Brazing of pressureless-sintered SiC using Ag-Cu-Ti alloy

A pressureless-sintered SiC was brazed to itself using Ag-Cu alloy foil to which titanium had been added. The results obtained revealed the following. (i) Increasing the titanium addition to the base metal from 2 to 8 wt % improved the wettability greatly, but the bonding generally became weaker. (ii) With 2 wt % Ti addition, a reaction layer about 1 m thick was formed, regardless of which brazing temperature was used, while bond strength reached was over linearly with temperature. The maximum room-temperature bend strength reached was over 350 MPa. (iii) In the case of the alloy with only 2 wt % Ti additive, bonding was greatly influenced not only by improvement of the wettability at high temperatures and longer holding times, but also the composition and thickness of the resultant reaction layer.     

Microstructure and strength of TiAl/steel joint induction brazed with Ag-Cu-Ti filler metal

Abstract

Intermetallic TiAl was induction brazed to steel in an induction furnace with Ag-Cu-Ti filler metal at 1143 K for 0·2–2·4 ks. Microstructural analysis indicates that Ti, Al, and C atoms in base metal diffuse to the interface and react strongly with the filler metal during brazing. The interface structure of the joint can be divided into three distinct zones: the reaction layer near TiAl, composed of Cu-Al-Ti compounds and Ag based solid solutions; the central zone of the interface, consisting of Ag based solid solutions in which Ag-Cu eutectic phases are dispersed; a TiC reaction layer adjacent to the steel. The relationship between brazing parameters and tensile strength of the joints is discussed, and the optimum induction brazing parameters obtained. When brazed at 1143 K for 0.9 ks, the tensile strength of the joint is 298 MPa.     

Characterization of SiC/C (B)/SiC microcomposites by transmission electron microscopy

Uniform or composition-graded C(B) (i.e., boron-containing carbon) interphases in SiC/SiC model microcomposites were characterized by transmission electron microscopy after tensile tests and thermal ageing in air. A specific method was developed to prepare thin longitudinal sections of the tested specimens. Deflection of matrix cracks occurs within the uniform C(B) interphase, as long as its anisotropy remains high enough (i.e., when the boron content is not too high). It takes place close to the most anisotropic layer (i.e. that containing 8 at% of boron) in composition-graded interphases. In both cases, the crack deflection path does not reach the fibre, a feature which is consistent with the good mechanical properties. After ageing in air under tensile loading beyond the proportional limit (600°C; σ=800 MPa), the composition-graded interphase (made of five sublayers in which the boron content increases from 0 at% near the fibre to 33 at% near the matrix) was observed to act as a glass-forming protection, the pyrocarbon sublayer (at the fibre surface) remaining unoxidized. This revised version was published online in November 2006 with corrections to the Cover Date.     

Al2O3陶瓷/AgCuTi/可伐合金钎焊接头力学性能

为实现Al2O3陶瓷与可伐合金的可靠连接,分析影响接头力学性能的因素,测试了Al2O3陶瓷/AgCuTi/可伐合金钎焊接头的抗剪强度,通过光学显微镜、SEM及EDS对断口形貌、成分进行分析,确定了断裂路径.研究表明,钎焊温度为900 ℃,保温时间为5 min时,接头抗剪强度最高,达144 MPa.此时,断裂大部分发生在Al2O3陶瓷/钎料界面处,小部分发生在界面中的TiFe2、TiNi3金属间化合物层.钎焊温度升高,保温时间延长时,界面上出现大量的TiFe2、TiNi3金属间化合物,界面性能弱化,断裂发生在TiFe2、TiNi3金属间化合物层,造成Al2O3陶瓷/AgCuTi/可伐合金接头连接强度降低.     

High-resolution electron microscopy study of a high-copper variant of weldalite 049 and a high-strength AI-Cu-Ag-Mg-Zr alloy

A high-resolution electron microscopy study was preformed on two high-strength aluminium alloys, an Al-6.2Cu-0.4Ag-0.4Mg-0.2Zr alloy and a similar alloy but having a low lithium addition (1.25 wt%), named Weldalite 049, in order to identify their principal strengthening phase. The lattice images of the principal strengthening phase in these alloys were found to be different. The former alloy had the so-called phase, which agrees with previous publications, whereas Weldalite 049 had a phase similar to, but not exactly like, the so-called T₁ phase which is the principal strengthening phase of some high lithium (2 wt%) AlCuLi alloys.     

