In-situ combustion synthesis and densification of TiC–xNi cermets

The combustion synthesis reaction was combined with quasi-isostatic pressing (QIP) technique to fabricate full density TiC–xNi composites in a single processing operation. Combustion wave velocity and temperature of Ti–C–Ni were measured and the microstructure of the product was characterized by X-ray diffraction and scanning electron microscopy. With increasing Ni content in TiC–xNi, both the combustion wave velocity and temperature decrease. The Ni additive, mainly as a diluent and the binder of TiC grains in a matrix, formed a quasi-continuous phase enveloping spheroidal TiC particles and brought about a grain size decrease from 9 to 1 μm. TiC-20 wt% Ni cermet produced by the combustion synthesis/quasi-isostatic pressing process under 160 MPa for 20 s show near full density, high hardness and transverse rupture strength (1024.2 MPa).     

超声化学原位合成纳米Al_2O_3/6063Al复合材料组织及高温蠕变性能

为研究纳米颗粒增强铝基复合材料的高温蠕变特性,基于6063Al-Al2(SO4)3体系,采用超声化学原位合成技术,制备出不同Al2O3体积分数(5%、7%)的纳米Al2O3/6063Al复合材料,通过高温蠕变拉伸试验测试其高温蠕变性能,利用XRD、OM、SEM及TEM分析其微观形貌。结果表明:施加高能超声可显著细化增强体颗粒并提高其分布的均匀性,所生成的Al2O3增强颗粒以圆形或近六边形为主,尺寸为20~100nm;纳米Al2O3/6063Al复合材料的名义应力指数、表观激活能和门槛应力值与基体相比大幅提高,均随着增强体体积分数的增加而提高,表明纳米Al2O3/6063Al复合材料的抗蠕变性能提高;纳米Al2O3/6063Al复合材料的真应力指数为8,说明复合材料蠕变机制符合微结构不变模型,即受基体晶格扩散的控制;纳米Al2O3/6063Al复合材料的高温蠕变断口特征以脆性断裂为主,高应力下形成穿晶断裂,低应力下形成沿晶断裂和晶界孔洞;纳米Al2O3/6063Al复合材料的主要强化机制为位错强化与弥散强化。     

Improvements in mechanical properties of TiB2 ceramics tool materials by the dispersion of Al2O3 particles

TiB₂–Al₂O₃ composites with Ni–Mo as sintering aid have been fabricated by a hot-press technique at a lower temperature of 1530 °C for 1 h, and the mechanical properties and microstructure were investigated. The microstructure consists of dispersed Al₂O₃ particles in a fine-grained TiB₂ matrix. The addition of Al₂O₃ increases the fracture toughness up to 6.02 MPa m^1/2 at an amount of 40 vol.% Al₂O₃ and the flexural strength up to 913.86 MPa at an amount of 10 vol.% Al₂O₃. The improved flexural strength of the composites is a result of higher density than that of monolithic TiB₂. The increase of fracture toughness is a result of crack bridging by the metal grains on the boundaries, and crack deflection by weak grain boundaries due to the bad wetting characters between Ni–Mo and Al₂O₃.     

PVDF/PZNZT压电复合材料的结构与性能

为了扩展压电复合材料的应用领域,首先,通过固相合成法制备了0-3型聚偏氟乙烯(PVDF)/Pb(Zn_1/3Nb_2/3)_0.05Zr_0.47Ti_0.48O₃ (PZNZT)压电复合材料;然后,研究了PVDF含量对PVDF/PZNZT复合材料物相、显微结构及性能的影响。结果表明:PZNZT陶瓷粉料与PVDF粉料混合后,其平均粒度接近于纯PVDF粉料的。于220℃下烧结后, PVDF/PZNZT复合材料在XRD谱图中主要显现出PZNZT钙钛矿结构的衍射峰。当PVDF含量较低时, PZNZT陶瓷晶粒间的结合较松散;随着PVDF含量的增加,陶瓷晶粒几乎都被PVDF相包围。因显微结构不同,不同PVDF含量的PVDF/PZNZT复合材料在极化电场中呈现出不同的串、并联电路。极化后, 5wt% PVDF/PZNZT复合材料的电性能最佳,其介电常数为116、介电损耗tan δ为0.04、压电常数为48 pC/N且机电耦合系数为0.28。随PVDF含量的增加, PVDF/PZNZT复合材料的居里温度降低,维氏硬度有所增加,但仍小于纯PZNZT压电陶瓷的硬度。所得结论显示PVDF/PZNZT压电复合材料的性能可以满足水声、电声及超声换能器等的要求。      

