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.     

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.     

Microstructural changes and estimated strengthening contributions in a gamma alloy Ti–45Al–5Nb pack-rolled sheet

Thin sheets made of a gamma-titanium aluminide alloy, Ti–45Al–5Nb, produced by a pack-rolling process, were evaluated for microstructure variation and evolution taking place during aging and annealing treatments. The as-received sheet material was characterized by remarkably high yield strength, 810 MPa, and a complex bimodal microstructure. The microstructure consisted of a matrix of twinned gamma-phase grains and fine-lath lamellar grain microconstituent, together with a dispersed ultra-fine-grained gamma + alpha-2 mixture microconstituent. High-temperature isothermal aging treatments changed the microstructure to a stable mixture of gamma-phase grains (matrix) and coarse alpha-2-phase particles, having size distributions and volume fractions that were specific to the aging temperature. A concurrent strength loss reflects this trend and results in a stable strength level of 550 MPa upon aging at 1000 °C for 144 h. Using composition estimates from the phase-boundary shifts that occur from the Nb addition to a Ti–45Al base alloy and, the rule of mixtures, an analysis was made to show that the gamma-phase matrix has an intrinsic strength of 178 MPa. This is a significant intrinsic strength level, well over that of 70 MPa for the Ti–45Al binary alloy. This is rationalized as the solid-solution strengthening effect from shifts of the Ti and Nb levels in the gamma phase and, by an added effect due to increased oxygen solubility in the gamma phase. The overall strength of Ti–45Al–5Nb, however, is roughly the same as that of Ti–45Al, and this is explained by a drastic reduction in the volume fraction of alpha-2-phase in Ti–45Al–5Nb alloy, which is a result of the Nb-induced phase-boundary shifts.     

Influence of cooling rate on microstructure and mechanical properties of a Ti–48Al alloy

The microstructure of a Ti–48Al alloy cooled after a solution treatment in the -field is influenced by the cooling rate. In order to characterize the lamellar structure and to know the values of ₂-volume fraction, width of ₂-lamellae, and interlamellar spacing, we developed a method of measurement based on an observation of laths in the scanning electron microscope. The study revealed the heterogeneous distribution of the lamellar structure in a commercial purity Ti–48Al alloy cooled from the -field at a rate of 35°C/min. The evolution of ₂-volume fraction with the cooling rate showed that the microhardness is determined by the quantity of ₂-phase. The values of yield stress also increased and were all the less scattered as the structure became more perfectly lamellar. Yield stress and interlamellar spacing are linked by a Hall–Petch relation.     

Characteristics of the tensile flow behavior of Ti–46Al–9Nb sheet material – Analysis of thermally activated processes of plastic deformation

Flow behavior, strain hardening and activation parameters, i.e. activation volume, stress exponents and normalized free enthalpy of activation, of Ti–46Al–9Nb sheet with near-gamma microstructure have been investigated in tension tests between 700 and 1000 °C. The dependence of yield stress on temperature and strain rate, the course of the strain hardening curves and the values of activation parameters show that thermally activated dislocation mechanisms are mainly involved in the tensile deformation process of the investigated material. At constant temperature the value of the activation volume depends both on plastic strain and strain rate. The activation volume generally decreases with increasing strain. The decrease is particularly well observable for higher strain rates, thus indicating a growing role of thermally activated climb mechanisms governing the process of dynamic recovery. The activation volume calculated for a constant plastic strain (2% in case of this study) is a function of temperature and strain rate. At lower deformation rates, or alternatively at higher temperatures, the activation volume increases. Such behavior indicates a decrease in dislocation density due to the onset of dynamic recrystallization. The analysis of stress exponents and the obtained free enthalpy of activation confirm that different thermally activated processes are acting during deformation under the tensile test conditions studied.     

