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• Creep samples exhibit a higher γʹ dissolution rate than thermal exposure. • Creep samples show Co depletion and Ti enrichment in deformation twins. • A linear model is proposed for Vickers hardness changes. • Dissecting the impacts of temperature and stress on microstructure. The study provided a systematic investigation into the creep behavior of a γʹ-strengthened Co-based single crystal superalloy Co-7Al-8W-1Ta-4Ti (at.%) under compressive creep conditions at 1000 °C and 137 MPa. Simultaneously, the microstructural changes during thermal exposure at 1000 °C were thoroughly examined. The microstructure analysis of the creep samples revealed the presence of γ/γʹ rafts, twins, stacking faults, and dislocation networks. Statistical analyses were conducted to assess the dimensions and volume fractions of γʹ, alongside Vickers hardness tests and nanoindentation experiments. Results show that γʹ dissolution in the creep samples with substantially higher dissolution rates compared to thermal exposure. Localized Co depletion and Ti enrichment in twin is observed in creep sample, potentially initiating localized phase transformations. Concurrently, lattice rotation occurred during creep, resulting in a progressive deviation of crystal orientation away from the [001] direction. Stacking faults transitioned from network-like structures to parallel configurations, and twins played a significant role in creep deformation, particularly in samples subjected to 382 h creep. The study also explored the evolution of Vickers hardness and the elastic modulus, introducing a systematic linear model to describe Vickers hardness changes during both thermal exposure and creep conditions. [Display omitted] [ABSTRACT FROM AUTHOR]

• The effect of composition on the heat resistance of dual-phase Al-Zn-Mg-Cu alloys was investigate. • Alloys A1and A2 with a proper Zn/Mg ratio (1.50−2.14) and relatively lower (Zn + Mg) content (7−8.8 wt.%) showed better heat resistance. • The high proportion of T phase was beneficial to improve the thermal stability. • The coarsening mechanism of precipitates was revealed as IDC (150 °C) and MDC (200–300 °C). High-strength Al-Zn-Mg-Cu alloys are widely utilized, but their strength deteriorates as strengthening precipitates coarsen rapidly at elevated temperatures, limiting their applications above 150 °C. This study systematically investigates the microstructure evolution and its impact on the properties of peak-aged Al-Zn-Mg-Cu alloys with varying Zn/Mg ratios during thermal exposure at a series of temperatures from 150 to 300 °C for 500 h. The results reveal that alloys A1 and A2 with an optimal Zn/Mg ratio (1.50–2.14) and relatively lower (Zn + Mg) content (7.0–8.8 wt.%), exhibit superior heat resistance properties compared to the other three alloys. Despite having lower strength relative to alloys with higher solute content, peak-aged alloys A1 and A2 retain the highest strength after thermal exposure. This performance is attributed to the high proportion (over 80%) of T′/T phases in the precipitates for alloys A1 and A2, which demonstrate better thermal stability in comparison to η′/η phases. Additionally, the lower solute content reduces the driving force for diffusion of Zn and Mg atoms, thus inhibiting the coarsening of precipitates. Moreover, the study elucidates that the coarsening mechanism of precipitates transitions from interfacial diffusion control at 150 °C to matrix diffusion control at 200–300 °C. These insights into the composition-dependent coarsening behavior of precipitates in dual-phase strengthened Al-Zn-Mg-Cu alloys offer valuable guidance for designing heat-resistant aluminum alloys with enhanced performance at elevated temperatures. [Display omitted] [ABSTRACT FROM AUTHOR]

• Excess α 2 phases in the as-cast high-Nb-containing TiAl alloy were decomposed during thermal exposure at 950 °C. • Ti 2 Al phases precipitated at lamellar interfaces during thermal exposure at 950 °C. • Stacking faults generated from the α 2 → γ phase transformation provided nucleation sites for Ti 2 Al phases. The thermal stability of microstructures is crucial for determining the performance of alloys in extreme environments. In the present work, the microstructural evolution and precipitation behavior in a high Nb-containing Ti45Al8Nb alloy during thermal exposure at 950 °C were investigated. It was found that excess α 2 phases in the as-cast microstructure were unstable and tended to decompose during thermal exposure. Hexagonal Ti 2 Al phases precipitated at lamellar interfaces and had a [ 1 1 ¯ 0 ] γ ∥[ 11 2 ¯ 0 ] α2 ∥[ 11 2 ¯ 0 ] Ti2Al , (002) γ ∥(1 1 ¯ 00) α2 ∥(1 1 ¯ 00) Ti2Al crystallographic orientation relationship (OR) with the matrix. Stacking faults (SFs) generated in α 2 phases during the α 2 → γ phase transformation provided favorable nucleation sites for Ti 2 Al phases. [Display omitted] [ABSTRACT FROM AUTHOR]

