Calculation of the long-range order parameter from atom probe data

In addition to the known diffraction techniques, field-ion microscopy with atom probe is well established for determining the long-range order parameter. The site occupation probabilities of the chemical species on the different sublattices may be estimated from experimental profiles. However, this evaluation method demands that the superstructure planes be identified unequivocally from the data. This condition is not fulfilled in all cases. We propose a new analytical method for which this condition need not be met.     

On the peritectoid formation of Ni5Al3

The peritectoid formation of Ni₅Al₃ from the two phases NiAl and Ni₃Al was studied in a Ni---Al alloy containing 66 at% Ni by means of transmission electron microscopy. The product phase does not form as a uniform layer between the two initial phases as expected and already observed in a few systems. In the system studied here there are only very few nucleation sites located at the NiAl/Ni₃Al interface. The further growth of Ni₅Al₃ takes place only into one of the initial phases which is NiAl. A strict orientation relationship between NiAl and Ni₅Al₃ was observed; the growth direction was [221]. The transformation is presumably diffusion controlled; it is very sluggish and it can be described by a nucleation and growth process. From the study presented here we conclude that the formation of Ni₅Al₃ proceeds by a micromechanism which differs from that normally assumed for peritectoid reactions.     

Alloy design of gamma titanium aluminides based on phase diagrams

Phase stability in the Ti---Al---X (X = Cr, Mo and W) system has been investigated at 1473 and 1573 K by the following methods: microstructure observation of quenched specimens, diffusion coupling experiments, thermal analysis of DTA and in situ observation of X-ray diffraction. The ternary phase diagrams of Ti---Al---X system are proposed; additions of Cr, Mo and W stabilize the β phase in the ternary phase diagrams. The tendency of ‘β stabilizer’ is in the following order: 6 at% Cr = 3 at% Mo = 1 at% W 10 vol% β phase. In these Ti-Al-X ternary systems, the + β + γ three-phase coexisting region is close to the Ti---Al binary line and sifts slightly toward a higher concentration of Al as temperature increases. Based on the ternary phase diagrams, (γ + β) TiAl having a super-plastic capability and partial lamellar (P-L) microstructure which shows relatively well balanced mechanical properties from room temperature to elevated temperatures have been rationalized.     

Phase equilibria and microstructural control of gamma TiAl based alloys

Phase equilibria among the β (bcc or B2), (hcp) and γ (L1₀) phases in Ti---Al---M ternary systems at elevated temperatures have been studied, where the M is β stabilizing element for pure titanium. The β(B2) + + γ three-phase coexisting region exists at temperatures above 1473 K, and it moves towards a direction of high aluminum and high M concentrations with increasing temperature. The change in the phase equilibria by the addition M is associated with the ↔ β allotropic transformation temperature of pure titanium and can be thermodynamically interpreted in terms of the lattice stability ratios of M to Ti. The change in the phase equilibria results in new reaction pathways peculiar to the ternary systems, thereby opening more possibility for microstructure control of the gamma based alloys. The vertical section drawn based on these studies demonstrates that the γ phase can be in equilibrium with the β(B2) phase at relatively low temperatures in the ternary systems and create a reaction pathway of → β(B2) + γ. Upon cooling along the same pathway of → γ as in the binary alloy, the addition of third element affects the transformation rate, and the massive γ structure is found to be formed even under slow cooling rate.     

Microstructure and mechanical properties of orthorhombic alloys in the Ti---Al---Nb system

The current understanding of the metallurgy of the orthorhombic alloys in the Ti---Al---Nb system is reviewed with emphasis on tensile and creep properties of ternary alloys. It is shown that increasing the Nb content of alloys from 15 to 27 at% at a constant Al level significantly increases both the tensile and creep properties of equiaxed as well as lath structures, while small changes in Al content have a large effect on creep. For a given alloy composition, the amount of B2(β) phase and its distribution and the scale of O laths influences tensile properties, while creep properties depend on the volume fraction of equiaxed ₂/O phase present in the structure as well as the size of O laths.     

Concentration and temperature dependence of titanium self-diffusion and interdiffusion in the intermetallic phase Ti3Al

Tracer diffusion of ⁴⁴Ti was measured between 1373 and 1126 K in four alloys in the composition range from 25 to 35 at% Al in ordered Ti₃Al using the standard precision grinding sectioning technique and polycrystalline samples. The self-diffusion coefficient D^*_Ti was found to be lower than self-diffusion in pure -Ti and to increase very little with Al concentration. D^*_Ti reveals Arrhenius behaviour which is described by the frequency factor D₀ = (2.44 _−1.04^+1.81) · 10⁻⁵ m²/s and the activation enthalpy Q_Ti = (288.2 ± 5.7) kJ/mol. Especially, the stoichiometric composition shows no distinguished diffusion behaviour.

