Synthesis of Al2O3 matrix composites by reactive infiltration

Reactive infiltration of a NiO-base blended powder with molten aluminium was attempted at 1673 K in order to obtain Al2O3 matrix composites containing a dispersion of Al3Ni, AlNi and/or AlNi3. The NiO powder was barely infiltrated by the molten aluminium after a 3600 s holding time at 1673 K. A continuous layer of Al2O3 was observed to exist at the infiltration front, which prevented any further infiltration. TiB2 particles were added to the NiO powder in order to absorb the heat of reaction between NiO and aluminium. When the TiB2 particle content in the [NiO+TiB2] powder blend was greater than 20 vol%, spontaneous infiltration occurred completely. Thus, it was shown that the addition of the TiB2 particles assisted in the spontaneous infiltration. The specimens produced by the in situ reaction consisted of Al2O3, TiB2 and Al3Ni. Al3Ni was mainly located between the TiB2 and Al2O3. The effect of the TiB2 addition on the infiltration kinetics was to decrease the maximum attainable temperature caused by the exothermic reaction. This in turn prevented the formation of a continuous Al2O3 film at the infiltration front. This resulted in the production of pathways for the infiltration of the molten aluminium and made possible the complete infiltration. This revised version was published online in November 2006 with corrections to the Cover Date.     

Microstructure Evolution of Ti/BN Powder Blend during Ball Milling and Heat Treatment

Ball milled Ti/BN composite powder was prepared by high energy ball milling for 40 h, using Ti and BN (the molar ratio of Ti/BN is 3:2) as starting materials. The as-milled composite powder consists of TiN, Ti and amorphous phase. TiN formed while the milled powder was annealed at 400℃. The heat treatment at 700℃ led to the formation of TiB2 and TiB. The nanocrystalline Ti and amorphous phase converted to TiN and TiB2 when the powder was heated to 1300℃.     

高比表面积AlN粉末的制备

以硝酸铝和蔗糖为原料,采用溶胶-凝胶法制备碳铝干凝胶,通过碳热还原反应制备了高比表面积的AlN粉末,探讨了反应温度,硝酸镍的添加量对合成AlN粉末的影响.用XRD,SEM和低温N2物理吸附等手段表征发现,制备的AlN球形粒子粒径均一,约40～50nm左右,具有规则形貌,高比表面积和狭窄的孔径分布;镍盐的掺杂可以显著提升生成AlN的速率,使AlN球形粒子形貌发生很大的改变,其粒子变得粗大,甚至发生了部分团聚.     

Tensile properties and microstructures of Al composite reinforced with BN particles

Al–4.5 wt% Mg alloys reinforced with and without BN were fabricated by pressureless infiltration method and then their tensile properties and microstructures were analyzed. Since the spontaneous infiltration of molten metal occurred at a temperature of 800 °C for 1 h under nitrogen atmosphere, it was possible to fabricate the Al composites reinforced with BN and control alloy without BN. A large increase in strength in the case of the control alloy is related to the in situ formation of AlN particles as well as grain size refinement. In the composite reinforced with BN, additional AlN was formed by the interfacial reaction of the BN and Al melt as well as AlN by the in situ reaction. In addition, the grain size in the composite reinforced with 7.5 vol% BN further decreased to about 4.0 μm and exhibited approximately 3.5 times reduction in the grain size of the commercial alloy. Consequently, a further strengthening in the composite resulted from both the BN artificially added and AlN formation (by the in situ reaction and interfacial reaction) and grain refinement.     

