Hydrogen evolution from cathodically charged titanium aluminide alloy Ti–24Al–11Nb

A Ti₃Al-based titanium aluminide alloy, Ti–24Al–11Nb, was cathodically charged with hydrogen in a 5% H₂SO₄ aqueous solution for various charging times, and the formation and dissociation of the hydride, the hydrogen evolution behavior and the total hydrogen uptake were investigated mainly by means of X-ray diffractometry and thermal desorption spectroscopy (TDS). The same kind of hydride phase as observed previously in Ti–25Al alloy (hexagonal hydride) was presumably formed in the Ti–24Al–11Nb alloy after cathodic charging. No damage, such as cracks, was induced by hydrogen charging. Two kinds of TDS peaks, one probably corresponding to hydride dissociation and the other to hydrogen dissolution in the normal lattice site, were found after longer hydrogen charging. It is suggested that niobium addition to Ti₃Al-based titanium aluminide alloy may reduce hydrogen susceptibility during cathodic charging.     

Effect of fabrication process on microstructure of high Nb containing TiAl alloy

Ti–45Al–8.5Nb–(W,B) (at.%) alloys were fabricated by ingot metallurgy (IM) and elemental powder metallurgy (EPM) processes, respectively. The effect of fabrication process on microstructure of high Nb containing TiAl alloy was investigated. The results showed that IM alloy has near lamellar (NL) microstructure composed of coarse lamellar colonies with a lamellar spacing of about 700 nm and equiaxed γ phase existing at the boundaries of lamellar colonies. While EPM alloy has fully lamellar (FL) microstructure with a lamellar spacing of about 180 nm. A small amount of β phase exists either at the boundaries of the lamellar colonies or in the lamellar colonies in platelet or blocky morphology in IM alloy. However, the micro-segregation does not exist in EPM microstructure. Borides appear in both the microstructures, but their sizes are smaller in EPM microstructure. HIP treatment can significantly eliminate pores and decrease the β phase in IM alloy. It is regretful that HIP treatment can only decrease porosity to some extent, but cannot completely eliminate pores in EPM alloy.     

The performance of Al–Ti–C grain refiners in twin-roll casting of aluminium foilstock

The master alloys based on the Al–Ti–B system have been used extensively for refining the grain structure of aluminum alloys. The quality-related problems linked with the TiB₂ particles, however, have generated an interest in the Al–Ti–C grain refiners as an acceptable replacement for Al–Ti–B master alloys. TiC particles are smaller than the TiB₂ particles and are less prone to agglomeration. Al–3Ti–0.15C grain refiners have been in use for some time in several alloy systems. Much of the work reported on this alloy, however, has been from DC casting while performance data in strip casting is not available. In the present work, a commercial Al–3Ti–0.15C grain refiner was employed in the twin-roll casting of AA8111 foilstock. Its grain refining efficiency was compared with that of the Al–5Ti–0.2B master alloy, the standard grain refiner in aluminium industry for the manufacture of aluminium foil products.     

Beneficial effects of O-phase on the hydrogen absorption of Ti–Al–Nb alloys

The beneficial effects of O-phase on hydrogen absorption/desorption were demonstrated in three Ti–Al–Nb alloys with compositions in the vicinity of Ti₂NbAl. The alloys were first quenched from high temperature and then aged for a certain period of time at a lower temperature to get O-phase precipitation in the matrix before pressure–composition (P–C) isotherm measurements. All three alloys absorb hydrogen to β hydride at a very low equilibrium pressure. Further absorption to γ hydride is difficult for the quenched O-phase free specimens. β->γ hydride transformation occurs in all the aged specimens and the reversible absorption/desorption between β and γ hydrides is observed in some aged specimens with relatively high volume fraction of O-phase in the matrix. The hydrogen absorption/desorption abilities of these alloys become poor when the Nb content is decreased. The determined formation heat for β->γ hydride transformation is in the range of −30–45 kJ/mol H₂, which is larger than that of binary Ti₃Al. It is found that the value of formation heat becomes more negative with the increasing amount of O-phase and is relatively independent of the interface between O-phase and matrix. This indicates the beneficial effect is related to the O-phase structure itself.     

