1. Introduction
As a kind of superlight aluminium (Al) alloys, Al-Li alloys have become an excellent candidate engineering material for aerospace vehicles and thin-wall missile shells due to the advantages of low density, high specific strength and specific stiffness [1]. Improving the mechanical properties is the endless pursuing for the research about Al-Li alloys.
Al3Li (δ′), as the main strengthening phase with high Young's modulus in Al-Li alloys, is L12 structure and coherent with α-Al matrix (the mismatch between Al3Li and matrix ranges from 0.08 % to 0.3 %), triggering that the coherent scope remains up to a radius of 200 nm [2]. However, the coherency strain and interfacial energy between Al3Li and α-Al matrix is so small that Al3Li particles are easily sheared by dislocations, that is, the so-called planar slip phenomenon [3,4]. Consequently, Al-Li alloys invariably possess poor ductility due to the strain localization. To overcome this shortcoming, microalloying is a common strategy, which can efficiently reduce the trend of the planar slip by the formation of non-shearable particles [1,5]. With the additions of Sc and Zr, the precipitation of Al3Li enveloping Al3Sc and Al3Zr phases occurs due to the heterogeneous nucleation of Al3Li around Al3Sc and Al3Zr [[6], [7], [8], [9], [10]]. According to the literature [6,8], the two-stage aging treatment (450 °C/18 h + 190 °C/ 4 h) can trigger the formation of core/shell composite Al3(Sc, Zr, Li) precipitates in quaternary Al-Li-Sc-Zr alloy. The morphology, mean size and size distributions of core-shell Al3(Sc, Zr, Li) particles have been elucidated [9,[7], [8], [9], [10]]. The composite particle shows a regular elemental variation from the interfaces of α-Al/particle to the core of the particle [[9], [10], [11]]. However, the nucleation, coarsening kinetics and strengthening mechanisms of composite particles are still indistinct. Additionally, according to the literature [[12], [13], [14], [15]], Yb and Er in Al-Sc-Zr system can increase the creep resistance due to the larger lattice misfit between Al3Yb (Al3Er) and α-Al (7.4 % and 4.08 %, respectively).
It can be speculated that the tailored nanoparticles with a complex core/shell structure will satisfy a variety of demands, such as lattice misfit, shearability and coarsening resistance. In this communication, Yb and Er are added into the quaternary Al-Li-Sc-Zr system to obtain a new alloy system. In Al-Li-Yb-Er-Sc-Zr hexahydric alloy, novel core/shell composite particles will be precipitated under the corresponding specific aging parameters. The aims of this work are to investigate: (1) the effects of Yb, Er, Sc and Zr on precipitate behavior in Al-2Li alloy; (2) the coarsening kinetics of novel Al3(Yb, Er, Sc, Zr, Li) core/shell composite precipitates; and (3) the strengthening mechanisms of Al-Li-Yb-Er-Sc-Zr in the different heat-treatment states.
2. Experimental procedure
Al-Li-Yb-Er-Sc-Zr alloy was prepared from pure Al, pure Li, and master alloys of Al-10Yb, Al-10Er, Al-2Sc and Al-10Zr (wt%). The alloy was prepared in an electrical resistance furnace protected by argon atmosphere. After all the required charges were loaded in a graphite crucible placed in the melting furnace, the furnace was heated to 760 °C and the temperature was hold for 60 min. Then the temperature of melt was lowered to 720 °C and the melt was poured into a permanent mold preheated at 200 °C and then was cooled to ambient temperature. The as-cast Al-2.0Li-0.1Yb-0.1Er-0.1Sc-0.1 Zr (wt%) alloy was homogenized in the resistance furnace at 550 °C/16 h and then water-cooled. Subsequently, three different aging processes were conducted: (1) Isochronal aging from 100 °C to 550 °C with 50 °C increment per hour; (2) Isothermal aging at 300 °C/0-144 h; (3) Two-stage isothermal aging: 300 °C/24 h followed by 150 °C/0-144 h. After aging, the specimens were also water-quenched to ambient temperature. According to GB/T 4340 standard, the microhardness was measured by HXS-1000Z hardness tester (200 gf/10 s). The average value was calculated from at least fifteen random indentations and reported with standard deviations. The unit of microhardness is converted to MPa (1 HV≈10 MPa) [16]. The morphology and precipitate size of nanoscale precipitates were observed by STEM (FEI Talos F200X G2). To the accuracy for the precipitate size measurement, each average value of precipitate radius was deduced from a measurement of more than 200 precipitates and 5 dark-field TEM images.
