Microstructure and creep behavior of directionally solidified TiAl-base alloys

Tensile creep tests were conducted on directionally solidified TiAl alloys to discern the effect of alloying and lamellar orientation. A seeding technique was used to align the TiAl/Ti₃Al lamellar structure parallel to the growth direction for alloys of Ti–47Al, Ti–46Al–0.5Si–0.5X (X=Re, W, Mo, and Cr), and Ti–46Al–1.5Mo–0.2C (at.%). Tensile creep tests were performed at 750 °C using applied stresses of 210 and 240 MPa. Aligning the lamellar microstructure greatly enhances the creep resistance which can further be improved by additional alloying.     

Creep behaviour of Fe3Al-based alloys in DO3 phase field

A study is made to bring out the effect of alloying with Cr, Ti or Mn on the creep behaviour of Fe₃Al. Impression creep experiments have been carried out in the DO₃ phase field. In all the alloys, power law creep behaviour is observed in the stress range covered. The stress exponent for steady state creep rate and the activation energy for creep indicate that the creep rate is controlled by the dislocation climb process. Among the alloying elements studied, addition of Ti is most effective in improving the creep resistance.     

Effect of si addition on mechanical alloying behavior and creep properties of Al-10Ti-xSi alloys

Al-10Ti-xSi alloys (x=0∼6wt.%) have been mechanically alloyed under Ar atmosphere using an attritor and the alloying process has been investigated. From Al-10Ti composite powders, supersaturated Al(Ti) powders were obtained after mechanical alloying. In the ternary mixture, fine Si particles were observed to be distributed in the Al(Ti) matrix due to both the negligible solid solubility of Si in the Al matrix and the weaker chemical interaction of Si with Al, as compared with Ti. The sealed compacts were hot extruded to full density at 450°C with an extrusion ratio of 12:1. The microstructures and creep properties of the hot extruded alloys were examined. During consolidation, Si particles were dissolved in Al₃Ti up to 4 wt.% Si to form the (Al(Si))₃Ti phase, and the Ti₇Al₅Si₁₂ phase was formed beyond the solubility limit of Si in Al₃Ti. The transition from the Coble creep mechanism at low stresses and temperatures to dislocation one at high stresses and temperatures was observed. The stress and temperature of the transition from diffusional to dislocation creep became higher as Si concentration increased. This was due to an enhancement of Al₃Ti particle strength with increasing Si content as a result of Si incorporation. Thus, the addition of Si enhances the creep resistance of the MA Al-10Ti alloy.     

Effect of Mg addition on the creep and yield behavior of an Al–Sc alloy

The relationships between microstructure and strength were studied at room temperature and 300 °C in an Al–2 wt% Mg–0.2 wt% Sc alloy, containing Mg in solid-solution and Al₃Sc (L1₂ structure) as nanosize precipitates. At room temperature, the yield strength is controlled by the superposition of solid-solution and precipitation strengthening. At 300 °C and at large applied stresses, the creep strength, which is characterized by a stress exponent of ~5, is significantly improved compared to binary Al–Sc alloys, and is independent of the size of the Al₃Sc precipitates. At small applied stress, a threshold stress exists, increasing from 9% to 70% of the Orowan stress with increasing Al₃Sc precipitate radius from 2 to 25 nm. An existing model based on a climb-controlled bypass mechanism is in semi-quantitative agreement with the precipitate radius dependence of the threshold stress. The model is, however, only valid for coherent precipitates, and the Al₃Sc precipitates lose coherency for radii larger than 11 nm. For semi-coherent precipitates with radii greater than 15 nm, the threshold stress remains high, most likely because of the presence of interfacial misfit dislocations.     