Thermophysical properties of a Ti–44%Al–8%Nb–1%B alloy in the solid and molten states

The families of titanium aluminide intermetallic alloys have attractive high temperature mechanical properties which make them potential candidate materials for a wide range of applications, particularly in the aeronautic and automobile sectors. The development of appropriate manufacturing techniques is an essential stage in the engineering exploitation of these materials, e.g., Induction Skull Melting is one of the techniques which needs to be optimised for the casting of titanium aluminides. Research is underway to develop a computer model of this process but data are required for the key thermophysical properties. Pulse-heating techniques have been used to measure properties for the Ti–44Al–8Nb–1B system. Rectangular samples have been prepared and are resistively heated as part of a fast capacitor discharge circuit. Time-resolved measurements with sub-μs resolution of currents through the specimen were made with a Pearson probe current monitor using the induction principle. Voltages across the specimen were determined with knife-edge contacts and voltage dividers, and radiance temperatures of the sample were measured with a pyrometer. These measurements allow the calculation of specific heat and dependencies between enthalpy, electrical resistivity and temperature of the alloy up into the liquid phase. Data for thermal diffusivity have been obtained by using the Wiedeman–Franz relation. The results are compared with those obtained using DSC and the four-probe method to measure the temperature dependence of the resistivity.     

Effect of directional solidification and heat treatments on the microstructure and mechanical properties of multiphase intermetallic Zr-doped Ni–Al–Cr–Ta–Mo alloy

The effect of directional solidification and heat treatments on the microstructure and mechanical properties of intermetallic Ni–21.29Al–7.04Cr–1.46Ta–0.64Mo–0.57Zr (at.%) alloy was studied. Increasing growth rate is found to decrease primary dendrite arm spacing and to increase volume fraction of β(NiAl)-based dendrites and low melting point γ′(Ni₃Al)/Ni₅Zr eutectic. Room-temperature tensile yield strength and ultimate tensile strength increase and plastic elongation to fracture decreases with the increasing growth rate. Two types of heat treatments of directionally solidified (DS) specimens including two-step ageing at temperatures of 1273 and 1123 K and two-step solution annealing at 1373 and 1493 K were performed. Ageing at 1273 and 1123 K decreases volume fractions of the dendrites and eutectic regions and leads to a coarsening of spherical -Cr and needle-like γ′ precipitates within the β-phase. Annealing at 1373 K for 100 h is shown to be sufficiently long to completely dissolve the eutectic regions. Compressive yield strength increases with increasing temperature reaching a peak value at about 1023 K and then decreases at higher temperatures. Minimum creep rate is found to depend strongly on the applied stress and temperature according to a power law. The power law stress exponent n is determined to be 5.1 and apparent activation for creep Q_a is measured to be 326 kJ/mol.     

Characterization of the hot deformation behavior of a Ti-22Al-25Nb alloy using processing maps based on the Murty criterion

Isothermal compression testing of Ti-22Al-25Nb alloy was carried out at deformation temperatures between 940 and 1060 °C with strain rate between 0.001 and 10 s⁻¹, and a height reduction of 50%. The hot deformation behavior of Ti-22Al-25Nb alloy was characterized based on an analysis of the stress-strain behavior, kinetics and the processing map, for obtaining optimum processing windows and achieving desired microstructures during hot working. The constitutive equation was established, which described the flow stress as a function of the strain rate and deformation temperature. The apparent activation energies were calculated to be 788.77 kJ/mol in the α₂ + β/B2 + O phase region and 436.23 kJ/mol in the α₂ + B2 phase region, respectively. Based on Dynamic Material Model and the Murty instability criterion, the processing map for the Ti-22Al-25Nb alloy was constructed for strain of 0.6. The map exhibits a stable domain for the temperature range of 940-1060 °C and strain rate range of 0.001-0.1 s⁻¹ with two peaks in power dissipation of 51 and 56%, occurring at 940 °C/0.001 s⁻¹ and 1060 °C/0.001 s⁻¹, respectively. One is associated with lamellar globularization, and the other displays a phenomenon of recrystallization. Therefore, the desired processing condition of the Ti-22Al-25Nb alloy is 940 °C/0.001 s⁻¹ in the α₂ + β/B2 + O phase field. Moreover, the material also undergoes flow instabilities at strain rates higher than 1 s⁻¹. This instability domain exhibits flow localization and adiabatic shear bands which should be avoided during hot processing in order to obtain satisfactory properties.     

