•Anisotropy of fatigue crack propagation of Al-Cu-Li alloy thick plate is investigated.•Effect of T1 precipitates on fatigue crack propagation in L-S and S-T orientations with pan-caked grain structure•Discussion about the relationship of Brass grains, T1 precipitates and fatigue crack propagation in Al-Cu-Li alloyAnisotropy in fatigue crack propagation (FCP) behavior of Al-Cu-Li-T87 alloy thick plate is investigated by means of optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron back scattering diffraction (EBSD) in the present work. Results show that FCP rates of the three orientations show distinct difference. Grain size and boundary, coarse inclusion particles, precipitates and grain orientations are capable of affecting the fatigue performance, but coarse inclusion particles and T1 precipitates are found to be the main factors in determining the fatigue performance. Grain boundaries of pan-caked grain structure fail to impede the fatigue crack growth effectively because of the coarse inclusion particles and precipitates along grain boundaries. Fatigue cracks easily propagate along the grain boundaries, therefore result in a low FCP resistance. In addition, Brass grains can induce crack deflection due to the relative large twist angle of grain boundaries between Brass grain and neighboring grain when considerable T1 precipitates are observed in Al-Cu-Li alloy.

•Quantitative analyze the dissolution of various size clusters near a fatigue crack tip.•The number density of small clusters increased and that of large clusters reduced during cyclic deformation.•The driving force for dissolution of clusters was increased with the clusters size.•The dissolution of large clusters in 170 °C/1 h sample was more violent than 170 °C/8 h sample.Quantitative and qualitative analyses on the dissolution behavior of various size Cu-Mg co-clusters near a fatigue crack tip of underaged Al-Cu-Mg alloy during cyclic loading were performed by means of transmission electron microscopy (TEM) and atom probe tomography (APT). Results showed that the driving force for dissolution of Cu-Mg co-clusters was increased with the increase of the co-clusters size. Therefore, Cu–Mg co-clusters in smaller sizes were easier to dissolve. The number density of small Cu–Mg co-clusters (20–50 atoms) increased and that of Cu–Mg co-clusters (50–200 atoms) reduced in the fatigue crack tip region during cyclic deformation. This was because that the large cluster could be repeated cut by moving dislocations to two or more parts of smaller clusters. Besides, the small clusters containing 20–100 atoms also might re-form after fatigue crack propagation (FCP) test in the fatigue crack tip region. The average equivalent radius of large Cu–Mg co-clusters (100–200 atoms) decreased from 1.38 nm to 1.31 nm during fatigue deformation in 170 °C/1 h sample. But there was no obvious difference of that in 170 °C/8 h sample. This result indicated that the dissolution of the large co-clusters in 170 °C/1 h sample was more violent than 170 °C/8 h sample.

•An innovative RRA treatment was designed to enhance fatigue crack resistance.•An almost complete transgranular crack propagation was found in Paris regime.•The coarse precipitates in the grain interior determined the transgranular crack path.A new retrogression and re-aging (RRA) treatment was designed to enhance the fatigue crack propagation (FCP) resistance of a superhigh strength Al–Zn–Mg–Cu alloy in this work. As compared to traditional RRA treatments, a lower retrogression temperature and a longer retrogression and re-aging time were employed. This modified RRA processing obtained a coarse precipitates in matrix, and a slightly increased precipitate free zone (PFZ) width at grain boundary. This led to an almost complete transgranular crack propagation, rather than a partial intergranular crack propagation as traditional RRA tempered samples. This kind of fatigue crack propagation manner gave rise to an ultralow FCP rate, comparable to fatigue resistant 2000 series aluminum alloy, in an Al–Zn–Mg–Cu alloy while keeping superhigh tension strength.

Dislocation mechanism operating in dynamic recrystallization (DRX) during hot compression of Mg–5.51Zn–0.49Zr alloy was investigated by X-ray diffraction, optical microscopy and transmission electron microscopy. The results showed that the continuous DRX occurred at a low strain rate of 1×10−3 s−1, which was associated with the operation of the single gliding dislocation climbing. At the intermediate strain rate of 1×10−2 s−1, the continuous DRX was associated with the climbing of the gliding dislocation array as deformed at an elevated temperature of 350 °C, and in contrast, the discontinuous DRX was observed and associated with the bulging of subgrain boundaries as the deformation temperature was raised to 400 °C. The continuous DRX was associated with the climbing of the leading dislocation ahead of pile-ups, and resultant rearrangement of misorientated flat dislocation pile-ups as the strain rate was increased to 1×100 s−1. It is suggested that the mechanism predominating the dislocation climbing was changed from the vacancy migration to the stress acting on the leading dislocation ahead of the pile-up as the strain rate was gradually increased.