TiB_2粉料的还原合成与酸洗纯化研究

用SHS还原合成法合成了TiB2 混合粉料 ,并通过酸洗处理获得了高纯TiB2 粉料。研究了合成过程和酸洗条件对TiB2 纯度的影响 ,对合成粉料酸洗过程进行了热力学分析。试验结果表明 ,TiO2 B2 O3 Mg三元系统的化学反应由两部分组成 ,第一部分由金属Mg还原出B和Ti单体 ,第二部分由这两种单体化合反应生成TiB2 。TiB2 粉料的纯度主要取决于酸洗条件。提高盐酸浓度、增加酸过量和延长酸洗时间可以提高TiB2 粉料的纯度 ,而升高酸洗温度还可以大大加快酸洗速度。     

氮化铝粉末制备工艺的研究

本文介绍了氮化铝(AlN)陶瓷的特性,并以金属铝直接氮化法和氧化物高温碳还原氮化法为重点,阐述了AlN粉末的各种制备工艺及反应机理、主要工艺参数的影响,对各种方法的优缺点进行了评述。提出:还原法和直接氮化法是目前较成熟的方法,而气相反应法具有较好的推广应用前景。     

Growth of highly c-axis oriented aluminum nitride thin films on β-tantalum bottom electrodes

We have investigated the influence of tantalum (Ta) bottom electrodes on the crystallinity and crystal orientation of aluminum nitride (AlN) thin films. AlN thin films and Ta electrodes were prepared by using rf magnetron sputtering method. The crystal structure of the Ta electrodes was tetragonal (β-Ta, a metastable phase) at room temperature. The crystallinity and orientation of the AlN thin films and Ta electrodes strongly depended on sputtering conditions. Especially, the crystallinity and crystal orientation of the Ta electrodes were influenced by their film thickness and the substrate temperature. When the thickness of the Ta bottom electrodes was 200 nm and the substrate temperature was 100 °C, the AlN thin films indicated high c-axis orientation (the full width at half maximum of rocking curve of 3.9°). The crystal orientation of the AlN film was comparable to that of AlN thin films deposited on face centered cubic (fcc) lattice structure metal, such as Au, Pt and Al, bottom electrodes.     

熔体反应法制备Al-4.5Cu/TiB2复合材料的热力学

用冶金热力学理论计算出TiO2-H3BO3-Na3AlF6-Al-4.5Cu体系的起始反应温度,为950℃,并根据溶液热力学分析了TiB2颗粒形成的可能性.当熔体中[Ti]、[B]的摩尔比为1/2时,在熔体中形成TiB2颗粒;当熔体中[Ti]、[B]的摩尔比大于1/2时,熔体中除生成TiB2颗粒外,还形成TiAl3组织.     

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.     

Fabrication of (TiB/Ti)-TiAl composites with a controlled laminated architecture and enhanced mechanical properties

The(TiB/Ti)-TiAl composites with a laminated structure composing of alternating TiB/Ti composite layers,α₂-Ti₃Al interfacial reaction layers of andγ-TiAl layers were successfully pre pared by spark plasma sintering of alternately stacked Tib₂/Ti powder layers and TiAl powder layers.And the influence of thickness ratio of Tib₂/Ti powder layers to TiAl powder layers on microstructure evolution and mechanical properties of the re sulting(TiB/Ti)-TiAl laminated composites were investigated systemically.The results showed that the thickening ofα₂-Ti₃Al layers which originated from the reaction of Ti and TiAl was significantly hindered by introducing Tib₂particles into starting Ti powders.As the thickness ratio of Tib₂/Ti powder layers to TiAl powder layers increased,the bending fracture strength and fracture toughness at room temperature of the final(TiB/Ti)-TiAl laminated composites were remarkably improved,especially for the(TiB/Ti)-TiAl composites prepared by Tib₂/Ti powder layers with thickness of 800μm and TiAl powder layers with thickness of 400μm,whose fracture toughness and bending strength were up to 51.2 MPa·m^1/2and 1456 MPa,respectively,293%and 108%higher than that of the monolithic TiAl alloys in the present work.This was attributed to the addition of high-performance network TiB/Ti composite layers.Moreover,it was noteworthy that the ultimate tensile strength at 700℃of(TiB/Ti)-TiAl composites fabricated by 400μm thick Tib₂/Ti powder layers and 400μm thick TiAl powder layers was as high as that at 550℃of network TiB/Ti composites.This means the service temperature of(TiB/Ti)-TiAl laminated composites was likely raised by 150℃,meanwhile a good combination of high strength and high toughness at ambient tempe rature could be maintained.Finally,the fracture mechanism of(TiB/Ti)-TiAl laminated composites was proposed.     

