Fracture characteristics of metal/intermetallic laminar composites produced by reaction sintering and hot pressing

Metal/intermetallic layered composites were formed by a process recently developed in which a self-propagating, high-temperature synthesis reaction was initiated at the interface between dissimilar metal foils. After the reaction, one of the metal foils was entirely consumed, resulting in a metal/intermetallic laminar composite. This study details the tensile fracture characteristics of these unique composites. Fracture mechanism and failure energy were controlled by varying the intermetallic-to-metal volume ratio. Failure initiated with the formation of cracks in the intermetallic layer. For high intermetallic-to-metal ratios, the intermetallic crack release energy was too great to prevent cracks from propagating through the metal layer and propagating the crack into the adjacent intermetallic layers, leading to a fast, low energy fracture. For lower intermetallic-to-metal ratios, the metal layers adsorbed the intermetallic crack release energy and blunted the propagating crack. Final failure resulted by ductile fracture of the metal layer after extensive intermetallic cracking.     

Microstructural evolution during mechanical milling of Ti/Al powder mixture and production of intermetallic TiAl cathode target

Titanium aluminides are of great technological interest because of their attractive mechanical properties. Mechanical milling/alloying is a promising powder metallurgical technique, which can achieve ultrafine, uniform and manipulable microstructures. In this study, we employed a recently revisited discus mill to produce a composite Ti–(50–57) at.%Al powder feedstock, which is suitable for hot consolidation to produce bulk cathode targets for physical vapour deposition (PVD) coatings. The effects of milling time, quantity of process control agent (PCA) and discus-to-powder weight ratio (DPR) on the microstructure evolution of the attendant Ti/Al composite powder were investigated in detail. It was found that to produce Ti/Al composite powders with a fine particle size and a uniform microstructure, the practicable processing parameters should be 2 or 3% isopropanol addition as PCA, 12 h of milling time and at least 13:1 DPR weight ratio. Cathode targets were produced by hot isostatic pressing (HIPing) the as-milled powders. The targets were then used to produce a PVD TiAlN coating which had an average microhardness of 2400 HV.     

反应热压烧结法制备SiC/Ti3SiC2复合材料及其性能

采用反应热压烧结法制备了SiC/Ti3SiC2复合材料,研究了热压温度、SiC含量及粒度对SiC/Ti3SiC2复合材料相组成、力学性能以及应力-应变行为的影响.结果表明:热压温度影响SiC/Ti3SiC2复合材料相组成;随着热压温度的提高,复合材料的弯曲强度和断裂韧性提高;随SiC含量的增加,SiC/Ti3SiC2复...     

Microstructural evolution during creep of a hot extruded 2D70Al-alloy

Microstructural evolution during creep of a hot extruded Al–Cu–Mg–Fe–Ni (2D70) Al-alloy was investigated in this study using transmission electron microscopy (TEM). The samples for creep test were carried out two-stage homogenization, followed by extruding. The creep ultimate strength dropped and the temperature increased gradually from 312 to 117 MPa and from 423 to 513 K, respectively. The microstructural observation for the crept samples showed that the S′ phase coarsened with increased creep temperature and the aging precipitates transformed from S″ phase to S′ phase during creep process. Meanwhile, excess solute atoms in supersaturated solid solution dynamically precipitated to further form finer S′ phase and S″ phase, which pinned the dislocations and impeded the dislocation movements. Large amount of dislocations piled up around the micron-scale Al₉FeNi phase, and a lot of dislocation walls were generated along 〈220〉 orientation. S phase accumulates around these defects. The interaction between dislocations and precipitates was beneficial for the improved performances at elevated temperature.     

Temperature fluctuation/hot pressing synthesis of Ti3SiC2

A novel temperature fluctuation synthesis and simultaneous densification process for the preparation of Ti₃SiC₂ was developed. The advantages of this novel method include low synthesis temperature, short reaction time and simultaneous densification. The microstructure and room temperature mechanical properties of the Ti₃SiC₂ synthesized using this method were investigated. The result demonstrated that the Ti₃SiC₂ ceramic consisted of mainly laminated grains. It was found, with the aid of computer simulated crystallite shape, that the laminated Ti₃SiC₂ grains were composed of thin hexagonal plates. These laminated grains characterized the Ti₃SiC₂, and were responsible for the mechanical properties of the polycrystalline Ti₃SiC₂ ceramic. The measured flexural strength and the fracture toughness were 470 ± 26 MPa and 7.0 ± 0.2 MPa·m^1/2, respectively. The high toughness was attributed to the contribution of crack deflection, crack bridging, delaminating and grain pull-out of laminated Ti₃SiC₂.     