The role of Fe and Ti additions in the microstructure of Nb–18Si–5Sn silicide-based alloys

The effects of Fe and Ti on the microstructure and hardness of the as cast and heat treated Nb–24Ti–18Si–5Fe–5Sn (NV8) and Nb–45Ti–15Si–5Fe–5Sn (NV4) alloys were studied. The microstructure of NV8-AC consisted of (Nb,Ti)_ss, (Nb,Ti)₃Sn, (Nb,Ti)₅Si₃, (Nb,Ti)₃Si, FeNb₄Si, and Fe₂Nb₃ and a Ti rich oxide. The microstructure of NV8-HT consisted of (Nb,Ti)₃Si, (Nb,Ti)₃Sn and the Ti rich oxide. In NV8 the formation of Nb₅Si₃ was destabilised, the stability of Nb₃Si was enhanced and the eutectic between Nb₅Si₃ and the solid solution was suppressed. The microstructure of NV4-AC contained Ti rich and Nb rich solid solutions, 3-1 and 5-3 silicides. The FeNb₄Si and Fe₂Nb₃ phases and the Ti rich oxide observed in NV8-AC were not formed in NV4-AC. The microstructure of NV4-HT consisted of (Ti,Nb)₃Sn, β(Ti,Nb)_ss, (Ti,Nb)₃Si and (Ti,Nb)₅Si₃ phases. The solubility of Fe in the Ti-based 3-1 silicide was significantly lower than in the Nb-based 3-1 silicide. The β(Ti,Nb)_ss + (Ti,Nb)₅Si₃ → (Ti,Nb)₃Si transformation was enhanced in NV4. The effects of Fe and Ti on the hardness of Nb–18Si–5Sn-based alloys, and of alloying elements on the hardness of Nb₃Sn, Ti₃Sn, and Nb₃Si, Ti₃Si, and Ti and Nb base 5-3 silicides are discussed.     

Investigation of the titanium–indium system

The phase diagram of the Ti–In system was determined using DTA, XRD and EDX analyses. The existence of the phases Ti₂In₅ [Mn₂Hg₅ type structure, space group P4/mbm, a=0.99995(3), c=0.29960(2) nm] and Ti₃In [Ni₃Sn type structure, space group P6₃/mmc, a=0.5978(1), c=0.4812(1) nm] was confirmed. The phase previously labeled Ti₃In₂ was found to exist in a narrow homogeneity region near Ti₅₆In₄₄. Rietveld refinement of the XRD powder pattern yielded solutions compatible with a Cu₃Au-type or a BiIn-type crystal structure, but not with a CuAu-type crystal structure. Furthermore, at 38.5 at.% In, a new phase was observed having a γ-brass related crystal structure [Ti₈In₅, space group , a=0.99578(6) nm]. The intermetallic phases were formed by a cascade of peritectic reactions ending in a eutectic at >99 at.% indium between Ti₂In₅ and (In) at 0.4 K below the melting temperature of pure indium.     

Production of Al–Ti–B master alloys from Ti sponge and KBF4

It is of great interest to replace the K₂TiF₆ salt so as to reduce the volume of fluoride-bearing particulate material to be added to the aluminium melt in the popular “halide salt” process. Ti sponge was used in the present work as the source of Ti in the production of an Al–5Ti–1B grain refiner. Addition of Ti granules into molten aluminium, either premixed with or before KBF₄ salt, has produced Al–5Ti–1B alloys where the boride particles were relatively few and predominantly of the AlB₂ type. The grain refining efficiency of these alloys were far from satisfactory. TiB₂ was the dominant boride phase with sufficient number of blocky aluminide particles when Ti, in excess of the TiB₂ stoichiometry was supplied before hand and the balance was reserved for co-addition with KBF₄. Al₃Ti particles were generated soon after the Ti solubility limit was exceeded in the first step while the boride particles were subsequently produced by the reaction between molten aluminium, KBF₄ and K₂TiF₆. The Al–5Ti–1B master alloy thus produced provided an adequate grain refining performance while the amount of particulate material to be added to the aluminium melt was reduced by nearly 30%.     