Microstructure of alumina reinforced with tungsten carbide

Carbides and nitrides reinforced alumina based ceramic composites are generally accepted as a competitive technological alternative to cemented carbide (WC-Co). The aim of this work was to investigate the effect of dispersed tungsten carbide (WC) on the microstructure and mechanical properties of alumina (Al₂O₃). Micron size alumina and tungsten carbide powders were mixed in a ball mill and uniaxially pressed at 1600°C under 20 MPa in an inert atmosphere. The hardness of WC reinforced alumina was 19 GPa and fracture toughness attained up to 7 MPa m^1/2. It was demonstrated by TEM analysis that coarse, micrometersized tungsten carbide grains were located at grain boundaries of the alumina matrix grains. Additionally, sub-micrometer tungsten carbide spheres were found inside the alumina particles. Crack deflection triggered by the tungsten carbide at the grain boundaries of the alumina matrix is supposed to increase fracture toughness whereas the presence of intergranular and intragranular hard tungsten carbide particles are responsible for the increase of the hardness values of the investigated composite materials.     

Changes in the mechanism of heat transfer in passing from microparticles to nanoparticles

On the basis of experimental data on thermal conduction and sound velocity in composites obtained by sintering detonation nanodiamonds with the crystallite size of 4–5 nm and diamond micropowders with a grain size of about 10 μm at a high pressure (5–7 GPa) and high temperature (1200–1800°C), mechanisms of heat transfer in such structures are suggested. These mechanisms are shown to be different in composites of micro- and nanoparticles. In composites of micrometer particles, the conventional macroscopic mechanism of phonon propagation is active. In composites with a grain size of a few nanometers, the main contribution comes from thermal resistance on grain boundaries.     

Nb掺杂TiO2压敏陶瓷的施主掺杂固溶度

研究了不同烧结温度TiO2压敏陶瓷的显微结构、施主掺杂固溶度和电学性能。采用SEM和EDS测试了其显微结构和晶粒化学组成。随烧结温度增加,TiO2晶粒迅速长大,显微结构均匀致密,但过高的烧结温度将导致较多气孔。1350℃为比较合适的烧结温度,其晶粒大小为15μm左右。在施主掺杂浓度一定的条件下,施主掺杂离子Nb5+在TiO2晶粒中的固溶度、晶粒电子浓度和电导率随烧结温度上升而增加,晶粒电阻率随烧结温度上升而下降。以能谱中O为参考确定TiO2晶格数量计算得到的固溶度及其电学性能更符合实验测试结果。     

液相浸渍C/C复合材料反应生成TaC的形貌及其形成机制

采用Ta有机溶剂浸渍C/C复合材料,经固化、热处理制备C/C-TaC复合材料.研究发现:在2MPa浸渍压力下,Ta有机溶剂易于浸渍C/C复合材料和固化;1500℃热处理后,Ta有机溶剂全部转变为TaC,其尺寸细小,结晶度高,呈颗粒状或聚集成团簇均匀分布在热解炭层面上;1800和2000℃热处理后的TaC形貌与1500℃热处理后的相似,Tac颗粒无明显长大现象.Ta有机溶剂转化生成Tac的机理研究表明:热处理过程中,Ta有机溶剂先生成中间相的氧氟化钽,转变为Ta₂O₅后,再与C还原-化合生成TaC.     

不同自润滑相尺寸和分布位置对自润滑表面膜形成的影响

采用分步高能球磨、快速感应烧结制备Ni基耐高温自润滑涂层IS304(IS:Induction Sintering), 涂层中高温润滑相BaF₂/CaF₂尺寸仅为1mm左右、低温润滑相Ag尺寸小于2mm, 并且BaF₂/CaF₂主要分布于强化相Cr₂O₃中.摩擦实验中当温度为室温~250℃时,IS304涂层的摩擦系数较高, 由于润滑相尺寸的细化以及润滑相BaF₂/CaF₂分布位置的改变,使得摩擦副在高速瞬时接触中产生的瞬态温升可以有效地激活BaF₂/CaF₂的自润滑性能,最终在压力及摩擦力的作用下形成自润滑表面膜, 使得IS304涂层在280℃时即可具有较低的摩擦系数     

