CeO2 induced dispersive distribution of TiB2 particles in in situ TiB2/Al composite

Abstract

The roles of CeO₂ additive during preparation of in situ TiB₂/Al composite, alleviating particle settlement in composite melt and significantly improving particle dispersion in final microstructure, are studied in this paper. It is evidenced that the CeO₂ additive reacts with Al melts to release Ce solute into the melts, and the released surface active Ce is absorbed in the Al/TiB₂ interfaces without any other reaction products. First principles calculations show that the interfacial energy of Al/TiB₂ interfaces is reduced owing to the presence of Ce in Al/TiB₂ interfacial area. Therefore, the wettability of molten Al on TiB₂ surface is increased and the dispersion of TiB₂ particles in Al matrix is eventually improved.     

Nucleation behaviour of diamond particles on silicon substrates in a hot-filament chemical vapour deposition

Diamond films and particles have been deposited on a silicon substrate using a hot-filament chemical vapour deposition (CVD) method in order to study the effect of hydrogen on the behaviour of diamond nucleation. The nucleation density of diamond was affected by both hydrogen treatment prior to deposition and filament temperature,T _f. The nucleation density was decreased markedly with increasing hydrogen-treatment time. The nucleation density also changed with increasingT _f, which increased initially and then reached a maximum at 2100°C and decreased thereafter. Etching of the substrate surface was observed and enhanced with both increasing hydrogen-treatment time and increasingT _f. The changes in nucleation behaviour were related closely to the etching of substrate surface. These results are explained in terms of the etching of nucleation sites.     

Fracture behaviour of a TiB2-based ceramic composite material

The fracture behaviour of a multicomponent TiB₂/BN/AlN material was studied in both the double cantilever beam (DCB) and single-edge-notched beam (SENB) geometries. Crack growth in the SENB geometry at various stages of the test was monitored using optical and electron microscopes. R-curve behaviour was found in both specimen geometries for this material. Evidence of crack-interface tractions were found in the form of (i) ligamentary bridges and (ii) frictional interlocks, along with evidence for microcracking about the crack tip. Measurements of crack-opening displacement with distance back from the crack tip were made. The inflection point of such data is in agreement with the range of the measuredR-curve.     

Nucleation behaviour and microstructure of single Al-Si12 powder particles rapidly solidified in a fast scanning calorimeter

The understanding of rapid solidification behaviour, e.g. the undercooling versus growth velocity relationship, is crucial for tailoring microstructures and properties in metal alloys. In most rapid solidification processes, such as additive manufacturing (AM), in situ investigation of rapid solidification behaviour is missing because of the lack of accurate measurement of the cooling rate and nucleation undercooling. In the present study, rapid solidification of single micro-sized Al-Si12 (mass%) particles of various diameters has been investigated via differential fast scanning calorimetry employing controllable cooling rates from 100 to 90,000 K s^?1 relevant for AM. Based on nucleation undercooling and on microstructure analysis of rapidly solidified single powder particles under controlled cooling rates, two different heterogeneous nucleation mechanisms of the primary α-Al phase are proposed. Surface heterogeneous nucleation dominates for particles with diameter smaller than 23 μm. For particles with diameter larger than 23 μm, the nucleation of the primary α-Al phase changes from surface to bulk heterogeneous nucleation with increasing cooling rate. The results indicate that at large undercoolings (>?95 K) and high cooling rates (>?10,000 K s^?1), rapid solidification of single particle can yield a microstructure similar to that formed in AM. The present work not only proposes new insight into rapid solidification processes, but also provides a theoretical foundation for further understanding of microstructures and properties in additively manufactured materials.

     

Microstructure and mechanical behavior of AlSiCuMgNi piston alloys reinforced with TiB2 particles

AlSiCuMgNi piston composites reinforced with in-situ TiB₂ particles were fabricated by mixing salts reaction process successfully. Microstructures of the composites were observed by mean of scanning electron microscope (SEM) and transmission electron microscope (TEM). X-ray diffraction (XRD) was used to identify the phases in the composites. TiB₂ reinforcement grows in equiaxed or near equiaxed shape and the interfaces between reinforcements and matrix are clear. Compared with the matrix alloys, the composites show an obvious aging peak and an incubation time in the hardness. The aging is accelerated in the composites reinforced with TiB₂. At room temperature, the ultimate tensile strength (UTS) of the composites increases as the percentage of TiB₂ reinforcement increases. When the temperature is beyond 250°C, the ultimate tensile strength of the piston composites decreases sharply. The fracture surfaces of the piston composites are analyzed.     

