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Al-Mg-Mn-Er-Zr alloy, produced through a combination of cold rolling and heat treatment process conditions, has been studied on its microstructure evolution, strengthening mechanism, and corrosion properties. The results demonstrated that, the dispersion of coherent Al3 (Er, Zr) nanoscale particles effectively pinned dislocations and increased the strength. The dislocation strengthening was considered as the primary strengthening method for Al-Mg-Mn-Er-Zr alloy. The β phase was preferentially precipitated at the grain boundary and the tip of the Al6Mn phase during annealing. The corrosion resistance of stabilized alloys was degraded after cold rolling, which can be attributed to the increase in the proportion of sub-grain stripe and high-angle grain boundaries. The alloy prepared by the process (50% deformation + 270 ˚C/4 h annealing + 10% deformation) exhibited a UTS of 434 MPa and a YS of 323 MPa; the degree of sensitization was quantified by the ASTM G67 test and the susceptibility to intergranular corrosion was in unsusceptible categories; this alloy thus achieves excellent mechanical properties and presents sufficient corrosion resistance.

With the rapid development of the machinery industry, bearing materials with better tribological properties are demanded. Al alloys containing Erbium (Er) and Zirconium (Zr) have better comprehensive performance, whose tribological properties have hardly been reported before. In this work, dry sliding wear tests were performed on the Al–Sn and Al–Sn–Er–Zr alloys in two different annealing states to study the effect of trace Er, Zr on the tribological properties of the Al–Sn alloys. The Al–Sn–Er–Zr alloys showed better wear resistance than the Al–Sn alloys, which was mainly due to the higher hardness and strength of the Al–Sn–Er–Zr alloys. The wear losses of the alloys annealed at 375 °C for 100 h at the load of 70 N were significantly lower than those of the alloys annealed at 300 °C for 4 h. The wear coefficient K of the alloys annealed at 375 °C for 100 h decreased with the increasing load. The Al–Sn–Er–Zr alloys showed smoother worn surfaces than the Al–Sn alloys. With the load increasing, the wear mechanism changed from abrasive wear, mild adhesive wear and slight delamination wear to severe adhesive wear and delamination wear. The wear scars of different samples under the same load were similar, indicating that the addition of trace Er, Zr and annealing process did not change the wear mechanism. All samples experienced cracks and voids in the tribolayers. And oxygen accumulated in these defects. Most wear debris were lamellar, which was related to the adhesive wear and consist with the delamination theory.

The thermodynamics of five Al3Er compounds were investigated through first-principles density-functional theory (DFT) and experimental analysis. The Al3Er compounds with Al3Ho.hR20 (prototype Al3Ho, Pearson symbol hR20), Cu3Au.cP4, AlNd3.hP8, Ni3Ti.hP16 and Al3Gd.hR12 structures exhibited formation energies of −0.412(−0.417), −0.411(−0.416), −0.400(−0.413), −0.399(−0.345) and −0.342(−0.345) meV/atom when using DFT with “standard” potential (“frozen core” potential) of Er. The results indicated that the Al3Ho.hR20 structure was the thermodynamic stable phase and the other structures were metastable. The formation energy of Cu3Au.cP4 structure was only 1 meV/atom less than that of Al3Ho.hR20. Experimentally, the Al-30 wt.% Er alloys were cooled from 900 °C to 500 °C at the rate of 5 ± 2 °C/h and 60 ± 2 °C/h, respectively. The corresponding XRD analysis showed that the Al3Ho.hR20 was formed at the cooling rate of 5 ± 2 °C/h and the Cu3Au.cP4 was formed at the cooling rate of 60 ± 2 °C/h, which indicated that the Al3Ho.hR20 was in a thermodynamic stable phase and the Cu3Au.cP4 was in a metastable phase with high stability. The structural analysis indicated that the tiny energy difference between Al3Ho.hR20 and Cu3Au.cP4 might be attributed to a similar structure with varied stacking sequences.

The effect of various retrogression and re-aging (RRA) treatments on mechanical properties and corrosion behavior of a new type Al–Zn–Mg–Cu–Er–Zr alloy was investigated by microhardness testing, tensile testing, intergranular corrosion (IGC) and exfoliation corrosion (EXCO) testing. The results show that the RRA treatment can effectively improve the IGC and EXCO resistance with less strength sacrificing because the grain interior precipitates and the grain boundary precipitates are similar to that of T6 temper and T73 temper. Meanwhile, as the microhardness of the retrogressed alloy reaches the peak value during the retrogression at 170 °C, the corresponding RRA-treated alloy possesses the better strength, but the corrosion resistance is poor in comparison with that of the retrogression at 190 °C. The optimal combination of strength, IGC and EXCO resistance is obtained after retrogression at 180 °C for 60 min. Moreover, for different heat treatment tempers, the corresponding microstructural evolution of the precipitates including Al3(Er, Zr) particles was also discussed in detail.