微波碳热还原法制备氮化铝粉末的工艺研究

采用微波碳热还原法制备了氮化铝粉末,研究了铝源、碳源和添加剂对制备氮化铝粉末的影响. 通过对所合成的产物进行XRD检测分析表明,氢氧化铝和乙炔黑是最合适的铝源和碳源、单质添加剂的氮化催化效果最明显. 以氢氧化铝和乙炔黑为原料,加入单质添加剂,在氮气气氛下反应温度为1300℃、反应时间为1h时能获得完全氮化的氮化铝粉末,可见微波碳热还原工艺能够大大降低碳热还原法制备氮化铝粉末的反应温度,并缩短反应时间.     

A transmission electron microscope study of a cubic boron nitride-based compact material with AIN and AIB2 binder phases

The microstructure of a commercial cubic boron nitride compact, with aluminium nitride and aluminium diboride binder phases, has been characterized by transmission electron micro-scopy. Within individual grains significant plastic deformation is observed in the form of dis-locations and microtvvin lamellae. There is no evidence of recrystallization. Grains have been forced into direct contact, but there is no evidence of new growth of cubic boron nitride between the grains. Some grains have obviously broken or shattered during manufacture. Aluminium nitride is present mostly as an oriented rind around the cubic boron nitride grains where they are not in direct contact, while aluminium diboride forms a largely continuous network in the channels between them. Our observations suggest that the two aluminium compounds are responsible for most of the bonding in the compact. The orientation relation-ships between the cubic boron nitride and As aluminium nitride rind are, on (1 1 1) facets of BN, BN(1 1 1) AIN(0 0 0 1) and BN[1 1 0] AIN[1 1 – 0], and on (0 0 1) facets of BN, BN(0 0 1) AIN(0 0 0 1) and BN[1 1 0] AIN[l 1 ¨ 0]. The aluminium diboride is mostly unoriented. The results are discussed in the context of previous work on diamond-based compact materials.     

Behavior of magnesium and silicon in the formation of MgO/AlN composite

MgO/AlN composites have been fabricated by directed metal nitridation of Al–Si alloy in flowing N₂ at 1473 K. A mixture of magnesia particles and chemically pure magnesium powder was placed on the surface of Al–Si alloy block as reinforcement materials. Mg powder initiates the infiltration and nitridation of Al alloy melt by eliminating protective Al₂O₃ film at the reaction frontier. New Mg vapor from the interface reaction between Al and MgO particles, keeps as continuous deoxidization agent as the added Mg powder. The spinel layer thickness due to the reaction of Al melt with MgO particles is controlled by Mg content. Si not only reduces the surface tension and viscosity of Al alloy melt, but also leads to increase in N₂ content.     

Investigation of the Young''s modulus of TiB needles in situ produced in titanium matrix composite

In situ Ti/TiB composites with different volume fractions of discontinuous TiB reinforcements were produced by powder metallurgy. After compacting Ti+TiB₂ powders by hot unidirectional pressure, heat treatments led to the in situ formation of distinctive needles of TiB, randomly distributed in the titanium matrix. The Young's modulus of TiB was evaluated using the ASW computation method and experimental Vickers micro-indentation. Three point bend tests were performed on Ti/TiB composites as a function of the TiB volume fraction in order to extract the Young's modulus of TiB from the elastic properties of the composite. The different values obtained according to these three methods were discussed and compared with the literature.     

TiB2-TiC复合粉的自蔓延高温还原合成

热分析结果表明,对于B₂O₃-TiO₂-Mg-C体系,可利用SHS还原技术合成出TiB₂-TiC陶瓷复合粉。其化学反应机理为:Mg先还原B₂O₃和TiO₂,新生的Ti与B和C反应生成TiB₂和TiC; TiO₂的还原经历了TiO₂→TiO→Ti的逐步过程。采用一定的酸洗工艺得到了纯净的TiB₂-TiC陶瓷复合粉。复合粉中包含六方片状TiB₂和圆球状TiC;复合粉中1μm以下颗粒质量百分数超过45%,87%以上的颗粒大小在3μm以下。在TiB₂-TiC中,TiC_<em>y以一种贫碳结构存在,物料中Ti被B或C结合形成TiB₂和TiC_<em>y,y的值为0.7483。     

