wt% Fe powders was studied, with particular emphasis on droplet size, undercooling, and phase stability. The atomized Al–Fe powders exhibited four microstructural features, i.e., Al3Fe phase (now known as Al13Fe4), Al + Al6Fe, α–Al dendrite, and a predendritic microstructure. The presence of these phases was noted to depend on alloy composition and a kinetic phase competitive growth mechanism due to the initial undercooling experienced by the powders. The occurrence of structures of the predendritic, cellular, and/or dendritic type was properly predicted by the theory of dendrite growth into undercooled alloy melts for the case of large undercoolings.

A novel microstructure in the Al-La-Ni ternary eutectic alloy

The morphology of growth of phases involved in eutectic solidification continues to attract interest because of its impact on properties and performance of materials containing eutectic constituents. Al-11.3La-5.4Ni (wt%) forms a ternary eutectic at growth velocities V in the range 0.01 to 2mm/s comprising fragmented pseudo-lamellar elements each made of odd numbers of alternate Al{sub 11}La{sub 3} and Al{sub 3}Ni fibers usually with Al{sub 11}La{sub 3} as the end member of each element. Formation of {alpha}Al dendrites within this eutectic matrix at V exceeding 2mm/s suggests an asymmetrical zone of coupled eutectic growth similar to Al-Al{sub 11}Ln{sub 3} (Ln = La, Ce or Nd). The eutectic interphase spacing {lambda} is given by {lambda}{radical}V = 8.1 {+-} 1.4 {micro}m{sup 3/2}s{sup {minus}1/2} which is {approximately} 30% smaller than found for the constituent Al-based binary eutectics. Hardness at the same value of {lambda} is larger by some 50% than for either of the constituent binary eutectics.

Growth morphology and electronic properties of ultrathin Al films on Pt(111)

The growth morphology and electronic properties of Al films on Pt(111) have been investigated by STM, XPS and AES at 160 K and 300 K. Film growth is kinetically controlled at both temperatures, with formation of 2D Al dendrites during the initial stages of deposition. At 300 K, completion of the first monolayer is followed by 3D island formation, while at 160 K layer-by-layer growth persists for Al coverages ≥ 5 monolayers. Pt core-level and valence-band X-ray photoelectron spectra show growth of a high binding energy interfacial Pt state during 300 K deposition, indicative of Pt Al surface alloying. No significant alloying occurs during 160 K growth. At 300 K, deposition of second-layer Al signals the onset of adatom place exchange with the underlying Pt, reflecting a rather delicate balance between surface energy and free energy of alloy formation. The limiting Pt Al surface alloy stoichiometry corresponds to Pt 2Al 3, and the associated Pt core level shift is in quantitative accord with an interesting linear correlation which is found between the composition and associated core-level shifts of other Al-transition metal alloys.

The Solidification Microstructure of Ai-Cu-Si Alloys Metal Matrix Composites

ABSTRACTThe relationship between solidification microstructure and fluidity in MMC was studied. The composites were fabricated by infiltration of liquid metal into a alumina SAFFIL fibers preform under a gas pressure, using alloys of the AlCuSi system as matrices. The fluidity was measured in terms of classic foundry practice (i.e., the distance of flow liquid metal into the preform, while solidifying). The characterization of solidification microstructure in the cast composite was analysed and correlated with the results of fluidity. The attention was particularly focused on such effects as the presence or absence of selective nucleation, the refinement of certain solidifying phases in the presence of fibers and their influence on microstructure formation and segregation of certain elements present in the liquid at the fiber matrix interface. By comparing reinforced and non reinforced zones, it was shown that the presence of fibers resulted in a refinement of the dendritic arm spacing of the α-Al phase, with nucleation of Si on the fibers and without nucleation of primary Al dendrites. The results were discussed and compared with the microstructures and fluidity test of the unreinforced Al-Cu-Si alloys.

Nucleation characteristics and microstructure in off-eutectic Al-Zn alloys

The undercooling required for eutectic nucleation was measured using differential scanning calorimetry for Al-Zn alloys containing 2–8 wt% Al. While an average undercooling of only 0.25°C was required for the nucleation of Zn by proeutectic Al, an undercooling of 4.0°C was necessary for the nucleation of Al by primary Zn. The non-reciprocal nucleation characteristics of this system results in the formation of Zn halos around proeutectic Al dendrites and a difference in the eutectic structure when comparing hypo- to hypereutectic alloys in terms of interlamellar spacing, second phase volume fraction, and eutectic morphology.

