Relationship between microstructure,properties and reaction conditions for Cu–TiB2 alloys prepared by in situ reaction

Three Cu–TiB₂ alloys (0.45, 1.6, 2.5 wt.% TiB₂) have been prepared by a combination of in situ reaction and rapid solidification under optimized conditions. The relationship between microstructure, properties and in situ reaction condition for these Cu–TiB₂ alloys was investigated and analyzed by modeling. It is shown that the distribution and size of TiB₂ particles are strongly dependent on the choice of in situ reaction conditions and solute concentration; specifically, the size and aggregation level of TiB₂ particles tend to increase as the volume per cent of TiB₂ in the three Cu–TiB₂ alloys increases when the same in situ reaction conditions are used. The forming, coarsening and aggregation models of TiB₂ particles are established. The mechanical properties of three Cu–TiB₂ alloys are directly related to the distribution and size of TiB₂ particles, the change in the grain size and the residual solute concentration. A composite model reveals the separate contributions of these parameters to the mechanical properties.     

Microstructure and properties of Fe–Cr–C hardfacing alloys reinforced with TiC–TiB2

Abstract

Different amounts of TiB₂ powder were added to flux cores of wear resistant hardfacing flux cored wires for the preparation of new flux cored wires. Fe–Cr–C hardfacing alloys reinforced with TiB₂ were produced by arc hardfacing. The microstructure, hardness and wear resistance behaviour of the hardfacing alloys were investigated using an optical micrograph, scanning electron micrograph (SEM), X-ray diffractometer, macrohardness tester, microhardness tester and abrasive wear tester. The results showed that, among the hardfacing alloys, a new hard phase, i.e. TiC–TiB₂ composite compound particles, was formed and dispersed in the primary carbides and matrix structures. The TiC–TiB₂ reinforced Fe–Cr–C hardfacing alloys imparted greater hardness and better wear resistance. The presence of TiC–TiB₂ hard phase particles is the main reason for the improvement in hardness and wear resistance of Fe–Cr–C hardfacing alloys.     

冷却速率对含TiB2的TiAl合金凝固组织和力学性能的影响

通过熔模精密铸造制备不同厚度的Ti-48Al-2Cr-2Nb和Ti-48Al-2Cr-2Nb-0.25TiB2合金铸板,研究冷却速率和TiB2添加对合金凝固组织和力学性能的影响.实验结果表明,当凝固速率从37增加至2×102 K/s时,合金的凝固路径并未发生改变.基体合金的晶粒从650细化至300μm,Ti-48Al-...     

Effects of trace TiB2 on microstructure in cast titanium alloys

Abstract

Effects of trace TiB₂ on solidification microstructure of Ti–6Al–4V alloy were investigated. The result shows that the microstructure of the ingot was refined by the added TiB₂. The grain size and α lath size were reduced gradually with the increase in TiB₂. The grain size is reduced by about an order of magnitude with an addition of 0·96 wt-%TiB₂, and the dendrite appears in the ingot. Such a tendency is similar to the change of the microstructure with boron addition. Ti–6Al–4V–B phase diagram and growth restriction factor Q are applied to analyse the influence mechanism of trace TiB₂ on the microstructure.     

A mechanism for the poisoning effect of silicon on the grain refinement of Al–Si alloys

The poisoning effect of excess Si solute on the grain-refining potency of Al–Ti–B grain refiners in Al–Si casting alloys has been studied in a crystallographic investigation. The edge-to-edge matching model was used for investigating and comparing the possible poisoning effects of several binary and ternary intermetallic compounds containing Si and Ti. The results show that the poisoning effect is probably due to the formation of a Ti₅Si₃ coating on the surface of TiAl₃, because the Ti₅Si₃ phase has a much better crystallographic matching with TiAl₃ than it does with the Al matrix. However, TiB₂ particles appear to survive because an excessively large misfit prevents the Ti₅Si₃ phase from forming on the surface of TiB₂. The implications of this proposed mechanism are discussed in the light of current practical casting solutions and the continuing debate on the grain refinement mechanism.     

Kinetic behaviour of TiB2 particles in Al melt and their effect on grain refinement of aluminium alloys

Solidification experiments were carried out to investigate the kinetic behaviour of TiB₂ particles in Al melt and their effect on the grain refinement of commercially-pure Al. A model was proposed to describe the kinetic behaviour of TiB₂ particles during the whole process from the addition of TiB₂ to the melt to the freezing of the melt. The results indicate that TiB₂ particles are not stable in Al melt. They may dissolve and coarsen during the holding period and grow during the cooling period of the melt. The kinetic behaviour of TiB₂ particles in the melt has a great influence on their number density and the grain refinement. Solute Ti addition can suppress the dissolution, Ostwald ripening and growth behaviours of TiB₂ particles.     

