Characterization of Al2O3 composites with Mo particulates,II. Densification and mechanical properties

Three processes were employed to mix MoO₃ or Mo powder with submicrometric Al₂O₃ powder in the preparation of the Mo/Al₂O₃ precursors. The first two processes employed a dissolution route which dissolved MoO₃ powder in ammonia solution, followed by spray-drying or hot-plate-drying to obtain ammonium molybdate/Al₂O₃ granules. Then, the granulated powders were reduced in H₂ atmosphere and hot-pressed to obtain Mo/Al₂O₃ composites. The third process involved mechanically mixing metallic Mo particles with Al₂O₃. The composites normally can achieve 99% relative density after hot-pressing. Microstructural results of hot-pressed composites reveal that intergranular Mo grains of micrometer size dispersed at the grain boundaries of dense Al₂O₃ matrix, and some fine grain Mo grains of nanometric size were entrapped within the Al₂O₃ grains in the composite. The four-point bending strength and the toughness of the dense composites increases 26% and 32% respectively, with the addition of Mo content, but are controlled by the uniformity of the microstructure of the composites. The hardness of the composites basically obeys the prediction of the rule of mixture. The strengthening and toughening mechanisms of the composites with respect to the microstructure are analyzed.     

Effect of Al2O3 particulates as reinforcement in age hardenable aluminium alloy composites

Abstract

This paper investigates the behaviour of aluminium based composites, reinforced with aluminium oxide particulates, manufactured using a powder processing route and various sintering times. It is found that a softening effect, compared with the monolithic material, coincides with migration of magnesium from the matrix to the matrix/reinforcement interface and to the specimen surface. The addition of 5 wt-%Si to the matrix alloy results in a retention of hardness in the specimens given short sinter times (0.5 and 1 h). This coincides with reduced migration of magnesium from the matrix. It is postulated that the excess silicon in the matrix results in sufficient magnesium being retained in the matrix during short sinter times to take part in the formation of Mg₂Si precipitates necessary for the hardening of the Al alloy 6061 matrix.     

Al2Mo3O12/polyethylene composites with reduced coefficient of thermal expansion

Recently, polymer composites reinforced with low fractions of thermomiotic nanoceramics have triggered a lot of research. The efforts have been focused on achieving considerable reduction of the coefficient of thermal expansion (CTE) of polymeric materials without deterioration of other physical properties. In this context, polyethylene (PE) composites reinforced with different loads of Al₂Mo₃O₁₂ nanofillers (0.5–4 mass %) were fabricated by micro-compounding. To enhance the interfacial interaction between the two components, chemical functionalization of Al₂Mo₃O₁₂ was performed with vinyltrimethoxysilane (VTMS) prior to micro-compounding. Infrared spectroscopy and thermogravimetry demonstrated the successful grafting of VTMS on the Al₂Mo₃O₁₂ surface. The composites showed strongly decreased CTEs, up to 46 % reduction for loadings of 4 mass % compared with neat PE, suggesting intimate filler–matrix interactions. The variation of CTEs of the composites in terms of the filler fraction was successfully described by Turner’s model allowing calculation of the bulk modulus of monoclinic Al₂Mo₃O₁₂ (13.6 ± 2.6 GPa), in agreement with the value obtained by an ultrasonic method. The thermal stability of the composites was improved, although the addition of functionalized fillers decreased the degree of crystallinity of the PE to a small extent. The Young’s modulus and yield strength of the composites increased from 6.6 to 19.1 % and 4.0–6.0 %, respectively, supporting the existence of strong filler–matrix interactions, contributing to an efficient load transfer. Finite element analysis of thermal stresses indicated absence of plastic deformation of the matrix or fracture of the nanofillers, for a 100 K temperature drop.     

The effect of Al2O3 particulates on the precipitation behaviour of 6061 aluminium-matrix composites

The effect of angular and spherical shaped Al₂O₃ particulates on the precipitation behaviour of 6061 aluminium-matrix composites has been studied using microhardness testing, differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). The evolution of precipitates at each stage of precipitation corresponding to the DSC-peak was monitored through careful TEM observations. Both the formation and dissolution behaviour of the precipitates in the ageing process in the composites were compared with the unreinforced 6061 matrix alloy. Although an overall accelerated ageing response in the composites was reported, not all stages in the ageing process were influenced by the addition of particulates. However, the precipitation sequence was not altered by the presence of the particulates. The degree of acceleration and the relative proportion of the phase/phases was found to depend on reinforcement parameters such as size, shape and volume fraction. In the composites, higher dislocation densities were observed in the immediate vicinity of the ceramic particles which may have formed due to the large difference in the coefficients of thermal expansion (CTE) between the ceramic particles and the matrix. These CTE-dislocation effects influence the kinetics of precipitation.     

Characterization of hot pressed Al2O3-platelet reinforced hydroxyapatite composites

Hydroxyapatite reinforced with monocrystalline Al₂O₃ platelets was densified by hot pressing. The effect of volume fraction and size of platelets on the microstructure, strength and toughness was investigated. It was demonstrated that no phase degradation occured during thermal treatments. A better homogeneity of composite mixtures was achieved when large platelets had been used. In return, the incorporation of small platelets appeared more favourable to increase the mechanical characteristics although limiting effect induced by microstructural defects. The flexural strength can reach 140 MPa with an associated fracture toughness of 2.5 MPa % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaOaaaeaaca% qGTbaaleqaaaaa!36F8!\[\sqrt {\text{m}} \] compared to 137 MPa and 1.2 MPa % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaOaaaeaaca% qGTbaaleqaaaaa!36F8!\[\sqrt {\text{m}} \] for dense monolithic HAP. The observation of crack propagation allowed us to point out the mechanisms responsible of this toughening. Crack deflection on disk faces should be considered as the initiating phenomenon leading to platelet debonding, crack bridging or branching. Crack deflection and branching may also result in the formation of unbroken ligaments of material which bridge the crack.     

