Thermal vacancies and the yield anomaly of FeAl

We review here the George-Baker model for the yield strength anomaly of FeAl which is based on hardening by thermal vacancies at intermediate temperatures and dislocation creep at high temperatures. Results of up-quenching and down-quenching experiments, which corroborate the vacancy hardening mechanism, are discussed. Some implications of the model are compared with available experimental results.     

The stress anomaly in FeAl–Fe3Al alloys

The anomalous stress peak observed near 500–600 °C in Fe–Al alloys has now been convincingly explained using a model of hardening by immobile thermal vacancies on the lower temperature side of the peak and the loss of hardening as these vacancies become mobile at higher temperatures. The large numbers of vacancies required for such hardening are associated with compositions close to stoichiometry, i.e. 40–50%Al, raising the question of whether such a vacancy hardening model can be adopted for Fe₃Al alloys, which show a similar stress peak anomaly. Examination of data on vacancy formation over the entire range of composition, Fe–Fe₃Al–FeAl, shows that, indeed, a vacancy hardening model appears capable of explaining the stress anomaly for both FeAl and Fe₃Al.     

Hydrogen permeation in Fe-40 at.% Al alloy at different temperatures

The electrochemical permeation technique for studying transport and trapping of hydrogen in Fe-40 at.% Al alloy at temperatures of 5, 25, 45 and 65 °C was used in the paper. The influence of temperature on the effective hydrogen diffusion coefficient, hydrogen permeation rate and hydrogen solubility was determined. The activation energy of hydrogen diffusion in iron aluminide in the studied temperature range was also determined.     

Modelling of defects and amorphisation by ball milling of γ-TiAl

The formation and site preference energies and volume changes of single and pair of defects of ternary alloying elements in γ-TiAl intermetallic compound were studied by the density functional theory. Slight tendency to clusterization of antisite defects has been found. This may lead to disorder in the system. The V and Cr atoms prefer to reside in the Ti sublattice. The formation energy for Cr–Cr, Cr–V and V–V nearest neighbour pairs are in the 1.3–2 eV range. The Al antisite in Ti sublattice requires much less energy than the Ti antisite in Al sublattice. The amorphisation process of TiAl alloy was studied by means of high energy ball milling of Ti and Al elemental powders, which produces amorphous structure after 40 h. The amorphous states were studied by the DFT calculations of many random atomic configurations and the results were compared with the NiAl compound. Possible explanation for the amorphisation of the TiAl compound is presented.     

Dependence of the brittle-to-ductile transition temperature (BDTT) on the Al content of Fe–Al alloys

The brittle-to-ductile transition temperature (BDTT) of binary Fe–Al alloys with between 9.6 and 45 at.% Al was investigated in the as-cast state by four-point bending tests. An increase of the BDTT was observed with increasing aluminium content between 9.6 and 19.8 at.%. Up to 41.3 at.%, the BDTT did not change significantly. A sharp increase of the BDTT occurred between 41.3 and 45 at.% Al. Transgranular cleavage was observed at a composition of 25 at.% Al, mixed-mode fracture between 39.6 and 41.3 at.% Al and intergranular fracture at 45 at.% Al. The results indicate that the increase in BDTT is correlated with the transition from mixed-mode to intergranular fracture.     

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.     

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.     

Thermophysical properties of FeAl (Fe-40 at.%Al)

The thermophysical properties — electrical resistivity, thermal conductivity, thermal expansion, and specific heat, of a B2 iron-aluminide (Fe-40 at.% Al) alloy are measured. The measured values of electrical resistivity indicate three distinct regions. An initial sharp rise below 400°C is followed by a gradual increase to near saturation around 900°C. Resistivity above this temperature exhibits an anomalous negative temperature dependence. The thermal conductivity displays a continuous rise as a function of temperature for T<800°C, beyond which it saturates to a value of 0.17 W/cm-°C. The relation between electrical resistivity and thermal conductivity obeys the Wiedemann-Franz law signifying the dominance of electrons in the heat transport. The measurements of specific heat indicate a complex behavior suggesting inseparable contributions of various temperature dependent phenomena arising from phonons, conduction electrons and magnons. Both the thermal expansion and mean coefficient of thermal expansion (MCT) exhibit a rising trend with temperature. The temperature dependence of the various modes of lattice, electronic, and magnetic excitations is invoked to explain the observed variations in properties. The role of the inherent electronic and magnetic structure on physical properties is highlighted.     