Interdiffusion was measured in single phase conditions by combining samples of 25 and 35 at% Al. The interdiffusion coefficient was evaluated by Boltzmann-Matano analysis between 26 and 34 at% Al at different temperatures. Again, only a weak concentration dependence of and an Arrhenius behaviour were detected. Applying Darken's equation, the self-diffusion coefficient of aluminium D^*_Ti was calculated by combining with the thermodynamic factor Φ(X_{Al, T}) in Ti₃Al. D^*_Al turned out to be smaller than D^*_Ti, e.g. by a factor of 6 at 1170 K. The Arrhenius relation is characterized by D₀ = (2.32 _−1.20^+2.48) · 10⁻¹ m²/s and Q_Al = (394.5 ± 7.5) kJ/mol. Different jump possibilities of the Al atoms are discussed. From the overall diffusion behaviour in the Ti₃Al-phase it is concluded that atomic migration proceeds via thermal vacancies. There is no indication regarding the formation of constitutional vacancies. Diffusion behaviour of the components Al and Ti in Ti₃Al is compared with our recent results of Al impurity diffusion (SIMS analysis) and self-diffusion in pure -Ti.     

Hot deformation behavior of a Fe3Al-binary alloy in the A2 and B2-order regimes

A binary Fe₃Al alloy is investigated with respect to hot deformation behavior and microstructural as well as microtextural modifications. Applying the hot deformation simulator (WUMSI) to hot rolling conditions in the A2 and B2-order regimes in combination with data analysis, significant changes in deformation behavior are identified. These conditions are selected for performing hot rolling experiments. The differences in microstructure are investigated. On the basis of microtexture investigations by means of electron backscatter diffraction (EBSD) differences concerning orientation gradients and sub-grain structures are found. A model of combined order-related and non-order related effects is proposed explaining the observed material behavior. The information gained is the basis for the optimization of the thermomechanical treatment to produce ductile Fe₃Al sheet material.     

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 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.     

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.     

Strengthening of iron aluminide alloys by atomic ordering and Laves phase precipitation for high-temperature applications

The ternary system Fe–Al–Ta allows the formation of the hard and brittle ternary Laves phase Ta(Fe_0.5+x,Al_0.5−x)₂ with hexagonal C14 structure. The present study concentrates on Fe–Al–Ta alloys with small Ta contents between 2 and 6 at.% and various Al contents between 0 and 45 at.%. The phase equilibria in the ternary Fe–Al–Ta system at 1000 °C are studied experimentally for determination of the solubility limits of Ta in iron aluminide matrices and types of phases and structures which may occur at high temperatures. It is observed that small amounts of Laves phase together with atomic ordering increase the yield stress and affect ductility in a complex way.     

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.     

The oxidation of nanocrystalline Ni3Al fabricated by mechanical alloying and spark plasma sintering

Nanocrystalline Ni₃Al was fabricated through mechanical alloying of elemental powders and spark plasma sintering. The nanocrystalline Ni₃Al has a nearly full density after being sintered at 1223 K for 10 min under a pressure of 65 MPa. Isothermal and cyclic oxidations of nanocrystalline Ni₃Al were tested at 1173–1373 K with intervals of 100 K. The results indicate that nanocrystalline Ni₃Al exhibits excellent isothermal and cyclical oxidation resistance. The oxide scales consist primarily of dense and continuous -Al₂O₃. The grain refinement is beneficial for improving the oxidation resistance of Ni₃Al by providing more nucleation centers for the Al₂O₃ formation, promoting the selective formation of Al₂O₃ and improving the adhesion of oxide scales to the matrix.     

Shear-induced chemical disordering in Ni3Al at different temperatures

Molecular dynamics simulations have been used to study shear-induced chemical disordering in Ni₃Al lattices at different temperatures and strain rates. Shearing determines the formation of an amorphous layer, the thickness of which increases linearly with the square root of time. The rate at which the amorphous layer grows is both shearing rate- and temperature-dependent. A linear correlation between the amorphous layer growth rate and the shear modulus is found. This suggests that mechanical properties could play a central role in shear-induced disordering processes.     