烧成温度对Al2O3/Ni复合材料的致密化、物相组成和显微结构的影响

首先采用非均相沉淀合成出Ni包裹Al₂O₃粉体,然后热压烧结包裹粉体制备了Al₂O₃/Ni复合材料。本文作者主要研究了不同烧成温度对复合材料致密化、物相组成和显微结构的影响。结果表明:在1400℃保温1h,烧结体获得了最大相对密度,而致密度随Ni含量的增加反而降低;高于1350℃时,除Al₂O₃和Ni相外,在烧结体的表层生成一种由Al,Ni,O,C四种元素组成的新相;随着温度的升高,包裹层的纳米Ni颗粒聚合、长大,并退缩至三角晶界处,在适当的烧结温度(1400℃),少数小的纳米Ni颗粒被卷入Al₂O₃晶内,大的位于三角晶界,当烧成温度为1450℃时,不仅观察到Al₂O₃/Ni界面存在空隙,也发现了Al₂O₃晶粒异常长大现象。     

原位Al2O3片晶/Ce-TZP和SrO·6Al2O3棒晶/Ce-TZP复合材料的制备与显微结构

通过在Ce-TZP基体中加入AlOOH及矿化剂TiO₂或反应剂SrCO₃制备了原位Al₂O₃片晶/Ce-TZP复合材料和原位SrO·6Al₂O₃棒晶/Ce-TZP复合材料。在烧结过程中TiO₂促进Al₂O₃晶粒发生显著的各向异性生长原位生成的片晶、Al₂O₃与SrCO₃发生反应,原位生成的高度各向异性的棒晶,它们在基体中分布均匀,具有较大的纵横比。烧结温度对片晶/棒晶的大小和含量有明显影响。通过在基体中原位形成片晶或棒晶,材料的力学性能有明显的改善。     

Interaction of Cu–Al–Ni shape memory alloys particles with molten In and In + Sn matrices

Metal matrix composites for high-damping application were produced by embedding soft metallic matrices (pure In, In–10 wt.% Sn and In + Sn eutectic alloys) in powders of Cu–Al–Ni shape memory alloys (SMAs). During the composite production, the shape memory alloy particles interact with the molten matrices giving place to Cu dissolution from the shape memory alloy particles to the matrices, grain boundary penetration, and formation of intermetallic compounds. Adhesion, wetting and interfacial reaction are crucial for the final composites properties. Preliminary results on microstructural investigations performed applying optical and electron microscopy are presented in this contribution. The influence of thermal treatments on the microstructure of one composite is also discussed.     

The role of metal–ceramic interfaces on the high temperature mechanical response of nanostructured nickel–yttria tetragonal zirconia polycrystals (Ni–YTZP)

High temperature creep of a metal–ceramic composite consisted of yttria tetragonal zirconia polycrystals (YTZP) with a certain amount of nickel grains (Ni–YTZP, 2.5% vol.) has been studied. The average grain size of YTZP grains was 0.20 μm, whereas that of the Ni grains was 50–70 nm. This work reports the mechanical response under creep of such composite materials. It is found that the activation energy for creep is significantly higher than that reported in similar systems with much larger nickel grains. This fact has been explained in terms of the chemical nature of the metal–ceramic interfaces.     

Structural and morphological investigations on DC-magnetron-sputtered nickel films deposited on Si (100)

Pure nickel thin films were deposited on Si (100) substrates under different conditions of sputtering using direct current magnetron sputtering from a nickel metal target. The different deposition parameters employed for this study are target power, argon gas pressure, substrate temperature and substrate-bias voltage. The films exhibited high density of void boundaries with reduction in <111> texture deposited under high argon gas pressures. At argon gas pressure of 5 mTorr and target power of 300 W, Ni deposition rate was ~40 nm/min. In addition, coalescence of grains accompanied with increase in the film texture was observed at high DC power. Ni films undergo morphological transition from continuous, dense void boundaries to microstructure free from voids as the substrate-bias voltage was increased from −10 to −90 V. Furthermore, as the substrate temperature was increased, the films revealed strong <111> fiber texture accompanied with near-equiaxed grain structure. Ni films deposited at 770 K showed the layer-by-layer film formation which lead to dense, continuous microstructure with increase in the grain size.     

Pore formation during oxidative annealing of Al2O3 −∶Fe and slowing of grain growth by precipitates and pores

High-density samples of polycrystalline Al₂O₃ doped with iron retain their high density when annealed at high temperature in a reducing atmosphere, but pores are formed, both inside grains and at grain boundaries, and the density decreases upon annealing in oxidizing atmospheres. An explanation for these effects is proposed. The presence of pores and second-phase particles of FeAl₂O₄ is found to affect the physical properties and to slow grain growth. Grain size is proportional to the number of second-phase particles to the power –1/3 indicating that the number of second-phase particles per grain remains constant during grain growth.     