3. Results
3.1. Isochronal aging
Fig. 1 illustrates the evolution of the microhardness in Al-Li-Yb-Er-Sc-Zr during isochronal aging from 100 °C to 550 °C for 50 °C/1 h. The precipitation behavior can be reflected by the evolution of microhardness during isochronal aging. As shown in Fig. 1, three peaks in microhardness are obtained at 250 °C for 645 MPa, 350 °C for 768 MPa and 500 °C for 743 MPa, respectively. According to the previous research in Al-Sc-Zr and Al-Li-Mg-Sc-Zr alloys [17], the second peak at 350 °C can be attributed to the precipitation of Sc. The third peak at 500 °C can be related to the precipitation of Zr. This can be interpreted as the result of atom diffusivity difference of Sc and Zr: DSc>DZr [17]. However, in this research, the temperature of first peak is different from the previous literature [12,15,17,18]. In the isochronally aged Al-Li-Mg and Al-Li-Mg-Sc-Zr alloys [17], the first peak attributed to the formation of Al3Li is achieved at 150 °C. In the isochronally aged Al-Er-Sc-Zr [12,18] and Al-Yb-Er-Sc-Zr alloys [15], the first peak related to the formation of Al3Yb and Al3Er is obtained at 200 °C and 250 °C, respectively. Therefore, the first microhardness peak at 250 °C in this research may be attributed to the simultaneous precipitation of Li, Yb and Er from 100 °C to 250 °C.
Fig. 1.
Fig. 1.
Evolution of the microhardness for Al-Li-Yb-Er-Sc-Zr at isochronal aging in the stages of 50 °C /h.
3.2. Isothermal aging
Precipitation hardening response during isothermal aging at 300 °C/0-144 h in Al-Li-Yb-Er-Sc-Zr is exhibited in Fig. 2. The microhardness in Al-Li-Yb-Er-Sc-Zr significantly increases to 725 MPa at 24 h and then remains plateau value before 144 h. Based on the result of aging at 300 °C/0-144 h, a two-stage aging treatment (300 °C/24 h +150 °C) is performed to further improve aging hardening response by precipitating the core-shell Al3(Yb, Er, Sc, Zr, Li) composite particles. The first aging stage is designed at 300 °C/24 h so as to precipitate core-Al3(Yb, Er, Sc, Zr) particles and avoid the formation of AlLi phase [19]. The second aging stage is at 150 °C designed to reduce in the thermodynamic driving force for homogeneous nucleation of Al3Li phase, triggering the enhancement of the heterogeneous nucleation of shell-Al3Li precipitates on the pre-existing core-Al3(Yb, Er, Sc, Zr) particles [20,21].
Fig. 2.
Fig. 2.
Evolution of the microhardness for Al-Li-Yb-Er-Sc-Zr at isothermal aging for 300 °C/0-144 h and 300 °C/24 h + 150 °C/96 h.
3.3. Core-shell Al3(Yb, Er, Sc, Zr, Li) composite particles
The core-shell Al3(Yb, Er, Sc, Zr, Li) composite particles in Al-Li-Yb-Er-Sc-Zr aged isothermally at 300 °C/24 h + 150 °C/144 h are observed by STEM, as shown in Fig. 3. The dark-field TEM image and selected area diffraction pattern (SADP) are presented in Fig. 3(a). The core-shell particles with a special morphology of a dark core surrounded by a bright shell are homogeneously distributed in the α-Al matrix. As shown in the inset SADP in the [001]Al zone axis, strong fcc reflections and L12 superlattice reflections are observed, suggesting that the core-shell particles are L12 cubic crystal structure. The mean radius of overall particles is about 8.5 nm, measured by Image Pro Plus software. Nine core-shell particles with doughnut-shape are presented in HAADF-STEM image, as shown in Fig. 3(b). The brightness difference between core and shell is attributed to the difference in atomic number, indicating that the atomic number of elements in the core is larger than those in the shell. As shown in Fig. 3(c), EDS chemi-STEM mapping scanning results of Yb, Er, Sc and Zr demonstrate that the core is (Yb, Er, Sc, Zr)-rich. However, Li element cannot be detected by EDS due to the small atomic number. V. Radmilovic et al. [6] observed the precipitation of core/shell Al3 (Sc, Zr, Li) particles, using TEM and APT analysis. The core-shell Al3 (Sc, Zr, Li) particles were also created via a two-stage heat treatment (450 °C/18 h + 190 °C /4 h). After the second aging stage, the Li-rich shell was observed by energy filtered jump ratio TEM. Therefore, in combination of the previous research [6,9,21], the conclusion can also be drawn that the dark shell is Li-rich. As shown in Fig. 2, the microhardness in Al-Li-Yb-Er-Sc-Zr is obviously increased under the two-stage aging treatment. The microhardness in Al-Li-Yb-Er-Sc-Zr continually increases to almost 910 MPa under the two-stage aging of 300 °C/24 h + 150 °C/96 h, indicating that the microhardness of Al-Li-Yb-Er-Sc-Zr can be improved by the precipitation of core-shell Al3(Yb, Er, Sc, Zr, Li) composite particles.
