The microscopic mechanism of silicon precipitation in Al/Si system (original) (raw)
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Microstructural Changes at the Initial Stage of Precipitation in an Aluminum–Silicon Alloy
Materials transactions, 2005
In this study, we use a transmission electron microscope (TEM) to observe the microstructure of an Al-Si alloy from immediately after quenching to the initial stage of precipitation. Results reveal the presence of patches immediately after quenching that are thought to be Si clusters, and these are coherent with the {111} matrix . At the initial stage of aging, Si crystals with a plate-like structure are observed, contradicting the round shape previously described. We propose that the Si clusters that appear immediately after quenching act as nuclei for the plate-like Si crystals. As the aging time increases, these increase in length and thickness, becoming thick plate-like Si phase precipitates.
Physica Status Solidi (a), 1984
Doppler broadening of the positron annihilation line was measured for quenched and aged specimens of an aluminium–silicon (1.29 at%Si) alloy. One set of specimens was aged at room temperature (set A) and one set was isochronally (t = 30 min) aged at temperatures ranging from 347 to 884 K (set B). The lineshape parameter S measured in the as-quenched condition was larger than that of well-annealed pure aluminium. S decreased with time of ageing at room temperature, which might be ascribed to the formation of vacancy loops decorated with silicon atoms. After 1320 h at room temperature the specimens of set A were aged for 30 min at 353 K in vacuo. Then, thc lineshape parameter S decreased further, which is interpreted as due to a further disappearance of the earlier formed vacancy loops. The behaviour of the lineshape parameter S for set B can be described as follows: 1. 350 to 450 K. Constant value of S, about the same as found for pure aluminium. 2. 450 to 580 K. Increase of S, ascribed to precipitation of vacancies near the Si aggregateslmatrix interface relieving transformation strains. 3. above 580 K. Decrease of S to the pure-aluminium value. This effect is ascribed to the growth of silicon precipitates accompanied by annihilation of vacancies. The misfit between the silicon precipitates and the Al-rich matrix is accommodated by dislocations.Die Dopplerverbreiterung der Positron-Annihilations-γ-Linie wird fur abgeschreckte und gealterte Proben einer Aluminium-Silizium-(1,29 Atyo Si) Legierung gemessen. Ein Probensatz wird bei Zimmertemperatur gealtert (Satz A) und ein Satz wird isochron (t = 30 min) bei Temperaturen von 347 bis 884 K gealtert (Satz B). Der Linienformparameter 8, der sofort nach dem Abschrecken gemessen wird, ist größer als der für gut getempertes reines Aluminium. S nimmt mit der Alte-rungsdauer bei Zimmertemperatur ab, was der Bildung von Leerstellenloops zugeschrieben werden kann, die mit Siliziumatomen dekoriert sind. Nach 1320 h bei Zimmertemperatur werden die Proben des Satzes A für 30 min bei 353 K im Vakuum gealtert. Dann sinkt der Linienformparameter S weiter ab, was als Folge eines weiteren Verschwindens der vorher gebildeten Leerstellenloops interpretiert wird. Das Verhalten des Linienformparameters 8 für den Satz B kann wie folgt be-schrieben werden: 1. 350 bis 450 K. Ein konstanter Wert für 8, etwa der gleiche wie für reines Aluminium. 2. 450 bis 580 K. Ein Anstieg von 8, der der Präzipitation von Leerstellen in der Nähe der Grenzfläche Si-Aggregate–Matrix zugeschrieben wird und Transformationsspannungen beseitigt. 3. oberhalb 580 K. Die Abnahme yon S auf den Wert des reinen Aluminium. Dieser Effekt wird dem Wachstum von Siliziumprazipitaten zugeschrieben und ist begleitet von Leerstellenannihila-tion. Das Misfit zwischen Siliziumpräzipitaten und Al-reicher Matrix wird durch Versetzungen ausgeglichen.
On precipitation in rapidly solidified aluminium-silicon alloys
Journal of Materials Science, 1983
The precipitation of silicon in rapidly solidified AlSi alloys was studied. For alloys with 2.4 and 11.0 wt % Si (2.3 and 10.3 at % Si, respectively) the lattice parameters of the Alrich and of the Si-rich phases were measured after ageing at 397,425 and 448 K. For alloys with 2.6 and 13.0 wt % Si crystallite sizes and lattice strains were determined by analysis of the X-ray diffraction line broadening. After ageing the lattice parameters of the Al-rich and the Si-rich phases were influenced by the difference in thermal expansion between both phases. After correction for this effect the amount of silicon dissolved in the Al-rich phase was estimated as a function of ageing time. Quenched-in (excess) vacancies influenced the precipitation kinetics. Activation energies for precipitation appeared to depend on the extent of transformation. Further, quenched-in vacancies caused anomalous maxima in the lattice parameter curves. The behaviour of the lattice microstrains on ageing was explained as a result of the disappearance of stresses due to quenching and the introduction and subsequent dissipation of stresses due to precipitation. After completed precipitation stresses due to the difference in thermal expansion between both phases still exist at room temperature.