Property Characteristics of a TiB2P/AI Composite Fabricated by Squeeze Casting Technology

TiB2P/Al composite was successfully fabricated by squeeze casting technology. Its mechanical and tribological properties were evaluated. The elimination of Ti-Al intermetallic compound was confirmed by X-ray diffraction (XRD) studies. At 45% volume fraction, the bending strength at ambient temperature was 934 MPa. And the fracture modes included ductile failure of Al matrix and brittle fracture of TiB2 particles. In dry sliding wear mode, severe plastic deformation and adhesive wear were found on the worn surfaces of the SiCP/Alcomposite. But no obvious characteristics of adhesion or abrasion wear were observed on that of the TiB2P/Al composites. At the steady stage, the friction coefficient of the SiCP/Al composite was about 0.6. While that of TiB2P/Al composite was only about 0.16～0.17.     

Wear behaviour of aluminium matrix TiB2 composite prepared by in situ processing

Abstract

The wear behaviour and microstructure of aluminium and Al-12Si alloy (A413) matrix composites containing 1 and 5 vol.-%TiB₂ particles have been investigated. The composites were prepared by an in situ reactive slag technique. The wear surfaces and wear products were studied after reciprocating and rolling - sliding tests. Wear resistance increased with increasing particle content, and the Al-12Si composites were more wear resistant than those with Al matrixes. The wear mechanisms are briefly discussed.     

The effect of vanadium additions on the precipitation behaviour of Ni-Co-Cr-Nb alloys

Four Ni-Co-Cr-Nb-V alloys containing varying amounts of vanadium were solution treated and then aged at 923, 1023 and 1123 K for various lengths of time. The structure of the solution treated alloys was single phase f c c. The presence of 1 wt% vanadium was adequate to promote the formation of metastable phase with a DO₂₂ type of ordered b c t structure. An increase in ageing time and temperature, and a decrease in the vanadium/niobium ratio caused an increased amount of stable phase, with a Cu₃Ti-type orthorhombic structure, to precipitate. The precipitation of the metastable phase has been explained in terms of the Engel-Brewer correlation.     

Nucleation behaviour of poly-3-hydroxy-butyrate

Poly-3-hydroxy-butyrate (PHB) is a thermoplastic polyester produced by bacterial fermentation. Because of this bacterial origin PHB is a very pure polymer. This high purity in turn leads to very few (if any) heterogeneous nuclei, which gives a much wider scope for a systematic study of nucleation behaviour and the effect of nucleating agents than was possible before. It is shown that in pure PHB nucleation is sporadic. The nucleation rate may be measured over a temperature range of some 100° C. The nucleation rate data are recorded in the range where homogeneous nucleation is possible and are at least consistent with it. When foreign particles are added the nucleation rate is modified. Two distinct types of behaviour are observed. One may be interpreted quantitatively as the local raising of the crystal melting point due to the constraints of the actual presence of a surface; and the other as being due to epitaxial growth on the foreign surface. Detailed kinetic data are presented to support these conclusions.     

Thermodilatometric behaviour of pure and doped ZrTiO4-SnO2

Thermodilatometric tests have been performed to investigate the shrinkage of green compacts of pure and ZnO-doped 4ZrO₂-5TiO₂-SnO₂ prepared by dry ball milling, flo-deflocculation and coprecipitation. Experiments have shown that the powder preparation procedure has a significant influence on the sintering process. Optimizing the homogeneity composition, which is enhanced from dry ball milling to coprecipitation, raises the starting sintering temperature. The reduction of the dimension of the starting particles increases the sintering rate and the addition of ZnO favours the shrinkage of the green bodies. Coprecipitated products lead to the highest final density because the evaporation of tin oxide on firing is reduced.     

