查看更多>>摘要：? 2022 Elsevier Inc.Aluminum alloy (AA2219) is widely used for the fabrication of earth storable and cryogenic propellant tanks. To ensure flight safety, it is important to understand the effect of long-term storage on the mechanical properties, which are dependent on the microstructure. Therefore, the present work is carried out to understand the effect of long term (11 years) ambient storage on the tensile and fracture properties of AA2219-T87 in parent material and weld joint. The tensile test results of naturally aged AA2219-T87 material for 11 years in both parent and weldments indicate an increase in tensile strength at room and cryogenic (77 and 20 K) temperatures. STEM-HAADF and TEM analysis revealed the presence of well-oriented densely populated CuAl2 precipitates (plate-like θ’ and θ” precipitates). The changes in mechanical and fracture properties were understood from the quantification of precipitate distribution, that can be attributed to the natural aging phenomenon taking place at ambient conditions, during the extended period of storage. Here, we report the mechanism towards understanding the increase in strength of AA2219-T87, by the growth of the finer θ” precipitates during natural aging. Since tensile (RT, 77 K, and 20 K) and fracture properties (RT) of welds have improved, with no significant reduction in ductility, long-term naturally aged (up to 11 years) fabricated products of AA2219-T87 are flightworthy.
查看更多>>摘要：? 2022The corrosion behavior of Al–Zn–Mg–Cu alloys with different components in NaCl solution were studied experimentally in this work and the composition changes of micron and nano secondary phases during corrosion were semi-quantitatively characterized. Meanwhile, the effect of the type and quantity of the secondary phases caused by different compositions on the corrosion resistance and its mechanism were discussed. The results show that localised self-corrosion was noted in the S-phase for AA7050 and the corrosion rate of S-phase around Cu-rich corrosion products was more than one order of magnitude higher than that in the area where no corrosion products were formed. With the progress of corrosion, the Cu content in the area forming corrosion products first decreased and then increased, while the O content first increased and then decreased. In addition to self-corrosion of the σ-phase, severe corrosion occurred in the interface between matrix and σ-phase for AA7136, and the corrosion rate of interface was more than one order of magnitude higher than that of σ-phase. Due to the absence of the secondary phases (the volume fraction is only 0.02%) above micron-scale (S- and σ-phase), the pitting resistance of E2 alloy designed by authors was significantly better than AA7050 and AA7136. Zn and Mg elements were enriched around the nano dispersoids formed by Cr and Mn in E2 alloy, which reduced the content of Zn and Mg elements near the grain boundary, resulting in the potential difference between intergranular and intragranular were 10 mV ~ 40 mV lower than AA7050 and AA7136. Meanwhile, the dispersoids hindered the intergranular corrosion and improved the intergranular corrosion resistance of E2 alloy.
查看更多>>摘要：? 2022 Elsevier Inc.Understanding the orientation of the product phase during β to α phase transformation, i.e., the transformation texture is important for optimizing the microstructure and mechanical properties of titanium alloys. As the α phase prefers to nucleate from the triple junction of β grain boundaries (GBs), it is essential to understand the orientation preference, i.e., the variant selection of the α formed at the triple junction (denoted as the αTJ). Recent work indicated that in theory the αTJ can keep strict Burgers orientation relationship (BOR) with all the adjacent β grains for some special types of triple junctions. However, the experimental studies are very limited so far. In this work, a number of α precipitates formed at the triple junctions were analyzed. It was shown that at the triple junctions close to the special types, the observed αTJ obeyed the triple BOR criterion, i.e., tended to keep the BOR with all the adjacent β grains. However, at the triple junctions greatly deviated from the special types, the selection of the αTJ variants obeyed the double BOR criterion of one GB. Moreover, it was revealed that the special low angle GB, at which a double BOR α can in theory be possible, had no advantage for the selection of αTJ variants compared to the general high angle GBs forming the triple junction. This work sheds light on understanding the general rule governing the transformation texture in Ti alloys.
