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.     

Reinforcement of Al–Fe–Ni alloys with the in situ formation of composite materials

One of the most effective methods for the improvement of the mechanical properties of metals is their reinforcement with non-metallic materials. In the present work powder of K₂TiF₆ and KBF₄ was added in an Al–Fe–Ni alloy while the alloy was in liquid form at 1060 °C with a 5 wt.% mixture of powders and with simultaneous stirring for 30 min. The liquid was squeeze-casted at 150 bar. The as-cast specimens were examined with electron microscopy and X-ray diffraction. SEM analysis revealed that the as-formed material is composed by needle-like crystallites along with a dentritic form and an interdendritic phase. The composition of the needle-like crystallites may presumably be expressed by the formula (Fe-Ni)Al₃. The rest of the matrix consists of almost pure Al grown dentritically, while the interdendritic phase contains Fe and Ni dissolved in Al. EDS analysis also proved the existence of spots with high Ti concentration, which probably refer to the Ti–B compounds. Finally TEM verified the presence of nanocrystals in the matrix.     

Combustion synthesis of TiC–TiB2 composites

An experimental study on formation of TiC–TiB₂ in situ composites with a broad range of compositions was conducted by self-propagating high-temperature synthesis (SHS) using the reactant compacts from different combinations of Ti, B₄C, C, and B powders. Direct reaction of Ti with B₄C at stoichiometry of Ti:B₄C = 3:1 yields a TiB₂-rich composite with TiC:TiB₂ = 1:2. Formation of the products containing 20, 33.3, and 50 mol% of TiB₂ was achieved by the Ti–B₄C–C reactants. In addition, the test specimen composed of Ti, B₄C, and B was employed for the synthesis of a composite with 80 mol% TiB₂. Among three different types of the powder compacts, the boron-containing sample was characterized by the fastest combustion wave and the highest reaction temperature. The lowest combustion temperature and wave velocity were observed in the Ti–B₄C compact. When fine Ni particles were added to the Ti–B₄C reactant, it was found that the propagation rate of the reaction front was increased and the densification of the end product was enhanced significantly. This was attributed to formation of the Ti–Ni eutectic liquid during the reaction. As a result, the relative density of a TiC + 2TiB₂ composite increases from 30 to 86% with the Ni content from 0 to 20 mol%. Based upon the XRD analysis, small amounts of TiNi₃ and TiB were detected in the Ni-reinforced TiC–TiB₂ composites.     

The microstructures and mechanical properties of cast Mg–Zn–Al–RE alloys

The microstructures and mechanical properties of cast Mg–Zn–Al–RE alloys with 4 wt.% RE and variable Zn and Al contents were investigated. The results show that the alloys mainly consist of α-Mg, Al₂REZn₂, Al₄RE and τ-Mg₃₂(Al,Zn)₄₉ phases, and a little amount of the β-Mg₁₇Al₁₂ phase will also be formed with certain Zn and Al contents. When increasing the Zn or Al content, the distribution of the Al₂REZn₂ and Al₄RE phases will be changed from cluster to dispersed, and the content of τ-Mg₃₂(Al,Zn)₄₉ phase increased gradually. The distribution of the Al₂REZn₂ and Al₄RE phases, and the content of β- or τ-phase are critical to the mechanical properties of Mg–Zn–Al–RE alloys. The Mg–6Zn–5Al–4RE alloy with cluster Al₂REZn₂ phase and low content of β-phase, exhibits the optimal mechanical properties, and the ultimate tensile strength, yield strength and elongation are 242 MPa, 140 MPa and 6.4% at room temperature, respectively.     

Strong bonding of infrared brazed α2-Ti3Al and Ti–6Al–4V using Ti–Cu–Ni fillers

Infrared dissimilar brazing of α₂-Ti₃Al and Ti–6Al–4V using Ti–15Cu–25Ni and Ti–15Cu–15Ni filler metals has been performed in this study. The brazed joint consists primarily of Ti-rich and Ti₂Ni phases, and there is no interfacial phase among the braze alloy, α₂-Ti₃Al and Ti–6Al–4V substrates. The existence of the Ti₂Ni intermetallic compound is detrimental to the bonding strength of the joint. The amount of Ti₂Ni decreases with increasing brazing temperature and/or time due to the depletion of Ni content from the braze alloy into the Ti–6Al–4V substrate during brazing. The shear strength of the brazed joint free of the blocky Ti₂Ni phase is comparable with that of the α₂-Ti₃Al substrate, and strong bonding can thus be obtained.     