Solid-solution hardening and softening by Fe additions to NiAl

Solid-solution hardening in the case of a ternary alloy addition to a B2 compound with the triple defect structure has been investigated. The fact that the ternary element may occupy either of two sublattices or may affect the concentration of other types of point defects present in the material makes this a very interesting problem to consider. Ni-rich (40 at% Al), stoichiometric (50 at% Al), and Al-rich (52 at% Al) alloys were doped with up to 12 at% Fe. Lattice parameter, bulk density and hardness measurements were performed on samples quenched from 1000 °C. It was found that solid-solution softening actually occurs in the Ni-rich alloys, while hardening was observed in the stoichiometric and Al-rich alloys. The vacancy concentration was determined from the experimental data, and the site occupancies of the Fe atoms were estimated from a thermodynamic model. Through careful consideration of all point defect concentrations the solid-solution hardening and softening behaviors could be effectively rationalized.     

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.     

Mechanical properties and oxidation behaviour of two-phase iron aluminium alloys with Zr(Fe,Al)2 Laves phase or Zr(Fe,Al)12 τ1 phase

Two-phase Fe-rich Fe–Al–Zr alloys have been prepared consisting of binary Fe–Al with a very low solubility for Zr and the ternary Laves phase Zr(Fe,Al)₂ or τ₁ phase Zr(Fe,Al)₁₂. Yield stress, flexural fracture strain, and oxidation behaviour of these alloys have been studied in the temperature range between room temperature and 1200 °C. Both the Laves phase and the τ₁ phase act as strengthening phases increasing significantly the yield stress as well as the brittle-to-ductile transition temperature. Alloys containing disordered A2+ ordered D0₃ Fe–Al show strongly increased yield stresses compared to alloys with only A2 or D0₃ Fe–Al. The binary and ternary alloys with about 40at.% Al and 0 or 0.8at.% Zr show the effect of vacancy hardening at low temperatures which can be eliminated by heat treatments at 400 °C. At higher Zr contents this effect is lost and instead an increase of low-temperature strength is observed after the heat treatment. The increase of the high-temperature yield strength of Fe-40at.% Al by adding Zr is much stronger than by other ternary additions such as Ti, Nb, or Mo. Tests on the oxidation resistance at temperatures up to 1200 °C indicate a detrimental effect of Zr already for additions of 0.1at.%.     

Point defects and diffusion in ordered alloys: An ab initio study of the effect of vibrations

We present a detailed investigation of the influence of atomic vibrations on the point defect and diffusion properties of ordered metallic alloys, by means of ab initio calculations with density-functional theory. Considering the case of Ni₂Al₃ which provides a rich panel of defect-related properties, our study reveals that the behaviour of this compound is largely monitored by self-interstitials, whereas such defects are usually ignored in metallic compounds. The vibration free energies are obtained for the full set of point defects of Ni₂Al₃, showing that these quantities are strongly defect-dependent, and significantly modify the free energy of the compound in an intricate composition-dependent manner. The second key-issue is the first ab initio full analysis of attempt frequencies, via the coupling of vibration analysis and saddle-point search for significant atomic jumps. This analysis indicates that attempt frequencies range over several orders of magnitude and exponentially increase with migration energies. We show the importance of these factors in reaching realistic composition-dependent diffusion coefficients.     

Two-phase intermetallic alloy Ni3(Al, Si):microstructure and mechanical properties

Yield stress in compression (0.2% flow stress) from ambient temperature up to 800 °C has been studied on Ni₃(Al, Si) alloy with the atomic composition Ni₇₈Al₁₁Si₁₁. When annealed at 1000 °C, the alloy has a pure L1₂ (γ′) ordered structure. After subsequent annealing at 750 °C, the disordered solid solution of Al and Si in Ni (face centred cubic, γ) precipitates in fine coherent particles. Calorimetry helps to describe the various phase transformations necessary to obtain the last microstucture. Solute addition of Si, which replaces Al atoms, increases the 0.2% flow stress of Ni₃Al in the fully γ′ microstructure. The γ precipitation shifts the peak stress towards higher temperatures and stresses.     

