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.     

Response of an Al—Cr alloy towards grain refinement by Al—5Ti-1B master alloy

Grain refining of an Al–2%Cr alloy by Al–5Ti-1B master alloy (5/1 TiBAl) has been studied as a function of the addition rate of the grain refiner in the range 0.2–1.2% and holding times up to 120 minutes. This study revealed that an effective grain refinement of Al–2%Cr requires a minimum addition of 1.2% grain refiner as against the 0.2% which is normally required for most of the conventional Al alloys. The work further revealed that the effect of grain refiner starts fading after holding the melt for about 5 minutes, irrespective of the addition rate of grain refiner. The formation of complex aluminides in the presence of Cr seems to be the primary reason for the poor grain refining response of this alloy at low addition rates of grain refiner. At high addition rates, the better grain refining behaviour of the present alloy was found to be essentially due to the availability of finer unreacted nucleating sites (TiAl₃ and TiB₂).     

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-...     

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.     

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.     

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.     

Ca对碳质孕育含Fe的Mg-3%Al合金晶粒细化的“免毒化”作用(英文)

利用Ca和碳质孕育对Mg3%Al合金进行复合变质处理,并研究了Fe及其Fe的添加顺序对细化效果的影响。与碳质孕育相比,Ca和碳复合孕育后的晶粒细化效果更为显著,Fe及其添加顺序对复合孕育细化效果无显著影响。Ca的添加可有效避免碳质孕育MgAl合金中因Fe所导致的晶粒粗化作用。在Ca和碳质复合孕育试样中可观察到大量的AlCO颗粒,这些颗粒实际应为Al4C3,并作为α-Mg的形核核心。然而,在熔体中添加Fe后,试样中可观察到大量在AlFe或AlCFe表面吸附Al4C3的双相粒子,该双相粒子可作为α-Mg的形核核心并导致晶粒细化。Ca有利于该双相粒子的生存,并与工艺条件无关,从而使得Fe对晶粒细化的"毒化"作用得到有效抑制,即Ca对Fe有"免毒化"效果。     

激光焊接颗粒增强铝基复合材料中TiB2颗粒的演变行为（英文）

研究大功率激光器焊接TiB2颗粒增强铝基复合材料时TiB2粒子的演变行为。采用X射线衍射(XRD)、扫描电镜(SEM)及能谱(EDS)分析焊缝内粒子的物相、热力学过程及形貌特征;同时对TiB2和铝基体的界面反应进行讨论。结果表明:当TiB2团簇尺寸大于激光光斑直径时,焊缝中部的TiB2粒子会熔融在一起,较大尺寸的TiB2会发生断裂;当与铝熔体接触后,熔化后的TiB2粒子会与Al发生反应生成Al3Ti和AlB12,并且焊缝中部的界面反应比焊缝边缘的剧烈。     

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%.     

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.     

Real-time observation of grain nucleation and growth during solidification of aluminium alloys

The crystallisation kinetics of liquid aluminium–titanium alloys with microscopic TiB₂ particles added to refine the grain size in the solidified material was studied by X-ray diffraction measurements at a synchrotron source. Real-time observation of the formation and growth of individual grains reveals the central role played by the added TiB₂ particles during solidification. Prior to the main transformation, weak reflections of a metastable TiAl₃ phase were detected. This observation finally pinpoints the highly debated mechanism responsible for enhanced grain nucleation in Al–Ti–B alloys.     

Effect of trace boron on grain refinement of commercially pure aluminum by Al–5Ti–1B

The effect of boron content on grain refinement of commercially pure aluminum by Al–5Ti–1B was quantitatively assessed. When the boron content is less than 0.03 wt.%, the refining performance of Al–5Ti–1B gradually is weakened as the boron content increases, which is attributed to the reaction of boron with the Al₃Ti interlayer on TiB₂ and the consumption of solute Ti. On the contrary, when the boron content exceeds 0.03 wt.%, the refining performance of Al–5Ti–1B gradually recovers with increasing boron content, which is related to the formation of primary AlB₂ particles that provide additional nucleant substrates.     

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.     