Effect of Si3N4-particles addition in Ag-Cu-Ti filler alloy on Si3N4/TiAl brazed joint

A novel composite filler alloy was developed by introducing Si₃N_4p (p = particles) into Ag-Cu-Ti filler alloy. The brazing of Si₃N₄ ceramics and TiAl intermetallics was carried out using this composite filler alloy. The typical interfacial microstructure of brazed joints was: TiAl/AlCu₂Ti reaction layer/Ag_(s,s) + Al₄Cu₉ + Ti₅Si_3p + TiN_p/TiN + Ti₅Si₃ reaction layer/Si₃N₄. Effects of Si₃N_4p content in composite filler alloy on the interfacial microstructure and joining properties were investigated. The distribution of Ti₅Si_3p and TiN_p compounds in Ag-based solid solution led to the decrease of the mismatch of the coefficient of thermal expansion (CTE) and the Young's modulus between Si₃N₄ and TiAl substrate. The maximum shear strength of 115 MPa was obtained when 3 wt.% Si₃N_4p was added in the composite filler alloy. The fracture analysis showed that the addition of Si₃N_4p could improve the mechanical properties of the joint.     

High-resolution electron microscopy of detonation nanodiamond

The structure of individual nanodiamond grains produced by the detonation of carbon-based explosives has been studied with a high-vacuum aberration-corrected electron microscope. Many grains show a well-resolved cubic diamond lattice with negligible contamination, thereby demonstrating that the non-diamond shell, universally observed on nanodiamond particles, could be intrinsic to the preparation process rather than to the nanosized diamond itself. The strength of the adhesion between the nanodiamond grains, and the possibility of their patterning with sub-nanometer precision, are also demonstrated.     

High-resolution electron microscopy study of belite

The microstructures of belite examined by high-resolution electron microscopy (HREM) show the (1 0 0) and (0 0 1) twin lamellae which are introduced by shear stresses during the transformation of–C₂S to-C₂S.-C₂S formed in the larnite was identified and the orientation relationship between them was also determined by a composite electron diffraction pattern (EDP), which is different from that suggested earlier by Groves. The EDP and the HREM image showed the existence of the unstable phase _L–C₂S.     

High-resolution electron microscopy characterization of partially crystallized Fe78Si12B10 alloy as a catalyst

As a catalyst, partially crystallized Fe₇₈Si₁₂B₁₀ alloy shows a three times higher activity than totally crystallized Fe₇₈Si₁₂B₁₀ in the hydrogenation of CO. High-resolution electron microscopy (HREM) reveals that the catalyst contains a great number of minute (less than 10 nm), highly dispersed -Fe particles which act as the major active component. Many tiny B and Si oxide clusters also exist in the amorphous matrix. After being rinsed by a NaOH solution, the catalyst exhibits a decreasing selectivity to methane due to the dissolution or aggregation of B and Si species. After reaction, the sizes of the active particles increase and an overlayer of B or Si species is found over the surface of some -Fe particles. This coverage is thought to be the primary origin of the deactivation of the catalyst.     

电子束钎焊接头组织分析

采用自主开发的电子束钎焊系统,对不锈钢毛细管板结构进行钎焊,通过电子探针显微分析仪研究了不同电子束钎焊规范下BNi-2钎料与管壁基体界面合金元素的分布,分析了钎料和界面区各相的化学组成.研究表明:在加速电压60kV,束流6.5 mA,加热时间37 s,扫描幅值O.5的电子束钎焊规范下,管板接头质量满足技术规范要求;随着电子束输入功率或功率密度增大,钎料和管壁的相互扩散作用增强,导致过渡层厚度增加,毛细管壁显著减薄;母材和钎料中合金元素的相互扩散导致过渡层的形成,过渡层主要由硼化铬、硼化镍和镍的固溶体组成.