The texture evolution of AA2524 cold-rolled sheet during annealing process and texture effect on fatigue crack propagation (FCP) behavior were investigated. Results showed the sample annealed at 400 °C for 45 min and solid solution-treated at 490 °C for 20 min had the strongest Goss and P textures, and possessed the lowest FCP rates. EBSD examination indicated Goss, P, Q, Cube, R and S grains induced great crack deflection to retard crack growth, when having large twist and tilt angle component boundary or pure twist boundary with neighboring grains. In the case of small angle boundary or relatively large angle tilt boundary with adjacent grains, Cube, Copper and R grains induced little crack deflection. Additionally, SEM results showed, in the presence of large twist angle component boundary or pure twist boundary, more severely plastic deformation presenting numerous slip bands occurred in Cube, R and S grains than that in Goss, P and Q grains during crack propagation. In contrast, no evident slip bands formed in Cube, Copper and R grains when having small angle boundaries or relatively large tilt angle boundary.

The thickening of Ω phase in Al–Cu–Mg alloys containing various bulk Ag contents during stress aging at 200 °C with a tensile stress of 240 MPa was investigated by a combination of transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM) and atom probe tomography (APT). TEM characterization confirmed preferred orientation of Ω phase in all stress-aged samples. Corresponding quantitative TEM calculations revealed the thickening kinetics of Ω phase was significantly accelerated during stress aging as compared to that during stress-free aging at 200 °C. HRTEM analysis on the α/Ω interfacial structure confirmed that the applied tensile stress facilitated the rapid nucleation of the growth ledge on the broad face of Ω phase, thereby resulting in the accelerated plate thickening during stress aging at 200 °C. Meanwhile, quantitative TEM analysis highlighted the stress-induced thickening of Ω phase at 200 °C was affected by the bulk Ag content. This was consistent with the HRTEM observation as the ledge nucleation was found to be suppressed with increasing Ag addition. Our APT analysis on different stress-aged samples further suggested the progressive enrichment of Ag atoms in the segregation layer helped to stabilize the interfacial structure and was responsible for the lowest nucleation rate of the ledge in 1.77Ag alloy as compared to that in 0.46Ag alloy.

The fatigue crack propagation behavior in the overaged Al-Zn-Mg-Cu alloy was characterized by optical microscopy, scanning electron microscopy, transmission electron microscopy and electron backscatter diffraction. The results revealed that a fatigue crack tended to transgranularly propagate in the near-threshold regime, whereas intergranular crack propagation was dominant at the high ΔK regime. The transition of crack propagation from a transgranular to an intergranular path that occurred in the Paris regime was strongly influenced by the misorientation of adjacent grains and precipitate free zones. In addition, a crystallographic model of crack propagation was proposed to interpret the transition. The fatigue short crack propagation on a single slip plane was responsible for the formation of a transgranular propagation path in the near-threshold regime. The fatigue long crack propagation, which was conducted by a duplex slip mechanism in the Paris regime, led to the formation of fatigue striations. The formation of a zigzag crack in the near-threshold regime was ascribed to the high misorientation of adjacent grains.

The microstructure and Vickers hardness behavior of Al–4 wt.% Cu alloy bearing θ′ precipitates, subjected to different pass of multi-axial compression (MAC) and re-aging treatments, has been studied. θ′ precipitates are almost completely dissolved after 12 passes (ɛ = 4.8) of MAC, and a non-equilibrium supersaturated solid solution is obtained. When this solid solution is re-aged at 100 °C, a slight increase of the hardness occurs due to the formation of GP zones. Precipitation of equilibrium θ phase is observed along the grain boundaries during re-aging. That is completely different from that of the coarse grained materials. The discrepancy for this unique re-aging behavior is attributed to extremely increasing of nucleation sites for θ phase at non-equilibrium grain boundaries (GB), as well as a large number of dislocations and vacancies acting as diffusion channel for Cu atoms.Highlights► Different passes of MAC has been carried out on Al–Cu alloy bearing θ′ particles. ► Reprecipitation hardening is not apparent during subsequent re-aging process. ► GP zones are formed at the early stage of re-aging. ► Most equilibrium θ precipitates appear preferentially on the grain boundaries.