The dispersion mechanism of TiB2 ceramic phase in molten aluminium and its alloys

Titanium diboride (TiB₂) ceramic particulates are dispersed in molten aluminium and its alloys for grain refining and for making cast metal–matrix composites. For producing cast MMC, the dispersion of the ceramic phase via in-situ aluminothermic reduction of K₂TiF₆ and KBF₄ flux mixture with molten aluminium and, via the addition of exogenously formed TiB₂ with the fluoride flux has been studied at 900°C. In this article, the aspects of interfacial energy that govern the dispersion and agglomeration of TiB₂ particulates are examined. The Gibbs-adsorption interface equation is particularly employed to define and to quantify the change in the surface energy as a function of the alloying element concentration and, consequently the effect of interfacial energy on the nucleation rate of TiB₂ formed via metallothermic reduction reaction and the size of the ceramic phase is also explained.     

Microstructural characteristics of alumina-based composite prepared by in situ reaction of alumina-silicon carbide-nickel system

 In situ reaction of nickel and silicon carbide has been attempted to prepare alumina-based composites containing some kinds of dispersed phases. The composites were fabricated by reducing and sintering of Al₂O₃/NiO/SiC mixtures. Reaction products (Ni₃Si and C) and metallic Ni were found to disperse at the matrix grain boundaries, while Ni was partly trapped into Al₂O₃ grains. In addition, carbon nanoballs encapsulating Ni₃Si were produced and dispersed in the composites. The carbon cages were approximately 80–100 nm in diameter with polyhedral shape, and had lattice spacing of 0.35 nm that was typical for the graphite. Encapsulated Ni₃Si had facet planes which were parallel to the carbon layers surrounding. Production of metal encapsulated carbon nanoball within ceramic materials is the first successive result that might promote researches on such novel ceramic composites. Received: 2 January 1997 / Accepted: 15 March 1997     

Magnetic properties and microstructure of FePt–SiO2 nanocomposite films fabricated by nanocluster beam technology

Nanocluster beam technology combined with conventional sputtering was used to fabricate FePt–SiO₂ nanocomposite films in this present work. The post-deposition annealing affected the final particle size and size distribution of FePt nanoclusters. The effects of both volume fraction of SiO₂ matrix and annealing temperature on magnetic and microstructural properties were studied. Partial ordering, grain growth and agglomeration of FePt particles in FePt–SiO₂ nanocomposite films occurred during annealing. A higher volume fraction of SiO₂ matrix was effective in suppressing diffusion of atoms and magnetic exchange coupling of FePt grains. Excessive SiO₂ however resulted in a lower degree of FePt ordering and thus lower coercivity.     

Microstructural development during thermomechanical processing of particulate metal-matrix composites

Abstract

The development of microstructure during the deformation and annealing of discontinuously reinforced metal-matrix composites is discussed with particular reference to composites having a pure aluminium matrix. The deformation inhomogeneities induced by the ceramic particles are examined, and the role of the particles in the nucleation and growth of recrystallised grains during subsequent heat treatment is considered. The effect of the ceramic particles on the microstructures developed during multipass hot rolling is shown to be complex, depending on particle size and volume fraction. Superplasticity is shown to occur by several mechanisms in particulate composites. The effect of deformation processing on the ceramic is discussed, with emphasis on the fracture and realignment of ceramic particles, platelets, and whiskers.

MST/1299     

Compressive fracture processes in an alumina-glass composite

At atmospheric pressure, a commercial 85% alumina fails by granulation, breaking into small flakes and splinters with long dimensions parallel to the load axis. Replica transmission electron microscopy examination of polished surfaces on specimens loaded to 89% (or more) of the fracture stress reveals the formation of microcracks at crystal-glass interfaces subparallel to the load axis. Scanning microscopy of granulated samples indicates that pores in the material either initiate or channel the cracks which dissect the ceramic into small particles during granulation failures. Thus, the interpore spacing seems to be an important factor in the particle size distribution of the broken ceramic. In the faulting mode of failure, which occurs under confining pressure, the sliding faces of the ceramic are slickensided. However, most of the loose fracture particles do not display slickensides; in size and shape they resemble the powder produced by granulation failures. True fault gouge material, consisting of comminuted particles mashed together, tends to adhere to the fault faces. The gouge has a broad particle size distribution, including particles much finer than the interpore spacing of the ceramic. At the highest loading rate (0.81 sec^–1 ) under atmospheric pressure, the particles produced by granulation fracture are slickensided and show evidence of either localized melting or plastic grooving induced by friction between adjacent particles.