Texture of TiNi shape memory alloy sheets produced by roll-bonding and solid phase reaction from elementary metals

We have developed a new process to produce Ti–Ni shape memory alloys from titanium and nickel sheets. This process is composed of repetitive roll-bonding for producing a Ti/Ni laminated sheet from them, and of subsequent heat treatment for forming a TiNi intermetallic phase by reactive diffusion. The Ti/Ni laminated sheet changed to a nearly single phase TiNi material through the formation of Ti₂Ni and Ni₃Ti intermediate products. TiNi sheets obtained by this process consisted of a near <111>//ND texture such as {223}<110> in the B2 austenite phase. Recoverable strain associated with martensitic transformation of TiNi was fairly isotropic in the sheet plane due to the relatively isotropic character of {111}<uvw> texture. The formation of such texture through phase transformations by reactive diffusion has been discussed in view of texture inheritance.     

Production of Al–Ti–B grain refining master alloys from Na2B4O7 and K2TiF6

It is very desirable to replace the KBF₄ salt in the popular “halide salt” process to reduce the volume of fluoride salts to be added to molten aluminium in the production of Al–Ti–B grain refiners. Being over 2 times richer in B, Na₂B₄O₇ is a promising replacement for KBF₄, and is used in the present work to produce Al–Ti–B grain refiner master alloys. A fraction of the aluminide particles were entrapped in the spent salt giving a relatively lower Ti recovery when KBF₄ was replaced by Na₂B₄O₇. The grain refining performance of the Al–Ti–B grain refiner alloy thus produced was nevertheless acceptable. The spent salt became too viscous with the oxides, aluminides and borides to be removed by decanting when Na₂B₄O₇·5H₂O was used to supply boron. The viscous spent salt, entrained in the grain refiner alloy, did not only impair its performance, but also hurt the fluidity of the molten alloy and made pouring difficult.     

Influence of tin on the structure and properties of as-cast Ti-rich Ti–Si alloys

By the methods of DTA, X-ray diffraction, metallography and microprobe analysis, phase equilibria in the Ti-corner (more than 50 at.% Ti) of the Ti–Si–Sn system were studied. The solidus projection and the melting diagram (solidus+liquidus) were constructed. A new ternary compound T of composition Ti₅Si_1.2–1.6Sn_1.8–1.4 was found to form with the crystal structure of W₅Si₃-type. The ternary eutectic equilibrium L↔β-Ti+Ti₅Si₃+Ti₃Sn was established to occur at 1460 °C with the composition of the invariant point E at 77Ti–9Si–14Sn. Microhardness measurements were carried out for the primary grains of the alloys with 5 at.% Si.     

Thermodynamic assessment of the Ti–B system

A consistent thermodynamic data set for the Ti–B system is obtained by means of CALPHAD technology. The sublattice model is used to describe the solid solution phases: (Ti%)₁(B, Va%)_0.5 and (Ti%)₁(B, Va%)₃ for the terminal solution (Ti) and (βTi), and Ti₁(B%, Ti)₁ and (B, Ti%)₁(B%, Ti)₂ for the compound solution TiB and TiB₂, respectively. The intermetallic compound Ti₃B₄ is treated as a stoichiometric compound. The liquid solution phase is assumed to be a substitutional solution with Redlich–Kister formula for the expression of its excess Gibbs energy. The complete T–x phase diagram for the Ti–B binary system is given. The calculation results agree well with experiments.     

Titanium-boride eutectic materials: Phase equilibria and properties of alloys in the Ti-rich portion of the Ti–Sn–B system

Phase equilibria in the Ti–TiB–Ti₃Sn region of the Ti–Sn–B system were experimentally investigated in the melting (solidification) temperature range using metallography (including EPMA), XRD, DTA, and pyrometry. The phase diagram is presented as solidus and liquidus surfaces projections and a vertical section through the 7.5 at.% B. The Sn alloying was found to have a profound hardening effect on the eutectic titanium-boride alloys.     

Investigation on the microstructure and mechanical properties of a cast Mg–6Zn–5Al–4RE alloy

Mg–6Zn–5Al–4RE (RE = Mischmetal, mass%) alloy was prepared by metal mould casting method. The microstructure and mechanical properties of the as-cast and heat-treated alloys were investigated. The results show that the phase compositions of the as-cast state alloy are supersaturated solid solution -Mg, lamellar β-Al₁₂Mg₁₇, polygonal Al₃RE and cluster Al₂REZn₂ phases. The mechanical properties, especially the ultimate tensile strength and elongation of the alloy were significantly improved by the heat treatment. Fracture surface of tensile specimens was analyzed by optical microscope and scanning electron microscope.     