Fig. 3.
Fig. 3.
(a) Dark-field TEM image of core-shell Al3(Yb, Er, Sc, Zr, Li) composite particles aged isothermally at 300 °C/24 h + 150 °C/144 h in Al-Li-Yb-Er-Sc-Zr and the inset selected area diffraction pattern in [001] zone axis. (b, c) HAADF-STEM of composite particles and corresponding element mapping distribution image of Yb, Er, Sc and Zr: (b) HAADF-STEM image; (c) element mapping distribution image.
High-resolution TEM images of a core-shell Al3(Yb, Er, Sc, Zr, Li) particle in [001] zone axis are manifested in Fig. 4(a) and (b). As shown in Fig. 4(a), three regions of the core, shell of particle and matrix in Fig. 4(a) can be observed. The three regions can be easily distinguished by the inset diffractograms transformed by FFT. Strong fcc reflections correspond to the matrix. Additional characteristic L12 superlattice reflections can be clearly observed in the core of particle observed in the diffractogram. The L12 superlattice reflections in the shell of particle are weaker in the diffractogram. Moreover, both the core and shell of composite particle are coherent with the matrix due to the non-existence of interfacial dislocations in Fig. 4(b).
Fig. 4.
Fig. 4.
(a, b) HRTEM images of a core-shell Al3(Yb, Er, Sc, Zr, Li) composite composite particle aged isothermally at 300 °C/24 h + 150 °C/144 h in Al-Li-Yb-Er-Sc-Zr.
4. Discussion
4.1. Coarsening kinetics of core-shell Al3(Yb, Er, Sc, Zr, Li) composite particles aged at 150 °C
The growth rate of spherical precipitates with nano-scale in a supersaturated solid solution can be expressed as [22]:
where γ is interfacial energy, Vm is molar volume of precipitates, c∞ is solute concentration, D is solute diffusion coefficient, kd is rate constant of a first-order deposition reaction and rB(cB) is a characteristic radius.
When D << kdr, the Eq. (1) can be simplified to:
where <r0> is average particle size at t = 0 and k1 is rate constant. The growth rate of precipitates is limited by diffusion. Eq. (2) can also be called as the Lifshitz-Slyozov-Wagner (LSW) model [23,24] for binary-alloy systems and the modified LSW model (called as BW model) for ternary-alloy systems [25]. Fuller et al. [26] reported the coarsening kinetics of Al3(Sc, Zr) at 300 °C, 350 °C and 375 °C. Analyses of the coarsening results indicated that diffusion-limited coarsening was occurring, which was supported by the agreement of the activation energy values. Karnesky et al. [27] and Gao et al. [28] found that the coarsening of Al3(Er, Sc) and Al3(Er, Zr) was also controlled by diffusion. Krug et al. [29] reported the PSDs of Al3(Re, Sc) (Re = Tb, Ho, Tm and Lu) at 300 °C. The agreement between the PSDs measured in the experiment and predicted by the BW model was observed, indicating diffusion-limited coarsening was the most probable mechanism. Dalen et al. [30] reported the time exponents of the evolution of average precipitate radius in Al-Yb-Sc-Zr at 300 °C. Although the time exponents were smaller than the 1/3 model value, the coarsening was occurring at the diffusion-limited rate. Therefore, with the additions of Yb and Er, the time exponents of radius evolution of precipitates can be slightly changed (>1/3 or <1/3), diffusion-limited coarsening is the most probable mechanism. However, the coarsening kinetics of core-shell Al3(X, Li) is unknown.