查看更多>>摘要：? 2022The microstructural characterization and mechanical behavior of the extruded ZK61 alloy were examined under dynamic and quasi-static loading at 623 K. The microstructure showed that an adiabatic shear band (ASB, ~10 μm in width) was formed under dynamic loading at 3.6 × 103 s?1 due to severe local plastic deformation and it was composed of ultra-fine grains (~0.46 μm). By comparison, the shear deformation of quasi-static loading was located in an area (~200 μm in width) with equiaxed grains (~4 μm). The texture analysis of the shear regions showed a stable orientation that all the (0002) basal plane was parallel to the shear direction (SD) and the orientation mainly aggregated near 〈10?13〉//SD, while a weak peak around 〈0001〉//SD appeared with a strain rate of 1.0 × 10?3 s?1. The main recrystallization mechanism during quasi-static deformation was discontinuous dynamic recrystallization (DDRX) which nucleated at the grain boundaries by absorbing surrounding high-density dislocations. However, the formation of the DRXed grains within the ASB during dynamic deformation was attributed to rotational dynamic recrystallization (RDR) that the sub-grain boundaries tilted nearly 30° to generate new grains driven by the grain boundary energy. For mechanical responses, the stress-strain curve of dynamic loading showed high yield stress and long working-hardening stage, while the strain-hardening and thermal-softening in quasi-static curves reached a dynamic balance.
查看更多>>摘要：? 2022 Elsevier Inc.An ultrafine grained Al-Mg-Sc-Mn-Zr alloy fabricated by laser powder bed fusion (L-PBF) was prepared to investigate the effect of different annealing heat treatments on the mechanical properties and corrosion behaviors. The results show that the optimized heat treatment is determined to be annealing at 300 °C for 2 h, which achieves highest hardness (142 HV) and best corrosion resistance (6.2 μA/cm2) simultaneously. The improvement of the mechanical properties and the corrosion resistance is mainly attributed to the precipitation of the secondary Al3(Sc, Zr) precipitates with sub-nano size. Moreover, the size of the nanoscale Al3(Sc, Zr) precipitates and the micropores also play a role in the corrosion resistance of the L-PBF Al-Mg-Sc-Mn-Zr alloy
查看更多>>摘要：? 2022 Elsevier Inc.A microstructural characterization of the early-stage steam oxidation behaviors of a γ-TiAl alloy was performed. The γ-TiAl alloy samples were oxidized in water steam at 900, 1000, 1100, and 1200 °C for 1 h, and multilayered structures were observed. A compact Ti3Al layer was formed next to the TiAl substrate. An outer layer of TiO2 and Al2O3 mixed oxides was observed, where the oxide grains are transferred from long-columnar grains at 900 °C to the randomly scattered grains at 1200 °C. The ribbon shaped Al2O3 grains extend inward into the Ti3Al layer at 1100 °C and above. An intermediate layer with dark speckles and bright background was formed at 1200 °C. Fine pores existed in the near-surface areas of the oxide scales for all specimens.
查看更多>>摘要：? 2021Selective laser melting (SLM) refers to a laser additive manufacturing technology. It shows its advantages of high efficiency and is capable of processing arbitrary complex structural parts. However, the SLM of magnesium alloy is highly challenging and should be studied in depth due to the low melting and boiling point of magnesium alloy. In this study, selective laser melting (SLM) technology was used to manufacture the Mg-Y-Sm-Zn-Zr alloy. Microstructure characteristics and performance mechanism of the SLMed samples were investigated. As revealed by the results, samples characterized by relatively high densities and low surface roughness could be produced when the energy density was 83.3–166.7 J/mm3. The highest density of 97.8% could be obtained when the energy density was 125.7 J/mm3. The molten pool was found to consist of slender columnar grains at the edge and a small amount of equiaxed grains at the top, while the grains below the molten pool were coarsened under the action of the thermal influence. The role played by Y2O3 in the solidification of SLM was characterized using the degree of lattice mismatch. Impacted by the high lattice mismatch between Y2O3 and Mg, Y2O3 could not serve as an effective heterogeneous nucleation particle. The highest comperssive performance was obtained at energy density of 125.7 J/mm3 (YS = 304 ± 5 Mpa, UTS = 394 ± 5 Mpa). The main strengthening mechanism was fine-grain strengthening, followed by precipitation strengthening and the effect of solid solution strengthening was not obvious. This work provides a certain guiding significance for the follow-up research of SLMed rare earth magnesium alloy.