Cu + TiB2 composite filler for brazing Al2O3 and Ti-6Al-4V alloy

Al₂O₃ and Ti-6Al-4V alloy were brazed using Cu + TiB₂ composite filler, which manufactured by mechanical milling of Cu and TiB₂ powders. Typical interface microstructure of joint was Al₂O₃/Ti₄(Cu,Al)₂O/Ti₂Cu + Ti₃Al + Ti₂(Cu,Al)/Ti₂(Cu,Al) + AlCu₂Ti/Ti₂Cu + AlCu₂Ti + Ti₃Al + Ti₂(Cu,Al) + TiB/Ti(s.s) + Ti₂Cu/Ti-6Al-4V alloy. Based on temperature- and time-dependent compositional change, the formation of intermetallics in joint was basically divided into four stages: formation of interfacial Ti₄(Cu,Al)₂O in Al₂O₃ side, formation of Ti₂Cu, Ti₃Al, TiB, Ti₂Cu, and AlCu₂Ti in layers II and IV, formation of Ti₂(Cu,Al) and AlCu₂Ti in layer III, formation of Ti + Ti₂Cu hypereutectoid organization adjacent to Ti-6Al-4V alloy. TiB in situ synthesized in joint not only acted as low thermal expansion coefficient reinforcement to improve the mechanical properties at room temperature, but also as skeleton ceramic of joint to increase high temperature mechanical properties of Al₂O₃/Ti-6Al-4V alloy joint increasing. When the joint containing 30 vol.% TiB brazed at 930 °C and 10 min of holding time, the maximum room temperature shear strength of joint was 96.76 MPa, and the high temperature shear strength of joint was 115.16 MPa at 800 °C.     

Microstructure and grain refining performance of a new Al–Ti–C–B master alloy

A kind of Al–Ti–C–B master alloy with a uniform microstructure is prepared using a melt reaction method. It is found that the average grain size of α-Al can be reduced from 3500 to 170 μm by the addition of 0.2 wt.% of the prepared Al–5Ti–0.3C–0.2B and the refining efficiency does not fade obviously within 60 min. It is considered that the TiC_xB_y and TiB_2−mC_n particles found at the grain center are the effective and stable nucleating substrates for α-Al during solidification, which accounts for the good grain refining performance.     

Differential thermal analysis (DTA) on the reaction mechanism in Fe–Ti–B4C system

Differential thermal analysis (DTA) was undertaken to determine the reaction mechanism in the Fe–Ti–B₄C system under argon. When the mixtures were heated to about 786 °C, Fe₂B and C appeared as a result of Fe reacting with B₄C. As the temperature continued to increase, FeTi formed by an interdiffusion between Fe and Ti. When the mixtures were heated to 1089 °C, FeTi reacted with Ti, leading to the formation of a Fe–Ti melt, into which the displaced C and B from B₄C dissolve, forming a Fe–Ti–C–B melt. Finally, when the concentration of C and B attained a certain value, Ti reacted with C and B, yielding TiC and TiB₂ in the melt, and simultaneously considerable heat released.     

The phase equilibria of the Al–Ce–Ni system at 500 °C

The phase equilibria at 500 °C in the Al–Ce–Ni system in the composition region of 0–33.3 at.% Ce are investigated using XRD and SEM/EDX techniques applied to equilibrated alloys. The previously reported ternary phases and the variation of the lattice parameters versus the composition for different solid solution phases are investigated. It is confirmed that τ₂(Al₂CeNi) exists at 500 °C, while τ₃(Al₅Ce₂Ni₅) does not exist at 500 °C. A new compound τ₉ with composition of about Al₃₅Ce_16.5Ni_48.5 is found. The solubility of Ni in Al₁₁Ce₃ and αAl₃Ce is generally about 1 at.%, while the solubility of Ni in Al₂Ce is measured to be 2.7 at.%. The solubility of Ce in Al₃Ni, Al₃Ni₂, AlNi and AlNi₃ is all less than 1 at.%. The solubility of Al in CeNi₅, Ce₂Ni₇ and CeNi₃ is measured to be 30.4, 4.8 and 9.2 at.%, respectively, while there is no detectable solubility for Al in CeNi₂. A revised isothermal section at 500 °C in the Al–Ce–Ni system has been presented.     