Microstructural evolution of Y123/Ag composites during sintering

The grain-growth kinetics of YBa2Cu3O7–xX (Y123) and the coarsening kinetics of silver inclusions in Y123/Ag composites during sintering were investigated. The sintering was carried out in the temperature range 900–950°C. The addition of silver lowered the formation of a liquid phase and the grain growth of Y123 in Y123/Ag composites was thus enhanced at the beginning of sintering. However, as the effective silver content was increased, more silver inclusions became interconnected, and the grain growth kinetics and coarsening kinetics slowed down significantly. This may be due to the mass transportation paths progressively changing from the grain boundaries to interfacial boundaries as the amount of interconnected silver networks is increased. The grain growth kinetic constant was calculated and compared with other published data. Because the inclusion was ripened with the growth of matrix grains, the coarsening of silver inclusions was deduced to be a coalescence process.     

Microstructural evolution of carbon fiber reinforced SiC-based matrix composites during laser ablation process

In this work, four different carbon fiber reinforced SiC-based matrix composites (C/SiC) were prepared, and microstructure evolution during laser ablation process was characterized. Laser irradiation provided a special high-temperature environment up to 3500 °C. For all four composites, different morphologies can be obtained in the transition region due to the oxidation of different matrices. While only needle-shaped carbon fiber and nanolayered carbon without any matrix remained in the central region, indicating that graphitization process occurred in the center, resulting from the high-temperature and low-oxygen environment in the laser process. Therefore, the laser ablation of C/SiC composites is controlled by chemical and physical erosion, and mainly by the physical erosion in the center.     

Effect of Nb2O5 on the microstructure and mechanical properties of TiAl based composites produced by hot pressing

In situ composites of TiAl reinforced with Al₂O₃ particles are successfully synthesized from an elemental powder mixture of Ti, Al and Nb₂O₅ by the hot-press-assisted reaction synthesis (HPRS) method. The as-prepared composites are mainly composed of TiAl, Al₂O₃, NbAl₃, as well as small amounts of the Ti₃Al phase. The in situ formed fine Al₂O₃ particles tend to disperse on the matrix grain boundaries of TiAl resulting in an excellent combination of matrix grain refinement and uniform Al₂O₃ distribution in the composites. The Rockwell hardness and densities of TiAl based composites increase gradually with increasing Nb₂O₅ content, and the flexural strength and fracture toughness of the composites have the maximum values of 634 MPa and 9.78 MPa m^1/2, respectively, when the Nb₂O₅ content reaches 6.62 wt.%. The strengthening mechanism was also discussed.     

原位热压TiC/Ti/Al合成Ti2AlC的研究

以TiC/Ti/Al为原料,采用热压工艺在1400℃原位合成和烧结了含少量第二相Ti3AlC2的Ti2AlC材料.通过不同温度和不同热压时间下合成试样的XRD分析探讨了Ti2AlC的合成过程.结果表明,高温下Ti与Al反应生成中间相TiAl金属间化合物,然后TiC与TiAl金属间化合物反应生成Ti2AlC.初期反应非常迅速,大部分Ti2AlC在此阶段生成.反应后期反应物减少,速度变慢,同时生成少量第二相Ti3AlC2.不同温度下合成的Ti2AlC颗粒具有不同的形貌特征.     

In situ TiC particles reinforced Ti6Al4V matrix composite with a network reinforcement architecture

TiC particles reinforced Ti6Al4V (TiCp/Ti6Al4V) composite with a network TiCp distribution has been successfully fabricated by reaction hot pressing of coarse Ti6Al4V particles and fine carbon powders. TiC particles are in situ synthesized around the boundaries of the Ti6Al4V particles, and subsequently formed into a TiCp network structure. Contrary to the typical Widmanstätten microstructure for the monolithic Ti6Al4V alloy, an equiaxed (α + β) microstructure for the Ti6Al4V matrix of the composite is formed. This is due to the isotropic tensile stress generated by the network TiCp structure and the mismatch of coefficients of thermal expansion (CTE) during the phase transformation. The prepared composite exhibits superior compressive strength before and after heat-treatment due to the reinforcement network architecture and the relatively large matrix region with an equiaxed microstructure.     

