The effect of substituting 0.01 or 0.02
at.% Er for Sc in an Al–0.06 Zr–0.06 Sc
at.% alloy was studied to develop cost-effective high-temperature aluminum alloys for aerospace and automotive ...applications. Spheroidal, coherent, L1
2-ordered Al
3(Sc, Zr, Er) precipitates with a structure consisting of an Er-enriched core surrounded by a Sc-enriched inner shell and a Zr-enriched outer shell (core/double-shell structure) were formed after aging at 400
°C. This core/double-shell structure strengthens the alloy, and renders it coarsening resistant for at least 64
days at 400
°C. This structure is formed due to sequential precipitation of solute elements according to their diffusivities,
D, where
D
Er
>
D
Sc
>
D
Zr at 400
°C. Zr and Er are effective replacements for Sc, accounting for 33
±
1% of the total precipitate solute content in an Al–0.06 Zr–0.04 Sc–0.02 Er
at.% alloy aged at 400
°C for 64
days. Er accelerates precipitation kinetics at 400
°C, resulting in: (i) strengthening due to the elimination of lobed-cuboidal precipitates in favor of spheroidal precipitates; and (ii) a decrease in the incubation time for nucleation because
D
Er
>
D
Sc. Finally, a two-stage aging treatment (24
h at 300
°C
+
8
h at 400
°C) provides peak microhardness due to optimization of the nanostructure.
A bulk nanostructured alloy with the nominal composition Cu–30Zn–0.8Al wt.% (commercial designation brass 260) was fabricated by cryomilling of brass powders and subsequent spark plasma sintering ...(SPS) of the cryomilled powders, yielding a compressive yield strength of 950MPa, which is significantly higher than the yield strength of commercial brass 260 alloys (∼200–400MPa). Transmission electron microscopy investigations revealed that cryomilling results in an average grain diameter of 26nm and a high density of deformation twins. Nearly fully dense bulk samples were obtained after SPS of cryomilled powders, with average grain diameter 110nm. After SPS, 10vol.% of twins is retained with average twin thickness 30nm. Three-dimensional atom-probe tomography studies demonstrate that the distribution of Al is highly inhomogeneous in the sintered bulk samples, and Al-containing precipitates including Al(Cu,Zn)–O–N, Al–O–N and Al–N are distributed in the matrix. The precipitates have an average diameter of 1.7nm and a volume fraction of 0.39%. Quantitative calculations were performed for different strengthening contributions in the sintered bulk samples, including grain boundary, twin boundary, precipitate, dislocation and solid-solution strengthening. Results from the analyses demonstrate that precipitate and grain boundary strengthening are the dominant strengthening mechanisms, and the calculated overall yield strength is in reasonable agreement with the experimentally determined compressive yield strength.
To provide insight into the relationships between precipitation phenomena, grain size and mechanical behavior in a complex precipitation-strengthened alloy system, Al 7075 alloy, a commonly used ...aluminum alloy, was selected as a model system in the present study. Ultrafine-grained (UFG) bulk materials were fabricated through cryomilling, degassing, hot isostatic pressing and extrusion, followed by a subsequent heat treatment. The mechanical behavior and microstructure of the materials were analyzed and compared directly to the coarse-grained (CG) counterpart. Three-dimensional atom-probe tomography was utilized to investigate the intermetallic precipitates and oxide dispersoids formed in the as-extruded UFG material. UFG 7075 exhibits higher strength than the CG 7075 alloy for each equivalent condition. After a T6 temper, the yield strength (YS) and ultimate tensile strength (UTS) of UFG 7075 achieved 734 and 774MPa, respectively, which are ∼120MPa higher than those of the CG equivalent. The strength of as-extruded UFG 7075 (YS: 583MPa, UTS: 631MPa) is even higher than that of commercial 7075-T6. More importantly, the strengthening mechanisms in each material were established quantitatively for the first time for this complex precipitation-strengthened system, accounting for grain-boundary, dislocation, solid-solution, precipitation and oxide dispersoid strengthening contributions. Grain-boundary strengthening was the predominant mechanism in as-extruded UFG 7075, contributing a strength increment estimated to be 242MPa, whereas Orowan precipitation strengthening was predominant in the as-extruded CG 7075 (∼102MPa) and in the T6-tempered materials, and was estimated to contribute 472 and 414MPa for CG-T6 and UFG-T6, respectively.
Microstructure, phase composition and mechanical properties of a refractory high entropy superalloy, AlMo0.5NbTa0.5TiZr, are reported in this work. The alloy consists of a nano-scale mixture of two ...phases produced by the decomposition from a high temperature body-centered cubic (BCC) phase. The first phase is present in the form of cuboidal-shaped nano-precipitates aligned in rows along -type directions, has a disordered BCC crystal structure with the lattice parameter a1 = 326.9 ± 0.5 pm and is rich in Mo, Nb and Ta. The second phase is present in the form of channels between the cuboidal nano-precipitates, has an ordered B2 crystal structure with the lattice parameter a2 = 330.4 ± 0.5 pm and is rich in Al, Ti and Zr. Both phases are coherent and have the same crystallographic orientation within the former grains. The formation of this modulated nano-phase structure is discussed in the framework of nucleation-and-growth and spinodal decomposition mechanisms. The yield strength of this refractory high entropy superalloy is superior to the yield strength of Ni-based superalloys in the temperature range of 20 °C to 1200 °C.