A comprehensive study on the microstructural evolution of a new type Al–Zn–Mg–Cu–Er–Zr alloy during homogenization was conducted by optical microscope, scanning electron microscope, transmission electron microscopy and X-ray diffraction analysis. The results show that serious segregation exists in as-cast alloy, and the primary phases are T (AlZnMgCu), S (Al 2 CuMg) and Al 8 Cu 4 Er, which preferentially locate in the grain boundary regions. The soluble T (AlZnMgCu) and S (Al 2 CuMg) phases dissolve into the matrix gradually during single-stage homogenized at 465 °C with prolonging holding time, but the residual Al 8 Cu 4 Er phase cannot dissolve completely. Compared with the single-stage homogenization, both a finer particle size and a higher volume fraction of L1 2 -structured Al 3 (Er, Zr) dispersoids can be obtained in the two-stage homogenization process. A suitable homogenization scheme for the present alloy is (400 °C, 10 h)+(465 °C, 24 h), which is consistent with the results of homogenization kinetic analysis.

An abnormal reverse transition of intergranular corrosion from sensitization to stabilization was observed in the Al-6.4 Mg-0.43Mn-0.37Er-0.17Zr alloy at low annealing temperatures between 220 and 260 °C for the first time. The reverse stabilized alloy annealed at 240 °C for 5 h had excellent long-term corrosion resistance in the deformed state. The corresponding transmission electron microscopy analysis showed that Al3Mg2 precipitates distributed separately along grain boundary, at triple junctions and the Al6Mn phase boundary. The combination of the low temperature reverse stabilization and retarded recrystallization by addition of Er and Zr caused the high Mg aluminum alloy to exhibit high corrosion resistance and strain hardening.

The orientation, hardness and Young's modulus of Al3Yb single crystal were analyzed using the electron back scatter diffraction (EBSD) and nanoindentation techniques, combined with the first principles calculations. The Al3Yb single crystal of about 1 mm size could be obtained by heating hypereutectic Al-38.5 (wt%)Yb alloy to liquid state and cooling to room temperature at 60 °C/h. The results showed that the hardnesses of the Al3Yb single crystals oriented in (318), (023), (213), (112) and (122) directions varied between 5.4 ± 0.1 and 5.7 ± 0.1 GPa, and the reduced modulus were between 131.9 ± 1.6 and 140.5 ± 3.9 GPa. First principles calculations by assuming standard Yb pseudopotential with 4f electrons showed that the elastic constants of C11, C12 and C44 were 149.4, 32.94 and 66.59 GPa, respectively, and the Poisson's ratio was 0.161. Based on these calculated elastic constants and Poisson's ratio and the reduced modulus values, the Young's modulus of the Al3Yb single crystal was determined to be between 148.4 ± 2.3 and 154.9 ± 2.3 GPa for different orientations and the anisotropy coefficient (A) was 1.17 ± 0.01.

Isothermal compression tests of a new type Al-Zn-Mg-Er-Zr alloy are carried out on a Gleeble-3800 thermal simulator at temperatures varying from 573 to 733 K and strain rates ranging from 0.001 to 10 s −1 . Processing maps have been constructed to study on hot workability characteristics of the alloy. The results show that the stable and unstable regions in the processing maps of Al-Kn-Mg-Er-Zr alloy should be identified based on the comprehensive consideration of the strain rate sensitivity m , power dissipation efficiency η and instability criterion ξ ( e ˙ ) . The processing maps exhibit two regions with high efficiency of power dissipation: one is at temperature of 620–680 K and strain rate of 0.001–0.003 s −1 ; another is at temperature of 680–733 K and strain rate of 0.001–0.1 s −1 . The highest η value occurs at about 653 K/0.001 s −1 and the peak η values are all above 37%. According to the microstructure observation, the flow instability is manifested as adiabatic shear bands and flow localization, while the microstructures characteristics in the “safe” domains mainly exhibit dynamic recovery. The presence of coherent L1 2 -structured Al 3 (Er,Zr) particles effectively pin the dislocation movement and the subgrain boundary slide so as to restrain the occurrence of dynamic recrystallization behavior, and the dominant softening mechanism is dynamic recovery during isothermal compression.

The healing kinetics of nanometer-scale voids in Al-Mg-Er and Al-Mg-Zn-Er alloy systems were investigated with a combination of in situ transmission electron microscopy and electron tomography at different temperatures. Mg was observed completely healing the voids, which were then rejuvenated to the alloy composition with further aging, in the Al-Mg-Er alloy. On the contrary, Mg51Zn20 intermetallic compound was formed in voids in the Al-Mg-Zn-Er alloy, which leads to complete filling of the voids but not rejuvenation for the material. For voids with different geometrical aspects, different evolution processes were observed, which are related to the competition between bulk and surface diffusion of the alloys. For voids with a large size difference in their two ends, a viscous flow of surface atoms can be directly observed with in situ electron microscopy, when the size of one end becomes less than tens of nanometers.

