The formation of coarse intermetallics in rapidly solidified AlCo alloys

doi:10.1016/0025-5416(83)90011-3

Materials Science and Engineering

Volume 60, Issue 3, September 1983, Pages 269-274

https://doi.org/10.1016/0025-5416(83)90011-3 Get rights and content

Abstract

Rapidly solidified hypereutectic AlCo alloys cast by melt spinning may contain coarse intermetallic Al₉Co₂, fine Al₉Co₂ or primary aluminum dendrites with cobalt segregated interdendritically. A metallographic investigation of as-cast ribbons indicated that the volume fraction and size of the intermetallics are determined by the cooling rate and the solute content; decreasing the cooling rate and increasing the solute content tended to increase the fraction of Al₉Co₂ precipitates. The intermetallic Al₉Co₃ does not appear to result from a breakdown of primary aluminum solidification as has been previously proposed but rather from the direct nucleation of Al₉Co₂ from the melt followed by subsequent growth.

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Supersonic cruise vehicle technology assessment study of an over/under engine concept, advanced P/M aluminum alloy development for damage tolerance and high strength structural applications
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Fatigue resistance of aluminum P/M alloy development
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Elevated temperature alloy development

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Influence of cold rolling and thermal treatment on microstructure and texture evolution, and tensile behaviour of high strength Al-Co-Sc-Zr alloys
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The present work is an effort to develop a set of newer aluminum alloys with a minor amount of alloying additions such as Co, Sc and Zr to pure Al in order to achieve higher strength along with a reasonable tensile ductility and improved microstructural stability at elevated temperatures. The alloys were melted using vacuum arc melting and were produced in rectangular slabs by suction casting. The slabs were cold rolled at room temperature up to ~80% reduction followed by direct aging at two different temperatures of 375 °C and 450 °C for 24 h. The effects of alloying additions, cold rolling and subsequent thermal aging were investigated by detailed microstructural examination and tensile testing at room temperature. Microstructural analysis revealed that the alloy contained significant volume fraction of second phase particles such as Al₉Co_2, Al₃Sc and Al₃(Sc_1−xZr_x) and were preferentially distributed in supersaturated equiaxed cells of Al matrix. The existence of Al₃(Sc_1−xZr_x) precipitates together with large fraction of Al₉Co₂ phase led to higher yield strengths of ~180–250 MPa and 17–20% tensile elongation. Electron backscattered diffraction observations showed equiaxed cells extensively deformed columnar grains and subgrain formation within the elongated grains in the rolling direction. The cold rolled plus aged samples exhibited a major fraction of rotated cube texture along with improved microstructural stability after high-temperature aging. This could be related to the existence of fine and stable Al₉Co₂ phase and Al₃(Sc_1−xZr_x) precipitates, which control grain growth and thereby these alloys thermally stable at high temperatures.
Sliding wear performance of Al–Co alloys fabricated by vacuum arc melting and correlation with their microstructure
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Citation Excerpt :
% Co alloy can be explained by the high solidification cooling rate of the richest Al composition. Garrett and Sanders [55] stated that during solidification of a hypereutectic Al-alloy, it is either primary Al or the primary intermetallic that will nucleate first. At high rates of solidification (high undercoolings), the nucleation of the primary intermetallic (that would precede under equilibrium conditions) can be suppressed; instead, nucleation and growth of primary aluminum can take place.
Al–Co alloys of various compositions (2–20 wt% Co) have been fabricated by vacuum arc melting and evaluated with respect to their microstructure and dry sliding wear performance. A variety of microstructures have been attained, ranging from that of a dual fibrous and lamellar Al/Al₉Co₂ eutectic (Al-2 wt. % Co), to that of coarse primary Al₉Co₂ plates within an Al-matrix (Al-20 wt% Co). Despite their hypereutectic compositions, the alloys behave as in-situ Al₉Co₂/Al composites: with increasing the Co content and, consequently, the Al₉Co₂ volume fraction, the hardness of the alloys increases and their wear rate decreases. The alloys exhibit lower wear rates compared to widely used Al-alloys, as-fabricated and heat-treated. The friction behavior has been analyzed and correlated with the good interfacial Al₉Co₂/Al bonding and the promotion of abrasive wear by the reinforcement. Raman spectroscopy of wear debris manifested its contribution to a mild wear mode through the absolute predominance of oxide-based phases with lubricating properties. A wear mechanism has been formulated based on the wear track morphology, debris nature and friction coefficient trends. Adhesive wear and abrasive wear assisted by oxidative wear have been identified as the main wear modes. The predominance of adhesion decreases and that of abrasion increases as the Co content increases.
Microstructure, phase morphology, eutectic coupled zone and hardness of Al–Co alloys
2020, Materials Characterization
Citation Excerpt :