The strengthening effect of (Ni,Fe)Al precipitates on the mechanical properties at high temperatures of ferritic Fe–Al–Ni–Cr alloys

Strengthening through a homogeneous distribution of a second phase is a concept that is widely employed in high-temperature materials. The most prominent among this group are nickel-based superalloys which owe their high-temperature strength to finely dispersed Ni₃Al particles. Similar microstructures can be obtained in the Fe–Al–Ni–Cr system with B2-ordered (Ni,Fe)Al precipitates in a ferritic matrix. These precipitates lead to an increase of high-temperature strength compared to conventional iron-base high-temperature alloys. However, secondary precipitates form during air cooling from high temperatures and affect the ductility. The results show that the ductility can be improved by a two-step aging treatment. Within the stress and temperature range investigated, the dependence of the secondary creep rate on the applied stress of aged alloys can be described by a power law if a threshold stress is introduced.     

Effective formation energies of atomic defects in D03Fe3Al: an ab-initio study

The effective formation energies of atomic defects in D0₃---Fe₃Al are determined within the framework of a grand-canonical statistical method. The grand-canonical defect formation energies entering this method are calculated by the ab-initio mixed-basis pseudopotential method. Thermal vacancies appear on the sublattice of the Fe atoms with an effective formation energy of 1.25 eV and on the Al sublattice (about 1.4 eV). The structural defects are Al antisite atoms on the γ sublattice of the Fe atoms or Fe antisite atoms on the Al sublattice.     

Dynamic dislocation behaviour in the intermetallic compounds NiAl, TiAl and MoSi2

The dynamic behaviour of dislocations in NiAl, TiAl and MoSi₂ on ‘easy’ slip systems is studied by in situ straining experiments in a high-voltage electron microscope. At elevated temperatures, the dislocations are smoothly bent as in NiAl and TiAl or sometimes show superkinks as in MoSi₂, and they move in a viscous way. It is suggested that this dynamic behaviour as well as the flow stress anomaly are connected with the formation of atmospheres around the dislocations. A model is proposed assuming that the lowest energy configuration of a dislocation may require a certain number of antisite defects or other point defects in the dislocation core. This cloud of disordered structure may follow partly the moving dislocations to induce an additional friction, analogous to other diffusion controlled mechanisms. The view of atmospheres controlling the dislocation mobility in intermetallics at elevated temperatures is supported by measurements of the dependence of the strain rate sensitivity on the strain rate itself.     

Thermal cycling of Ti46Al8Nb1B

The effects of thermal cycling on the mechanical properties and microstructure of a TiAl-based alloy (Ti46Al8Nb1B) have been investigated for samples with and without an imposed external stress. The temperature range (300–800 °C) and stress (0 or 300 MPa) were chosen to reflect likely industrial usage. Unstressed samples showed no significant effect of thermal cycling. Thermal cycling with external stress is found to reduce the yield stress, ductility and UTS and to increase the amount of primary creep. The microstructural changes include dislocation and twin generation, ₂/γ ledge formation, dissolution of ₂ lamellae and microcrack formation. The changes in mechanical properties are interpreted in terms of the changes in microstructure.     

An atomic study of the transitional region between γ/γ laths in γ-TiAl

In the present study, a transitional region between two thick γ laths is investigated with TEM, HAADF-STEM, EDX and HRTEM. The results show that the transitional region consists of three thinner γ laths, and all the thick and thin laths have a true twin relationship. The interface between γ/γ laths is basically a twin-related interface but involving a rigid body translation of the adjacent lamellae, and misfit compensating has been observed along the interface. Ti segregates into the transitional region, which is essential to formation of the three thin γ laths. Based on the characterization of the transitional region, the formation mechanism of the transitional region is revealed.     

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.