Precipitation strengthening behavior during aging of an Al-0.014Sc-0.008Er-0.08Zr-0.10Si (at.%) alloy was investigated utilizing microhardness, electrical conductivity and scanning electron ...microscopy. This new composition, with a Sc/Zr ratio (in at.%) smaller than 1/5 represents a significant reduction of the alloy's cost, when compared to more usual Al-0.06Sc (at.%) based alloys with typical Sc/Zr ratios of 3. The research presented herein focuses on identifying the optimal homogenization duration at 640 °C and additionally the temperature range at which a single-step aging treatment will achieve the highest possible microhardness in the shortest time. Due to a compromise between dissolution of Er-Si rich primary precipitates, homogenization of the Zr distribution and precipitation of large Al3Zr precipitates, 8 h at 640 °C appears to be the optimal homogenization duration for this alloy, leading to an hardness of 571 ± 22 MPa after aging for 24 h at 400 °C. To study the precipitation behavior of this low-Sc concentration alloy, isochronal aging to 575 °C with two different heating rates was performed. The small Sc concentration, compensated by a high Zr concentration, permits the alloy to achieve a similar peak microhardness during isochronal aging (587 ± 20 MPa) as the corresponding Sc-richer and Zr-leaner alloys. The isochronal aging experiments permits us to identify the best aging temperature as between 350 and 425 °C.
As demonstrated by isochronal aging experiments, the newly developed low Sc alloy (Al-0.014Sc-0.008Er-0.08Zr-0.1Si at.%) achieves comparable hardness as the more expensive high Sc alloy (Al-0.055Sc-0.005Er-0.02Zr-0.05Si at.%) with 4 times more Sc. Display omitted
Combined microadditions of 0.09 at.% Mo and 0.4 at.% Mn to a dilute Al-0.10Zr-0.01Sc-0.007Er-0.10Si (at.%) alloy lead to increases in strength upon peak-aging, and improved over-aging resistance at ...400, 425 and 450 °C for at least 6 months. These improvements are related to four cumulative effects. Firstly, Mn and Mo provide, in the as-cast state, a solid-solution-strengthening contribution of ∼90 MPa. The solid-solution contribution from Mo (∼80 MPa) remains essentially unchanged during aging at 400–450 °C, due to its extremely small diffusivity and precipitation. Secondly, Mn and Mo partition to the cores and shells, respectively, of the nano-size coherent, L12 (Al,Si)3(Zr,Sc,Er) nanoprecipitates, which nucleate in <1 h at 400–450 °C. This is associated with an increase in their number density and a reduction in their growth and coarsening kinetics, thus delaying the loss of strength upon over-aging. Third, Mn and Mo provide precipitation strengthening via submicron α-Al(Mn,Mo)Si precipitates (which form between 1 and 11 days at 400 °C), which counterbalances the loss of Mn solid-solution strengthening. Finally, the α-Al(Mn,Mo)Si precipitates scavenge Si from the matrix, which is then not available to accelerate the coarsening of the L12 precipitates, thereby improving the over-aging resistance of the alloy. Iron additions (0.015 at.%), expected to replace some Mn in the α-phase Al(Fe,Mn,Mo)Si, does not affect the aging behavior.
The new developed Mo and Mn modified Al-Zr-Sc-Er-Si alloy achieves high hardness and improved coarsening resistance due to the formation of L12 (Al,Si)3(Sc,Er,Zr) nano-size precipitates, with a core enriched in Sc, Er and Si, surrounded by a Mo enriched Al3Zr shell that slow down coarsening. A secondary α-Al(Mn,Mo)Si phase further strengthen the alloys after extended aging durations. Display omitted
Precipitation strengthening is investigated in binary Al–0.1Sc, Al–0.1Zr and ternary Al–0.1Sc–0.1Zr (at.%) alloys aged isochronally between 200 and 600
°C. Precipitation of Al
3Sc (L1
2) commences ...between 200 and 250
°C in Al–0.1Sc, reaching a 670
MPa peak microhardness at 325
°C. For Al–0.1Zr, precipitation of Al
3Zr (L1
2) initiates between 350 and 375
°C, resulting in a 420
MPa peak microhardness at 425–450
°C. A pronounced synergistic effect is observed when both Sc and Zr are present. Above 325
°C, Zr additions provide a secondary strength increase from the precipitation of Zr-enriched outer shells onto the Al
3Sc precipitates, leading to a peak microhardness of 780
MPa at 400
°C for Al–0.1Sc–0.1Zr. Compositions, radii, volume fractions and number densities of the Al
3(Sc
1−
x
Zr
x
) precipitates are measured directly using atom-probe tomography. This information is used to quantify the observed strengthening increments, attributed to dislocation shearing of the Al
3(Sc
1−
x
Zr
x
) precipitates.