Texture and microstructure evolution during tempering of gamma-massive phase in a TiAl-based alloy

This work focuses on the mechanisms of microstructure and crystallographic texture evolution during gamma-massive transformation and subsequent tempering treatments, in the case of the Ti–48Al–2Nb–2Cr (at%) TiAl-based alloy. A complete massive structure is obtained by ice-water quenching. The temperature range for the destabilization of this massive structure is estimated from DSC measurement. Then, tempering is performed in both α₂–γ and α–γ phase fields. Microstructure evolution is studied by transmission and scanning electron microscopy observation of samples from different stages of the heat treatments. Isotropic samples resulting from a powder-metallurgy route are used for this purpose. Then, working on the samples from investment casting, the reduction of solidification texture through massive then tempering treatments is analyzed at the light of the mechanisms of microstructure evolution identified previously.     

Effect of Nb particles on the flow behavior of TiAl alloy

High temperature compressive deformation behaviors of PM-TiAl alloy containing Nb particles (Ti–45Al–5Nb–0.4W/2Nb (at. %)) were investigated at temperatures ranging from 1050 °C to 1200 °C, and strain rates from 0.001 s⁻¹ to 1 s⁻¹. The flow curves were employed to develop constitutive equations, and the apparent activation energy of deformation Q was determined as 447.35 kJ/mol. A revised processing map was constructed on the basis of the flow stress, which can accurately describe the deformation behaviors and predict the optimum hot forging condition. The addition of 2% Nb particles reduces the peak stress and increases the activation energy of TiAl-based intermetallic, however, it increases the instable domain in the processing map.     

Deformation behaviour and oxidation resistance of single-phase and two-phase L21-ordered Fe–Al–Ti alloys

Single-phase Fe–Al–Ti alloys with the Heusler-type L2₁ structure and two-phase L2₁ Fe–Al–Ti alloys with MgZn₂-type Laves phase or Mn₂₃Th₆-type τ₂ phase precipitates were studied with respect to hardness at room temperature, compressive 0.2% yield stress at 20–1100 °C, brittle-to-ductile transition temperature (BDTT), creep resistance at 800 and 1000 °C and oxidation resistance at 20–1000 °C. At high temperatures the L2₁ Fe–Al–Ti alloys show considerable strength and creep resistance which are superior to other iron aluminide alloys. Alloys with not too high Ti and Al contents exhibit a yield stress anomaly with a maximum at temperatures as high as 750 °C. BDTT ranges between 675 and 900 °C. Oxidation at 900 °C is controlled by parabolic scale growth.     

Non-equilibrium microstructures in TiAl–2Fe alloy

Microstructure evolution in Al-49.6 at% Ti-1.9 at% Fe alloy during cooling from 1400 and 1300°C leading to non-equilibrium structure is presented and discussed. ©     

The ternary system Al–Ni–Ti Part II: thermodynamic assessment and experimental investigation of polythermal phase equilibria