Microstructural characterization of NiCr1BSiC laser clad layer on titanium alloy substrate

Laser cladding of NiCrBSiC powders on Ti–6Al–4V alloy substrate was carried out, and the microstructure of the laser clad layer was characterized by TEM and SEM. Results show that the phases of TiC, TiB₂, CrB and M₂₃C₆ were formed in situ in the clad layer. The TiC phase exists in the form of dendrites with two types of interface morphology including the non-faceted and the faceted one. The TiB₂ phase nucleates on the facets of TiC dendrites, and can grow to form a special morphology of microstructure in which the TiC dendrite is encased by the TiB₂ phase. The CrB and M₂₃C₆ phases exist in the form of rod-shaped morphology, inside which stacking faults could be observed. The clad layer matrix consists of primary γ_p-Ni dendrites and lamellar eutectics of γ_e-Ni+Ni₃B. The formation mechanism of the microstructure of the clad layer was discussed.     

Structural chemistry and phase relations in intermetallic systems Ti–{Pd,Pt}–Al

Phase relations in the ternary systems Ti–{Pd,Pt}–Al have been experimentally established for the partial isothermal sections at 950°C in the Pd/Pt-poor region (<25 at.% Pd/Pt). The investigation is based on X-ray powder diffraction, metallography, SEM and EMPA techniques on about 45 alloys, which were prepared by various methods employing arc melting, levitation melting under argon or by powder reaction sintering in closed crucibles. Three ternary compounds were observed at 950°C in the Ti–Pd–Al system: τ₃-(Ti,Pd)(Ti,Pd,Al)₂ with Laves-MgZn₂-type, τ₂-(Ti,Al)₆(Ti,Pd,Al)₂₃₊₁ with a filled Th₆Mn₂₃₊₁-type and τ₁-(Ti,Pd,Al)(Ti,Pd,Al)₃ with AuCu₃-type. Due to the wide extension of the Laves phase field, there is no compatibility among γTiAl and τ₂-(Ti,Al)₆(Ti,Pd,Al)₂₃₊₁. The Ti–Pt–Al system at 950°C contains three ternary compounds: τ₃-(Ti,Al)(Ti,Pt,Al)₂ with Laves-MgZn₂-type, τ₂-(Ti,Al)₆(Ti,Pt,Al)₂₃₊₁ with the filled Th₆Mn₂₃₊₁-type and τ₁-(Ti,Pt,Al) with Cu-type. Compatibility exists for Al-rich γTiAl and τ₂-(Ti,Al)₆(Ti,Pt,Al)₂₃₊₁. The typical feature for both alloy systems studied is the three-phase equilibrium: ₂Ti₃Al+γTiAl+τ₃-(Ti,Pd/Al)(Ti,Pd/Pt,Al)₂. The solid solubility of palladium and platinum in the binary titanium aluminides, as observed from EMPA and X-ray data, is rather small and at 950°C accounts to about 2.5 at.% Pd and 2.0 at.% Pt. Two new oxide compounds Ti₃PdAl₂O_x and Ti₃PtAl₂O_x with a filled Ti₂Ni-type are observed in both quaternary systems.     

A fundamental study on Ti–6Al–4V's thermal and electrical properties and their relation to EDM productivity

There are strong needs for productive/quality machining strategies of notoriously “difficult-to-machine” aerospace materials. The current means of machining these materials is dominated by mechanical cutting methods, which are costly due to high tooling costs, poor surface quality and limitations in the workpiece features and operations that can be machined. The newest EDM technology may be able to circumvent problems encountered in mechanical machining methods. In this paper, the EDM technology has been used to machine titanium alloy Ti–6Al–4V to investigate the effect of Ti–6Al–4V's thermal and electrical properties on the EDM productivity. In the study, temperature measurements have been made for Ti–6Al–4V workpieces with various duty factors to clarify the essential causes of difficulty in machining titanium alloys and observe the optimal duty factor in terms of productivity and quality.     

