Contribution to ternary system Al-Ti-B Part 1: Study of diborides present in the alulminium corner

Abstract

Diborides present in Al-Ti-B alloys with a weight ratio TilB < 2.2 are investigated in the arc melted and annealed conditions as well as in the controlled heated and cooled specimens by means of light and scanning electron microscopy, energy dispersive spectrometry and X-ray diffraction. In Al- Ti-B alloys with composition close to the AlB₂-TiB₂ tieline; apparently pure spheroidal and platelike TiB₂ particles are present. In the as cast condition of all other investigated alloys the formation of AlB₂ on primary TiB₂ particles is observed regardless of the applied cooling rate. In the investigated alloys apparently pure TiB₂ and AlB₂ coexist even after 1000 h exposure at 800° C, and the formation of mixed diboride (Al, Ti) B₂ was never observed. The results of this work are discussed together with the results of previous work in this area and it seems very likely that the mixed diboride (Al, Ti)B₂ is not a thermodynamically stable phase in the aluminium rich corner of the Al-Ti-B system, but onlyapparently pure AlB₂ and TiB₂ are present.     

Relationships of diboride phases in Al–Ti(Zr)–B alloys

Abstract

The relationships of diboride phases in Al–Ti(Zr)–B alloys with a variable Ti/B ratio close to the stoichiometry of TiB₂ were studied. The formation of diboride solid solutions was confirmed. A grain refinement mechanism is proposed as that diboride particles in the Al–Ti–B master alloys reacting with aluminium upon adding into an aluminium melt and release titanium into the melt through forming a (Ti,Al)B₂ solid solution and maintain a thin dynamic Ti rich layer on the surfaces of the (Ti,Al)B₂ particles, which nucleates α-Al grains in solidification. The poisoning effect of zirconium on grain refinement of aluminium by Al–Ti–B master alloys is also discussed.     

Microstructures of in situ Al/TiB2 MMCs prepared by a casting route

In situ Al/TiB₂ metal matrix composites (MMCs) have been successfully produced by Salt-Metal reactions. This is a novel low-cost reactive approach, which involves adding Ti and B bearing salts to molten Al. The reactions between the salts lead to the formation of the reinforcing TiB₂ particles in the Al matrix. The in situ formed TiB₂ particles are very fine (below 1 m in size). Strings and clusters of particle agglomerates are distinct microstructural features of all the composites with pure Al as the matrix. The effects of processing parameters on the kinetics of TiB₂ formation and on the final microstructures are studied in detail. Besides, efforts are made to improve the distribution of TiB₂ particles in the Al matrix by means of chemical additions; it is found that a homogeneous distribution is obtained by using a eutectic Al-Si alloy as the matrix material.     

Formation of TiB2 particles during dissolution of TiAl3 in Al–TiB2 metal matrix composite using an in situ technique

A new technique has been developed in an aluminum based metal matrix composite in order to reveal the mechanism of formation of TiB₂ particles by mixing molten master alloys i.e., Al–8Ti and Al–4B in the Ti:B weight ratio of 5:2. A composite containing fine TiB₂ particles produced by this technique. In this approach, the progress of In situ formation of TiB₂ was carried out using hot stage microscope, energy dispersive X-ray analysis, scanning electron microscopy and X-ray diffraction analysis. From the experimental observations obtained; it was proposed that the formation of TiB₂ particles occurred via diffusion of Boron atoms through TiAl₃ particles interface, thereby reacting to form fine TiB₂ particles. Studies indicate that since the primary TiB₂ particles on the surface of TiAl₃ are appreciably free and movable and because of boron diffusion across boundary layer towards TiAl₃, TiB₂ particles produced during growth with the primary ones formed agglomeration rings. A model was schematically developed to explain the formation of TiB₂.     