• The Ta, Nb and/or Re elements were used to increase the microstructural stability of high temperature Ti alloy. • The Ta and Nb exhibited weak β-stabilizing ability, while the Re was a strong β-stabilizer and obviously increased the content of β phase. • A trace Re addition retarded the growth of ordered α 2 precipitates due to the increase of Ta and Nb concentrations in the α phase. Mechanical properties and microstructural stability under the service temperature are important to the high temperature titanium alloy. In order to evaluate the potential in increase the service temperature of Ti alloy, two near-α Ti alloys with high content of Al as α-stabilizer and Ta, Nb and/or Re as β-stabilizers were designed and prepared by ingot metallurgy and thermomechanical processing, and the microstructure and mechanical properties before and after thermal exposure at 650 °C for 100 h were characterized. The results indicated that due to the weak β-stabilizing ability of Ta and Nb elements, only a small amount of β phase was formed in Ti-10Al-4Ta-2Nb alloy. With a trace Re addition, the β phase was obviously increased in Ti-10Al-4Ta-2Nb-0.25Re, indicating that the Re was a strong β-stabilizer. Under the same thermomechanical conditions, the Re addition decreased the volume fraction of primary α (α p) phase and refined the secondary α (α s) phase evidently. The primary α phase presented an obvious core-shell structure in the Ti-10Al-4Ta-2Nb alloy, with higher Al concentration in the shell. While the core-shell structure was not obvious in the Re-containing alloy due to the Re decreases the diffusion of Al, Ta and Nb elements. A large number of ordered α 2 precipitates can be observed in the α p and α s phases of two alloys. The α 2 precipitates continuously grew up during thermal exposure, however, their growth rate in the α s phase of Re-containing alloy were lower than that of Ti-10Al-4Ta-2Nb alloy. Although plenty of ordered α 2 precipitates formed in the Ti-10Al-4Ta-2Nb alloy, the alloy had a certain plasticity at room temperature. The trace Re addition evidently increased the tensile strength but caused the decrease of the plasticity. After thermal exposure, the strength was further increased, while the plasticity was decreased for both of alloys. [ABSTRACT FROM AUTHOR]

[Display omitted] • High strength solid-state welds were demonstrated between Ti and stainless steel through an impact welding method. • By incorporating a Nb interlayer these impact welds are stronger and more thermally stable. • With modest thermal exposure, Ti/Nb/SS welds presented improved toughness. • The effective operation temperature ranges are suggested for Ti-SS and Ti-Nb-SS spot impact welds. Stainless steel (SS) and titanium alloys can be welded in the solid-state by high speed oblique impact. Here, the effect of a niobium (Nb) interlayer on weld strength and thermal stability is evaluated. Both Ti/SS and Ti/Nb/SS welds were subjected to thermal exposure ranging from 300 °C to 950 °C for 1.5 h. Thermal exposure monotonically decreases the strength and toughness of the Ti/SS pair with a dramatic falloff in strength and change in failure mode from partial pullout failure to full interfacial fracture at 600 °C. With the interlayer, toughness was increased versus baseline up to 700 °C thermal exposure and then intermetallic formation again caused falloff in properties. Guidelines for the production, properties and applications of these classes of welds are provided. [ABSTRACT FROM AUTHOR]