Microstructural development and mechanical properties of pressureless-sintered SiC with plate-like grains using Al2O3-Y2O3 additives

Dense SiC ceramics with plate-like grains were obtained by pressureless sintering using -SiC powder with the addition of 6 wt% Al₂O₃ and 4 wt% Y₂O₃. The relationships between sintering conditions, microstructural development, and mechanical properties for the obtained ceramics were established. During sintering of the -SiC powder compact the equiaxed grain structure gradually changed into the plate-like grain structure that is closely entangled and linked together through the grain growth associated with the phase transformation. With increasing holding time, the fraction of phase transformation, the grain size, and the aspect ratio of grains, increased. Fracture toughness increased from 4.5 MPa m^1/2 to 8.3 MPa m^1/2 with increasing size and aspect ratio of the grains. Crack deflection and crack bridging were considered to be the main operative mechanisms that led to improved fracture toughness.     

Microstructural evolution during liquid-phase sintering of high-speed steel-based composites containing TiN-coated Al2O3, TiC,or Al2O3 particles: influence on wear properties

A high-speed steel containing 10 vol% TiN-coated Al₂O₃, Al₂O₃, or TiC particles was liquid-phase sintered with the addition of copper-phosphorus. The mixtures were sintered in a furnace with a controlled axial temperature gradient, and their microstructural evolution during sintering was studied using quantitative image analysis. Additional samples were fully densified and tested using a pin-on-disc tribometre. Interfaces were characterized by transmission electron microscopy. The final microstructure depended on the behaviour of the liquid phases surrounding the ceramic particles during sintering, which was different for the three types of particles used, and influenced the wear resistance.     

Characterization of domain configurations in LiTaO3/Al2O3 ceramic composites

Characterization of domain configurations in 15 vol% LiTaO₃/Al₂O₃ ceramic composites hot-pressed sintered at 1300 °C was carried out using transmission electron microscopy and high-resolution transmission electron microscopy. Mainly banded- and wedge-shaped 90° domains, with few 180° domains, were formed in the LiTaO₃ grains within the composite. The formation of 90° domains was attributed to the large stress caused by residual polarization. Many dark dots, distributed along the 90° domain boundaries, were detected and it was confirmed that they were not a deposited second phase but a type of contrast induced by high strain. The 90° domain boundaries presented an α-type fringe contrast and no δ-type fringes were observed, indicating that they were of the non-conventional type.     

Slip casting of Al2O3 and Al2O3/ZrO2 composites

Al₂O₃ and Al₂O₃/ZrO₂ composites have been fabricated by slip casting from aqueous suspensions. The physical and structural characteristics of the starting powders, composition of the suspensions, casting behaviour, microstructure of the green and fired bodies and the mechanical properties of the products were investigated. The addition of ZrO₂ to Al₂O₃ leads to a significant increase in fracture toughness when ZrO₂ particles are retained in the tetragonal form (transformation-toughening mechanism) but when microcracking (due to the spontaneous transformation of ZrO₂ from the tetragonal phase to the monoclinic one) is dominant, an excellent toughness value is accompanied by a drastic drop in strength and hardness.     

Microstructural evolution and mechanical properties of SiC/Al2O3 particulate-reinforced spray-deposited metal-matrix composites

In this study, aluminium-copper-based metal-matrix composites were synthesized utilizing the spray atomization and co-deposition technique. Microstructural characterization studies were carried out with an emphasis on understanding the effects associated with the co-injection of silicon carbide and aluminium oxide particulates. The results demonstrate the ageing kinetics of the spray-deposited and hot-extruded metal-matrix composites to be the same as those of the monolithic aluminium-copper material. Results of ambient temperature mechanical tests demonstrate that the presence of particulate reinforcement in the metal matrix does little to improve strength, and degrades the ductility of the matrix material. A model is formulated to compute the critical volume fraction of reinforcement. The results obtained using this model suggest that an optimum volume fraction of silicon carbide is essential in order to realize a strength improvement in the metal-matrix composite, relative to their monolithic counterpart.     

Fabrication of Al/Al2O3 composites and FGM

Al/Al2O3 composites of different ratios were hot-press sintered at 575 similar to 640℃ under a pressure of 30 MPa for 2 h in a vacuum furnace. It was found that the relative density of the Al/Al2O3 composites could be increased evidently with the rise of sinter temperature. No reaction occurred between Al and Al2O3 at the sinter temperatures. Under 640℃-30 MPa-2 h experimental condition, Al/Al2O3 system FGM was successfully fabricated, and its density range changed quasi-continuously from 2.887x10(3) kg/m(3) to 3.1909x10(3) kg/m3 within the middle 1.0 mm thickness range.     

CuAl_2O_3复合材料中Al_2O_3的长大动力学分析

通过分析铜铝颗粒表面铝与氧的反应,计算铜铝合金中铝的沉淀析出量、铝在铜基体中的扩散速率以及Al2O3颗粒大小,研究了喷射沉积内氧化法制备CuAl2O3复合材料中Al2O3的长大动力学行为。结果表明,在反应合成制备过程中,氧化铝颗粒长大动力学行为满足抛物线规律;铜铝合金表面铝与氧的反应是一种铝扩散控制型反应,该氧化铝颗粒的长大与铝在铜铝合金中的含量、扩散速率和所处位置(晶内、晶界)有关。计算得到的Al2O3颗粒大小与实际获得的氧化铝颗粒大小相吻合。