The microstructure and mechanical properties of TiC and TiB2-reinforced cast metal matrix composites

TiC and TiB₂ particles have been spontaneously incorporated into commercial purity aluminum melts through the use of a K-Al-F-based liquid flux that removes the oxide layer from the surface of the melt. The combination of spontaneous particle entry and close crystal structure matching in the Al-TiB₂ and Al-TiC systems, results in low particle-solid interfacial energies and the generation of good spatial distributions of the reinforcing phase in the solidified composite castings. The reinforcement distribution is largely insensitive to the cooling rate of the melt and the majority of the particles are located within the grains. Modulus increases after TiC and TiB₂ particle additions are greater than those for Al₂O₃ and SiC. It is thought that interfacial bonding is enhanced in the TiC and TiB₂ systems due to wetting of the reinforcement by the liquid and particle engulfment into the solid phase. TiC-reinforced composites exhibit higher stiffnesses and ductilities than TiB₂-reinforced composites. This has been attributed to stronger interfacial bonding in the Al-TiC system, due to the increased tendency for nucleation of solid on the particle surfaces.     

Microstructural characterisation and thermal stability of in situ TiB2/60Si–Al composite synthesised by displacement reactions and spray forming

Abstract

A novel in situ reactive technique has been employed for preparing 2·0 wt-%TiB₂/60Si–Al composite. The kinetic equations and the Arrhenius type equation were applied to compute the coarsening rate constant and the activation energy for grain growth for the composite when it was heated at semisolid state for partial remelting. Experimental results have shown that the in situ TiB₂ particles can refine effectively the primary Si phase and restrain the Si phase growth. The cubic coarsening rate constant for the composite was computed to be in the range of 75–148 μm³ min^?1 at temperatures in the range of 600–700°C, which was much less than that for the 60Si–Al alloy (1323–4523 μm³ min^?1). The value of activation energy for grain growth for the composite was about twice of that for the 60Si–Al alloy. The composite exhibited a higher thermal stability than that of the 60Si–Al alloy, suggesting that the in situ TiB₂ particles can effectively pin the grain boundaries and arrest the migration of liquid film in the semisolid state of the composite.     

In situ stir cast Al–TiB2 composite: processing and mechanical properties

Abstract

Elemental Ti and B powders of stoichiometric composition were mixed and added to molten aluminium. In situ TiB₂ particles were formed in the aluminium melt. On casting, an Al–TiB₂ composite was produced. Despite the presence of the Al₃Ti phase associated with the Al–Ti–B ternary system, the in situ TiB₂ particles, with sizes of 1–3 µm formed in the composite was able to yield an improvement of 57% increase in tensile strength, 66% in yield strength and 22% in modulus in an Al–15 vol.-%TiB₂ composite. The extent of improvement in these properties depended on the volume fraction in the composite. Fractography showed a texture of dimples seated with hexagonal TiB₂ particles indicating retention of high ductility in the composite, despite the fact that the predominance of the coarse Al₃Ti in the composite had led to premature rupturing.     

Properties of reactively cast aluminium–TiB2 alloys

Abstract

A salt base reactive casting process has been employed to produce A356 aluminium casting alloys containing fine dispersions of TiB₂. These have been compared with commercially available Duralcan A356 material with SiC particles which have also been incorporated by a casting technique. Structural, mechanical, and wear properties have been measured. These show that TiB₂ is extremely effective in enhancing modulus in addition to significantly reducing the coefficient of friction when used against an alloy steel plate. The wear rate of the alloy is found to be independent of particle type but is governed mainly by volume fraction, as is the load transition from mild to severe wear. However, the wear mechanisms for TiB₂ and SiC composites are different when the steel disc is taken into account and the wear debris examined. The plate is effectively machined by SiC, resulting in extensive damage of the mating plate. Alloy A356 with TiB₂ additions is made by reactive casting a simple alloy to process and is thereby a viable alternative to low cost Al–SiC cast composites.

MST/1854     

Effect of silicon content in grain refining hypoeutectic Al–Si foundry alloys with boron and titanium additions

Abstract

The effect of Si content on the grain refinement of hypoeutectic Al–Si alloys was investigated. Alloying with Si refines the grain structure, which tends to be coarse and columnar in commercially pure aluminium. The smallest grain size occurs at ～2 wt-%Si, where the solidification interval of hypoeutectic Al–Si alloys is the largest. Grains become increasingly coarser with increasing Si starting from this point. The grains of Al–Si alloys with 500 ppm Ti are smaller than those cast without Ti regardless of the Si content of the alloy. The fivefold reduction in grain size in commercially pure aluminium upon Ti addition is gradually reduced with increasing Si. Finally, the grain refinement provided by Ti fails to meet the expectations once Ti starts to be removed from the melt via the formation of Ti–Si compounds above 5 wt-%Si. The B addition relies on the formation of AlB₂ particles to offer grain refinement. Analysis of the Al rich corner of the calculated Al–Si–B liquidus surface suggests that the primary AlB₂ is formed at a Si concentration of ～4 wt-%. While a perfect grain refiner for hypoeutectic Al–Si alloys with at least 4 wt-%Si, B fails to refine the grain structure when the Si content is less.     

