Zhiyi Liu

Retrogression and re-aging treatment of Al-9.99%Zn-1.72%Cu-2.5%Mg-0.13%Zr aluminum alloy

Abstract The effect of retrogression and re-aging(RRA) heat treatment on the microstructure and mechanical properties of a low frequency electromagnetic casting alloy as Al-9.99%Zn-1.72%Cu-2.5%Mg-0.13%Zr was investigated by tensile properties test, Vickers hardness, electrical conductivity test, DSC analysis, SEM and TEM observation. The results show that RRA heat treatment can improve the stress corrosion cracking(SCC) properties with retention of the high strength of T6 level. After preaging at 100 °C for 24 h, retrogression at 200 °C for 7 min, and then re-aging at 100 °C for 24 h, the alloy obtains tensile strength up to 795 MPa, yield strength up to 767 MPa, maintains 9.1% elongation, and electric conductivity of 35.6%IACS. TEM observation shows that the re-dissolution of GP zone and η′ phase in the early stage of regression leads to the decrease of hardness, then the increase in the volume fraction of η′ and η phases leads to the increase again in the peak value, and finally the general coarsening of all particles results in a softening of the alloy. Meanwhile it is found that the conventional T6 heat treatment as the preaging and re-aging regime is not the optimum regime to the RRA treatment of the high-zinc content super-high strength aluminum alloy.

Enhanced Fatigue Crack Propagation Resistance in an Al-Zn-Mg-Cu Alloy by Retrogression and Reaging Treatment

The microstructures and fatigue crack propagation (FCP) behavior of an Al-Zn-Mg-Cu alloy in T761 and retrogression and reaging (RRA) conditions were characterized by employing differential scanning calorimetry, optical microscopy, scanning electron microscopy, transmission electron microscopy, and electron backscatter diffraction. The results suggested that coarse η′ precipitates were present in T761-treated sample, while fine dispersed η′ precipitates and GP zones were uniformly distributed in RRA-treated ones. Besides, the width of precipitate-free zones (PFZs) in T761-treated sample was found to be much greater than that in RRA-treated ones. Compared with T761-treated sample, the enhanced FCP resistance of RRA-treated sample was attributed to the shearable particles in matrix and narrow PFZs.

Microstructure and properties of Al-Cu-Mg-Ag alloy exposed at 200 °C with and without stress

Abstract The effect of stress on the microstructure and properties of an Al-Cu-Mg-Ag alloy under-aged at 165 °C for 2 h during thermal exposure at 200 °C was investigated. The tensile experimental results show that the remained tensile strength of both specimens at room temperature after being exposed at 200 °C with and without applying stress rises firstly, and then drops with the increasing of exposure time. The peak value of the remained strength reaches 439 MPa for non-stress-exposure for 10 h, and 454 MPa after being exposed with stress loaded for 20 h at 220 MPa. The elongation change is similar to that of strength. After being exposed for 100 h, specimen exposed at 220 MPa still remains a tensile strength of 401 MPa, larger than that exposed without applying stress. TEM shows that the microstructure of under-aged alloy is dominated by Ω phase mainly and a little ϑ ′ phase. The ϑ ′ and Ω phases are believed competitive with increasing exposure time. The width of precipitation free zone(PFZ) increases and the granular second phase precipitates at grain-boundary correspondingly. It is shown that the mechanical properties of alloy decrease slightly and present good thermal stability after thermal exposure at 200 °C and 220 MPa for 100 h.