The mean radius of Al3(Yb, Er, Sc, Zr, Li) particles at 150 °C for 48 h, 96 h and 144 h previously aged at 300 °C/24 h is about 5.4 nm, 7.2 nm and 8.5 nm, respectively. The corresponding measured dark-field TEM images are shown in Fig. 5. The average radius of Al3(Yb, Er, Sc, Zr, Li) particles increases with increasing aging time. The curve of <rt>3 vs. t is plotted in Fig. 6 (black line). It seems to obey the Eq. (2). However, the negative value of intercept or <r0> is unrealistic. Unfortunately, the value of <r0> at 300 °C/24 h cannot be measured by dark-field TEM observation due to the special structure factor of zero [11]. Therefore, the negative value of <r0> fitted by Eq. (2) can be attributed to the two factors: either statistical scatter in the data or inappropriate for LSW theory in Al3(Yb, Er, Sc, Zr, Li) particles. Precipitate size distributions (PSDs) of Al3(Yb, Er, Sc, Zr, Li) particles aged at 150 °C are plotted as a function of normalized radius (r/<r>) in Fig. 7 to confirm the appropriateness of theory in Al3(Yb, Er, Sc, Zr, Li) particles. The gray solid bars are the experimental data. The predicted PSDs (solid line) by LSW theory is superimposed for comparison. The predicated sharp of PSDs by LSW model is time-invariant and changeless [26]. The PSDs sharp of Al3(Yb, Er, Sc, Zr, Li) particles in this research seems to self-similar. However, the experimental PSDs of Al3(Yb, Er, Sc, Zr, Li) particles show a nearly symmetrical relative to the average radius, that is the normalized radius (r/<r>) is 1. The combined PSDs (48 h + 96 h + 144 h at 150 °C) are constructed by plotting the PSDs function as a function of the normalized precipitate radius, as shown in Fig. 8. It is noted that the normalized PSDs rather than the original particle-size data are combined so that the individual PSDs contribute to the combined PSDs with equal weight. As is evident on viewing Fig. 8, the combined experimental PSDs have deviated the sharp predicted by LSW model and are highly symmetrical. Therefore, the negative value of intercept or <r0> cannot be simply attributed to the statistical scatter in the data and may be an indication of inappropriate for LSW theory in Al3(Yb, Er, Sc, Zr, Li) particles in our research, that is the coarsening of particles is not controlled by diffusion.
Fig. 5.
Fig. 5.
The measured dark-field TEM images of Al3(Yb, Er, Sc, Zr, Li) composite particles at aged isothermally at 300 °C/24 h + 150 °C/48 h, 96 h and 144 h and the inset selected area diffraction pattern in [001] zone axis: (a) 48 h; (b) 96 h; (c) 144 h.
Fig. 6.
Fig. 6.
Average radius, <r>, of core-shell Al3(Yb, Er, Sc, Zr, Li) particles as a function of isothermal aging at 150 °C previously aged at 300 °C for 24 h.
Fig. 7.
Fig. 7.
PSDs of Al3(Yb, Er, Sc, Zr, Li) aging at 150 °C previously aged at 300 °C for 24 h: (a) 150 °C/48 h; (b) 150 °C/ 96 h; (c) 150 °C/144 h. The predictions of LSW theory (solid line) and Gaussian fitting (dotted line) are shown for comparison.
Fig. 8.
Fig. 8.
Combined PSDs of Al3(Yb, Er, Sc, Zr, Li) for all times (48 h + 96 h + 144 h) at 150 °C. The predictions of LSW theory (solid line) and Gaussian fitting (dotted line) are shown for comparison.