Interface microstructures in Ti-based composites using TiB2/C-coated and uncoated SiCf after short-term thermal exposure

The extent of interfacial reaction after short-term thermal exposure during vacuum plasma spraying (vps) and vacuum hot-pressing (vhp) of Ti-based metal-matrix composites (mmcs) using TiB₂/C-coated and uncoated SiC fibres has been investigated by a combination of scanning and transmission electron microscopies. There is no interfacial reaction after short-term thermal exposure during vps manufacture of SiC_f/Ti mmcs using either TiB₂/C-coated or uncoated SiC_f. There is only limited interfacial reaction after short-term thermal exposure during vhp manufacture of SiC_f/Ti-6Al-4V mmcs using TiB₂/C-coated SiC_f. In the initial stage of the interfacial reaction, TiB needles are formed by preferential nucleation and growth at β particles and grain boundaries in the Ti-6Al-4V matrix.     

Understanding mechanisms of grain refinement of aluminium alloys by inoculation

Abstract

For over half a century, grain refinement of aluminium alloys has been achieved by chemical inoculation; current grain-refinement practice involves the addition of master alloys (e.g. Al – Ti – B, Al – Ti – C) before casting, introducing inoculant particles to the melt. These particles act as nucleation points for α-Al grains, resulting in a uniformly fine, equiaxed as cast microstructure. Despite the ubiquity of this process, its underpinning science was not fully understood, hindering development of the area. From the 1950s onwards, the phase responsible for nucleation in alloys refined by Al – Ti – B was fiercely disputed. The debate focused so closely on this issue that other important factors were frequently ignored. During the 1990s, this debate was resolved through careful thermodynamic reasoning and novel experiments that derived their inspiration far from the foundry. This review focuses on subsequent experimental work and modelling: the expansion of the current understanding of grain refinement to include effects relating to the release of latent heat, the size distribution of inoculant particles and the alloying elements present in the melt (through poisoning and growth-restriction effects). The current contention regarding the nature of the interface between the nucleant phase (TiB₂) and the melt is also discussed. These recent advancements have lead to improvements in grain refining practice resulting in savings in the foundry and development of alternative master alloys for aluminium systems. The ideas have also been successfully applied to other metallic systems, notably magnesium and zinc alloys.     

Mechanical properties of high performance lightweight steels

Abstract

The incorporation of low density, high modulus ceramic particles into a steel matrix is a potential route to improve the mechanical performance of steels. A powder metallurgy, mechanical blending route has been adopted to produce a homogeneous distribution of TiB₂ particles in both pure Fe and 316L stainless steel matrices. This approach gave large increases in both the static and fatigue strength with increasing TiB₂ volume fractions, in comparison with the matrix material. Additions of TiB₂ also resulted in reduced density and increased stiffness in the composite or lightweight steel materials, giving a specific stiffness increase of 52%with a fraction of 30 vol.-%TiB₂ in Fe, compared with the matrix.     

Microstructure and Wear Resistance of Semisolid TiB2/7050 Composites Produced by Serpentine Tube Pouring Technique

TiB₂/7050 (3, 6, and 9 wt%) composites slurries with globular were synthesized by in situ reaction and serpentine tube pouring techniques. The results showed that the semisolid 7050 alloy and 3, 6, and 9 wt%TiB₂/7050 composites with average grain diameter of 28, 25, 20, and 19 µm and shape factor of 0.77, 0.83, 0.90, and 0.93, respectively, can be obtained at 660 °C pouring temperature. With increasing TiB₂ content and curves number, the α-Al grain size was decreased. The composite melts have an effect of “self-stirring” when they flow through the serpentine tube, which is beneficial to make the primary nuclei with globular grains. Moreover, the wear resistance of TiB₂/7050 composites improved obviously with increasing in situ TiB₂ particles content and that the wear rate of 9 wt%TiB₂/7050 composite was 79% lower than that of 7050 matrix alloy under 100 N applied load, 30 min sliding time, and 0.15 m/s sliding velocity.     