In this sense, Hung et al. [27] identified AlCo alloys as promising materials with potential hardness advantages after rapid quenching techniques as those found, for instance, in a typical metal additive manufacturing environment. Even without having studied it systematically, Garrett and Sanders [28] demonstrated that AlCo alloys may generate eutectic microstructures comprising intermetallic phases and α-Al over a broad range of processing conditions. They demonstrated the formation of exceptionally fine cellular microstructures in melt-spun AlCo alloys, containing Al9Co2 intermetallic particles embedded into the cells.
This study examines the roles played by crystal growth rates (V) and cooling rates ( $\dot{T}$ ) on the microstructural evolutions of hypoeutectic (0.7 wt% Co), eutectic (1.0 wt% Co) and hypereutectic (1.5 wt% Co) AlCo alloys. For this, these alloys have been directionally solidified by using a transient heat flow setup. It is shown that the microstructures of the 0.7 and 1.0 Co alloys are formed by α-Al matrices. In these alloys, a morphological reverse transition from α-Al matrix dendrites to cells is observed while a α-Al/ Al₉Co₂ eutectic mixture is also contained. In the Al- 1.5 wt% Co alloy, the prevalence of eutectic features occurs. Experimental growth laws relating cellular, dendritic and eutectic spacings to V and $\dot{T}$ are determined. Plots relating V and $\dot{T}$ to the alloys Co contents are proposed. The latter allow the range of coupled growth during transient solidification of AlCo alloys to be examined. The hypereutectic alloy (1.5 wt% Co) demonstrates increase of about 41% in hardness as compared to the other examined alloys.
Eutectic microstructures in dilute Al-Ce and Al-Co alloys
2019, Materials Characterization
Citation Excerpt :
For the binary Al-Co and Al-Ce systems, eutectic microstructures comprising intermetallic phases and alpha-Al matrix develop over a broad range of processing conditions. For example, Garrett and Sanders observed a very fine cellular microstructure in melt-spun Al-Co alloys that contained Al9Co2 intermetallic particles and alpha-Al [12]. The objective of the current work is to characterize Al-Co and Al-Ce eutectic microstructures of arc-melted samples, with emphasis on the phase morphologies and interfacial characteristics.
Al-Ce and Al-Co binary alloys with compositions of 0.5, 1.0, and 3.0 at.% were arc-melted, allowed to solidify, and then characterized by a combination of X-ray diffraction and electron microscopy techniques. In case of the Al-Ce system, α-Al and Al₁₁Ce₃ phases were observed for all three compositions. The 0.5% and 1.0 at.% Al-Ce alloy microstructures contain pro-eutectic α-Al and a α-Al/Al₁₁Ce₃ eutectic, whereas the 3 at.% Al-Ce alloy is fully eutectic. These eutectics are lamellar in each case. The Al-Co system reveals pro-eutectic α-Al phase and an Al/Al₉Co₂ eutectic for the 0.5 at.% composition, a largely eutectic microstructure for the 1 at.% alloy, and dendrites of the Al₉Co₂ phase, surrounded by a sheath of α-Al, with pockets of eutectic between these features for the 3 at.% composition. For the 0.5 at.% and 1 at.% Al-Co alloys the eutectic is rod-like. The orientation relationships between the intermetallic and α-Al phase were determined for the eutectics using electron diffraction, and the morphology was related to the interfacial lattice matching between the phases.
The influence of the fabrication route on the microstructure and surface degradation properties of Al reinforced by Al<inf>9</inf>Co<inf>2</inf>
2017, Materials Chemistry and Physics
Citation Excerpt :
As the primary intermetallic growth progresses and the temperature drops, the transport of solute may become rate limiting, at which point the continued growth of the intermetallic is not favored. Primary aluminum, therefore, begins to form, in compatibility with Garrett & Sanders [24]. Fig. 2b shows that the microstructure of Al-7Co_VAM consists of large elongated “colonies” that appear to exhibit a preferential alignment towards the direction of heat extraction, yet not being entirely parallel to.
Al-7 (wt. %) Co alloys have been prepared by conventional casting (Cast), vacuum arc melting (VAM) and free sintering (FS), with the objectives to: a) determine an effective fabrication route (in terms of low cost, ease of manufacture and property boosting) and b) exploit the benefits that the combination of a complex metallic alloy (Al₉Co₂) with Al at the Al-rich end of the Al-Co phase diagram can offer, corrosion- and wear-wise, whilst at the same time, specific weight and raw material costs remain at relatively low levels. All the attained microstructures consist of Al₉Co₂ in an Al matrix. However, the main microstructural features are strongly affected by the preparation method. The VAM alloy presents the finest microstructure, which is also highly directional. All the microstructures are analytically being discussed and explained with respect to the fabrication method. Porosity and hardness data are correlated with the fabrication route and the corresponding microstructures. All alloys, regardless of the preparation method, exhibit high resistance to localized corrosion in 3.5 wt% NaCl. The potentiodynamic and potentiostatic polarization performances are analysed. The sliding wear resistance of the Al-Co alloys is notably improved as compared to commercially pure Al and common Al-alloys. A wear mechanism is proposed and the role of Al₉Co₂ is clarified. The superiority of the VAM alloy with respect to the surface degradation behavior is noted and correlated with its microstructure.

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The formation of coarse intermetallics in rapidly solidified AlCo alloys

Abstract

Mater. Sci. Eng.

Supersonic cruise vehicle technology assessment study of an over/under engine concept, advanced P/M aluminum alloy development for damage tolerance and high strength structural applications

Fatigue resistance of aluminum P/M alloy development

Elevated temperature alloy development