The study on the coarsening process and precipitation strengthening of Al₃Er precipitate in Al–Er binary alloy

Highlights

• The critical radius of Al₃Er from coherent to semi-coherent was 8.0–9.1 nm.

• The diffusion activation energy of Er in Al was evaluated to be 77.2 ± 5.3 kJ/mol.

• The misfit of Al₃Er with matrix would be reduced at small particle range.

• The APB energy of Al₃Er was estimated to be about 0.60 ± 0.03 J/m².

Abstract

The aging strengthening behavior and coarsening process of Al₃Er precipitates were investigated in Al–0.045Er (in at.%) alloys aged isothermally at 300–400 °C, using Vickers micro-hardness measurement and transmission electron microscopy (TEM) observation. The isothermal hardness curves showed that the peak hardness decreased slightly with the increase of temperature. From the TEM observations, the radius of Al₃Er precipitates at peak hardness was measured to be 2.4 ± 0.4 nm. The coherency of Al₃Er precipitates started to get lost at the radius of 8.0–9.1 nm, which were much larger than the predicted value. The difference was speculated to be due the reduction of misfit between Al₃Er precipitates and matrix when particles were very small. From the analysis of the coarsening of coherent Al₃Er particles using LSW theory, the diffusion activation energy and the diffusivity pre-exponential constant of Er in Al were deduced to be 77.2 ± 5.3 kJ/mol and (4.3 ± 2.2) × 10⁻¹² m² s⁻¹, respectively. Finally, the strength increment caused by Al₃Er particles was evaluated using precipitation strengthening theory. It was found that when the particle radius was larger than 2.4 nm, the Orowan bypass mechanism dominated. In radius of 0–1.4 nm, a less coherency strengthening contribution indicated a reduction of lattice misfit between particles and matrix. From the strength plateaus in radius of 1.4–2.4 nm, the anti-phase boundary (APB) energy of Al₃Er precipitates was deduced to be 0.60 ± 0.03 J/m².

Introduction

The Al₃M trialuminide compounds with L1₂ structure in aluminum alloys have attracted much attention recently [1], [2], [3], [4], [5], [6] due to their particularly attractive characteristics including low density (nominally 75 at.% Al), high specific strength, very high melting points and excellent oxidation resistance [4]. Among them, Al₃Sc is the most studied intermetallic compound so far, since it has the thermodynamically stable L1₂ structure, relatively large volume fraction and modest coarsening rate [4], [7], [8]. But the high cost of Sc limits its industrial applications. In the periodic table of elements, Er is another element which can form thermodynamically stable L1₂-structured trialuminide compound. Furthermore its maximum solubility in Al is only second to Sc. Nevertheless its maximum solubility is only 1/5 of Sc, namely 0.0461 at.%, which corresponds to 0.14% volume fraction of Al₃Er precipitate in the conventional solidification situation [8]. Such a small volume fraction would result in a limited increment of strength in Al–Er alloys [8]. The coarsening resistance of Al₃Er precipitates is very weak due to the larger diffusivity of Er in Al (4 ± 2) × 10⁻¹⁹ m² s⁻¹ at 300 °C, compared with that of Sc in Al, 9.0 × 10⁻²⁰ m² s⁻¹ at the same temperature [3], [9]. However, lots of works [1], [3], [9], [10], [11], [12], [13] have shown that, in multicomponent alloy systems, the addition of Er can play a unique role. In Al–Er–Zr alloys, Er could stimulate the decomposition of Al–Zr alloy [1], [12], and Er and Zr had a synergetic effect on the precipitation hardening of this alloy [1]. When the Al–0.04Er–xZr (all compositions are in at.% unless otherwise noted) alloys were isothermally aged at 350 °C [1], the hardness curves of these alloys exhibited two peaks. The first peak of hardness was suppressed and the second one was enhanced with the increase of the Zr concentration, and the hardness of second peak could reach up to 560 MPa which was higher than that of Al–Er and Al–Zr alloys [1]. This characteristic of isothermal aging was obviously different from that of Al–Sc–Zr alloys, in which there was only one peak during isothermal aging [14]. Meanwhile in Al–Sc–Zr–Er alloys, by only a trace addition of 0.01 at.% Er, the micro-hardness could increase from 250 MPa to 450 MPa when aged at 400 °C [2]. Through investigations by three-dimensional atom probe (3DAP), it could be seen that, in Al–Er–Zr alloys, Al₃(Er, Zr) precipitates with Er and Zr segregated at the shells and cores [10], [12] was formed, and in Al–Er–Sc–Zr alloys, the Al₃(Er, Sc, Zr) with Er-enriched core surrounded by Sc-enriched inner shell and Zr-enriched outer shell [2] was precipitated. Compared with the Er-free containing particles [1], [2], both of them had smaller radius and larger number density when the alloys were aged by the same heat treatment condition. In conclusion, although the maximum solubility of Er in Al is extremely low, just about 1/5 of Sc, it is still a very effective element for the improvement of strength by further multi-microalloying with Sc or Zr. In other words, Er provides an alternative way for the development of high-performance Al alloys. The study of the mechanism in multicomponent systems is not only interesting due to its application potential, but also of scientific significance. Thereafter as prerequisites, the fundamental properties of Er and Al₃Er precipitates in the binary Al–Er alloy need to be clarified, for example the diffusivity of Er, the coarsening and precipitation strengthening of Al₃Er particles.