Influence of matrix alloying elements on reactive synthesis of 2124 aluminium alloy metal matrix composites

Abstract

In situ TiB₂ particle reinforced Al alloys are produced by reactive synthesis from elemental and prealloyed powders. The influence of 2124 alloying elements on the reactive synthesis is evaluated with a comparison of elemental AI, elemental AI-Cu mixture, and 2124 Al prealloyed powders as matrix materials. Experimental investigations by differential scanning calorimetry and dilatometry showed that the presence of Cu leads to an increase in the reaction rate during the formation of intermediate reaction products in comparison with the elemental Al matrix. X-ray diffraction of the reaction products showed a more complete conversion of the intermediate Al₃ Ti as a result of Cu addition. The Cu has no influence on the TiB₂ particle size, but the TiB₂ morphology changed from pure hexagonal to a more rounded morphology.     

Nano-structured intermetallic compound TiAl obtained by crystallization of mechanically alloyed amorphous TiAl, and its subsequent grain growth

The amorphization process during mechanical alloying (MA) was investigated for the Al-50at%Ti and Al-50at%Ti-10vol%TiB₂ powder mixtures. Pure metallic powders of Al and Ti were finely mixed and transformed to the amorphous phase after being milled for about 2880 ks. In the case of Al-50at%Ti-10vol%TiB₂ powder, the amorphous alloys with a fine dispersion of TiB₂ particles could be obtained for a shorter milling times than that required for the powders without TiB₂ ceramics. As a result of heat treatment for the mechanically alloyed amorphous powders, a nanocrystalline intermetallic compound of TiAl () could be produced. Subsequent grain growth of the phase during heat treatment was investigated by estimating the grain-growth exponent and the activation energy for grain growth. It was found from this estimation that the grain growth was further suppressed as the powders were mechanically alloyed for longer times. Furthermore, the addition of the TiB₂ particles that could be dispersed during MA finely and homogeneously in the amorphous matrix was found to be effective for suppression of the grain growth especially at elevated temperatures as well as for a long annealing.     

Performance of AlTi5B1, AlTi3B3 and AlB3 master alloys in refining grain structure of aluminium foundry alloys

Abstract

The capacity of AlTi5B1, AlTi3B3 and AlB3 grain refiners to refine the grain structures of AlSi7Mg and AlSi11Cu2 foundry alloys was investigated. The performance of AlTi5B1, well established to be the best grain refiner for wrought aluminium alloys, is not nearly as good with the AlSi7Mg and AlSi11Cu2 alloys. Relatively smaller grains are obtained with the AlTi3B3 grain refiner in both alloys. The AlB3 grain refiner, on the other hand, improves the grain structure only as much as the AlTi5B1 grain refiner does. With as much as 0·04–0·1 wt-%Ti, the commercial alloys cannot enjoy the outstanding potency of the AlB₂ particles since the B supply is readily transformed to TiB₂ particles. However, the grains of the Ti free AlSi7Mg and AlSi11Cu2 alloys (～0·005 wt-%Ti) are very small and nearly globular for the entire range of holding times when inoculated with AlB3, implying not only a remarkable grain refining capacity but also a strong resistance to fading of the grain refinement effect. The lack of Ti in the melt allows the entire B to form AlB₂ particles, the perfect substrates to promote the nucleation of α-Al crystals. Aluminium castings can enjoy grain sizes well below 200 μm, with an addition of 0·02 wt-%B, provided that they are Ti free. That the potent substrates are made available just before the nucleation of α-Al crystals avoids fading effects and is a further advantage of the AlB3 grain refiner in recycling operations.     