High-temperature mechanical properties of aluminium alloys reinforced with titanium diboride （TiB2） particles

The physical and mechanical properties of metal matrix composites were improved by the addition of reinforcements.The mechanical properties of particulate-reinforced metal-matrix composites based on aluminium alloys (6061 and 7015) at high temperatures were studied.Titanium diboride (TiB2) particles were used as the reinforcement.All the composites were produced by hot extrusion.The tensile properties and fracture characteristics of these materials were investigated at room temperature and at high temperatures to determine their ultimate strength and strain to failure.The fracture surface was analysed by scanning electron microscopy.TiB2 particles provide high stability of the aluminium alloys (6061 and 7015) in the fabrication process.An improvement in the mechanical behaviour was achieved by adding TiB2 particles as reinforcement in both the aluminium alloys.Adding TiB2 particles reduces the ductility of the aluminium alloys but does not change the microscopic mode of failure,and the fracture surface exhibits a ductile appearance with dimples formed by coalescence.     

Microstructures and mechanical properties of in-situ TiB2/Al−xSi−0.3Mg composites

In-situ 2 vol.% TiB₂ particle reinforced Al?xSi?0.3Mg (x=7, 9, 12, 15 wt.%) composites were prepared by the salt–metal reaction, and the microstructures and mechanical properties were investigated. The results show that the TiB₂ particles with a diameter of 20–80 nm and the eutectic Si with a length of 1–10 μm are the main strengthening phases in the TiB₂/Al?xSi?0.3Mg composites. The TiB₂ particles promote grain refinement and modify the eutectic Si from needle-like to short-rod shape. However, the strengthening effect of TiB₂ particles is weakened as the Si content exceeds the eutectic composition, which can be attributed to the formation of large and irregular primary Si. The axial tensile test results and fractography observations indicate that these composites show more brittle fracture characteristics than the corresponding alloy matrixes.     

Influence of TiB2 nanoparticles on elevated-temperature properties of Al-Mn-Mg 3004 alloy

Two contents (1.5% and 3%) of TiB₂ nanoparticles were introduced in Al-Mn-Mg 3004 alloy to study their effects on the elevated-temperature properties. Results show that TiB₂ nanoparticles were mainly distributed at the interdendritic grain boundaries with a size range of 20–80 nm, which is confirmed by transmission electron microscopy (TEM) and X-ray diffraction (XRD). Therefore, the volume fraction of the dispersoid free zones is greatly reduced and the motion of grain boundaries and dislocations is inhibited more effectively at elevated temperature. After peak precipitation heat treatment, the yield strengths in the alloy with 3% TiB₂ addition at room temperature and 300 °C were increased by 20% and 13% respectively, while the minimum creep rate at 300 °C was reduced to only 1/5 of the base alloy free of TiB₂, exhibiting a considerable improvement of elevated-temperature properties in Al-Mn-Mg alloys.     

A novel Al–Ti–B alloy for grain refining Al–Si foundry alloys

Al–Ti–B refiners with excess-Ti (Ti:B > 2.2) perform adequately for wrought aluminium alloys but they are not as efficient in the case of foundry alloys. Silicon, which is abundant in the latter, forms silicides with Ti and severely impairs the potency of TiB₂ and Al₃Ti particles. Hence, Al–Ti–B alloys with excess-B (Ti:B < 2.2) and binary Al–B alloys are favored to grain refine hypoeutectic Al–Si alloys. These grain refiners rely on the insoluble (Al,Ti)B₂ or AlB₂ particles for grain refinement, and thus do not enjoy the growth restriction provided by solute Ti. It would be very attractive to produce excess-B Al–Ti–B alloys which additionally contain Al₃Ti particles to maximize their grain refining efficiency for aluminium foundry alloys. A powder metallurgy process was employed to produce an experimental Al–3Ti–3B grain refiner which contains both the insoluble AlB₂ and the soluble Al₃Ti particles. Inoculation of a hypoeutectic Al–Si foundry alloy with this grain refiner has produced a fine equiaxed grain structure across the entire section of the test sample which was more or less retained for holding times up to 15 min.     