Microwave activated combustion synthesis of porous Al–Ti structures for composite reinforcing

A research on porous Al–Ti materials produced by combustion synthesis is presented. Some of the manufactured preforms could be infiltrated with liquid aluminium alloy to obtain locally reinforced castings. The synthesis progress was analysed, as well as a structure formation, its phase composition, homogeneity degree and type of porosity. Cylindrical green compacts were prepared of Al and Ti powders with various stoichiometric ratios. Synthesis was performed in a specially designed microwave reactor, by placing a compact in a waveguide, in intensely focused field. Temperatures recorded by a pyrometer showed that the synthesis propagation accelerated at the moment of Al₃Ti creation. The highest synthesis temperature was reached by the specimens containing 45–67% Al (all percentages atomic). Microscopic examinations revealed rounded grains, sometimes with cores of solid solution Ti(Al). The grains were basically composed of AlTi₃ that changed to AlTi at the edge. The grain structure was similar in almost all kinds of the specimens, especially in those with higher Al content. Around those grains, envelopes of AlTi and Al₂Ti were formed. Chemical composition of the matrix was approximately equivalent to stoichiometric ratio of the initial powder mixture. XRD investigations confirmed multiphase structure of the obtained compacts and a slight effect of the Ti powder granularity. Some of the specimens revealed regular structure with interconnected porosity. Those materials containing 75, 67 and 55% Al were used for producing the aluminium alloy-based composites.     

Phase constituents and microstructure of laser cladding Al2O3/Ti3Al reinforced ceramic layer on titanium alloy

Laser cladding of the Fe₃Al + TiB₂/Al₂O₃ pre-placed alloy powder on Ti-6Al-4V alloy can form the Ti₃Al/Fe₃Al + TiB₂/Al₂O₃ ceramic layer, which can greatly increase wear resistance of titanium alloy. In this study, the Ti₃Al/Fe₃Al + TiB₂/Al₂O₃ ceramic layer has been researched by means of electron probe, X-ray diffraction, scanning electron microscope and micro-analyzer. In cladding process, Al₂O₃ can react with TiB₂ leading to formation of amount of Ti₃Al and B. This principle can be used to improve the Fe₃Al + TiB₂ laser cladded coating, it was found that with addition of Al₂O₃, the microstructure performance and micro-hardness of the coating was obviously improved due to the action of the Al-Ti-B system and hard phases.     

In situ impedance spectroscopy study of the electrochemical corrosion of Ti and Ti–6Al–4V in simulated body fluid at 25 °C and 37 °C

The corrosion resistance of Ti and Ti–6Al–4V was investigated through electrochemical impedance spectroscopy, EIS, potentiodynamic polarisation curves and UV–Vis spectrophotometry. The tests were done in Hank solution at 25 °C and 37 °C. The EIS measurements were done at the open circuit potential at specific immersion times. An increase of the resistance as a function of the immersion time was observed, for Ti (at 25 °C and 37 °C), and for Ti–6Al–4V (at 25 °C), which was interpreted as the formation and growth of a passive film on the metallic surfaces.     

Self-propagating high-temperature synthesis of TiCxNy–TiB2 ceramics from a Ti–B4C–BN system

An experimental study on direct formation of TiC_xN_y–TiB₂ ceramics by self-propagating high-temperature synthesis (SHS) was conducted using a Ti–B₄C–BN system. The effects of the C/(C + N) ratio on the combustion behavior and reaction products were investigated. Experimental characterizations of quenched samples show that the combustion reaction started with the formation of highly substoichiometric TiN_y and TiB due to the solid-state reaction between Ti and BN; and then the TiN_y precursor and TiB dissolved back into the titanium melt, forming the Ti–B–N liquid, which in turn transformed to the Ti–B–N–C liquid due to the dissolution of the carbon atoms diffused away from B₄C. Finally, the TiC_xN_y and TiB₂ particles are formed from the melt through the dissolution-precipitation mechanism.     

The effects of lanthanum on microstructure and electrochemical properties of Al–Zn–In based sacrificial anode alloys

In order to improve the non-uniform corrosion of Al–0.5Zn–0.03In–1Mg–0.05Ti alloys, Al–5Zn–0.03In–1Mg–0.05Ti–xLa (x = 0.3, 0.5 and 0.7 wt.%) alloys were developed. Microstructures and electrochemical properties of the alloys were investigated. The results show that the optimal microstructures and electrochemical properties are obtained in Al–5Zn–0.03In–1Mg–0.05Ti–0.5La alloy. The main precipitate phase is Al₂LaZn₂ particles. The excellent electrochemical properties of Al–5Zn–0.03In–1Mg–0.05Ti–0.5La alloy is mainly attributed to fine grains and grain boundaries containing fine Al₂LaZn₂ precipitates. At the same time the fine grains can improve the non-uniform corrosion of Al–0.5Zn–0.03In–1Mg–0.05Ti alloy.     

Effects of CeO2 additive on the microstructure and mechanical properties of in situ TiB2/Al composite

In this paper, CeO₂ was investigated as an additive for in situ preparation of TiB₂/Al composite using an exothermic reaction process via K₂TiF₆ and KBF₄ salts. Experimental results indicated that when 0.5 wt.% CeO₂ additive was added, the dispersion of TiB₂ particles was improved significantly. Meanwhile, α-Al matrix grain was further refined. Compared with the composite without CeO₂, the ultimate tensile strength, yield strength, elastic modulus and tensile elongation increased by 8%, 7%, 26% and 14%, respectively in as-cast condition, and the tensile fracture behavior of the composite with CeO₂ belonged to a typical ductile fracture with microvoid coalescence.     