A comparative study of precipitation behavior of Heusler phase (Ni2TiAl) from B2-TiNi in Ni–Ti–Al and Ni–Ti–Al–X (X=Hf, Pd, Pt, Zr) alloys

In support of the design of high strength TiNi-based shape-memory alloys, the precipitation of L2₁–Ni₂TiAl phase from a supersaturated B2–TiNi matrix at 600 and 800 °C is studied using transmission and analytical electron microscopy (TEM/AEM), and 3D atom-probe microscopy (3DAP) in Ni–Ti–Al and Ni–Ti–Al–X (X=Hf, Pd, Pt, Zr) alloys. A B2/L2₁ fully coherent two-phase microstructure is confirmed to be analogous to the classical γ/γ′ system in terms of precipitate shape, spatial distribution and a minimum distance of separation between L2₁ precipitates as dictated by the interplay between strain and interfacial energies. The effects are also confirmed to disappear with loss of coherency. These results lend further support, at least qualitatively, to the theoretical predictions of microstructural dynamics of coherent aggregates. Selected cohesive properties of stable and virtual B2 compounds are calculated by an ab initio method, showing good agreement with measured site occupancy and lattice parameters. A simple analysis of the L2₁ precipitate size evolution suggests that in the case of alloys with Al, Zr or Hf substitution for Ti, the precipitates follow coarsening kinetics at 600 °C and growth kinetics at 800 °C, while for alloys with Pd or Pt substitution for Ni, precipitates follow one kinetic behavior at both temperatures. The temperature-dependent partitioning behaviors of Hf, Pd, Pt and Zr are established by quantitative microanalysis using AEM and nanoscale analysis using 3DAP. Both Hf and Zr prefer to partition to the B2 phase at 800 °C while they exhibit reverse behavior at 600°C. Pt also partitions to B2 at 800 °C, while Pd partitions to the L2₁ phase at both 600 and 800 °C. To describe the composition dependence of the lattice parameter of multicomponent B2 and L2₁ phases, the atomic volumes of Al, Hf, Ni, Ti and Zr in B2 and L2₁ phases are determined, providing a model for the control of interphase misfit in alloy design.     

Influence of Nb and Mo contents on phase stability and elastic property of β-type Ti-X alloys

The energetic, electronic structure and elastic property offl-type Ti-Xx （X=Nb and Mo, x=0.041 7, 0.062 5, 0.125 0, 0.187 5, 0.250 0, 0.312 5 and 0.375） binary alloys were calculated by the method of supercell and augmented plane waves plus local orbitals within generalized gradient approximation. The elastic moduli of the polycrystals for these Ti1-xXx alloys were calculated from the elastic constants of the single crystal by the Voigt-Reuss-Hill averaging method. Based on the calculated results, the influence of X content on the phase stability and elastic property offl-type Ti1-xXx alloys was investigated. The results show that the phase stability, tetragonal shear constant C, bulk modulus, elastic modulus and shear modulus offl-type Ti1-xXx alloys increase with an increase of X content monotonously. When the valence electron number offl-type Ti1-xXx alloys is around 4.10, i.e. the content of Nb is 9.87% （molar fraction） in the Ti-Nb alloy and Mo is 4.77% （molar fraction） in Ti-Mo alloy, the tetragonal shear constant is nearly zero. The Ti1-xXx alloys achieve low phase stability and low elastic modulus when the tetragonal shear constant reaches nearly zero. In addition, the phase stability offl-type Ti1-xXx alloys was discussed together with the calculated electronic structure.     

连接温度对TiAl/Ti3AlC2扩散焊接头界面结构及性能的影响

采用Ti/Ni复合中间层实现了TiAl合金和Ti₃AlC₂陶瓷的扩散连接,利用SEM,XRD等分析方法对接头界面结构进行了分析.结果表明,TiAl/Ti₃AlC₂接头典型界面结构为TiAl/Ti₃Al+Al₃NiTi₂/Ti₃Al/α-Ti+Ti₂Ni/Ti₂Ni/TiNi/Ni₃Ti/Ni/Ni₃(Ti,Al)/Ni₃Al+TiC_x+Ti₃AlC₂/Ti₃AlC₂.随着连接温度的升高,TiAl/Ti界面处的Ti_ss层逐渐减小,Ti₃Al化合物层逐渐变厚;TiNi化合物层厚度显著增加,Ti₂Ni和Ni₃Ti层厚度基本保持不变.接头抗剪强度随连接温度升高先增加后减小,当连接温度为850℃时,接头的抗剪强度最高可达到85.3 MPa.接头主要在Ni/Ti₃AlC₂界面及Ti₃AlC₂基体处发生断裂.