Production of in situ aluminum-titanium diboride master alloy formed by slag-metal reaction

Al-TiB₂ master alloys have received much attention in recent years owing to their potential as efficient grain refiners for aluminum foundry alloys. In this study, the process of production of master alloys was investigated to develop a low cost method, namely, slag-metal reaction. This method can be used to fabricate Al-TiB₂ master alloy in situ from the TiO₂-H₃BO₃-Na₃AlF₆ and Al system. Since the price of the raw materials is low and the technology is simple, the processing technique appears to reduce the cost of the master alloy. Because of exothermic reactions, not much energy is needed to melt materials. In this process, Titanium diboride particles were formed in situ through the reactions of TiO₂, H₃BO₃ and Na₃AlF₆. Results showed that when the aluminum melted, the condensed TiB₂ particles that formed in situ were spherical with an average diameter of 1 μm. Furthermore, these TiB₂ particles were distributed uniformly through the master alloy.     

Effects of nano-sized TiB2 particles and Al3Zr dispersoids on microstructure and mechanical properties of Al–Zn–Mg–Cu based materials

The effects of TiB₂ and Zr on the microstructure, aging response and mechanical properties of hot-extruded Al–Zn–Mg–Cu based materials were investigated and compared by multi-scale microstructure characterization techniques. The results showed that proper addition of TiB₂ particles could refine grain size during solidification, promote dynamic recrystallization during extrusion, and inhibit grain growth during solution treatment. Meanwhile, Zr addition had minor influence on the grain refinement during solidification, but could effectively suppress recrystallization and grain growth compared with the Zr-free alloy. Furthermore, the TiB₂ addition could simultaneously enhance the aging kinetics and peak-aged hardness of the materials. Comparatively, Zr addition could also improve the peak-aged hardness with minor effect on the aging kinetics of the materials. Finally, the quench sensitivity, elastic modulus and tensile properties of the materials were compared and studied. Specifically, the relationship between the microstructure and mechanical properties, and the strengthening mechanisms were discussed in detail.     

A novel fading-resistant Al-3Ti-3B grain refiner for Al-Si alloys

Al-Ti-B refiners with excess-Ti perform adequately for wrought aluminum alloys but inefficiently in the case of foundry alloys. The high content of silicon in the latter, which forms silicides with Ti and severely impairs the refining potency of the nuclei, is known to be responsible for the poor performance. Hence, new grain refiners, such as Al-3B and Al-3Ti-3B master alloys with excess-B have been developed with well documented advantages for Al-Si alloys. It is very desirable to involve TiAl₃ particles in the Al-3Ti-3B master alloy to maximize its grain refining efficiency. However, fading phenomenon is a key drawback for application of the TiAl₃-containing refiners in aluminum foundry. In the present work, new Al-3Ti-3B grain refiners, containing TiB₂, AlB₁₂ and TiAl₃ particles were developed with an aim to prolong the acting time after inoculation. The results showed that inoculation of Al-7Si alloy with thus meliorated Al-3Ti-3B grain refiner has produced a fine grain structure which was approximately maintained up to 30 min.     

Effect of TiB2 on univariant eutectic growth in a directionally solidified Al-alloy

Abstract

During directional solidification of a near-eutectic Al–Mn–Si alloy the univariant eutectic is formed, following the reaction: L→α-Al + α(AlMnSi) + L′ and reveals a non-faceted α-Al/faceted α(AlMnSi) structure. The addition of titanium diboride (TiB₂) promotes the nucleation of intermetallic α(AlMnSi) silicide phase at solid/liquid interface as particles, which are pushed and periodically engulfed by the advancing planar interface. This type of growth, called symbiotic growth, leads to a layered microstructure. To study the influence of TiB₂ upon the morphology of α(AlMnSi) phase various amounts (from 0·05 up to 2·00 wt-%) of TiB₂ were added to alloys with identical composition. The results show that the behaviour and the influence of the inoculant is not trivial. Both the presence of titanium diboride at the solid/liquid interface and its inoculation effect determine the final morphology of the silicide phase.     

Effect of SiC, Si3N4 and TiB2 additions on the oxidation resistance of TiAl alloys

The oxidation behavior of TiAl alloys containing dispersed particles of (5, 10, 15 wt.%) SiC, (3,5 wt.%) Si₃N₄ or (3, 5, 10 wt.%) TiB₂ was studied between 800 and 1200°C in atmospheric air. The TiAl−(SiC, Si₃N₄) alloys oxidized to TiO₂, Al₂O₃, and SiO₂. The TiAl−TiB₂ alloys oxidized to TiO₂, Al₂O₃, and B₂O₃ which evaporated during oxidation. Improvement in oxidation resistance accompanied by thin, dense scale formation due to the addition of dispersoids originated primarily from the enhanced alumina-forming tendency, improved scale adhesion by oxide grain refinement owing to the beneficial effect of dispersoids, and the incorporation of SiO₂ within the oxide scale in the case of TiAl−(SiC, Si₃N₄) alloys.     