In Eq. (1), when D >> kdr, the Eq. (1) can be simplified to:
where k2 is rate constant. The growth rate of precipitates is controlled by interface reaction [22]. The curve of <rt>2 vs t is plotted in Fig. 5 (red line). A reasonable positive value of intercept or <r0> are obtained. Moreover, the prediction PSDs (dotted line) calculated by normal distribution (Gaussian fitting) is also superimposed in Fig. 7. Compared to the PSDs predicted by LSW model, the perfect agreement with the PSDs sharp fitted by normal distribution and experimental PSD of Al3(Yb, Er, Sc, Zr, Li) particles is observed, as shown in Fig. 7, Fig. 8. According to the literature [[9], [10], [11]], with the addition of Li, the sizeable confined Li interfacial excess is observed. The interfacial free energy can be decreased. In this research, the coarsening of Al3(Yb, Er, Sc, Zr, Li) is inappropriate for LSW theory. Moreover, a good agreement with the PSDs sharp fitted by normal distribution and experimental PSDs of Al3(Yb, Er, Sc, Zr, Li) particles is observed. Therefore, the conclusion that the coarsening of Al3(Yb, Er, Sc, Zr, Li) particles is controlled by interface reaction and not diffusion can be drawn.
4.2. Strengthening mechanisms of core-shell Al3(Yb, Er, Sc, Zr, Li) composite particles aged at 150 °C
As shown in Fig. 4, Al3(Yb, Er, Sc, Zr, Li) particles aged at 150 °C for 144 h remain coherent with the matrix. Therefore, ambient-temperature strengthening mechanism of coherent precipitates is adopted. In precipitation strengthening, the mechanisms include shearing and bypass strengthening. In shearing strengthening mechanism, dislocations move towards the strengthening particles, triggering the modulus strengthening and coherency strengthening (Δσcoh+Δσmod), and dislocations shear the strengthening particles, leading to the order strengthening (Δσord) [12,13,17,31,32]. The dislocation bypass strengthening particles, resulting in the Orowan strengthening (ΔσOr) [12,13,17,31,32]. The operating strengthening is decided by the mechanism with the smallest stress. Therefore, to confirm the operating strengthening mechanism, the calculated strengthening increments using the equations listed in Refs. [12,13,17,31,32] and experimental increment, using the equation of ΔHV/3, in which ΔHV is the increment in imcrohardness of as-aged state and as-cast Al-2Li alloy (about 210 MPa), are listed in Table 1. Consequently, for Al3(Yb, Er, Sc, Zr, Li) particles aged at 150 °C for 48 h, 96 h and 144 h, the experimental increments are close to the predicated values of Orowan strengthening. Moreover, the critical radius from shearing mechanism to bypass mechanism of core-shell particles is about 2-4 nm [12,13,27]. Therefore, it can indicate that the Orowan bypasss mechanism is operating in Al-Li-Yb-Er-Sc-Zr at aging for 150 °C.
Table 1 Experimental and calculated strength increments aged at 150 °C for 48 h, 96 h and 144 h previously aged at 300 °C for 24 h.
| Aging Time | Δσord (MPa) | Δσcoh+Δσmod (MPa) | ΔσOr (MPa) | ΔHV/3 (MPa) |
|---|---|---|---|---|
| 48 h | 95 | 109 | 243 | 220 |
| 96 h | 109 | 138 | 234 | 237 |
| 144 h | 122 | 162 | 239 | 230 |
5. Conclusions
(1) In isochronally aged Al-Li-Yb-Er-Sc-Zr alloy, the precipitation of Li, Yb and Er occurs at 100-250 °C, triggering the first peak in microhardness. The precipitation of Sc and Zr commences at 350 °C and 500 °C, leading to the second and third peaks in microhardness, respectively.
(2) The core-shell Al3(Yb, Er, Sc, Zr, Li) composite particles are precipitated after the two-stage aging treatment, whereby the core of (Yb, Er, Sc, Zr)-rich formed at 300 °C and the shell of Li-rich formed at 150 °C.
(3) The coarsening kinetics of Al3(Yb, Er, Sc, Zr, Li) particles aged at 150 °C exhibits a better fit to the relation of <r>2 ∝ kt. The excellent agreement with the PSDs sharp of Al3(Yb, Er, Sc, Zr, Li) particles and the PSD predicated by Gaussian fitting indicates the coarsening of precipitates is controlled by interface reaction, not diffusion.
(4) The Orowan bypass strengthening is operative mechanism in Al-Li-Yb-Er-Sc-Zr aged at 150 °C due to the good agreement with the experimental and calculated increments in microhardness.