Sintering behaviour of Ti-2Ni and Ti-5Ni elemental powders

The effect of nickel powder additions (2 and 5 at.%) on the sintering behaviour of titanium powder has been investigated through the dilatometric sintering study. The sintering of titanium powder was found to be enhanced significantly and the activation energies were decreased with increasing nickel content. The sintering of Ti-2 at.% Ni system at lower temperature was found to be controlled by Ti-Ti and Ti-Ni solid state sintering, whereas inter-diffusion dominates in Ti-5 at.% Ni sintering. At higher temperatures, sintering was found to be controlled by mixed mechanisms, i.e. inter-diffusion, chemical reaction and transient liquid phase formation.     

Improvements in mechanical properties of TiB2 by the dispersion of B4C particles

Densities up to 99% of the theoretical value were achieved by hot-pressing of TiB₂-B₄C composites at 1700° C for 1 h using 1 vol % Fe as a sintering aid. The microstructure consists of dispersed B₄C particles in a fine-grained TiB₂ matrix. Addition of B₄C particles increases the fracture toughness of TiB₂ (to 7.6 MPa m^1/2 at 20 vol % B₄C) and yields high fracture strength (to 700 MPa at 10 vol % B₄C). Microstructural observations indicate that the improved strength is a result of a higher density, smaller grain size and intergranular fracture, and the toughness increase is a result of crack deflection around the B₄C particles.     

Sliding wear behaviour of Al 6063/TiB2 in situ composites at elevated temperatures

A low cost system of Al 6063 ? xTiB₂ (x = 0, 5, 10 wt.%) in situ metal matrix composites (MMCs) were prepared by the reaction mixture of K₂TiF₆ and KBF₄ with molten alloy. These in situ prepared composites were characterized by using scanning electron microscope, X-ray diffractometer, and microhardness analysis. The dry sliding wear behaviour of the prepared composite was investigated by using a Pin on Disc method at different applied loads of 9.8, 19.6 and 29.4 N for various temperatures (100, 200 and 300 °C). The study at room temperature was also carried out for comparison purpose. The results indicate that the wear rate decreases with the increase in the weight percentage of TiB₂, while it increases with the increase in the applied load.     

Nucleation of polystyrene latex particles in the presence of gamma-methacryloxypropyltrimethoxysilane: functionalized silica particles

Silica/polystyrene nanocomposite particles with different morphologies were synthesized through emulsion polymerization of styrene in the presence of silica particles previously modified by gamma-methacryloxypropyltrimethoxysilane (MPS). Grafting of the silane molecule was performed by direct addition of MPS to the aqueous silica suspension in the presence of an anionic surfactant under basic conditions. The MPS grafting density on the silica surface was determined using the depletion method and plotted against the initial MPS concentration. The influence of the MPS grafting density, the silica particles size and concentration and the nature of the surfactant on the polymerization kinetics and the particles morphology was investigated. When the polymerization was performed in the presence of an anionic surfactant, transmission electron microscopy images showed the formation of polymer spheres around silica for MPS grafting densities lower than typically 1 micromole x m(-2) while the conversion versus time curves indicated a strong acceleration effect under such conditions. In contrast, polymerizations performed in the presence of a larger amount of MPS moieties or in the presence of a non ionic emulsifier resulted in the formation of "excentered" core-shell morphologies and lower polymerization rates. The paper identifies the parameters that allow to control particles morphology and polymerization kinetics and describes the mechanism of formation of the nanocomposite colloids.     

Preparation of Fe3AI based iron aluminides from elemental and prealloyed povvders

Abstract

Iron aluminides were prepared by a powder metallurgy process from elemental powders, mixtures of prealloyed and elemental powders, and prealloyed powder. The sintering behaviour of various powders was studied using scanning electron microscopy, optical microscopy, and density measurement. It was found that sintering of elemental powder involved two distinct processes, i.e. alloying and densification, but sintering of prealloyed powder involved densification alone. The addition of prealloyed powder to elemental powders was helpful in restraining the swelling of sintered samples, the degree of swelling of sintered samples being reduced as the amount of prealloyed powder increased. For samples made from Fe-25 at.-%Al prealloyed powder, remarkable shrinkage was measured after sintering at 1250°C for 1 h. Within the correct range, their density increased with sintering temperature and time, but prolonged sintering at high temperature resulted in the loss of aluminium and a two phase microstructure. The difference in sintering behaviour between the various powders was discussed on the basis of thermodynamics.