基于CO_2连续激光器模拟的氮化物填充聚四氟乙烯复合材料耐烧蚀性能

通过在聚四氟乙烯(PTFE)基体中添加不同比例微米、纳米尺度氮化硼(BN)或氮化铝(AlN),以提高高压断路器PTFE喷口复合材料的耐电弧烧蚀性能。利用CO2连续激光器烧蚀PTFE喷口材料来模拟电弧烧蚀过程,分析了光反射率、热导率以及相对介电常数对烧蚀量的影响。通过比较复合材料烧蚀量大小和数值分析结果可知,材料热学参数(热导率和热扩散系数)对烧蚀量起主要作用,BN/PTFE复合材料的耐烧蚀能力优于AlN/PTFE复合材料,10.0%BN/PTFE复合材料的热导率可以达到0.46W/(m·K),比纯PTFE的提高了92%,相应烧蚀过程中的质量损失为21.8mg,比3.0%AlN/PTFE复合材料的质量损失降低了47%,有效提高了喷口复合材料的耐烧蚀能力。     

抗水性AlN粉体的电动特性研究

通过测量粉体在水溶液中的Zeta电位和颗粒尺寸,研究了引入不同的分散剂时抗水性AlN粉体的电动特性．结果表明,溶液的pH值和引入不同的分散剂均会导致抗水性AlN粒体的表面荷电状况及其在水溶液中的分散状况发生变化．NH4PA是一种适合抗水性AlN粉体水基流延用的有效分散剂,在碱性条件下添加该分散剂有利于抗水性AlN粉体在水溶液中的稳定分散．     

TiAl-B合金片状TiB2晶体表面结构及生长机理

用原位自生法制备了Ti-54Al-xB (at%)合金并利用XRD,SEM对合金的相组成和微观组织进行了研究。结果表明:片状TiB₂基面存在与[0001]晶向平行的细棒状分枝,表面存在与[0001]晶向垂直的薄片凸耳状分枝,它们的晶体学取向与母体的取向一致。片状TiB₂厚度一般小于0.3μm,表面上的凸耳状分枝厚度一般小于0.1 μm。分析表明,在不平衡凝固条件下,片状TiB₂固-液界面前沿产生的富Al边界层和随着片状TiB₂晶体的长大导致固-液界面上B原子过饱和度不均匀性增加的共同作用使固-液界面失稳,从而形成了片状TiB₂的这种表面结构。     

氮化铝陶瓷表面钛金属化的研究

利用熔盐热歧化反应在氮化铝陶瓷表面上成功地进行了钛金属化，成功地在氮化铝陶瓷表面上制备了钛金属化膜。还研究了温度、反应物浓度、反应时间对钛金属化膜厚度的影响。研究了金属膜的组成及界面反应机理和界面层的显微结构。研究发现，在热歧化反应沉积钛金属膜的过程中，沉积到AlN陶瓷表面的钛金属与AlN发生反应生成TiN0.3、TiN和Ti9Al23。     

反应烧结法制备(AlN,TiN)-Al2O3复合材料的研究

以Ti,Al,Al₂O₃为初始粉料,通过750~800℃氮气保护下的中温焙烧,然后在1420~1550℃在氮气氛下反应烧结,制备了不同配比的(AlN,TiN)-Al₂O₃复合材料。研究了组成及烧结工艺对复合材料力学性能、显微结构等的影响。用XRD,SEM等方法分析粉体及烧结体的相组成及微观结构。分析结果表明:AlN,TiN的形成,有助于材料的致密化并使其力学性能提高。组成为20wt%(Al,Ti)-Al₂O₃的粉体在1520℃、30MPa、保温、保压30min热压烧结条件下,与N₂气反应可得到硬度(HRA)为 94.1的高硬度的(AlN,TiN)-Al₂O₃复合材料,该材料的抗弯强度为687 MPa,断裂韧性(K_IC)为6.5MPa·m1/2。