The grain refinement mechanism of cast aluminium by zirconium

The mechanism underlying the grain refinement of cast aluminium by zirconium has been studied through examination of a range of Al alloys with increasing Zr contents. Pro-peritectic Al₃Zr particles are reproducibly identified at or near the grain centres in grain-refined alloy samples based on the observations of optical microscopy, scanning electron microscopy and X-ray diffraction. From the crystallographic study using the edge-to-edge matching model, electron backscatter diffraction and transmission electron microscopy, it is substantiated that the Al₃Zr particles are highly potent nucleants for Al. In addition, the effects of Al₃Zr particle size and distribution on grain refinement has also been investigated. It has been found that the active Al₃Zr particles are bigger than previously reported other types of active particles, such as TiB₂ for heterogeneous nucleation in Al alloys. Considering the low growth restriction effect of Zr in Al (the maximum Q-value of Zr in Al is 1.0 K), it is suggested that the significant grain refinement of Al resulting from the addition of Zr can be mainly attributed to the heterogeneous nucleation facilitated by the in situ formed Al₃Zr particles.     

Effect of Mg on the Microstructure and Mechanical Properties of Al0.3Sc0.15Zr-TiB2 Composite

Al-0.3Sc-0.15Zr-TiB₂ composites with varying additions of Mg were cast through a novel processing technique using oil Quenched Investment Casting (QIC). Addition of Mg resulted in grain refinement of the composite. Al₃(Sc, Zr) primary particles and TiB₂ are responsible for grain refinement in these composites. Presence of fine nanosized uniformly distributed precipitates of Al₃(Sc, Zr) at the peak age condition together with TiB₂ particles increase the strength and ductility of the composites. The presence of Sc and Zr reduces the size of TiB₂ particles down to 10 nm. The optimum magnesium content in the composites studied lies between 3.5 and 6%.     

Grain refinement of TiAl-based alloys: The role of TiB2 crystallography and growth

A crystallographic model is used to predict the nucleation potencies of TiB₂ particles during solidification of TiAl-based alloys. Two nucleation scenarios are investigated. In scenario 1, primary TiB₂ grows in the melt before formation of the body-centred cubic β phase. In scenario 2, secondary TiB₂ precipitates after the first β phase but before formation of the hexagonal close-packed α phase. The model predicts high α and β nucleation potencies of TiB₂ in both scenarios. However, pre-existing β grains in scenario 2 are predicted to be preferred α nucleation sites. The experimentally observed β refinement by primary TiB₂ agrees with the model predictions. Grain refinement in scenario 2 is attributed to α nucleation on β, which is interleaved with secondary TiB₂.     

激光熔覆B4C-TiO2-Al粉末制备原位TiB2+TiC/Fe复合涂层（英文）

采用B4C、TiO2、Al以及Fe基自熔合金粉末为前驱体,利用激光熔覆技术在钢基体上制备TiB2+TiC颗粒增强Fe基复合涂层。结果表明,激光熔覆过程通过B4C-TiO2-Al反应生成了均匀分布于基体的TiB2-TiC复合陶瓷相。TiB2颗粒呈长条块状,TiC以不规则形状分布于基体中。涂层具有比基材1045钢更好的耐磨性能,但涂层的摩擦因数小。     

Grain refining action of Al–5Ti–C and Al–TiC master alloys with Al–5Ti master alloy addition for commercial purity aluminium

Al–Ti–C master alloys have a great potential as efficient grain refiners for aluminium and its alloys. In the present work, the Al–5Ti–C, Al–TiC and Al–5Ti master alloys have been successfully prepared by a method of liquid solidification reactions. While the Al–5Ti–C master alloy consists of some strip- or needle-like TiAl₃, and in addition to TiC particles in the Al matrix, the Al–TiC master alloy revealed the presence of only TiC particles, and the Al–5Ti master alloy consists of only some blocky TiAl₃ particles. A united refinement technology by Al–5Ti–C+Al–5Ti and Al–TiC+Al–5Ti master alloys was put forward in this paper. The blocky TiAl₃ particles in Al–5Ti master alloy can not only improve the grain refinement efficiency of Al–5Ti–C and Al–TiC master alloys but also reduce the consumption because the blocky TiAl₃ particles improve the grain refinement efficiency of TiC particles in Al–5Ti–C and Al–TiC master alloys.