Interfacial structure and reaction mechanism of AlN/Ti joints

Bonding of AlN to Ti was performed at high temperatures in vacuum. The bonding temperature ranged from 1323 to 1473 K, while the bonding time varied from 7.2 up to 72 ks. The reaction products were examined using elemental analysis and X-ray diffraction. TiN, Ti3AlN (τ1), and Ti3Al were observed at the AlN/Ti interface, having various thickness at different bonding conditions. The thickness of TiN and Ti3AlN layers grew slowly with bonding time. On the other hand, growth of the Ti3Al layer followed Fick’s law. The activation energy of its growth was found to be 146 kJ mol-1. When thinner Ti foil (20 μm) was joined to AlN at 1473 K for a long time (39.6 ks), the Ti central layer has completely consumed and another ternary compound Ti2AlN(τ2) started to form. A maximum bond strength was achieved for an AlN/Ti (20 μm) joint made at 1473 K for 28.8 ks, after which the bond strength of the joint deteriorated severely. This revised version was published online in November 2006 with corrections to the Cover Date.     

掺Si对热压合成Ti2AlC/Ti3AlC2的影响及理论分析

采用热压工艺以Ti、Al、Si元素粉和活性炭为原料,分别以2.0Ti/1.1Al/1.0C(摩尔比)及以0.1和0.2mol的Si取代Al,合成了Ti2AlC/Ti3AlC2块体材料.通过建立Ti2AlC、Ti3AlC2和掺Si的计算模型,计算了平均原子净电荷和平均共价键键级.结果表明:以元素粉2.0Ti/1.1Al/1.0C为原料在1450℃热压60min合成只含有非常少量Ti3AlC2的Ti2AlC材料;当Si取代Al达到0.2mol时,作用非常明显,表现为使同一温度下Ti3AlC2含量增加而Ti2AlC含量减少.另外,应用掺Si后对原子净电荷和共价键键级的影响解释了实验结果.     

无压渗透法制备Al／AlN复合材料及其性能

在常压下通过熔渗工艺将AlSi7Mg合金渗入由AlN粉末模压成形、预烧所获得的预烧结坯中,得到了不同Al含量的Al／AlN复合材料。采用X射线衍射仪对复合材料的相组成进行了测试,采用金相显微镜和SEM对其显微组织进行了观测,并对不同Al含量的Al／AlN复合材料的维氏硬度、抗弯强度、热膨胀系数及导热系数等进行了测试分析...     

新型Ti_3AlC_2-Al_2O_3/TiAl_3复合材料的组织结构与性能

利用Al-TiO2-TiC体系,通过机械球磨和反应热压制备出Ti3AlC2与Al2O3两相原位内生成增强TiAl3金属基复合材料。借助DSC、XRD、SEM和TEM研究了复合材料的反应机制、显微组织、力学性能及抗氧化性能。结果表明,球磨50h后的复合粉末经1 250℃/50 MPa保温10min烧结后可得到组织均匀细小且致密的Ti3AlC2-Al2O3/TiAl3复合材料,其密度、维氏硬度、室温三点弯曲强度、断裂韧性及压缩强度分别为3.8g/cm3、8.4GPa、658.9 MPa、7.9 MPa·m1/2和1 742.0 MPa,1 000℃的高温压缩强度为604.1 MPa。Ti3AlC2-Al2O3/TiAl3复合材料的增韧机制主要包括Ti3AlC2和Al2O3颗粒的剥离、Ti3AlC2相导致的裂纹偏转和桥接以及Ti3AlC2颗粒的变形及层裂。Ti3AlC2-Al2O3/TiAl3复合材料在700~1 000℃温度区间内生成的氧化层虽不致密,但仍表现出优异的抗高温循环氧化性能。     

炭/炭-陶瓷/炭多元复合材料的原位热压制备与性能

以纤维素和凹凸棒石(PG)为原料,在220℃下水热24h制备凹凸棒石/炭(PG/C)复合材料。采用浸渍-炭化工艺在炭/炭(C/C)复合材料中引入PG/C作为添加剂,一步热压对材料最终成型,原位获得C/C-陶瓷/C复合材料。研究了添加PG/C对C/C力学性能和抗氧化性能的影响。结果表明:PG/C在热压过程中转变为顽辉石/C,顽辉石/C通过"填充"和"桥联"起增强作用,顽辉石陶瓷表面负载纳米炭层有效避免了陶瓷相与基体炭间弱结合的产生。随着PG/C中表面负载纳米炭含量的减少,C/C的强度逐渐增加。当炭含量为13%的PG/C作为添加剂时,C/C的抗弯强度为263MPa,弹性模量为47GPa,相对于没有添加剂的C/C抗弯强度提高了45%,弹性模量提高了42%;相对于以PG作为添加剂的C/C抗弯强度提高了16%,弹性模量提高了27%。添加PG/C使C/C抗氧化性得到了提高;1 000℃下C/C的质量损失降低了12%~18%。     

