Non-isothermal retrogression kinetics for grain boundary precipitate of 7A55 aluminum alloy

The retrogression kinetics for grain boundary precipitate （GBP） of 7A55 aluminum alloy was investigated by transmission electron microscopy （TEM） observation. The results reveal that the coarsening behavior of GBP obeys “LSW” theory, namely, the cube of GBP average size has a linear dependence relation to retrogression time, and the coarsening rate accelerates at the elevated retrogression temperature. The GBP coarsening activation energy Qo of （115.2±1.3） kJ/mol is obtained subsequently. Taking the retrogression treatment schedule of 190℃, 45 min derived from AA7055 thin plate as reference, the non-isothermal retrogression model for GBP coarsening behavior is established based on “LSW”theory and “iso-kinetics” solution, which includes an Arrhenius form equation. After that, the average size of GBP r（t） is predicted successfully at any non-isothermal process T（t） when the initial size of GBP r0 is given. Finally, the universal characterization method for the microstructure homogeneity along the thickness direction of TA55 aluminum alloy thick plate is also set up.

In situ thermomechanical processing to avoid grain boundary precipitation and strength-ductility loss of age hardening alloys

To avoid grain boundary(GB) precipitation during aging, a new strategy of in situ thermomechanical processing for age hardening alloys was proposed. Specifically, high-density nanoscale precipitates were introduced into ultrafine grain(UFG) interiors of 7075 Al alloy by equal-channel-angular(ECAP) processing at 250 ℃ for 8 passes, thus avoiding GB precipitation. Tensile test results indicated that the UFG 7075 Al alloy exhibits superior mechanical properties(yield strength of 350 MPa, ultimate tensile strength of 500 MPa, uniform elongation of 18% and tensile ductility of 19%) compared with the UFG 1050 Al counterpart(yield strength of 170 MPa, ultimate tensile strength of 180 MPa, uniform elongation of 2.5% and tensile ductility of 7%). Fracture surface morphology studies revealed numerous homogeneous micro shear bands in necking shrinkage areas of both UFG 7075 Al and 1050 Al alloys, which are controlled by cooperative GB sliding. Moreover, the introduction of nanoscale precipitates in UFG 7075 Al matrix weakened the tendency of shear fracture, resulting in a higher tensile ductility and more homogeneous deformation. Different from the GB precipitation during postmortem aging, in situ thermomechanical treatment dynamically formed GBs after precipitation, thus avoiding precipitation on GBs.

Influence of quench transfer time on microstructure and mechanical properties of 7055 aluminum alloy

The influence of quench transfer time on the microstructure and mechanical properties of 7055 aluminum alloy with and without zirconium was investigated by tensile properties test,optical microscopy,scanning electron microscopy and transmission electron microscopy.For the Zr-free alloy,the strength increases to the highest value at 20 s with transfer time,and then decreases slightly.The elongation decreases slowly with transfer time within 20 s,and more rapidly after 20 s.For the Zr-containing alloy,prolonging transfer time within 20 s results in slight decrease in the strength and elongation,and rapid drop of which is observed after 20 s.For the Zr-free alloy,prolonging transfer time can increase the percentage of intergranular fracture,which is mainly caused by wide grain boundary precipitate free zone.The failure mode of the Zr-containing alloy is modified from the predominant transgranular void growth and intergranular fracture to transgranular shear and intergranular fracture with increase in the transfer time,which is attributed to the wider grain boundary precipitate free zone and coarse equilibrium η phases in the matrix.

Two-stage double peaks ageing and its effect on stress corrosion cracking susceptibility of Al-Zn-Mg alloy

Different artificial two-stage ageing behaviors and their effect on stress corrosion cracking （SCC） susceptibility of AI-Zn-Mg alloy have been investigated. The experimental results show that two hardness peaks present on the second-stage ageing-hardening curve when the first-stage ageing is dealt with comparatively lower temperature than the conventional one. The first peak is caused by dispersive and evenly distributed G.P. zones, while η ′phases and coarsened G.P. zones contribute to the second peak. Tensile strength of experimental alloy raises 9.6% （33.2 MPa） and SCC susceptibility decreases 38.9% by applying the second peak ageing regime instead of conventional T73. AI-Zn-Mg alloy obtains high strength and SCC resistance due to its finely dispersive matrix precipitates （MPts）, coarsened and discontinuous grain boundary precipitates （GBPs）, as well as the narrow precipitate free zone （PFZ） in the second peak ageing condition. 2017 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science ＆ Technology.

Effect of Solution Annealing on Microstructure and Mechanical Properties of a Ni-Cr-W-Fe Alloy

The effect of solution annealing on the microstructure and mechanical properties of a Ni-Cr-W-Fe alloy developed for advanced 700？C ultra-supercritical power plants was investigated. Test samples in this study were subjected to different solution treatments and the same aging treatment（at 760？C for 1 h）.When solution annealing temperature was elevated from 1020？C to 1150？C, the stress-rupture life at750？C/320 MPa was increased from 60 h to 300 h, the stress-rupture elongation was enhanced from12% to 17%, and the elongation of the tensile at 750？C was improved from 11% to 24%. All tensile and stress-rupture samples displayed an intergranular dimple mixed fracture. Intergranular micro-cracks had a great relationship with the morphology of grain boundary carbides. Most carbides retained the morphology of globular shape and continuous thin plate. After tensile and stress-rupture tests, a few carbides were converted into lamellar. The results showed that intergranular micro-cracks were easier to form at continuous thin plate carbides than at globular shape carbides. Lamellar carbides hardly caused the nucleation of micro-cracks. Besides, grain boundaries sliding and elements diffusion during stressrupture tests led to the formation of precipitate free zones, which accelerated the extension of microcracks and influenced the stress-rupture life.

Inhibiting segregation enabled outstanding combination of mechanical and corrosion properties in precipitation-strengthened aluminum alloys

Segregation of solutes to grain boundary(GB)is the dominant restriction on enhancement of mechanical properties and corrosion resistance during aging in precipitation-strengthened aluminum alloys.Here,we innovatively introduce the cyclic deformation by the vibration during aging to eliminate the GB segre-gation,resulting in the formation of the narrow precipitate-free zone(PFZ)widths near the GB,as well as the fine and discontinuous grain boundary precipitations(GBPs).Compared with the traditional peak-aging,the 2014 aluminum alloy treated by thermal cooperative vibration aging(TCVA)exhibits a superb combination of impact toughness and corrosion resistance,and retains the strength and ductility.In ad-dition,the atomic simulations show that TCVA generates numerous vacancies near the GB,but does not change the dislocation density with the increase of cycle time.These results indicate that the vacancy significantly promotes the nucleation and growth of precipitates in the vicinity of GB,resulting in the narrow PFZs and fine discontinuous GBPs.The present work provides the fundamental knowledge and method to inhibit the equilibrium segregation of solutes to GB during aging,and further realizes the precipitation-strengthened aluminum alloys with excellent mechanical and corrosion properties.