High-performance Sc-containing aluminum alloys are limited in their industrial application due to the high cost of Sc elements. Er, Zr, and Y elements are candidates for replacing Sc elements. Combined with the first-principles thermodynamic calculation and the classical nucleation theory, the nucleation of L1₂-Al₃M (M = Sc, Er, Y, Zr) nanophases in dilutealuminum alloys were investigated to reveal their structural stability. The calculated results showed that the critical radius and nucleation energy of the L1₂-Al₃M phases were as follows: Al₃Er > Al₃Y > Al₃Sc > Al₃Zr. The Al₃Zr phase was the easiest to nucleate in thermodynamics, while the nucleation of the Al₃Y and Al₃Er phases were relatively difficult in thermodynamics. Various structures of Al₃(Y, Zr) phases with the radius r < 1 nm can coexist in Al-Y-Zr alloys. At a precipitate’s radius of 1–10 nanometers, the core–shelled Al₃Zr(Y) phase illustrated the highest nucleation energy, while the separated structure Al₃Zr/Al₃Y obtained the lowest one, and had thermodynamic advantages in the nucleation process. Moreover, the core–shelled Al₃Zr(Y) phase obtained a higher nucleation energy than Al₃Zr(Sc) and Al₃Zr(Er). Core–doubleshelled Al₃Zr/Er(Y) obtained a lower nucleation energy than that of Al₃Zr(Y) due to the negative ΔG_chem of Al₃Er and the negative Al₃Er/Al₃Y interfacial energy, and was preferentially precipitated in thermodynamics stability.

The interface structure and segregation behavior of L1₂-Al₃M (M = Sc, Zr, Er, Y) phases were investigated based on first-principles calculations. The results showed that the order of the thermodynamically stable interface was Al₃Zr/Al > Al₃Sc/Al > Al₃Er/ Al > Al₃Y/Al. The interfaces of Al₃Sc/Al₃Zr, Al₃Er/Al₃Zr, and Al₃Y/Al₃Er obtained negative interfacial energies and low coherent strain energies and were favorable to form a clear interface. Zr atom tended to segregate to the first atomic layer on the Al side of the Al/Al₃Sc, Al/Al₃Er, and Al/Al₃Y interfaces. The driving effect of the Zr atom segregation to the Al₃Y shows was stronger than that to Sc and Er atoms, whereas the high coherent strain energy hindered the formation of Al₃Y/Al₃Zr interface. Er atom tended to segregate at the Al/Al₃Y interface and accelerated the formation of core-shelled Al₃Y/Al₃Er. Furthermore, the formation of the double core-shelled Al₃Y/Al₃Er/Al₃Zr was discussed.

Titanium alloys additively manufactured by electron beam melting (EBM) inevitably obtained some pore defects, which significantly reduced the very high cycle fatigue performance. An ultrasonic fatigue test was carried out on an EBM TC21 titanium alloy with hot isostatic pressing (HIP) and non-HIP treatment, and the effect of pore defects on the very high cycle fatigue (VHCF) behavior were investigated for the EBM TC21 titanium alloy. The results showed that the S-N curve of non-HIP specimens clearly had a tendency to decrease in very high cycle regimes, and HIP treatment significantly improved fatigue properties. Fatigue limits increased from 250 MPa for non-HIP specimens to 430 MPa for HIP ones. Very high cycle fatigue crack mainly initiated from the internal pore for EBM specimens, and a fine granular area (FGA) was observed at the crack initiation site in a very high cycle regime for both non-HIP and HIP specimens. ΔK_FGA had a constant trend in the range from 2.7 $\mathrm{MPa}\sqrt{\mathrm{m}}$ to 3.5 $\mathrm{MPa}\sqrt{\mathrm{m}}$, corresponding to the threshold stress intensity factor range for stable crack propagation. The effect of pore defects on the very high cycle fatigue limit was investigated based on the Murakami model. Furthermore, a fatigue indicator parameter (FIP) model based on pore defects was established to predict fatigue life for non-HIP and HIP specimens, which agreed with the experimental data.