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The high-temperature oxidation resistance of AISI 321 stainless steel for solar thermal power generation heat exchanger highly determines its service life. Therefore, in this work, aluminizing treatment and aluminizing with subsequent laser shock peening (LSP) were employed to improve the high-temperature oxidation resistance of AISI 321 stainless steel at 620°C. The results showed that these two treatments decreased the oxidation rate as compared to the base AISI 321 steel. Concretely, the optimal oxidation resistance was observed in the aluminized steel before an oxidation testing time of 144 h due to the increased the entropy of the LSP-treated specimen. After 144 h, however, the LSP-treated sample showed the best oxidation resistance because of the formation of protective α-Al2O3. For the LSP-treated samples, the large amount of sub-grain boundaries formed on aluminized layer could act as the fast short-circuit path for the outward diffusion of Al element, facilitating the rapid nucleation of α- Al2O3. Meanwhile, the aluminized layer is able to isolate the contact between oxidation environment and matrix, thereby decreasing the oxidation rate. Further, the oxidation parabolic constant D(t) of LSP-treated steel was calculated to be minimum (6.45787×10–14), which is respectively 69.18% and 36.36% of aluminized steel and 321 steel during the whole oxidation process. Consequently, the combination of aluminizing and LSP can better improve the high-temperature oxidation resistance of 321 stainless steel.

The precipitation evolutions of Mg-Al-Zn alloys play essential roles in their mechanical properties, corrosion performance, formability, plastic deformation mechanisms and texture development. In the present work, the precipitation evolutions of AZ80 magnesium alloy during both non-isothermal and isothermal processes were unraveled by utilizing in situ electrical resistivity monitoring, hardness testing, differential scanning calorimetry and microstructural characterization. The results showed that discontinuous precipitation (DP) and continuous precipitation (CP) occurred competitively during non-isothermal and isothermal processes. The precipitation of dominant β-Mg17Al12 phase during non-isothermal processes was highly dependent on the thermal history. During isothermal processes, the precipitation behavior of AZ80 magnesium alloy could be considered as the functions of holding temperature and time. At lower temperatures, massive DP and CP were gradually formed to equally strengthen the alloy. At higher temperatures, the Ostwald coarsening was characterized in the later stages and indicated to slightly soften the alloy. Isothermal time-temperature-precipitation curves and quantitative precipitate evolution were estimated to unravel precipitation characteristics and their strengthening functions.

An Al–Cu–Mg–Zr alloy, which obtained different homogenization cooling rates by changing the heat-treated sample size, was compressed to various strains at the deformation temperature of 300 oC and strain rate of 0.01 s−1. The results showed that the homogenization cooling rate had strong effects on the hot deformation behavior of the alloy. The flow stress and relative dynamic softening rate of the alloy were significantly higher under a high cooling rate (HCR) than those under a low cooling rate (LCR). Furthermore, based on X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and thermodynamic equilibrium phase calculation, the substructure evolution in the grain interior, morphology, and spatial distribution of the precipitates were studied to determine the differences in the flow softening mechanism. The main softening mechanism could be summarized as dynamic recovery and precipitation coarsening for the LCR alloy and dynamic precipitation for the HCR alloy.