Acknowledgments
This work was supported financially by the National Natural Science Foundation of China (Nos. 51671063, 51771060, 51871068 and 51971071), the Domain Foundation of Equipment Advance Research of 13th Five-year Plan (No. 61409220118), the Fundamental Research Funds for the Central Universities (No. HEUCFG201834), the Harbin City Application Technology Research and Development Project (Nos. 2015RQXXJ001 and 2017RAQXJ032), the Ph.D. Student Research and Innovation Fund of the Fundamental Research Funds for the Central Universities (No. 3072019GIP1013).
Reference
Experimental characterization of microstructure evolution in three binary Al-Li alloys provides critical tests of both diffusion screening theory and multiparticle diffusion simulations, which predict late-stage phase-coarsening kinetics. Particle size distributions, growth kinetics and maximum particle sizes obtained using quantitative, centered dark-field transmission electron microscopy are compared quantitatively with theoretical and computational predictions. We also demonstrate the dependence on delta' precipitate volume fraction of the rate constant for coarsening and the microstructure's maximum particle size, both of which remained undetermined for this alloy system for nearly a half century. Our experiments show quantitatively that the diffusion-screening theoretical description of phase coarsening yields reasonable kinetic predictions, and that useful simulations of microstructure evolution are obtained via multiparticle diffusion. The tested theory and simulation method will provide useful tools for future design of two-phase alloys for elevated temperature applications. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd.
The precipitation behaviour in an Al-5 wt.% Mg-1.8 wt.% Li alloy has been systematically investigated for both isothermal and non-isothermal heat treatments, using atom probe tomography, transmission electron microscopy and in situ small-angle X-ray scattering. In the investigated temperature range, the delta' phase is shown to form, with a Mg content similar to that of the alloy, and a Li content of 18.5 at.%. The quantitative data on isothermal and non-isothermal kinetics from the in situ experiments are compared to the outcome of a precipitation class model adapted to the case of high volume fractions. The model parameters suggest that the main effect of Mg on the precipitation of the delta' phase in Al is to lower the solubility of Li in the alpha-Al solid solution, with little effect on the interfacial energy of the precipitates or on the Li diffusivity in Al. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd.
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The size distribution of particles, which is essential for many properties of nanomaterials, is equally important for the mechanical behaviour of the class of alloys whose strength derives from a dispersion of nanoscale precipitates. However, particle size distributions formed by solid-state precipitation are generally not well controlled. Here we demonstrate, through the example of core-shell precipitates in Al-Sc-Li alloys, an approach to forming highly monodisperse particle size distributions by simple solid-state reactions. The approach involves the use of a two-step heat treatment, whereby the core formed at high temperature provides a template for growth of the shell at lower temperature. If the core is allowed to grow to a sufficient size, the shell develops in a 'size focusing' regime, where smaller particles grow faster than larger ones. These results suggest strategies for manipulating precipitate size distributions in similar systems through simple variations in thermal treatments.
Two Al-Sc-based alloys (Al-0.12Sc and Al-0.042Sc-0.009Yb, at.%) and their counterparts with Li additions (Al-2.9Li-0.11Sc and Al-5.53Li-0.048Sc-0.009Yb, at.%) are aged at 325 C. For both base alloys, the addition of Li results in greater peak hardness from incorporation of Li in the L1(2)-structured alpha'-Al-3(Sc,Li) and alpha-Al-3(Sc,Li,Yb) precipitates, and a concomitant increase in number density and volume fraction of the precipitates and a reduction in their mean radius. These changes result from a combination of: (i) an increase in the driving force for precipitate nucleation due to Li; (ii) a decrease in the elastic energy of the coherent misfitting precipitates from a decrease in their lattice parameter mismatch due to their Li content; and (iii) a decrease in the interfacial free energy, as determined from measurements of the relative Gibbsian interfacial excess of Li. In Al-2.9Li-0.11Sc (at.%), the Li concentration of the precipitates decreases from 9.1 at.% in the peak-aged state (8 h) to 5.7 at.% in the over-aged state (1536 h). As a result, the precipitate volume fraction decreases from 0.56% at peak aging time to 0.45% at 1536 h. In Al-5.53Li-0.048Sc-0.009Yb (at.%), the relatively limited Li concentration produces only a small increase in Vickers microhardness from precipitation of metastable delta'-Al3Li upon a second aging at 170 degrees C following the primary aging at 325 degrees C. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd.