Influence of Zr on the Grain Refinement of Aluminium

The influence of Zr on the grain refinement of aluminium after additions of Ti and B is discussed for Zr concentrations both below and above the peritectic (0.11 mass%). For hypoperitectic concentrations it is found that Zr and Ti in solution cooperate in promoting a fine-grained structure. At hyperperitectic Zr concentrations a deterioration occurs in the grain refinement due to formation of a mixed aluminide phase (Ti[sub 1-x]Zr[sub x])Al[sub 3] that removes dissolved Ti from the melt. Presence of this phase results in a lowering of the total growth-restricting effect by solutes on the growth rate of the [alpha]-Al dendrites. (orig.)

Crystal structure of β-FeSiAl5 and (BeFe)BeSiFe2Al8 phases

Among the aluminum cast alloys, the precipitation hardenable Al-7Si-0.3Mg is perhaps the most widely used alloy. Iron is present as an impurity in this alloy, and an increase in iron content adversely affects the fracture toughness and other mechanical properties. Correlation with microstructure reveals that [beta]-FeSiAl[sub 5] intermetallic compound, mostly seen in interdendritic areas, is responsible for the poor tensile properties. Beryllium trace addition ties up iron, forming a new phase with altered morphology seen only inside the [alpha]-Al dendrites. The formation of the Be-Fe phase inside [alpha]-Al dendrites is speculated to be due to a peritectic reaction. Although analysis of the Be-Fe phase has been carried out in an earlier study, the crystal structure of this phase has not been reported. Generally, phases that act as nucleates (TiAl[sub 3] in aluminium alloys) by peritectic reaction have a crystal structure similar to that of the matrix, or certain crystallographic planes of the nucleates are coherent with the matrix. The [beta]-phase (monoclinic structure), when transformed to Be-Fe phase, could result in a crystal structure with higher symmetry. Therefore, the present study was carried out with a view to: (1) reexamining the crystal structure of [beta]-phase and; (2) identifying the crystal structuremore » of Be-Fe phase.« less

Quantitative Gefügeanalyse von Aluminium-Silicium- Gußlegierungen / Quantitative Microstructural Analysis of Aluminium-Silicon Cast Alloys

A method is described which, using the Quantimet 500, enables the semi-automatic quantitative characterisation of hypoeutectic Al-Si microstructures. Microstructural parameters are the volume fraction and mean chord length of the primary phase and eutectic phases, and the dendrite arm spacing. The difficulty in determining the microstructural parameters is that a portion of the eutectic Al phase crystallises on the primary Al dendrites. In this work the crystallisation is taken into consideration using image processing. The method used is also applicable to other alloy systems where crystallisation of a eutectic phase onto the primary phase occurs. The subjective measurement of the dendrite arm spacing is replaced by a measurement of the mean chord length of the primary dendrites and a proportionality constant is given for hypoeutectic Al-Si alloys wich relates the two parameters

The effect of hot-rolling on chill-cast Al, Al-2 wt % Ni and Al-4 wt % Ni alloys

The effect of hot-rolling on the mechanical properties and microstructure of directionallysolidified hypoeutectic Al-Al3Ni alloys has been studied. Chill-cast hypoeutectic alloys were produced by casting into pre-heated mild-steel moulds placed on copper chills. The chill-cast Al-2 wt% Ni and Al-4 wt% Ni hypoeutectic alloys can be hot-rolled at 500‡ C to reductions of greater than 95%. Deformation is achieved by deforming the aluminium-rich dendrites in the rolling direction, followed by interpenetration of the Al3Ni fibres into the dendrites resulting in a homogeneous microstructure. The variations of room-temperature tensile properties for the chill-cast hypoeutectic alloys were measured as a function of reduction of thickness during hot-rolling. The ultimate tensile strength and yield strength increase during rolling because of increasing Al3Ni fibre alignment, homogeneous dispersion of the Al3Ni fibres throughout the Al matrix, and work hardening in the Al matrix. The as-chill-cast alloys have strengths which agree with the composite law of mixtures for a combination of Al dendrites and Al-Al3Ni eutectic. After hot-rolling, the alloy strengths can be predicted from discontinuous fibre reinforcement theory.

Recrystallization behaviour of commercial Al-1%Mn alloy

Abstract The recrystallization behaviour of a commercial Al-1%Mn alley is discussed en the basis of microstructural changes during industrial precessing steps. The main effects result from the extended heat treatment at high temperatures normally used fer direct-chill-cast blocks. The precipitation of finely dispersed second-phase particles within the Al dendrites limits the nucleation of recrystallization to the deformation zone around the coarse primary particles. The formation of new grains at or near the large primary particles involves the migration and dissolution of subgrain boundaries. These reactions are enhanced by the presence of a precipitate-free zone around the primary particles, where the Mn content in solid solution has been reduced by the diffusion of Mn atoms into the particles. A fine final grain size is obtained provided that no decomposition occurs while heating to the annealing temperature, which should be chosen well above the recrystallization temperature of the cold-rolledmaterial.