Deformation behavior of an Al–Cu–Mg–Mn–Zr alloy during hot compression was characterized in present work by high-temperature testing and transmission electron microscope (TEM) studies. The true stress–true strain curves exhibited a peak stress at a critical stain. The peak stress decreased with increasing deformation temperature and decreasing strain rate, which can be described by Zener–Hollomon (Z) parameter in hyperbolic sine function with the deformation activation energy 277.8 kJ/mol. The processing map revealed the existence of an optimum hot-working regime between 390 and 420 °C, under strain rates ranging from 0.1 to 1 s−1. The main softening mechanism of the alloy was dynamic recovery at high lnZ value; continuous dynamic recrystallization (DRX) occurred as deformed at low lnZ value. The dynamic precipitation of Al3Zr and Al20Cu2Mn3 dispersoids during hot deformation restrained DRX and increased the hot deformation activation energy of the alloy.

Microstructures and mechanical properties of an Al-Cu-Mg-Ag alloy aged for 1 h at temperatures in a range 25 °C to 450 °C were characterized in the present work by means of hardness tests, electrical conductivity measurements, and transmission electron microscopy (TEM). In-situ X-ray diffraction (XRD) was also employed to examine the precipitation behavior of Ω phase in a temperature range of 25 °C to 400 °C The in-situ Xray diffraction peak at 2θ = 26°–28° detected at elevated aging temperatures between 165 °C and 400 °C was attributed to the formation of Ω phase. TEM observations demonstrated the existence of Ω phase in the alloy when aged for 1 h at temperatures in a range 145 °C to 450 °C.

The alloying behavior of rare earth erbium in an Al–Cu–Mg alloy was characterized by employing OM, SEM, EDS, XRD and TEM. The results suggested the erbium mainly segregated to the grain boundaries in a form of Al8Cu4Er phase during solidification, which was crushed up along with the grain boundaries after hot rolling. Moreover, Al3Er phase with the L12 structure was also observed in the Al–Cu–Mg–Er alloy in T351 condition. The dendritic structure of as-cast Al–Cu–Mg alloy was refined remarkably by erbium addition.

A Thermecmastor-Z hot deformation simulator, optical microscopy, XRD and TEM were employed to characterize the flow stress behavior and microstructure of twin roll cast ZK60 magnesium alloy during initial stage of hot compression at elevated temperature of 300 °C and 400 °C and a given strain rate of 10−2 s−1. The results suggest that flow stress drop during initial stage of hot compression at 300°C, generally led by dynamic recrystallization, is attributed to twinning, correspondingly to dynamic recrystallization as deformation temperature is raised to 400 °C.

Low-temperature dynamic recrystallization, which generally occurs below 200 °C, was found to occur at an elevated deformation temperature as high as 350 and 400 °C as the strain rate employed was increased up to 100 s−1 during hot compression of twin-roll-cast Mg–5.51Zn–0.49Zr (wt.%) alloy. New recrystallized grains formed at the junction of misorientated dislocation pile-ups due to the rearrangement of the dislocation pile-ups. Both the climbing force acting on the leading dislocation ahead of the pile-up and the free-energy change during this rearrangement were calculated.

The existing form and grain refining effects of small zirconium addition in pure Mg, Mg-Yb and Mg-Zn binary alloys, and Mg-Zn-Yb ternary alloy (ZK60-Yb) were investigated. The results show that Zr element exists mainly in single and cluster particles of pure α-Zr or Zn-Zr compounds inside grains and at grain boundaries. Only the particles located in the interior of grains can act as the nucleus for α-Mg growth and effectively promote the formation of fine equiaxed grains. The broken and dispersed Zr-rich particles produced during the hot extrusion process can form nebulous banded structure in which these fine particles may act as obstacles to dislocation motion in wrought magnesium alloys.