Grain refinement of AZ31 magnesium alloy by new Al-Ti-C master alloys

New Al₄C₃-containing Al-Ti-C master alloys (Al-0.6Ti-1C and Al-1Ti-1C) were developed by introducing Ti element into Al-C melt using melt reaction method, in which most of the TiC particles distribute around Al₄C₃ particles. It is believed that most of the C firstly reacts with Al melt and form Al₄C₃ particles by the reaction Al(l)+C(s)→Al₄C₃(s), and after adding Ti into the Al-C melt, the size of Al₄C₃ particles is decreased and the distribution of Al₄C₃ is improved through the reaction Ti(solute)+Al₄C₃(s)→TiC(s)+Al(l). With the addition of 1% Al-1Ti-1C master alloy, the average grain size of AZ31 is reduced sharply from 850 μm to 200 μm, and the grain morphology of α-Mg transits from a fully-developed equiaxed dendritic structure to a petal-like shape. Al-C-O-Mn-Fe compounds are proposed to be potent nucleating substrates for primary Mg. Appropriate addition of Ti is believed to increase the grain refinement efficiency of Al₄C₃-containing Al-Ti-C master alloys in AZ31 alloy.     

Ti 元素对B4C/Al复合材料力学性能的改善作用研究

B₄C/Al复合材料是目前最理想的中子吸收材料,广泛用于乏燃料储存。本文利用液态搅拌法制备B₄C/Al复合材料,通过添加Ti元素,探讨了界面反应对材料的界面结构和力学性能的影响。研究发现,Ti元素通过参与界面反应,改变了界面结构,在B₄C颗粒表面形成了紧密结合的纳米TiB₂界面层;Ti的添加消除了界面微裂纹、微孔、分离等缺陷。随着界面反应程度的加强,材料强度提高,尤其反应脱落的纳米TiB₂颗粒作为原位第二强化相进一步增强基体。B₄C/Al复合材料断裂过程表现为韧窝延性断裂;TiB₂界面层增强了B₄C颗粒与基体的结合,断裂行为从B₄C-Al界面脱落转变为B₄C颗粒断裂;但过渡的界面反应会形成微韧窝,引起材料延伸率下降。     

Al-5Ti-B中间合金的热机械处理对铸造铝合金晶粒细化的强化作用(英文)

采用等温液相退火和塑性变形工艺制备含不同尺寸Al3Ti和TiB2粒子的Al-5Ti-B(质量分数,%)中间合金。结果表明:随着中间合金中Al3Ti和TiB2粒子尺寸的减小,合金的晶粒细化作用得到增强,但不适当地减小Al3Ti和TiB2的粒径可能导致这些颗粒作用减弱和结块,从而降低合金的晶粒细化效率。     

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.     

Degradation of strength properties and its fracture behaviour of TiB2-TiC-based composite ceramic cutting tool materials at the high temperature

Strength retention is important for tool materials at high temperature because cutting temperature in machining is ranged from room temperature to 1000 °C. A study examining the strength properties and fracture behaviour of TiB₂-TiC-based composite ceramic cutting tool materials is presented at different temperatures. MoSi₂ and SiC additives are considered to investigate their effects on the density, microstructure, strength and failure mechanism of composites. It is found that the addition of SiC contributed more to the high-temperature strength of composites than MoSi₂, but it did not improve the room-temperature strength, despite grain refinement. The TBAVS8 composite has a flexural strength of 800 MPa at room temperature and can retain 75% at 900 °C. At room temperature, the fracture behaviour of composites was dominated by the strong bonding of the Ni binder phase. At high temperatures, the softer Ni binder phase was pinned, and its sliding was inhibited by SiC particles, which decelerated the strength degradation.     

原位生成铝基复合材料的激光焊接

采用大功率激光器研究新型铝基复合材料TiB2/ZL101的焊接性能,TiB2粒子的存在增加了焊缝熔池粘度降低了熔池流动性,影响了焊缝成形,增加了气孔敏感性.焊缝中气孔主要来源于氢和复合材料中的残留盐.激光焊接过程中较大的冷却速度使得焊缝晶粒非常细小,TiB2粒子在焊缝中分布更均匀,没有出现粒子偏析,主要是因为TiB2粒子是属于纳米级,在凝固过程中被凝固界面前沿所捕获而没有被推移.TiB2粒子没有与铝基体发生界面反应生成脆性相Al3Ti及AlB2,TiB2粒子与Al基体界面结合较好.结果表明,激光焊接后没有破坏TiB2粒子的增强效果.     

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.