Microstructural evolution of Ni-NaCl mixtures during mechanochemical reaction and mechanical milling

The evolution of microstructure of Ni and NaCl mixtures formed by mechanochemical reaction and mechanical milling has been studied using X-ray diffraction, electron microscopy and magnetic measurements. Separate nano-sized Ni particles were formed by continuous solid-state reaction of NiCl₂ + 2Na during mechanical milling. Further milling resulted in the growth of clustered particles due to inter-particle welding during collision events. On the other hand, milling of micron-sized Ni and NaCl powders resulted in a layered particle morphology and continuous decrease in particle size with increasing milling time.     

Properties of SiCf/SiC composites fabricated by slurry infiltration and hot pressing

Abstract

Two-dimensional SiC fibre reinforced SiC ceramic matrix composites (SiC_f/SiC) were fabricated by vacuum infiltration and hot pressing using a 200 nm thick pyrolytic carbon coated Tyranno SA3 fabric and 50 nm sized β-SiC powder. Hot pressing was carried out at 1750°C for 3 h in an Ar atmosphere under a pressure of 20 MPa. Al₂O₃–Y₂O₃–MgO sintering additive (10 wt-%) and polyvinyl butyral resin (45 wt-%) with respect to the matrix SiC were found to be the optimum contents for the high density composite. Vacuum infiltration with a force gradient produced much higher amount of slurry infiltration than simple dipping. Much improved density of 3·02 g cm^?3, compared to the previous reports, was achieved for the SiC–SiC_f containing approximately 67 vol.-% of fibre. This composite showed a step increase with a stress–displacement behaviour during the three-point bending test due to the fibre reinforcement. The displacement for failure and flexural strength were 0·58 mm and 342 MPa respectively, which were much larger than those for monolithic SiC.     

Microstructural evolution and deformation behavior of copper alloy during rheoforging process

A complete rheo-forming process was carried out to investigate the rheoforging process of C3771 lead brass valve,starting from the semi-solid billet preparation to rheoforging experiments and material performance tests.The near-spherical micro-grains with mean equivalent diameter of 56.3 μm,shape factor of 0.78 were obtained when the raw C3771 lead brass were rotary swaged to a radial strain of0.22 and then heated to 895℃ for 5 min.The Forge 3 D software was used to analyze the temperature,strain and strain rate distribution of copper valve for obtain the reasonable process parameters during the subsequent rheoforging process.The experiment results showed that near-spherical micro-grains were stretched and refined to about 35.7-43.4 μm in different positions due to the dynamic recrystallization during the rheoforging process.The cap thread and nut thread failure torque of the so-produced valve are also discovered to be higher than the traditionally forged copper valve with dendrite micro-grains,with an enhancement of the cap and thread failure torque of 42.2% and 28%,re s pectively.     

Micro-mechanism and mechanical properties of solid-powder hot isostatic pressing diffusion-bonded Ti2AlNb alloy

Achieving the near-net shaping of brittle and difficult-to-machine materials is still challenging. Thus, we explore a method to prepare Ti-22Al-25Nb alloy by solid-powder hot isostatic pressing (HIP) diffusion bonding. The grain size, microstructure, interface features and mechanical properties of the fabricated alloy were systematically investigated. The results show that the solid-powder interface realizes a complete metallurgical bonding, and the grain size, composition and microstructure in transition zone is formed on the side of preform through recrystallization. There is a huge difference for the grain size between the powder forming zone and the preforming zone. As a result, the fabricated sample for solid-powder transition zone exhibits an excellent mechanical properties, with a tensile strength of 940 MPa, elongation of 2.9% and torsional strength of 815 MPa, respectively. In response to the torsional force, the crack starts from the preforming zone, and the crack deflection and branching occurs in the transition zone, thereby preventing the crack from propagating to the powder forming zone. The torsional strength of the solid powder HIP diffusion bonding zone is basically the same as that of the preformed zone. This study proposes a new solution for fabricating brittle and difficult-to-process materials and is of great significance in the development of the overall near-net shaping technology for complex components of such material.