Role of silicon in accelerating the nucleation of Al3(Sc,Zr) precipitates in dilute Al–Sc–Zr alloys
Acta Materialia, 2012
The effects of adding 0.02 or 0.06 at.% Si to Al-0.06Sc-0.06Zr (at.%) are studied to determine the impact of Si on accelerating Al 3 (Sc,Zr) precipitation kinetics in dilute Al-Sc-based alloys. Precipitation in the 0.06 at.% Si alloy, measured by microhardness and atom-probe tomography (APT), is accelerated for aging times <4 h at 275 and 300°C, compared with the 0.02 at.% Si alloy. Experimental partial radial distribution functions of the a-Al matrix of the high-Si alloy reveal considerable Si-Sc clustering, which is attributed to attractive Si-Sc binding energies at the first and second nearest-neighbor distances, as confirmed by first-principles calculations. Calculations also indicate that Si-Sc binding decreases both the vacancy formation energy near Sc and the Sc migration energy in Al. APT further demonstrates that Si partitions preferentially to the Sc-enriched core rather than the Zr-enriched shell in the core/shell Al 3 (Sc,Zr) (L1 2 ) precipitates in the high-Si alloy subjected to double aging (8 h/300°C for Sc precipitation and 32 days/400°C for Zr precipitation). Calculations of the driving force for Si partitioning confirm that: (i) Si partitions preferentially to the Al 3 (Sc,Zr) (L1 2 ) precipitates, occupying the Al sublattice site; (ii) Si increases the driving force for the precipitation of Al 3 Sc; and (iii) Si partitions preferentially to Al 3 Sc (L1 2 ) rather than Al 3 Zr (L1 2 ).
Transition metal co-precipitation mechanisms in silicon
Acta Materialia, 2007
Formation mechanisms of precipitates containing multiple-metal species in silicon are elucidated by nano-scale morphology and phase investigations performed by synchrotron-based X-ray microprobe techniques. Precipitates formed at low (655°C) and high (1200°C+) temperatures exhibit distinguishing features indicative of unique formation mechanisms. After lower-temperature annealing, co-localized single-metal silicide phases are observed, consistent with classical models predicting that dissolved, supersaturated metal atoms will precipitate into solid second-phase particles. Precise precipitate morphologies are found to depend on the local crystallographic environment. In precipitates formed during slow cooling from higher-temperature anneals, nano-scale phase separation and intermetallic phases are evident, suggestive of a high-temperature transition through a liquid phase. Based on experimental results and phase diagram information, it is proposed that under certain conditions, liquid metal-silicon droplets may form within the silicon matrix, possibly with the potential to getter additional metal atoms via liquid-solid segregation.
Atomic-scale pathway of early-stage precipitation in Al–Mg–Si alloys
Acta Materialia, 2015
Strengthening in age-hardenable alloys is mainly achieved through nano-scale precipitates whose formation paths from the atomic-scale, solute-enriched entities are rarely analyzed and understood in a directly-verifiable way. Here, we discover a pathway for the earliest-stage precipitation in Al-Mg-Si alloys: solute clustering leading to three successive variants of FCC clusters, followed by the formation of non-FCC GP-zones. The clusters, which originally assume a spherical morphology (C1), evolve into elongated clusters and orient themselves on {111} Al (C2) and subsequently on {100} Al planes and <100> Al directions (C3). We also analyze the association of quenched-in dislocations with clustering phenomena. The results of this work can open a new frontier in advancing alloy-process-property design for commercially-important age-hardenable Al alloys.
Growth of silicon particles in an aluminum matrix
Metallurgical Transactions A, 1986
The growth of silicon particles in cast aluminum-silicon alloys, during isothermal heat treatment, has been studied three-dimensionally with the aid of the global parameters of quantitative microscopy an$ with the Coulter Counter. The measured particle volume distribution can be represented as being log-normal, its geometric standard deviation of the distribution maintaining a constant value throughout isothermal growth. Increase in the average particle volume is in direct proportion to time. Its rate is an Arrhenius function of temperature, with an activation energy of about 80 kilo-calories per mol.
Orientation of silicon particles in a binary Al–Si alloy
Journal of Materials Science, 2008
The orientations Si-crystals take in aluminium, in an alloy with composition Al-1.3at%Si, were investigated by transmission electron microscopy. Hardness was measured for isothermal heat-treatments at 175°C and 260°C. Conditions analysed by TEM were 17 h at 175°C and an additional 3 h at 260°C, both containing a high density of small Si-crystals, the finest corresponding to 175°C. Two main orientation relationships were found: ð001ÞAl jj ð001ÞSi; ½100Al jj ½100Si and ð001ÞAl jj ð110ÞSi; ½1 1 1Si jj ½010Al: The first accounted for approximately 60% of Si precipitates in condition 17 h_175°C. Despite a high number density and well-aligned interfaces, the Si precipitates have negligible influence on hardness. Findings are consistent with Ge particles in Al-Ge alloys.
Heterogeneous nucleation of solidification of Si in Al-Si and Al-Si-P alloys
Acta Metallurgica et Materialia, 1995
Heterogeneous nucleation of solidification in melt spun AI-Si and AI SiP has been studied using differential scanning calorimetry, and transmission, scanning transmission and high resolution electron microscopies. The microstructures of the heat treated melt spun alloys all consist of an Al matrix, AI-Si eutectic distributed along the AI grain boundaries, and Si embedded in the Al matrix. The Si microstructure depends on the level of P: coarse faceted Si particles are nucleated by AlP particles in Al Si containing 2 ppm P and Al-Si P containing 35 ppm P whereas eutectic droplets of fine Si particles are nucleated by the surrounding AI matrix at a high undercooling in AI-Si containing 0.25 ppm P. The Si nucleation onset temperature remains approximately constant while the peak and end temperatures both decrease with increasing cooling rate, in agreement with classical nucleation theory. Kinetic analysis, using the spherical cap model gives contact angles of l0 °, 43 ° and l0 ° for Si nucleation in low and high purity AI-Si and Al-SiP respectively.