The aging strengthening behavior and coarsening process of Al 3 Er 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 Al 3 Er precipitates at peak hardness was measured to be 2.4 ± 0.4 nm. The coherency of Al 3 Er 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 Al 3 Er precipitates and matrix when particles were very small. From the analysis of the coarsening of coherent Al 3 Er 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 m 2 s −1 , respectively. Finally, the strength increment caused by Al 3 Er 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 Al 3 Er precipitates was deduced to be 0.60 ± 0.03 J/m 2 .

The effect of Yb and Si on precipitation hardening and recrystallization of Al–Zr alloys have been investigated. The results show that Yb is more effective than Si to accelerate the precipitation kinetics and stimulate the precipitation of Al–Zr alloy. The peak hardness values for the Al–Yb–Si–0.08Zr are 510 MPa and 515 MPa after isochronal aging to 425 °C and isothermal at 350 °C for 200 h, respectively, which are significantly larger than that of the Al–Yb–Zr and Al–Si–Zr alloys. The precipitates in alloys with addition of Zr are more thermally stable than that of the Al–Yb alloy. The recrystallization temperatures of Al–Zr, Al–Si–Zr, Al–Yb–Zr and Al–Yb–Si–Zr alloys are 250, 400, 425 and 450 °C, respectively, increase with the increase of the corresponding peak aging hardness. The recrystallization temperature of Al–Yb alloy is 325 °C, which is much lower than that of the Al–Yb–Zr alloy due to the coarsening of the Al3Yb at temperature larger than 300 °C.

Using in situ transmission electron microscopy and electron tomography, we have studied the healing process of submicron-scale voids embedded in a cold-rolled Al–Mg–Er alloy. The results show that voids are healed successfully within 50 min at a relatively low temperature of 453 K. Quantitative analysis of the in situ micrographs reveals that the void-healing process involves several stages: an initial fast-healing stage, then a slow-healing stage, and finally a rapid healing near the end. The different healing rates are likely related to varying surface curvatures due to the evolution of void geometry during the healing process. Because the voids are embedded inside Al alloy grains, lattice diffusion is considered to dominate the entire healing process. Mg enrichment was observed at the healed voids immediately after the healing. This indicates that the faster diffusion of Mg atoms in the Al matrix enhances void healing in the Al–Mg–Er alloy. Post-mortem experiments verify that voids in bulk cold-rolled Al–Mg–Er alloy can also be healed by 1 h of annealing at 473 K, consistent with the in situ experiments. The fatigue crack growth resistance and plasticity of the cold-rolled Al alloy are improved significantly after annealing at 473 K.

The solubilities of Er and Yb in Al at high temperature were evaluated from the electrical resistivity measurement, from which the Δ S and Δ H were estimated to be 3.0 ± 0.1k and −0.86 ± 0.01 eV for Al–Er alloys, 3.7 ± 1.0k and −0.93 ± 0.07 eV for Al–Yb alloys, respectively. The solubility curves of Er and Yb therefore could be obtained. According to the solubility curves of Er and Yb, on the one hand, the largest volume fractions of Al 3 Er, Al 3 Yb at room temperature were calculated to be 0.18%, 0.10%, respectively, which were obviously less than that of Al 3 Sc, i.e., 0.92%. On the other hand, the free energy per unit volume for the nucleation of trialuminides was evaluated with the consideration of Gibbs–Thomson effect. According to the obtained chemical driving force, the corresponding critical radius, critical nucleation energy and the steady state nucleation rate could be further calculated. The results indicated that in the binary alloys with the same concentration at the same temperature, comparing with Al 3 Sc, Al 3 Yb and Al 3 Er showed smaller critical radius and larger steady state nucleation rate, which implied that in Al–Yb and Al–Er alloys, higher number density of fine precipitates would be possibly existed.

The temporal evolution of the Al3(ErZr) precipitates is investigated in Al–0.04Er–0.04Zr and Al–0.04Er–0.08Zr alloys during aging at elevated temperature of 450 and 500 °C. The results show that the coarsening behavior of the precipitates in both alloys is similar, due to the core–shell structure with Zr segregated in the shell of the Al3(ErZr) precipitates. With the increase of the amount of Zr addition, the misfit between the precipitate and the Al matrix decreases, consequently the coherency transition size of the precipitates in Al–0.04Er–0.08Zr is larger than that of Al–0.04Er–0.04Zr. After aging at 500 °C for 100 h, some of the precipitates in Al–0.04Er–0.08Zr transform to D023 structure due to the high Zr/Er ratio, while the precipitates in Al–0.04Er–0.04Zr keep the L12 structure.

The precipitation hardening of dilute Al–Er, Al–Zr and Al–Er–Zr alloys was investigated by microhardness measurements and transmission electron microscopy. The Al–0.04Er (at.%) alloy obtains its maximum aging hardness of 400 MPa due to the precipitation of Al3Er. The Al–0.04Er–0.08Zr obtains a maximum hardness of 560 MPa, which is significant greater than that of the Al–Er and Al–Zr alloys. The synergetic effect of Er and Zr on the precipitation hardening of Al–Er–Zr alloys is discussed.