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 degrees C. This core/double-shell structure strengthens the alloy, and renders it coarsening resistant for at least 64 days at 400 degrees 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 degrees 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 degrees C for 64 days. Er accelerates precipitation kinetics at 400 degrees 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 degrees C + 8 h at 400 degrees C) provides peak microhardness due to optimization of the nanostructure. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd.
The effect of substituting 0.01 at.% Er for Sc in an Al-0.06Zr-0.06Sc-0.04Si (at.%) alloy subjected to a two-stage aging treatment (4 h/300 degrees C and 8 h/425 degrees C) is assessed to determine the viability of dilute Al-Si-Zr-Sc-Er alloys for creep applications. Upon aging, coherent, 2-3 nm radius, L1(2)-ordered, trialuminide precipitates are created, consisting of an Er- and Sc-enriched core and a Zr-enriched shell; Si partitions to the precipitates without preference for the core or the shell. The Er substitution significantly improves the resistance of the alloy to dislocation creep at 400 degrees C, increasing the threshold stress from 7 to 10 MPa. Upon further aging under an applied stress for 1045 h at 400 degrees C, the precipitates grow modestly to a radius of 5-10 nm, and the threshold stress increases further to 14 MPa. These chemical and size effects on the threshold stress are in qualitative agreement with the predictions of a recent model, which considers the attractive interaction force between mismatching, coherent precipitates and dislocations that climb over them. Micron-size, intra- and intergranular, blocky Al3Er precipitates are also present, indicating that the solid solubility of Er in Al is exceeded, leading to a finer-grained microstructure, which results in diffusional creep at low stresses. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd.
Nanosized precipitates in an Al-0.06Sc at.% alloy containing various microalloying additions were studied, with the goal of developing cost-effective aluminum alloys for high-temperature applications, using micro-hardness, electrical conductivity and atom-probe tomography measurements. Substituting 0.005 at.% Er for the more expensive Sc maintains high ambient-temperature strength, and dramatically improves the high-temperature creep resistance, as anticipated from the increase in lattice parameter mismatch between the alpha-Al(fcc) matrix and the coherent L1(2)-ordered Al-3(Sc,Zr,Er) precipitates. A concentration of the slow-diffuser Zr as low as 0.02 at.% is sufficient to provide coarsening resistance at 400 C (an homologous temperature of 0.72) for up to 66 days by forming a Zr-enriched outer shell encapsulating the precipitates. Finally, adding 0.05 at.% Si enhances ambient-temperature strength by increasing the number density of precipitates, while decreasing the homogenization and peak-aging heat-treatment times, which is caused by the Si atoms accelerating the Er and Sc diffusion kinetics. Si-containing alloys are also cost effective, owing to the existence of Si in commercial purity Al. But addition of Si reduces the precipitate coarsening resistance: the magnitude of this effect is, however, determined by the Si concentration. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd.
An Al-6.3Li-0.07Sc-0.02Yb (at.%) alloy is subjected to a double-aging treatment to create nanoscale precipitates, which are studied by atom-probe tomography and transmission electron microscopy. After homogenization and quenching, Yb atoms form clusters exhibiting L1(2)-like order. A first aging step at 325 degrees C leads to a doubling of microhardness as a result of the formation of coherent precipitates with an Al(3)Yb-rich core and an Al(3)Sc-rich shell. The core and shell both exhibit the L1(2) structure and both contain a large concentration of Li, which substitutes for up to 50% of the Sc or Yb atoms at their sublattice positions. These core/single-shell precipitates provide excellent resistance to overaging at 325 degrees C. Subsequent aging at 170 degrees C increases the microhardness by an additional 30%, through precipitation of a metastable delta'-Al(3)Li second shell on the core/single-shell precipitates, thereby forming a chemically and structurally complex core/double-shell structure. The metastable delta'-Al(3)Li phase is observed to form exclusively on pre-existing core/shell precipitates. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd.