Formation and stability of refractory metal diborides in an Fe3Al matrix

The feasibility of forming refractory diboride particles in an Fe₃Al matrix by conventional casting techniques is examined; the microstructural stability of such particles upon subsequent exposure to elevated temperatures is discussed. Four different alloys were cast (Fe–Al–Ti–B, Fe–Al–Zr–B, Fe–Al–Nb–B, and Fe–Al–Ta–B); the nominal composition in each case was intended to yield stoichiometric Fe₃Al with 10 vol% of the stoichiometric refractory diboride. As-cast, annealed and heat treated alloys were examined using optical microscopy, X-ray diffraction, differential thermal analysis and scanning electron microscopy in conjunction with energy dispersive X-ray (EDX) spectroscopy to understand the formation and thermal stability of refractory diborides (TiB₂, ZrB₂, TaB₂, NbB₂) in Fe₃Al. In all four cases, refractory diborides formed; the Fe–Al–Ti–B and Fe–Al–Zr–B alloys yielded a two-phase microstructure of Fe₃Al and the diboride. The Ti and Zr diboride reinforcements were stable and showed minimal coarsening following high temperature exposure. The Nb- and Ta-containing quaternary alloys, in addition to the respective diborides, included a eutectic reaction at temperatures between 1150°C and 1200°C that results in the formation of an Fe-rich phase thought to be Fe₂B.     

Microstructures and mechanical properties of Fe–Al–Ta alloys with strengthening Laves phase

The addition of Ta to Fe–Al alloys results in the formation of a stable Ta(Fe,Al)2 Laves phase with hexagonal C14 structure in the Fe–Al phase at temperatures of 800, 1000 and 1150 °C. It was found that the solubility of Ta in Fe–Al is generally low and the solubility of Ta varies with Al content. Respective isothermal sections of the Fe–Al–Ta system have been established. Particular attention has been given to precipitation in the Fe₃Al phase with a small addition of Ta. At intermediate temperatures, 600–750 °C, an additional Heusler-type phase with L2₁-structure precipitates, which transforms at longer times and high temperatures to the stable C14 Laves phase. The yield stress in compression and the creep behaviour of the Fe–Al–Ta alloys with various microstructures were studied. Due to the presence of the L2₁-Heusler phase, the yield stress and the creep resistance at temperatures below 700 °C was increased considerably.     

Creep of Fe3Al-based alloys with vanadium and carbon additions

Creep experiments were conducted on Fe₃Al-based alloys with vanadium and carbon additions in the temperature range from 923 to 1023 K, corresponding to the occurrence of B2 lattice. The alloys contained (in atomic %) (i) 27.0 Al, 1.17 V, and 0.02 C and (ii) 27.0 Al, 1.13 V, and 0.73 C (Fe balance). The alloys were tested in the as-cast state and annealed at 1273 K for 50 h. Creep tests were performed in uniaxial compression at a constant load with stepwise loading. Stress exponents and activation energies for the creep rate were determined. The values of the stress exponent in low-carbon alloy correspond to a five-power-law creep. The activation energy is greater than the activation enthalpy of diffusion of both Fe and Al in Fe₃Al and is substantially greater than the activation enthalpy of diffusion of V in Fe₃Al. The creep rate is impeded in the high-carbon alloy by the presence of tiny carbide particles. Consequently, the creep resistance of the high-carbon alloy in the as-cast state is greater, especially for higher temperatures and lower stresses. The carbide particles coarsen during annealing at 1273 K and are unable to obstruct dislocation motion because the mean distance between them is too large. The high-carbon alloy is then creep-weaker due to the reduced amount of vanadium present in the matrix.     

Mechanical properties and lattice misfit of γ/γ′ strengthened Co-base superalloys in the Co–W–Al–Ti quaternary system

A continuous γ/γ′ two-phase field has been identified extending between the ternary Co–Al–W system to the binary Co–Ti system. The lattice misfits of two phase γ/γ′ alloys from the Co–Al–W–Ti quaternary system were measured by X-ray diffraction and found to be positive and vary linearly with composition. Differential scanning calorimetry measurements showed that the solidus and liquidus temperatures decrease from the W and Al rich end to the Ti rich end, whilst the γ′ solvus temperature increases. Long term heat treatments identified the occurrence of discontinuous precipitation at the grain boundaries in many of the alloys studied. The high temperature strength and creep resistance of the quaternary alloys in the intermediate composition range surpassed those of the binary and ternary alloys.     

