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.

In our study, a complete review of the literature, critical evaluation and thermodynamic assessment of the Cu-Zn, Cu-Se and Zn-Se binary systems were carried out. The modified quasi-chemical model (MQM) was applied to describe the Gibbs energy of the liquid phase. The Gibbs energies of all intermetallic compounds and terminal solid solutions were described using the compound energy formalism (CEF) model. The re-optimization of the Cu-Zn binary system was carried out by considering the ordered bcc_B2 crystal structure of the β’ phase. Moreover, the β and δ phases in the Cu-Zn binary system with the same bcc_A2 crystal structure were modeled as one single phase in the present work. A self-consistent thermodynamic database was constructed for the Cu-Zn, Cu-Se and Zn-Se binary systems, work that formed part of a comprehensive thermodynamic database development project researching zinc-based biodegradable materials.

Low cycle fatigue (LCF) behaviors of TC21 alloy with a bi-lamellar basketweave microstructure were investigated in this paper. The strain fatigue tests were carried out at total strain amplitudes of 1.4% to 2.0%. The cyclic stress response showed the cyclic softening behavior. In addition, the shape of the hysteresis rings exhibited a non-Masing model behavior. The cyclic stress–strain as well as the strain-life equations were obtained. The fatigue life decreased significantly with an increasing total strain from 1.4% to 2.0%. The cyclic softening behavior was interpreted by cyclic back stress and friction stress. Low cycle fatigue cracks were predominantly initiated on the surface of the samples. The relationship between the fatigue sub-critical crack and microstructure was also discussed. The cyclic deformation behavior and crack initiation mechanism were revealed on the basis of the deformation microstructure under different strain amplitudes.

Very high cycle fatigue (VHCF) tests were carried out under variable amplitude loading for TC21 titanium alloy. The first level of high amplitude loading was set as 950 MPa close to yield strength, and the second level of low amplitude loading was determined between 435 MPa and 500 MPa where fatigue cracks initiated at the specimen subsurface under constant amplitude. The results indicate that the high/low stress block significantly reduced the cumulative fatigue life of low stress amplitude, and the fatigue crack initiation site changed from the specimen subsurface under constant loading to the specimen surface under stress block. Based on continuum damage mechanics, the fatigue damage model of two-step stress block was established to estimate the fatigue damage process. The prediction of cumulative fatigue life generally agreed with the experimental data. The cumulative fatigue damage of the stress block was related to the stress amplitude and the cycle ratio, which determined the stress fatigue damage and its interaction damage. The surface crack initiation in the stress block accelerated fatigue damage of low stress amplitude, reducing the cumulative life.

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.

Low cycle fatigue (LCF) crack initiation, propagation and damage behaviors of TC21 alloy with basketweave microstructure were investigated. The process of LCF damage was observed by a long-focus optical microscopic imaging system, and fatigue crack propagation was analyzed through in-situ SEM fatigue. The results indicated that LCF crack damage displayed different sensitivity to cyclic stress. LCF microcracks initiated from slip bands and propagated through the microcrack coalescences at high stress, while LCF cracks tended to initiate at the α_L/β interface and connect with these interface microcracks. Furthermore, the LCF damage model was established on the basis of Lemaitre damage theory. When the maximum stress exceeded yield stress, LCF damage increased sharply and fatigue life decreased significantly, which agreed with experiment data.