Crystal growth of TiAl alloys

The growth of TiAl alloys over a range of Al concentrations has been considered for ingots processed by the floating zone technique. For Al-rich alloys, single crystals of γ-TiAl cannot be grown for compositions below Ti-54 at% Al since a banded microstructure forms due to the limitations imposed by the L + → γ peritectic reaction. However, near stoichemetric TiAl crystals can be grown by the traveling solvent method when using a TiAl---Co flux, although optimum processing conditions have not yet been realized. For Ti-rich alloys, evidence of a growth morphology consisting of β-phase dendrites embedded within a continuous matrix of the peritectic -phase is found for ingots processed up to 200 mm h⁻¹. The final lamellar orientation of the ingot is then determined by the orientation of the peritectic -phase and not by that of the leading β dendrites.     

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.     

Kinetics of hydrogen absorption and desorption of magnesium nickel alloy

Reaction rates in the Mg₂Ni---H₂ system are still unclear, although they are needed for the process/optimization of hydrogen storage reactors. In this article, the rates of hydrogen absorption and desorption of the magnesium nickel alloy were investigated at different pressures up to 2.0 MPa using a constant-temperature thermogravimetric method. The powder of the alloy was microencapsulated with copper to improve the alloy properties (such as cycling strength) as a pretreatment. In the experiments, weight changes of the smallest amount possible of samples were monitored with the help of an ultra-accurate thermobalance, in order to avoid changes of the sample temperature due to the heat of the reaction. The reaction curves obtained revealed unique dependence of temperature and pressure and influence of thermal hysterisis. It is also possible that hydrogen-absorption and-desorption rates of this hydrogen storage alloy are expressed by the rate equation of first-order reversible reaction.     

Densification and microstructural evolution of a high niobium containing TiAl alloy consolidated by spark plasma sintering

Gas-atomized Ti–45Al–7Nb–0.3W alloy powders were consolidated by the spark plasma sintering (SPS) process. The densification course and the microstructural evolution of the as-atomized powders during SPS were systematically investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and electron back-scattered diffraction (EBSD) techniques. As a result of SPS densification, special (α + γ) precipitation zones are formed in the initial stage of sintering, and the residual β phases in the microstructure of the powders are fragmentated. During the following SPS course, α₂/γ lamellar colonies at the edge of the precipitation zone, α₂ and B2 phase as well as dynamic recrystallized γ grains are found to form. For the as-atomized powders sintered at 1000 °C, the densification is preceded by the early rearrangement of the powder particles and the following formation of sintering necks. For the powders sintered at 1200 °C, plastic deformation plays an important role in densification. Local melting and surface bulging between two adjacent particles can also serve as one of the densification mechanisms. In the later stage of sintering, the growth of sintering necks controlled by diffusion and the pore closure would make important contributions to the densification.     

Reaction behavior and pore formation mechanism of TiAl–Nb porous alloys prepared by elemental powder metallurgy

Ti–48Al–6Nb (at.%) porous alloys are fabricated by elemental powder metallurgy to study the pore formation and propagation mechanism. Reactive diffusion, pore formation process, and pore characteristics of the porous TiAl–Nb alloys are investigated at different temperatures. It is found that the porous alloys exhibit a uniform, maze-like network skeleton, viz., a typical α₂-TiAl₃/γ-TiAl fully lamellar microstructure. The reactive diffusivities between Ti and Al powders are dominant during the Ti–Al–Nb powder sintering. Gas release during sintering also plays an important role in the pore propagation and the compact expanding process. In addition, a pore-formation model is proposed to interpret the growth mechanism of pores and skeletons.     

Effect of lamellar plates on creep resistance in near gamma TiAl alloys

Effect of interlamellar spacings on creep rate in Ti-48 at% Al alloy with fully transformed lamellar structure (FL) has been investigated at 1123 K/98 MPa. In addition, the correlation between creep rate and lamellar orientation to stress axis was elucidated by conducting creep tests at 1148 K/68.6 MPa for three single crystal (SC) specimens of Ti-48 at% Al with different lamellar orientations to stress axis. The difference in creep rate among FL specimens having different interlamellar spacings could not be defined in the early stage of transient creep. The onset of accelerating creep was slightly retarded by decreasing the inter-lamellar spacing. While the creep rate of the SC specimen whose lamellar orientation to stress axis 30–63 ° gradually decreases with increasing strain, and is larger than that of the FL specimen, the creep rate of the SC specimen with lamellar orientation nearly parallel to stress axis drastically decreases within small strain, and is 1/50 smaller than that of the FL specimen. This strong lamellar orientation dependence of creep rate is interpreted by the correlation between dislocation slip system and the lamellar plate.