Effects of alloying elements on phase stability in Nb–Si system intermetallics materials

The effects of alloying elements, such as Ti, Cr, Al and Hf on phase stability in Nb–Si system intermetallics materials have been investigated by scanning electron microscopy (SEM), X-ray energy dispersive spectrum (EDS) and X-ray diffraction (XRD). The binary, ternary and multicomponent alloys have been fabricated by vacuum non-consumable arc melting method. The results showed that Ti and Hf tend to stabilize Nb₃Si phase to lower temperatures, while Cr and Al promote the direct formation of Nb₅Si₃ phase. The coordinate effects of alloying elements on microstructures of Nb–Si system intermetallics materials have also been examined.     

Diffusion creep of intermetallic TiAl alloys

The creep behaviour of γ-TiAl with L1₀ structure without second phases, γ-TiAl with precipitated particles of α₂-Ti₃Al with D0₁₉ structure, and γ-TiAl with the H-phase Ti₂AlC has been studied at low stresses in the temperature range 900–1200°C. The obtained data allow the construction of creep deformation mechanism maps for the studied alloys which may be used for an extrapolation of the observed creep behaviour. At higher stresses dislocation creep occurs in all alloys, which is well described by the Dorn equation with stress exponents in the range 3–5. Extended Coble creep with threshold stress was observed only for the studied two-phase alloys. A strong temperature dependence of the threshold stress for Coble creep was found for the TiAl alloy with carbide particles.     

Sulfidation processing and Cr addition to improve oxidation resistance of TiAl intermetallics in air at 1173 K

High temperature oxidation properties of TiAl- (1,2,4 and 10) Cr and 40Ti-56Al–4Cr alloys, which were sulfidized at 1173 K for 86.4 ks at 1.3 Pa sulfur partial pressure in a H₂–H₂S gas mixture, were investigated at 1173 K in air for up to 2.7 Ms. The sulfidation processing formed a (Cr,Ti)Al₂ layer between a TiAl₃ (TiAl₂ included) layer and a Ti-rich sulfide scale by selective sulfidation of Ti. Oxidation of the sulfidation-processed alloys was examined for up to 2.7 Ms in air under isothermal and room temperature to 1173 K heat cycle conditions. In both oxidation experiments the sulfidation processed TiAl–10Cr alloy showed very good oxidation resistance up to 2.7 Ms, due to the formation of a continuous Ti(CrAl)₂ Laves layer, which was changed from (Cr,Ti)Al₂ and has a composition of 28.7Cr–36.2Al–35.1Ti, between the layers of protective Al₂O₃ (TiO₂ included) and TiAl₂, which was changed from TiAl₃. The sulfidation processed TiAl, TiAl–4Cr, and 40Ti–56Al–4Cr alloys showed better oxidation resistance than conventional TiAl based alloys, but displayed localized oxidation. The Ti(Cr,Al)₂ Laves on the sulfidation processed TiAl–4Cr alloy was discontinuous, leading to a localized oxidation after long oxidation. The sulfidation processed 40Ti–56Al–4Cr alloy oxidized faster than the sulfidation processed TiAl–10Cr alloy due to the formation of an Al₂O₃ and TiO₂ mixture, although the TiAl₂ layer remains. It was concluded that the Ti(Cr,Al)₂ Laves layer between the oxide scale and alloy substrate caused the good oxidation resistance.     

A study of the microstructures and oxidation of Nb–Si–Cr–Al–Mo in situ composites alloyed with Ti,Hf and Sn