Hot workability, which indicates the ability of metallic materials to be deformed at elevated temperatures, is generally governed by the microstructure-dependent strength and ductility. In this work, the influence of three types of pre-precipitation microstructure, tailored through different cooling conditions (i.e., air cooling, water quenching and furnace cooling) after heat treatment, on hot workability in an Al-Cu-Mg-Zr alloy was studied by means of constitutive analysis, processing maps and microstructure characterizations using scanning electron microscopy, electron backscatter diffraction and transmission electron microscopy. The results showed that pre-precipitation microstructure had a considerable influence on flow stress, hot deformation activation energy, dynamic softening and hot workability. The highest flow stress accompanied by a more notable dynamic softening was shown in the water-quenched alloy, whereas the flow stress in the furnace-cooled alloy was the lowest. The flow stresses of air-cooled, water-quenched and furnace-cooled alloys could be described well by the hyperbolic sine equation with hot deformation activation energies of 180.5, 298.9 and 161.5 kJ/mol, respectively. Dynamic softening mechanisms were found to be associated with different contributions of dynamic recovery, dynamic precipitation and/or dynamic recrystallization in various tempers of the present alloy. Further, the air-cooled alloy showed minimal instability regimes and an optimal pre-precipitation microstructure for hot forming. The deteriorated hot workability in the water-quenched alloy was attributed to dynamic precipitation effects. However, coarse constituent phases in the furnace-cooled alloy were easily broken during deformation under a higher strain rate, which led to deformation instability.

Modern aluminum industries need an in-depth understanding and a more accurate prediction of the flow stress and microstructure evolution during multi-stage thermomechanical processing. The underlying mechanisms of Al alloys are distinct from that of other metallic materials (e.g., steels and copper) owing to the very high stacking fault energy. In the hot forming process of ultra-high strength Al-Zn-Mg-Cu alloys, the high alloying element additions have a more complex effect on multiple static softening mechanisms during post-deformation, i.e. the coupled recovery, recrystallization, precipitation and their interactions, which have not been well revealed and included in the existing plasticity models. In the present work, an integrated physically based model based on the observed microstructural characteristics and static softening behavior was developed to unravel the multiple static softening mechanisms following multi-stage hot deformation of Al-Zn-Mg-Cu alloys. By incorporating the multicomponent effects, i.e. process variables and chemical compositions, into static recovery, static recrystallization and precipitate coarsening models, the evolutions of stress, microstructure and static softening fraction could be accounted reasonably during post-deformation holding. A special attention was paid to model the functions of various alloy solute contents (i.e. Zn, Mg, Cu and Zr) on the precipitation thermodynamic, recovery and recrystallization kinetics in Al-Zn-Mg-Cu alloys. After validating by experimental data, the integrated physically based model could predict the effects of alloying elements on microstructural evolution, recrystallization and static softening kinetics of Al-Zn-Mg-Cu alloys. It was found that the different precipitation behaviors due to the addition of various alloying elements have a considerable influence on recovery, recrystallization and coupled static softening process. The solid solution atoms remaining in the matrix basically contributed to grain boundaries mobility and then slow recrystallization process, which depended on the interaction parameter, binding energy and diffusion rates of various alloying elements. This work offers an in-depth understanding of static softening mechanisms and provides a potential way for future development of advanced models and design strategies for multi-stage thermomechanical processes in Al-Zn-Mg-Cu alloys.

The complex precipitates have been found to work on the hot workability of heat treatable Al alloys considerably. In this work, to include the functions of interrupted holding following multistage hot working, double-stage hot compression tests were performed on an Al–Cu–Mg–Zr alloy with different pre-precipitation microstructures which were tailored through air cooling, water quenching and furnace cooling after solution heat treatment. Microstructural characterizations and physically-based modeling were employed to investigate the static softening behaviors during interval holding. The results indicated that static softening fraction increased with rising deformation temperature, strain rate and holding time. Under the same deformation conditions, the furnace-cooled alloy presented the highest static softening, while the lowest in water-quenched alloy. Particularly, double plateaus were presented in the static softening curve of water-quenched alloy when deformed at 300 °C and 0.1 s−1, which was interpreted by static recovery, static precipitation and its coarsening as well as the complete depletion of stored strain energy during post-deformation holding. When deformed at 450 °C and 0.1 s−1, higher static softening fraction and longer plateaus were observed in all alloy specimens due to the remarkable static recovery within a very short holding time after first-pass deformation. In addition, the established recovery model could generally rationalize the experimental results, and the direct static softening contribution of precipitates coarsening was indicated to be slight according to the results of integrated model.