Some works [8], [12] have been done about the diffusivity of Er and the coarsening of Al₃Er precipitates, however, any results on the precipitation strengthening of Al₃Er particles have not been reported. The temporal evolution of precipitate radii and matrix concentrations at 300 °C have been studied by TEM and 3DAP [8] respectively, from which the reciprocal of the temporal exponent was estimated to be 1/3, in agreement with that predicted by the LSW model for bulk diffusion, and the diffusivity of Er in Al at this temperature was determined to be (4 ± 2) × 10⁻¹⁹ m² s⁻¹. The diffusivities at a wider temperature range is essential for the investigation of Er containing precipitates evolution, which can be calculated using the pre-exponential diffusivity constant, D₀, and activation energy Q of Er according to the Arrhenius equation. The relevant Q can be determined from the isothermal evolution of mean radius. For example, in Al–Mn–Fe–Si–Zr–Sc and Al–Fe–Si–Zr–Sc alloys [15], the Q of Zr was calculated to be 285 ± 31 kJ/mol and 250 ± 43 kJ/mol which were in good agreement with the reported value 242 kJ/mol [4], [16], [17] determined by the isotopic tracer method in Al–Zr alloys. In a similar way, the Q of Sc in Al–0.17Sc was calculated to be 176 kJ/mol [7] which was agreed well with the value obtained by Fujikawa [18], who determined it to be 173 kJ/mol also by the isotopic tracer method in Al–Sc alloys. The next issue is about precipitation strengthening. According to precipitation strengthening theory, when the size of the precipitate is small, the shearing mechanism is dominating, which involves modulus mismatch strengthening, coherency strengthening and order strengthening [19]. When the precipitates are large enough so that the dislocation cannot shear them, the Orowan bypass process is activated [20]. Using these theories, the contributions to the strength of Al₃Sc precipitates in Al–0.18Sc have been evaluated [21], which predicted a transition from precipitate shearing to Orowan dislocation looping mechanisms at a radius of 2 nm, and increment in yield strength caused by Al₃Sc precipitates was about 200 MPa. All these predictions were in good agreement with experimental data.

In this study, an Al–0.045Er alloy was isothermally aged at 300–400 °C for different times. The Vickers micro-hardness was measured to reflect the precipitation strengthening of Al₃Er precipitates. It was determined by TEM observations the critical radius of Al₃Er precipitates changing from being coherent to semi-coherent with the matrix. In the coherent particle range, the Al₃Er precipitate radii with change of time at different aging temperatures were obtained, from which the diffusion activation energy Q and pre-exponential diffusivity constant D₀ of Er in Al were derived using LSW theory. Finally, the increment of strength caused by Al₃Er precipitates was evaluated by precipitation strengthening theory, and the predictions were compared with the experimental data.