First-principles study of point defects and Si site preference in Al3Ti

The formation energies of various point defects in Al₃Ti(D0₂₂) have been calculated by using first-principles calculation. The effects of vacancies on Si site preference were examined to better understand Si substitution behavior in Al₃Ti. The results show that, under Al-rich condition, the formation energy of antisite Al_Ti is the lowest than those of other point defects, and Ti vacancy is more preferred than Al vacancy. But Si prefers to occupy Al vacancy compared with Ti vacancy which causes a finite solubility of Si in Al₃Ti system. The calculation is instructive for the further improvements of process of Si removal.     

High temperature deformation with diffusional and plastic accommodation in Ti/TiB whisker-reinforce in-situ composites

A steady-state creep model of a metal matrix composite was proposed considering the accommodation processes of the misfit strain between the matrix and reinforcements in order to comprehensively explain dispersion strengthening, composite strengthening and composite weakening. The proposed model was verified experimentally using the model composites, Ti/TiB in situ composites of 5, 15 and 20vol.%TiB, which have a good interfacial bonding, suitable size and aspect ratio of TiB for a moderate diffusional accommodation rate, and no fine oxides. A sigmoidal curve in a double-logarithmic scale of stress and strain rate was observed in the creep of Ti/15TiB at 1123 K and was classified into a complete-diffusional-accommodation region corresponding to composite weakening, a diffusional-accommodation-controlled region and a plastic-accommodation-controlled region corresponding to composite strengthening with increasing stress. The activation energies in the three regions were close to those of the volume, interface and volume diffusion of Ti, respectively. In the plastic-accommodation-controlled region, the strain rate decreased with increasing volume fraction, while near the complete-diffusional-accommodation region, the strain rate scarcely changed. The strain rate of the transverse creep was ten times that of the longitude creep in the plastic-accommodation-controlled region, while the difference was scarcely observed near the complete-diffusional-accommodation region.     

Influence of impurities on hot-rolled molybdenum for high temperature applications

Refractory metals are crucial for high-temperature applications such as fusion reactors. However, no systematic research has been reported for the influence of the impurities in refectory metals, especially at high temperatures. Our goal is an investigation of the influence of dissolved impurities on the recrystallization behavior of hot-rolled molybdenum (Mo). Two sets of 80%-rolled Mo plates with different purity levels pure Mo and non-pure Mo with the content of impurities, O, C, N, etc. are analyzed. Both isothermal and isochronous annealing experiments demonstrated that the impurities resulted in a reduction of recrystallization temperature and acceleration of texture transformation in a Mo plate. The recrystallization temperature of as-rolled non-pure Mo is about 100 °C lower than that of pure Mo. EBSD results show deformed γ-fiber texture gets weakened while {001}〈110〉 component gets strengthened at 1250 °C for pure Mo and 1150 °C for non-pure Mo, leading to the destabilization of fibrous grains. The XRD and TEM results reveal that the dislocation density of as-rolled non-pure Mo is ~2.4 times that of pure Mo. This discovery deviates from the conventional understanding of impurities or second phase particles in metals, providing better practical knowledge and opportunities to establish the technical parameters of refractory metals for high temperatures applications.     

Properties of super heat-resistant silicon carbide fibres with in situ BN coating

An in situ BN coating was prepared on the surface of a nearly stoichiometric continuous SiC fibre with trademark Cansas-3301 (C3). The coated fibre was then subjected to continuous pyrolysis at 1800 °C, obtaining a fibre named Cansas-BN-1800 (C18). After annealing in Ar at 1500 °C for 1 h, the strength retention ratio of C3 was 49%, and that of C18 was almost unchanged. The strength decrease of the C3 fibre was mainly caused by the formation of surface defects resulting from fibre decomposition and active oxidation. However, the in situ BN coating on C18 protected the fibre from forming surface defects, resulting in high strength. Due to slight growth of the grain and purification of the grain boundary during fast heating at 1800 °C, C18 showed excellent creep resistance in the range of 1200–1500 °C.     

A physically-based model for global collision avoidance in 5-axis point milling

Although 5-axis free form surface machining is commonly proposed in CAD/CAM software, several issues still need to be addressed and especially collision avoidance between the tool and the part. Indeed, advanced user skills are often required to define smooth tool axis orientations along the tool path in high speed machining. In the literature, the problem of collision avoidance is mainly treated as an iterative process based on local and global collision tests with a geometrical method. In this paper, an innovative method based on physical modeling is used to generate 5-axis collision-free smooth tool paths. In the proposed approach, the ball-end tool is considered as a rigid body moving in the 3D space on which repulsive forces, deriving from a scalar potential field attached to the check surfaces, and attractive forces are acting. A study of the check surface tessellation is carried out to ensure smooth variations of the tool axis orientation. The proposed algorithm is applied to open pocket parts such as an impeller to emphasize the effectiveness of this method to avoid collision.     