AbstractThe effect of heat treatment on precipitation and growth of coherent nanometer-sized Al3(Sc,Zr) particles and the effect of these particles on tensile properties of a direct chill (DC) cast Al–Zn–Mg–Cu–Sc–Zr alloy were studied. The size distribution, average size, number density and volume fraction of the Al3(Sc,Zr) particles were determined as a function of the solution treatment temperature and time. An increase in the solution treatment temperature and time resulted in Al3(Sc,Zr) particles with a larger mean diameter, higher volume fraction and lower number density. The particle size distributions were described well by normal (Gaussian) distributions. The kinetics of the phase transformation followed the Kolmogorov–Johnson–Mehl–Avrami law, with the Avrami exponent m = 0.404. Room temperature tensile properties were evaluated in the as-solution treated and artificially aged conditions. The coherent nanometer-sized Al3(Sc,Zr) particles provided additional Orowan strengthening, which increased with increasing particle volume fraction and decreasing particle size, and varied from 75 to 118 MPa after different heat treatments.]]>
AbstractThe coarsening behavior of four Al(Sc,Zr) alloys containing small volume fractions (<0.01) of Al3(Sc1−xZrx) (L12) precipitates was investigated employing conventional transmission electron microscopy (CTEM) and high-resolution electron microscopy (HREM). The activation energies for diffusion-limited coarsening were obtained employing the Umantsev–Olson–Kuehmann–Voorhees (UOKV) model for multi-component alloys. The addition of Zr is shown to retard significantly the coarsening rate and stabilize precipitate morphologies. HREM of Al(Sc,Zr) alloys aged at 300 °C reveals Al3(Sc1−xZrx) precipitates with sharp facets parallel to {1 0 0} and {1 1 0} planes. Coarsening of Al-0.07 Sc-0.019 Zr at.%, Al-0.06 Sc-0.005 Zr at.% and Al-0.09 Sc-0.047 Zr at.% alloys is shown to be controlled by volume diffusion of Zr atoms, while coarsening of Al-0.14 Sc-0.012 Zr at.% is controlled by volume diffusion of Sc atoms.]]>
AbstractThe coarsening kinetics of nanoscale, coherent precipitates in α-Al during aging of a supersaturated Al–0.06 Sc–0.02 Er (at.%) alloy at 300 °C are studied using transmission electron microscopy and local-electrode atom-probe tomography. Erbium and Sc segregate at the precipitate core and shell, respectively. The matrix supersaturations of Er and Sc, as well as the mean precipitate radius and number density evolve in approximate agreement with coarsening models, allowing the determination of the matrix/precipitate interfacial free energy and solute diffusivities. At 300 °C, the α-Al/Al3(Sc1-xErx) interfacial free energy due to Sc is about twice as large as for α-Al/Al3Sc. The diffusivity of Er in the ternary alloy is about three orders of magnitude smaller than that of Er in binary Al–0.045 at.% Er and about two orders of magnitude smaller than the diffusivity of Sc in binary Al–Sc. The measured Sc diffusivity is consistent with the literature values.]]>
AbstractThe age-hardening response at 300 °C of Al–0.06Sc–0.02RE (at.%, with RE = Tb, Ho, Tm or Lu) is found to be similar to that of binary Al–0.08Sc (at.%), except that a shorter incubation period for hardening is observed, which is associated with nanoscale RE-rich Al3(RE1−xScx) precipitates. In addition, Al–0.06Sc–0.02Tb (at.%) has a much lower peak microhardness than that of Al–0.08Sc (at.%) due to the small solubility of Tb in α-Al(Sc). Peak-age hardening occurs after 24 h, and is associated with a high number density of nanoscale Sc-rich Al3(Sc1−xREx) precipitates. Analysis by three-dimensional local-electrode atom-probe tomography shows that x increases with increasing atomic number, and that the REs partition to the core of the precipitates.]]>
It is known that Zr and Yb partition to the Al(3)Sc precipitates created during aging when microalloyed separately in dilute binary Al-Sc alloys. Addition of Zr delays precipitate coarsening, thereby improving the coarsening resistance of the ternary Al-Sc-Zr alloys. Addition of Yb increases the resistance against dislocation climb, thereby improving the creep resistances of the ternary Al-Sc-Yb alloys. A combination of microhardness, creep, and atom probe tomography measurements provide evidence that these effects of Zr and Yb additions are cumulative in quaternary dilute Al-Sc-Yb-Zr alloys: Yb increases their creep resistance at 300 degrees C compared with ternary Al-Sc-Zr alloys and Zr improves their coarsening resistance at 300 degrees C compared with ternary Al-Sc-Yb alloys. Additionally, excellent coarsening resistance is observed at 350 and 375 degrees C. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd.
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