1Cr15Ni36W3Ti was thermally exposed at 580 ℃ and 680 ℃, respectively, up to 3000 h. The γ' phase and intergranular TiC carbides continuously coarsened during exposure. None of η , laves or σ phase was discovered in the exposed samples, indicating good microstructure stability under the present exposure conditions. The ripening process of the γ' phase could be well modelled utilizing the LSW theory. The evolutions of the yield and tensile strengths were monotonous during exposure at 580 ℃. However, a transition point in strengths was detected in the tensile samples exposed at 680 ℃ for 300 h. Accordingly, the critical γ' diameter was measured to be 13−14 nm. The γ'/dislocations interaction mechanism transformed from shearing to looping with the γ' diameter exceeding the critical point. The combination of the weakly coupled dislocations model and the Orowan looping model yielded a critical diameter of 13.1 nm which coincided well with the measured one, indicating the applicability of these two strengthening models for 1Cr15Ni36W3Ti. The present exposure conditions did not exert a profound effect on the fracture mode. All the tensile samples underwent a typically ductile fracture with a dimple pattern dominating the fracture surface. The dispersed deformation induced by the prevalence of dislocation looping in the over-aged tensile samples retarded the propagation of intergranular cracks. The declined precipitation hardening increment and the enhanced deformation homogeneity partially recovered the tensile ductility in the over-aged samples exposed at 680 ℃. [ABSTRACT FROM AUTHOR]

The effect of thermal exposure on microstructure and creep properties of a fourth-generation nickel-based single crystal superalloy was investigated. The thermal exposure of samples after the full heat treatment was carried out at 1000 °C, 1100 °C and 1140 °C for 100 h and 200 h. The γ′ coarsening, γ′ rafting and γ channel widening were observed in samples after thermal exposure. When the thermal exposure time was constant, the morphology of γ′ phase in the alloy evolved significantly with increasing aging temperature. The interfacial dislocation networks in aged samples after creep ruptured gradually became irregular and sparse with the increase of exposure temperature. When the higher exposure temperature was used, enlargement of the defect pores was observed in samples, the microcracks were more likely to initiate and propagate at the corner of these pores. After aging at 1000 °C for 100 h, the creep life at 1140 °C/137 MPa was slightly longer than that of heat-treated sample, which could be attributed to the slightly coarsened γ′ phase, homogenization of refractor elements. In contrast, the creep life of sample exposed at 1140 °C for 100 h was greatly decreased. The decrease of creep life was dominated by the rafting of γ′ phase, the irregular interfacial dislocation networks as well as the enlargement of homogenization pores. [ABSTRACT FROM AUTHOR]

To examine the influences of minor modification of Al content on the microstructural stabilities and stress rupture properties, two alloys with minor difference in Al content were exposed isothermally at 1100 °C for 100 h, 500 h, and 1000 h, respectively. The microstructures were characterized before and after thermal exposure. It was found that when Al content was decreased by 0.4 wt %, the volume fraction γ′ decreased by 4 %, the size of γ′ increased by 40 nm, the matrix channel width increased by 5 nm, and the misfit degree of γ/γ′ phases increased by 0.006 % after heat treatment (HT). During thermal exposure, the alloy with low Al content had a better resistance to coarsening of γ′ phase and precipitation of μ phase. The γ′ particles of the alloy with high Al content tended to connect each other and coarsened along <100>directions; however, the γ′ particles of the alloy with low Al content remained cubic after 500 h. A serious coarsening behavior took place in the two alloys after aging for 1000 h. The structural stabilities were significantly improved. However, the change of 0.4 wt % Al content was found to have little effect on the high temperature stress-rupture properties. [ABSTRACT FROM AUTHOR]

An attempt has been made here to evaluate the effect of thermal exposure on the mechanical behavior and failure mechanisms of carbon fiber composite sandwich panel with pyramidal truss core under axial compression. Analytical formulae for the collapse strength of composite sandwich panel after thermal exposure were derived. Axial compression tests of composite laminates and sandwich panels after thermal exposure were conducted at room temperature to assess the degradation caused by the thermal exposure. Experimental results showed that the failure of sandwich panel are not only temperature dependent, but are time dependent as well. The decrease in residual compressive strength is mainly attributed to the degradation of the matrix and the degradation of fiber–matrix interface, as well as the formation of cracks and pores when specimens are exposed to high temperature. The measured failure loads obtained in the experiments showed reasonable agreement with the analytical predictions. [Copyright &y& Elsevier]