Effects of co-addition of titanium and boron on microstructure of superplastic Cr–Mo steels

Abstract

The effects of titanium and boron on the microstructure of a low alloyed Cr–Mo steel with 0·6 wt-%C have been investigated by comparison with a steel containing only titanium and a steel free from both titanium and boron. Each of the steels was subjected to thermomechanical treatment and annealed at 700°C, resulting in small grains of size a few micrometres. The steel containing both titanium and boron possessed the smallest ferrite grains and M₃C carbides of the three examined. This is attributed to a fine dispersion of borides (TiB₂ ) and borocarbides (Ti(C,B)) of size 10 nm in the ferrite matrix through the pinning effect. At the grain boundaries small carbide particles were present which were effective in inhibiting grain boundary migration. The extremely fine borides and/or borocarbides were useful in suppressing intragranular deformation of ferrite grains due to precipitation hardening. This may have assisted in promoting grain boundary sliding, resulting in superior superplastic elongation.     

Microstructure and refinement mechanism of TiB2/TiAl3 in remelted Al-5Ti-1B system

In this work, we propose a new method (by remelting Al-5Ti-1B) to investigate the grain refinement mechansim. It is found that the morphology and size of TiAl₃ phase had little effect on the grain refinement of pure Al. Therefore, further experimental studies were carried out to understand the potency of TiB₂ particles. The high-resolution transmission electron microscopy (HRTEM) observation has confirmed the existence of an atomic layer on the surface of (0001) TiB₂, which is possibly a two-dimensional (2D) (1-12) TiAl₃. Crystallographic study indicates that it is a more suitable nucleation sites for α-Al than other particles. The TiB₂ particle with TiAl₃ 2D acts as the best effective nucleation sites for α-Al.     

The relationship between TiB<Subscript>2</Subscript> volume fraction and fatigue crack growth behavior in the in situ TiB<Subscript>2</Subscript>/A356 composites

The relationship between TiB₂ volume fraction and fatigue crack growth behavior in the A356 alloy matrix composites reinforced with 3, 5.6, and 7.8 vol% in situ TiB₂ particles has been investigated. The mechanisms of crack propagation in the TiB₂/A356 composites were also discussed. The results show that the 3 vol% TiB₂/A356 composite has nearly the same crack growth behavior as the matrix alloy, while the 5.6 vol% TiB₂/A356 composite exhibits a little bit faster crack growth rate. The 7.8 vol% TiB₂/A356 composite presents the lowest resistance to crack growth, indicating that the crack growth is accelerated by increasing TiB₂ volume fraction. Fractographies reveal that an increase in TiB₂ volume fraction results in a change from the formation of striation and slip to the failure of voids nucleation, growth, and coalescence. Cracks tend to propagate within the matrix and avoid eutectic silicon and TiB₂ particles in the intermediate ΔK region, while prefer to propagate along interfaces of eutectic silicon and TiB₂ particles and link the fractured eutectic silicon particles in the near fractured ΔK region. Furthermore, the propensity for the separation of TiB₂ increases with the increase in TiB₂ volume fraction. The massive voids caused by fractured eutectic silicon and separated TiB₂ particles propagate and coalesce, and then accelerates the crack growth in TiB₂/A356 composites.     

Synthesis of titanium diboride TiB2 and Ti-Al-B metal matrix composites

Titanium diboride TiB₂ and TiAl aluminide composites reinforced with in situ borites have been synthesized from the elemental powders of Ti and B, and Ti, Al and B respectively using mechanical alloying technique. No progressive diffusion between Ti and B was observed. The formation of TiB₂ was found to be governed by strong and fast exothermic heat release. This indicates that the formation of TiB₂ compound in local area of mechanically alloyed powder generated high energy which in turn ignited and promoted the formation of new compound in the rest of the area. Because of the presence of Al in Ti-Al-B system, the concentration of Ti or B was diluted. The exothermic reaction between Ti and B was consequently delayed. However, grain refinement of Ti and Al in this system down to nanometer scale is faster than that in Ti-Al system due to the contribution of B. Using X-ray analysis, strong but broad TiAl, and weak TiB and TiB₂ peaks had been detected at 50 h of mechanical alloying indicating the formation of nano TiAl composite reinforced by TiB and TiB₂. However, TiB was, however, not a stable phase; it later was transformed into equilibrium phase of TiB₂ after annealing at 800 °C.