Fe3O4/CNTs@Cf复合材料的制备及其吸波性能的研究

为了改善传统碳材料的吸波性能,获得具有多元吸波机理的吸波材料,本文通过化学气相沉积法在碳纤维表面原位生长碳纳米管,后采用溶剂热反应在CNTs@C_f上生成Fe₃O₄纳米颗粒,制备出了Fe₃O₄/CNTs@C_f复合材料,并对其吸波性能进行研究,分析了复合材料的合成机理和吸波机理。其反射损耗在C波段可达-43.02 dB,随着Fe₃O₄纳米颗粒含量的进一步增加,其吸波性能下降。     

Characterization of carbon/carbon composite/Ti6Al4V joints brazed with graphene nanosheets strengthened AgCuTi filler

Carbon/carbon (C/C) composite and Ti6Al4V alloy (wt%) were successfully brazed with graphene nanosheets strengthened AgCuTi filler (AgCuTiG). Graphene nanosheets (GNSs) with low CTE and high strength were dispersed into AgCuTi filler by ball milling. The interfacial microstructure was systematically characterized by varieties of analytical means including transmission electron microscopy (TEM). Results show that typical interfacial microstructure of the joint brazed at 880 °C for 10 min is a layer structure consisting of (Ti6Al4V/diffusion layer/Ti2Cu + TiCu + Ti3Cu4 + TiCu4/GNSs + TiCu + TiC + Ag(s,s) + Cu(s,s)/TiC/C/C composite). The interfacial microstructure and mechanical properties of brazed joints changed significantly as temperature increased. High temperature promoted the growth of TiCu and TiC phases, which were attached to GNSs. Meanwhile, the diffusion layer and primary reaction layers thickened as temperature increased, while the thickness of brazing seam decreased. The maximum shear strength of 30.2 MPa was obtained for the joint brazed at 900 °C for 10 min. GNSs decreased the thickness of brittle reaction layers and promoted the formation of TiCu and TiC phases in brazing seam, which caused the strengthening effect and decreased the CTE mismatch of brazed joints. The fracture modes are also discussed in this paper.     

Interfacial reaction behavior and bonding mechanism between liquid Sn and ZrO2 ceramic exposed in ultrasonic waves

Ultrasound-assisted dipping of ZrO₂ ceramics into molten Sn solder was performed to realize the low-temperature joining of ZrO₂ ceramics in this study. Scanning electron microscopy with energy dispersive spectrometer, X-ray diffraction and X-ray photoelectron spectroscopy were employed to study the effects of ultrasonic vibration on the microstructure of Sn/ZrO₂ interface, and to elucidate the joining mechanism between Sn coating layer and ZrO₂ ceramic. Results showed that, after ultrasonically dipping in molten Sn for 1200 s, a pure Sn solder layer with a thickness of approximately 8–9 µm was coated on the ZrO₂ surface. The Sn coating layer exhibited excellent metallurgic bonding with ZrO₂ ceramic. A nano-sized ZrSnO₄ ternary phase, which was beneficial to the smooth transition of the lattice from Sn solder to ZrO₂ ceramic, was formed at the Sn/ZrO₂ interface. The formation of ZrSnO₄ interlayer was ascribed to the acoustic cavitation induced high-temperature reaction of Sn, O and ZrO₂ at the molten Sn/ZrO₂ ceramic interface. The tested average shear strength of ZrO₂/Sn/ZrO₂ joints was approximately 32 MPa, and the shearing failure mainly took place within the Sn solder layer.     

Effect of brazing parameters on microstructure and mechanical properties of Cf/SiC and Nb-1Zr joints brazed with Ti-Co-Nb filler alloy

Reliable brazing of carbon fiber reinforced SiC (C_f/SiC) composite to Nb-1Zr alloy was achieved by adopting a novel Ti45Co45Nb10 (at.%) filler alloy. The effects of brazing temperature (1270–1320 °C) and holding time (5–30 min) on the microstructure and mechanical properties of the joints were investigated. The results show that a continuous reaction layer (Ti,Nb)C was formed at the C_f/SiC/braze interface. A TiCo and Nb(s,s) eutectic structure was observed in the brazing seam, in which some CoNb₄Si phases were distributed. By increasing the brazing temperature or extending the holding time, the reaction layer became thicker and the amount of the CoNb₄Si increased. The optimized average shear strength of 242 MPa was obtained when the joints were brazed at 1280 °C for 10 min. The high temperature shear strength of the joints reached 202 MPa and 135 MPa at 800 °C and 1000 °C, respectively.