Al–Mg–Zn alloys reinforced by T–Mg₃₂(Al,Zn)₄₉ phase had higher structure stability and strength than Al–Zn–Mg–(Cu) alloys reinforced by MgZn₂ phase, but the reasons for these two kind of alloys was not well-known. To reveal the discrepancy between T phase and MgZn₂ phase, the lattice parameters, cohesive energy, and electronic structure as well as the elastic properties were investigated based on density functional theory. Four types of T phase unit cell were employed according to symmetry of space group. The calculated lattice constants well-agreed with experimental data. Compared to MgZn₂ phase, T phases obtained lower cohesive energy owing to their partial covalent bond, which may result in a higher structure stability. The elastic modulus E of T phase depended on the occupation of Al atom, and the effect of the occupation of Al atom on the structure and properties of T phase was also discussed.

This paper discusses the effect of basketweave microstructure on the very high cycle fatigue behavior of TC21 titanium alloy. Ultrasonic fatigue tests at 20 kHz are done on a very high cycle fatigue (VHCF) property of the alloys with 60 μm and 40 μm basketweave size, respectively. Results show that the alloys illustrate step-wise S-N characteristics over the 10⁵–10⁹ cycle regimes and that fatigue fracture in both of the alloys occur beyond the conventional fatigue limit of 10⁷ cycles. Subsurface crack initiation occurs at low stress amplitude. A fine granular area (FGA) is observed along the α lamella at the subsurface crack initiation site. The mechanism for the subsurface crack initiation is revealed using layer-by-layer-polishing, due to the micro-voids that are introduced at the granular α phase. The colony of α lamella is due to the local stress concentrated between them under the cyclic load. The stress intensity factor range at the FGA front is regarded as the threshold value controlling the internal crack propagation. Furthermore, the effect of the baseketweave size on the very high cycle fatigue limits of the TC21 titanium alloy is evaluated based on the Murakami model, which is consistent with the experimental results. The fatigue life of TC21 titanium alloy is well predicted using the energy-based crack nucleation life model.

The effect of casting pores on the very high cycle fatigue (VHCF) behavior of a directionally solidified (DS) Ni-base superalloy DZ4 is investigated. Casting and hot isostatic pressing (HIP) specimens were subjected to very high cycle fatigue loading in an ambient atmosphere. The results demonstrated that the continuously descending S-N curves were exhibited for both the casting and HIP specimens. Due to the elimination of the casting pores, the HIP samples had better fatigue properties than the casting samples. The subsurface crack initiated from the casting pore in the casting specimens at low stress amplitudes, whereas fatigue crack initiated from crystallographic facet decohesion for the HIP specimens. When considering the casting pores as initial cracks, there exists a critical stress intensity threshold ranged from 1.1 to 1.3 $\mathrm{MPa}\sqrt{\mathrm{m}}$ , below which fatigue cracks may not initiate from the casting pores. Furthermore, the effect of the casting pores on the fatigue limit is estimated based on a modified El Haddad model, which is in good agreement with the experimental results. Fatigue life for both the casting and HIP specimens is well predicted using the Fatigue Indicator Parameter (FIP) model.

The effect of low cycle fatigue (LCF) predamage on the subsequent very high cycle fatigue (VHCF) behavior is investigated in TC21 titanium alloy. LCF predamage is applied under 1.8% strain amplitude up to various fractions of the expected life and subsequent VHCF properties are determined using ultrasonic fatigue tests. Results show that 5% of predamage insignificantly affects the VHCF limit due to the absence of precrack, but decreases the subsequent fatigue crack initiation life estimated by the Pairs law. Precracks introduced by 10% and 20% of predamage significantly reduce the subsequent VHCF limits. The crack initiation site shifts from subsurface-induced fracture for undamaged and 5% predamaged specimens to surface precrack for 10% and 20% predamaged specimens in very high cycle region. Furthermore, the predicted fatigue limits based on the El Haddad modified model for the predamaged specimens agree with the experimental results.

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.