Redistribution and re-precipitation of solute atom during retrogression and reaging of Al-Zn-Mg-Cu alloys

Abstract The redistribution and re-precipitation of solute atom during retrogression and reaging of three different Al-Zn-Mg-Cu aluminum alloys were investigated. The results of hardness and tensile strength test indicate that after pre-aging at 100 °C or 120 °C and retrogressing at 200 °C for various time and re-aging treatment, the hardness and strength of the alloys are all larger than those under pre-aging condition, some of them even exceed the value under peak aging(T6) condition. TEM observation shows that the PFZ formed during retrogressing in short time becomes narrow and even disappears after re-aging treatment. However, the PFZ formed during retrogressing for a long time does not narrow after re-aging treatment. It is suggested that the redistribution and re-precipitation of solute atom during re-aging treatment result in the narrowing and even disappearance of the PFZ formed during retrogression, which reinforces the grain-boundaries and presents the value of tensile strength exceeding peak-aging strength in the RRA condition, while the precipitates in the matrix of the alloys still keep or even exhibit a more dispersed distribution, and a greater effect of precipitation strengthening is obtained.

Multistage-aging process effect on formation of GP zones and mechanical properties in Al–Zn–Mg–Cu alloy

Abstract The tensile properties and fatigue behavior of an Al–Zn–Mg–Cu alloy were investigated by performing tensile tests and fatigue crack propagation (FCP) tests. The tensile results show that lower aging temperature modified retrogression and re-aging (RRA) process enhances the elongation, but reduces the strength of the alloy, as compared to conventional RRA process which employs peak aging temperature. Both ductility and strength, however, are increased by employing a natural aging prior to re-aging based on the former modified RRA process. Fatigue test results show that both routes reduce FCP rate. Especially, the lower re-aging temperature modified RRA process obtains the lowest FCP rate. Natural aging treatment could enhance the nucleation rate of GP zones. A large amount of GP zones could be cut by dislocations, which is responsible for the highest tensile strength and elongation, as well as lower FCP rate.

Dislocation mechanism for dynamic recrystallization in twin-roll casting Mg-5.51Zn-0.49Zr magnesium alloy during hot compression at different strain rates

Abstract 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 1×10 0 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.

Dependence of Competitive Grain Growth on Secondary Dendrite Orientation During Directional Solidification of a Ni-based Superalloy

The competitive grain growth in bicrystal samples during unidirectional solidification of a Ni-based superalloy was found to depend on secondary dendrites perpendicular to the grain boundary of bicrystal samples, rather than primary dendrites parallel to the thermal gradient as generally recognized. The primary dendrite orientation, however, had significance for the dendrite blocking in overgrowth processes and the resultant overgrowth rate during competitive grain growth.

Effects of Severe Cold Rolling on Exfoliation Corrosion Behavior of Al-Zn-Mg-Cu-Cr Alloy

The exfoliation corrosion (EXCO) behavior of Al-Zn-Mg-Cu-Cr alloy after severe cold rolling was investigated by optical microscope, transmission electron microscope, and electrochemical technique. The results show that the EXCO resistance decreased with increasing cold rolling reduction because of the grain boundaries which were decorated with the continuously distributed particles. After the solution treatment, the samples with different reductions retained the fibrous grains, and the elongated grains accelerated the growth of the corrosion cracks according to crack propagation analysis. Furthermore, the increase of deformation enhanced the degree of recrystallization, while the number of corrosion cells increased greatly when in the electrolyte, which tended to reduce the resistance to EXCO.

Effect of Si target sputtering power on diffusion barrier properties of Ta–Si–N thin films

Abstract Ta–Si–N thin films and Cu/Ta–Si–N thin films were deposited on p type Si(111) substrates by magnetron reactive sputtering. Then the films were characterised by four point probe sheet resistance measurement, AFM, SEM and XRD respectively. According to the XRD results, the authors found that the crystallisation of Ta nitrides in Ta–Si–N/Si thin films is suppressed effectively when fabricated by a high Si target sputtering power. As the Si target power varies, the failure temperature of Cu/Ta–Si–N/Si is changed. The sample fabricated by the Si target power of 200 W fails after 800°C rapid thermal annealing and it has the highest failure temperature. The investigation of failure mechanism shows that Cu atoms diffuse through grain boundaries or amorphous structure of the Ta–Si–N barrier, and react with Si to form Cu–Si phase. And it causes the failure of the barrier.

Transition of crack propagation from a transgranular to an intergranular path in an overaged Al-Zn-Mg-Cu alloy during cyclic loading

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.