The Al–Ni–Ti phase diagram has been thermodynamically assessed and a consistent set of thermodynamic functions has been developed. The thermodynamic modeling is based on an experimental investigation of the phase equilibria in the composition range of 0.1x_Al0.7. Alloys were prepared by argon-arc or vacuum-electron beam melting of elemental powder blends. X-ray powder diffraction, metallography, SEM and EMPA-techniques were employed to analyze the samples in the as-cast state as well as after annealing at 800, 900 and 1000°C. The existence of the four ternary compounds, τ₁ to τ₄, has been confirmed, although homogeneity regions differ significantly from reports in the literature. The homogeneous phase, previously claimed at “Al₂₃Ni₂₆Ti₅₁”, is shown by high resolution microprobe and X-ray diffraction measurements to be an extremely fine-grained eutectic structure. The congruent melting behavior of τ₄=AlNi₂Ti is confirmed, but, in contrast to earlier reports, primary crystallization and congruent melting have been observed for τ₁=Al₁₃Ni₂Ti₅ and τ₃=Al₃NiTi₂. In contrast to earlier assessments, τ₁,τ₂ and τ₃ are experimentally found to be stable at 800, 900 and 1000°C. The thermodynamic modeling of the ternary phases τ₂ and τ₃ is done with simplified sublattice models, considering their crystal structure and homogeneity ranges. The sublattice model for τ₄ is taken from an earlier asessment of the nickel-rich ternary phase equilibria. The present assessment covers the entire composition range. An application to the solidification behavior of ternary alloys is also exemplified.     

Microstructures and mechanical properties of Fe3Al-based Fe–Al–C alloys

In this paper results on the microstructures and mechanical properties of Fe₃Al-based Fe–Al–C alloys with strengthening precipitates of the perovskite-type κ-phase Fe₃AlC_x are presented. The alloys are prepared by vacuum induction melting and cast into Cu-moulds. The composition of the Fe₃Al matrix of the investigated Fe–Al–C alloys varies between 23 and 29 at.% Al. The ternary C-additions range from 1 to 3 at.%. The microstructures of the alloys are characterised by means of light optical microscopy (LOM). Phase identification is performed by means of X-ray diffraction (XRD). The strength of the alloys as a function of temperature is determined through compression tests. The room-temperature ductility is evaluated by tensile tests. The fracture surfaces of the tensile specimens are analysed using scanning electron microscopy (SEM).     

Structure of Al–Ni intermetallic composite layers produced on the Inconel 600 by the glow discharge enhanced-PACVD method

The Plasma Assisted Chemical Vapor Deposition (PACVD) treatment conducted under glow discharge conditions in an atmosphere of trimethylaluminum vapors applied to an Inconel 600 substrate yielded composite surface layers built of intermetallic phases of the Al–Ni system with the outer zone composed of aluminum oxides. Such layers have very advantageous performance properties, such as high hardness, good corrosion and frictional wear resistance and, good adherence to the substrate.The present study is dedicated to microstructure characterization of the layers. The layers were examined using a variety of methods. Based on the results of these examinations, the microstructure of the composite layers was described as a multizone one with an outer Al₂O₃ zone, an intermediate AlNi₃ + Al₂O₃ zone and a diffusion zone of type Ni(Al,Cr,Fe) + AlNi₃ + Cr₇C₃. The mechanism of layer formation as well as the correlation between the microstructure and the observed improvement of the surface properties of the Inconel 600 alloy are discussed.     

Martensite in a TiAl alloy quenched from beta phase field

Hexagonal martensite with well-developed midribs was obtained by iced-brine quenching of Ti44Al4Nb4Hf0.1Si from the beta phase field. Crystallographic characterisation shows great resemblances of the martensite in this alloy to the lenticular martensite in Fe–Ni alloys in the morphology and substructures of martensite plates. It also bears similarities with the martensite in titanium and its alloys in habit planes and orientation relationship with parent phase. Martensitic transformation under the conditions employed in this work was incomplete and the remaining beta phase after martensitic transformation, thus, underwent diffusional beta-to-alpha transformation or massive transformation, depending on the local circumstances.     