In situ XRD studies on Al–Ni and Al–Ni–Sr alloys prepared by rapid solidification

In this paper the structure and stability of Al–17 wt.%Ni(Al–17Ni) and Al–17 wt.%Ni–2 wt.%Sr alloys prepared by rapid solidification was investigated by means of XRD techniques. Our work demonstrates that both alloys are crystalline and composed of fcc (Al–Ni) solid solution and orthorhombic Al₃Ni phases. The ternary alloy shows in addition the presence of small amount of tetragonal Al₄Sr phase. In situ XRD experiment demonstrates the stability of the solute solution up to 650 °C, Al₃Ni above 750 °C while Al₄Sr overcomes melting of the major phases at 800 °C. High-temperature structure analysis proved strong bindings between Al and Ni atoms in Al₃Ni phase, corroborating its covalent nature, linear and faster increase of the fcc volume with annealing temperature. The linear correlation between constituting atoms decreases with increase of the temperature.The work also documents the applicability of pair distribution function (PDF) analysis to the study of multiphase crystalline systems.     

The third-element effect in the oxidation of Ni–xCr–7Al (x = 0, 5, 10, 15 at.%) alloys in 1 atm O2 at 900–1000 °C

The oxidation in 1 atm of pure oxygen of Ni–Cr–Al alloys with a constant aluminum content of 7 at.% and containing 5, 10 and 15 at.% Cr was studied at 900 and 1000 °C and compared to the behavior of the corresponding binary Ni–Al alloy (Ni–7Al). A dense external scale of NiO overlying a zone of internal oxide precipitates formed on Ni–7Al and Ni–5Cr–7Al at both temperatures. Conversely, an external Al₂O₃ layer formed on Ni–10Cr–7Al at both temperatures and on Ni–15Cr–7Al at 900 °C, while the scales grown initially on Ni–15Cr–7Al at 1000 °C were more complex, but eventually developed an innermost protective alumina layer. Thus, the addition of sufficient chromium levels to Ni–7Al produced a classical third-element effect, inducing the transition between internal and external oxidation of aluminum. This effect is interpreted on the basis of an extension to ternary alloys of a criterion first proposed by Wagner for the transition between internal and external oxidation of the most reactive component in binary alloys.     

Thermodynamic modeling of the Al–Cr system

By means of calculation of phase diagram (CALPHAD) technique, the Al–Cr system was critically assessed. Three solution phases (liquid, body-centered cubic, face-centered cubic) were modeled with the Redlich–Kister equation. The intermetallic compounds Al₇Cr, Al₁₁Cr₂, Al₄Cr, Al₈Cr₅, AlCr₂, which have a homogeneity range, were treated as the formulae Al₇(Al,Cr), Al₁₁(Al,Cr)₂, Al₄(Al,Cr), (Al,Cr)₈(Al,Cr)₅, (Al,Cr)(Al,Cr)₂ using two-sublattice model, respectively. A set of self-consistent thermodynamic parameters describing the Gibbs energy of each individual phase as a function of composition and temperature for the Al–Cr system was obtained.     

A contribution to the Al–Pd–Mn phase diagram

A part of the Al–Pd–Mn phase diagram in the vicinity of the icosahedral phase was refined. Partial isothermal sections of 710, 850, 870 and 880°C are presented. The overall compositional range of the icosahedral phase was found to be between 5.8 and 10.5 at.% Mn and between 69.5 and 71.5 at.% Al at these temperatures. It shifts to lower Mn concentration at lower temperatures. It was confirmed that the phase usually designated Al₃Pd has a lower Al concentration in the binary alloys than that according to the formula. It extends to the ternary compositions up to about 5 at.% Mn. The increase of the Mn content results in an increase of the Al concentration of this phase.     

Thermodynamic analysis of alloys Ti–Al, Ti–V, Al–V and Ti–Al–V

Thermodynamic analysis of three binary Ti-based alloys: Ti–Al, Ti–V, and Al–V, as well as ternary alloy Ti–Al–V, is shown in this paper. Thermodynamic analysis involved thermodynamic determination of activities, coefficient of activities, partial and integral values for enthalpies and Gibbs energies of mixing and excess energies at four different temperatures: 2000, 2073, 2200 and 2273 K, as well as calculated phase diagrams for the investigated binary and ternary systems. The FactSage is used for all thermodynamic calculations.