The structure of the unidirectionally solidified Al-Al2Au eutectic

The structure of a number of unidirectionally solidified Al-Al2Au alloys of eutectic and off-eutectic compositions has been investigated over a wide range of growth rates (1.6×10−4 to 1.66×10−2cm sec−1) using a thermal gradient of approximately 80 to 100‡ C cm−1. The system exhibits an asymmetric coupled growth zone, which widens at high thermal gradient-growth rate ratios. At high solidification rates a broken lamellar structure is obtained, while at low rates a mixed structure with broken lamellae and rods is present. The crystallographic orientation of the phases as determined on different eutectic grains is as follows: $$\begin{gathered} lamellar interface \left\| {(001)_{Al_{ 2} Au} } \right.\left\| {(01 1)} \right._{Al} \hfill \\ \left[ {1 1 0} \right]_{Al_{ 2} Au} \left\| {\left[ {1 0 0} \right]} \right._{Al} \hfill \\ growth direction of \hfill \\ lamellae and rods \left\| {\left[ {1 1 0} \right]_{Al_2 Au} \left\| {\left[ {1 0 0} \right]_{Al} } \right.} \right. \hfill \\ \end{gathered} $$ while the preferred growth direction for the Al dendrites as well as for the Al2Au dendrites has been found to be the [100] direction. The interlamellar spacing varies according to the well known relationship $$\lambda = A \cdot R^{ - n} $$ where R is the growth rate, with n-0.37 for the free-dendrites eutectic structures and n-0.49 for the inter-dendrite eutectic structures.

INFLUENCE OF Ca, Ti, P, S AND C ON SILUMIM

Commercial Silicon always contains Fe, Al, Ca, Ti, P, S and C as impurities, which tend to segregate as shown in Table. 1. Test bars of Silumin made from same Aluminium ingot and socalled same charge Silicon have often different values in their properties. Some proportion of impurities in Silicon is assumed to be alloyed in Silumin, but their influences on Silicon have not yet been studied. In this report influences of Ca, Ti, P, S and C on Silumin are investigated using standard Silumin and mother alloys shown in Table. 2. P and S were added in Silumin by melting the mixture of Silumin borings and P or S in hot graphite crucible, and also by charging in molten Silumin by phosphorizer, C was added in Silumin by violently starring the mixture of molten Silumin and graphite while cooling and heating again at 1000°C about one hour. Influences of Ca or Ti on mechanical properties of Silumin are shown in Table. 3, Fig. 1 and Fig. 3, Chemical compositions in Table. 4, and their microstructure and casting surface in Photo. 1-11. The results obtained are as follows:(1) Ca has modification effect on Silumin, but it is not so remarkable as that by Na. Tensile strength of Silumin containning 0.05% of Ca is equal as Silumin modified by Na, but elongation is lower. Ca over than 0.05% injures both tensile strength and elongation of Silumin, because of the appearance of CaSi2. Ca also makes casting surface of Silumin so-called toad skin disliked by founder. This phenomenon is a sort of modification in which primary Al dendrites crystalize in the normal direction to casting surface.(2) Ti less than 0.05% has little influence upon mechanical properties and microstructure.(3) P acts as nucleus of Si crystal as phosphide. C also acts as nucleus, but effect of S as nucleus is feeble.(4) Oxide that might be included, and observed in fact, above treatments, has little nucleation effect.

EFFECT OF 20 ADDING ELEMENTS ON SILUMIN

Effect of Ca, Ti, P, S and C, which will possiblly be alloyed in Silumin from the raw material; Silicon, on Silumin has already been reported. In this report effects of other 20 adding elements (Ag, B, Ba, Be, Bi, Cd, Co, Cr, Cu, Fe, Mg, Mn, Mo, Ni, Pb, Sb, Sn, V, W and Zn) on the hardness, microstructure and casting surface of Silumin has been studied. Relations between the content of additional elements and the Rockwell hardness (B scale) of Silumin are shown in Table 2 and Fig. 1a-d. Microstructures of slowly cooled or chillcast specimens are shown in Photo. 1-92, and the casting surfaces are also shown in Photo. 96-115. The results of observations may be summarized as follows;(1) The most effective element to increare the hardness of Silumin is found to be Mg followed by Cu, although the addition of Co, Cr, Ni and Mn are found also to be very effective, and the effect of Ba, Bi, Ag, Fe and Mo is not so conspicuous. Zn, Cd and Pb have little or no effect.(2) Ba, Sb, Bi and Mg modify the structure of Silumin especially as chilled but their modifying power is not so strong as Na. The microstrcture of Silumin modified by these elements are not so fine as modified by Na especially as slow cooled.(3) Ba, Sb, Bi and Mg make the casting surface of Silumin like toad's skin which is disliked by founder. This phenomenon is a result of feeble modification in which Al dendrites grow rectangular to the casting surface.