Lightweight, age-hardenable aluminum alloys are attracting increasing attention as a means to reduce vehicle mass and improve fuel economy. To accelerate the adoption of these alloys, knowledge of ...the complex precipitation processes that underlie their primary strengthening mechanism is essential. Here we employ a combination of atom-probe tomography (APT), differential scanning calorimetry (DSC), transmission electron-microscopy, X-ray diffraction and first-principles calculations to reveal the compositional evolution of Q-phase precipitates in a commercial, age-hardenable aluminum alloy, W319. Three different aging conditions are investigated: 438K/8h, 463K/8h and 533K/4h. Co-precipitation of θ′- and Q-phase precipitates is observed for all aging conditions, which, when combined with DSC analysis of the precipitation sequence, suggests that Q-phase precipitates serve as heterogeneous nucleation sites for θ′-platelets. Regarding composition evolution, aging at the lower temperatures yields Q-phase precipitates that are Cu-rich, yet deficient in Mg and Si: 44Al–22Cu–16Mg–16.5Siat.%. The composition evolves to become Mg-rich after aging at 533K: ∼28Al–9Cu–37Mg–26Siat.%. APT provides evidence for partitioning of Zn to the Q-phase precipitates. The energetics of Zn partitioning was evaluated using first-principles calculations, and suggests that this partitioning is a kinetic effect. Our analyses provide new insights into the complex precipitation processes in commercial Al alloys, and should foster the enhancement of alloy performance through optimization of aging conditions.
Gas-atomized powders of two ternary alloys, Al-3.60Mg-1.18Zr and Al-3.66Mg-1.57Zr (wt.%), were densified via laser powder bed fusion. At energy densities ranging from 123 to 247 J/mm3, as-fabricated ...components are near-fully densified (relative density 99.2–99.9%) as verified by X-ray tomography. While Mg acts a solid-solution strengthener, Zr creates two types of metastable L12 Al3Zr precipitates, each playing dual roles: (a) sub-micrometer Al3Zr particles form in the melt upon solidification and act as grain refining agents, nucleating fine aluminum grains, which (i) prevent hot-tearing during the rapid solidification inherent to laser melting and (ii) enhance tensile strength (Hall-Petch strengthening) and ductility (influence a heterogenous grain structure) after fabrication; (b) Al3Zr nano-precipitates form in the solid alloy during subsequent aging, which (i) precipitation-strengthen the alloy leading to an increase of >40% in strength over the as-fabricated value, and (ii) promote thermal stability of the fine grain size (and the associated Hall-Petch strengthening) after exposure to high temperature due to the slow kinetics of Al3Zr coarsening (from the sluggish diffusivity of Zr in solid Al-Mg). While the Zr-richer alloy shows higher yield and ultimate tensile strength in the as-fabricated state, both alloys have identical mechanical properties after peak aging. Interconnected bands of fine (∼0.8 μm), equiaxed, isotropic grains and coarser (∼1 × 10 μm), columnar, textured grains – both containing oxide particles and Al3Zr precipitates - provide a combination of high yield strength and high ductility (e.g., ∼354 MPa, and ∼20%, respectively) with isotropic values in both as-fabricated and peak-aged samples, unlike Al-Si alloys processed via laser fusion of commercial Al-Si-based powders. The pre-alloyed, gas-atomized Al-Mg-Zr powders do not contain expensive alloying elements such as Sc, nor do they require blending with a second powder to nucleate fine grains, making them excellent candidates for economical, large-scale additive manufacturing applications.
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Polycrystalline Co-10Ni-(9 – x)Al-(9 – x)W-2xTi at% (x = 0, 1, 2, 3, 4) alloys with γ(f.c.c.) plus γ′(L12) microstructures are investigated, where the γ′(L12)-formers Al and W are replaced with Ti. ...Upon aging, the initially cuboidal γ′(L12)-precipitates grow and develop a rounded morphology. After 256h of aging at 1000°C, the precipitates in the 6 and 8at% Ti alloys coalesce and develop an irregular, elongated morphology. After 1000h of aging, replacement of W and Al with Ti increases both the mean radius, , and volume fraction, ϕ, of the γ′(L12)-phase from = 463nm and ϕ = 8% for 2at% Ti to = 722nm and ϕ = 52% for 8at% Ti. Composition measurements of the γ(f.c.c.)-matrix and γ′(L12)-precipitates demonstrate that Ti substitutes for W and Al in the γ′(L12)-precipitates, increases the partitioning of W to γ′(L12), and changes the partitioning behavior of Al from a mild γ′(L12)-former to a mild γ(f.c.c.)-former. The grain boundaries in the aged alloys exhibit W-rich precipitates, most likely μ(Co7W6)-type, which do not destabilize the γ(f.c.c.) plus γ′(L12) microstructure within the grains. Four important benefits accrue from replacing W and Al with Ti: (i) the alloys’ mass density decrease; (ii) the γ′(L12)-solvus temperature increases; (iii) the γ′(L12) volume fraction formed during aging at 1273K (1000°C) increases; and (iv) the 0.2% offset flow stress increases.
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