查看更多>>摘要：? 2022 Elsevier Inc.The creep resistance of T5-treated Al-Mg-Si alloy with different Cu content was investigated in this study. Four T5-treated alloys containing no Cu, 0.4wt%Cu 0.6 wt%Cu and 0.8wt%Cu were provided and labeled as 0Cu, 0.4Cu, 0.6Cu and 0.8Cu samples. The tensile creep experiments were carried out at 250 °C. The results showed that 0.6Cu sample had the highest creep resistance, followed by 0.8Cu, 0.4Cu and 0Cu samples. Using various characterization methods, the effects of texture component, dislocation evolution and precipitation on the creep resistance were unveiled: (1) The high (Goss + R-Goss)/Brass texture ratio in 0Cu sample failed in delaying the dislocation motions. In contrast, the low ratio in 0.6Cu sample succeeded in reaching a balance between soft and hard grain orientations and enhancing the creep resistance. (2) Dislocation tangles significantly happened in 0.6Cu sample, which inhibited the continuous movement of dislocations. (3) The suitable addition of 0.6 wt%Cu behaved the most effective strengthening on the creep resistance. But unfortunately, the loss of solution strengthening occurred in 0.8Cu sample due to the over-added Cu element accelerated the precipitation of Mg and Si elements.
查看更多>>摘要：? 2022 Elsevier Inc.Micro-sized silicon carbide particles (SiCp) reinforced aluminum (Al) matrix composites have been widely used in the aerospace, electronics and transportation applications. However, due to the easily formed stress concentration in the SiCp/Al interfacial micro-zones, the large-sized SiCp may cause serious plastic loss of SiCp/Al composites that have limited their applications. In this study, the 14.5 μm SiCp was selected to prepare the SiCp(CNT) hybrid reinforcement, and then the 15 wt. % SiCp/Al and SiCp(CNT)/Al composites were fabricated by the vacuum hot-pressing sintering, respectively. Compared to the SiCp/Al composite, the SiCp(CNT)/Al composites own the larger mechanical properties such as the Young's modulus, the yield strength and the tensile strength. Among these composites, the SiCp(0.5CNT)/Al composite presents the best matching of strength and plasticity as a result of the existence of CNTs in the SiCp/Al interfacial micro-zones. Strengthening and toughening effects of CNTs in the SiCp/Al interfacial micro-zones can contribute to: 1) increasing the punched zone size around the SiCp; 2) increasing the dislocation density in the SiCp/Al interfacial micro-zones; 3) changing the dislocation distributions in the SiCp/Al interfacial micro-zones; 4) pinning the dislocation movements by CNTs in the SiCp/Al interfacial micro-zones. From the analysis above, the strengthening and toughening mechanism of the CNTs introduced in the SiCp/Al interfacial micro-zones of SiCp(CNT)/Al composites can be revealed, which can be further developed to guide and prepare the hybrid particles reinforced metal matrix composites.
查看更多>>摘要：? 2022 Elsevier Inc.A comparative study of structural, ferroelectric, magnetic and dielectric properties of Ca, Sr and Ba doped 0.2BiFeO3–0.8PbTiO3 (BF-PT) binary solid solutions synthesized via the solid-state reaction route has been reported. The XRD analysis confirms that all the samples possess a tetragonal structure and, after the BF-PT system, the maximum c/a ratio of ~1.070 has been observed in the BF-PCT system. The remnant polarization (Pr ~ 1.99μC/cm2) and the dielectric constant (~563 at 3 MHz) have also been observed to be the highest in the BF-PCT system as compared to BF-PST and BF-PBT solid solutions. The AC conductivity data of all the prepared samples obey Universal Joncher's Power law and was found to be increased from the BF-PT system to the BF-BT system. A simultaneous change in the hopping mechanism has also been observed from a translational type in the Pb-based system to a localized type in the Ba-based (non?lead) system. The NEXAFS study reveals that the best dielectric and ferroelectric results observed in the lead-based solid solutions have been attributed to the strong hybridization between the Pb 6 s and O 2p states of PbTiO3, which are absent in BaTiO3.
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