Ordering in the sublattices of Fe3Al during the phase transformation B2↔D03

The temperature dependence of the iron concentrations in the individual sublattices of hyperstoichiometric binary Fe₇₂Al₂₈ and ternary Fe₆₈Al₂₈Cr₄ alloys were obtained from X-ray diffraction data measured in a high temperature vacuum chamber during linear heating around the phase transformation B2↔D0₃. A method for the processing of the diffraction pattern based on the splitting of the diffraction lines of the structure D0₃ into three groups is presented. Applying this method it was found that the structure B2 was not well developed in both samples. The maximum value of c_C≈0.8 gives S_B2 equal to 0.4 and 0.3 for binary and ternary alloy, respectively. The D0₃-order was not well developed too, because structure D0₃ arises from the structure B2. D0₃-ordering, i.e. redistribution of atoms within the sublattices A and B, is given only by the total number of iron atoms in these sublattices before the phase transformation B2↔D0₃.     

The ternary system Al–Ni–Ti Part I: Isothermal section at 900°C; Experimental investigation and thermodynamic calculation

Phase relations in the ternary system Al–Ni–Ti have been experimentally established for the isothermal section at 900°C for concentrations 0.1x_Al0.7. The investigation is based on X-ray powder diffraction, metallography, SEM and EMPA-techniques on about 40 ternary alloys, prepared by argon-arc or vacuum-electron beam melting of proper elemental powder blends. The existence of four ternary compounds, τ₁ to τ₄, is confirmed, however, in contrast to earlier investigations at significantly different compositions and with different shape of the homogeneity regions. This is particularly true for the phase regions of τ₃-Al₃NiTi₂ with the MgZn₂-type structure ranging from Al₃₀Ni₂₈Ti₄₂ (composition lowest in Al) to Al₅₀Ni₁₆Ti₃₄ (composition richest in Al) and for τ₂-Al₂NiTi. The complex atom site substitution mechanism in τ₃ changing from Ti/Al exchange at Al-poor compositions towards Ni/Al replacement for the Al-rich part was monitored in detail by quantitative X-ray powder diffraction techniques (Rietveld analyses). In contrast to earlier reports, claiming a two-phase region Ni{Al_xTi_1-x}₂+τ₃, we observed two closely adjoining three-phase equilibria: ₂-AlTi₃+Ni{Al_xTi_1-x}₂+ τ₄-AlNi₂Ti and ₂-AlTi₃+τ₃-Al₂NiTi₂+τ₄-AlNi₂Ti. The earlier reported “homogeneous phase at Al₂₃Ni₂₆Ti₅₁′” was shown by high resolution microprobe and X-ray diffraction measurements to be an extremely fine-grained eutectic. The experimental results are in fine agreement with the thermodynamic calculation.     

On the four-phase reactions in the Ti–Ni–Al system

Two four-phase reactions of transition type in the Ti–Ni–Al system were studied on several alloys, which were annealed at carefully set temperatures and quenched. The phase constitution was established by XRD and EPMA analyses. Due to sluggish reaction kinetics, the transition temperatures were defined by annealing and quenching techniques as no DTA signals could be received. For the reaction NiAl + TiNiAl  TiNiAl₂ + TiNi₂Al, the transition temperature was found to be 925 °C ± 15 °C and for the reaction TiNiAl + Ti₃NiAl₈  TiAl₂ + TiNiAl₂, the transition temperature was found to be 990 °C ± 15 °C. Furthermore we confirmed the three-phase field TiNi₂Al + Ti₃Al + Laves phase (TiNiAl), as reported at 900 °C by Huneau et al. in 1999.     

Crystallography and phase transformation mechanisms in TiAl-based alloys – A synthesis

The variation in cooling rates of Ti–46.8Al–1.7Cr–1.8Nb (at. %) alloy from the high-temperature α domain produces lamellar, Widmanstätten, feathery-like (α₂ + γ) structures, as well as γ-massive phase, often coexisting together. Earlier reported crystallographic and morphological details of each of these structures are compiled together and a combined view on the generation of their crystallographic related possible variants and their solid phase transformation mechanisms is proposed. Sympathetic nucleation is suggested as a common mechanism for the lamellar structure at slow cooling rate, the Widmanstätten structure and the feathery-like structure.