The effects of alloying on the microstructures, solidification path, phase stability and oxidation kinetics of Nb_ss/Nb₅Si₃ base in situ composites of the Nb–Ti–Si–Al–Cr–Mo–Hf–Sn system have been investigated in this study. All the studied alloys are classified as hyper-eutectic Nb silicide base in situ composites and have lower densities compared to nickel-based superalloys. The Nb₃Si silicide formed in the Hf-free alloys and transformed to Nb_ss and αNb₅Si₃ during heat treatment at 1500 °C. This transformation was enhanced by the addition of Ti. The Nb_ss and Nb₅Si₃ were the equilibrium phases in the microstructures of the Hf-free alloys. In the presence of Ti, the βNb₅Si₃ only partially transformed to αNb₅Si₃, suggesting that Ti stabilises the βNb₅Si₃ to lower temperatures (at least to 1300 °C). Furthermore, alloying with Hf stabilised the hexagonal γNb₅Si₃ (Mn₅Si₃-type) silicide in the Hf-containing alloys. The addition of Sn promoted the formation of the Si-rich C14 Laves phase and stabilised it at 1300 °C. This is attributed to the Sn addition decreasing the solubility of Cr in the Nb_ss of the Nb–Ti–Si–Al–Cr–Mo–Hf–Sn system whilst increasing the Si solubility. The Si solubility in the C14 Laves phase was in the range ∼6.6 to 10.5 at%. The lattice parameter of the Nb_ss in each alloy increased after heat treatment signifying the redistribution of solutes between the Nb_ss and the intermetallic phases. The oxidation resistance of the alloys at 800 °C and 1200 °C increased significantly by alloying with Ti and Sn. Pest oxidation behaviour was exhibited by the Nb–18Si–5Al–5Cr–5Mo (as cast), Nb–24Ti–18Si–5Al–5Cr–5Mo (as cast), Nb–24Ti–18Si–5Al–5Cr–2Mo (heat treated) and Nb–24Ti–18Si–5Al–5Cr–2Mo–5Hf (heat treated) alloys at 800 °C. Pesting was eliminated in the alloy Nb–24Ti–18Si–5Al–5Cr–2Mo–5Hf–5Sn at 800 °C, indicating that the addition of Sn plays an important role in controlling the pest oxidation behaviour at intermediate temperatures. The oxidation behaviour of all the alloys at 800 °C and 1200 °C was controlled by the oxidation of the Nb_ss and was sensitive to the area fraction of Nb_ss in the alloy.     

Evaluation of elastic modulus and hardness of Fe–Al base intermetallics by nano-indentation techniques

Nanoindentation has been used to study Young's modulus and hardness of Fe–Al alloys with Al contents from 22 to 47 at%, and the influence of Cr additions to near Fe₃Al alloys. Materials are examined in the quenched state and the well-annealed state with, respectively, relatively poor order with retained thermal vacancies, and good order with few vacancies. Significant changes occur in both modulus and hardness, affected by Al content, Cr addition and heat treatment. These changes are discussed in terms of state of order, possible role of magnetisation, and vacancies.     

Stability of B2 ordered phase in the Ti-rich portion of Ti–Al–Cr and Ti–Al–Fe ternary systems

The stability region of the B2 phase at 1000°C in the Ti-rich part of the Ti–Al–Cr and Ti–Al–Fe ternary systems are investigated by energy dispersive spectroscopy (EDS) and transmission electron microscopy (TEM) using two-phase alloys and diffusion couples. It is established that the critical boundaries of the A2/B2 continuous ordering transition are functions of both the Al and Fe or Cr contents, and the phase equilibria between the α₂ and the β and between the β and FeTi (B2) phases are strongly affected by the A2/B2 order–disorder transition. By extrapolating these ternary data to the Ti–Al binary and using the Bragg–Williams–Gorsky approximation a metastable A2/B2 ordering boundary is postulated to exist at 1000°C in the vicinity of 23.5 at%Al in the Ti–Al binary system.     

Atomic site location by channelling enhanced microanalysis (ALCHEMI) in γ′-strengthened Ni- and Pt-base alloys

The additions of alloying elements to Ni- and Pt-base alloys influence the microstructure and thereby the creep properties, whereas the mechanism is uncertain. Therefore atomic site location by channelling enhanced microanalysis (ALCHEMI) was used to determine the site partitioning of ternary and quaternary alloying elements in the L1₂-ordered γ′-phase. Two ternary Ni–Al alloys with Cr and Ti additions were investigated. The measured site partitioning showed that Cr and Ti atoms prefer the Al-sublattice sites. For a ternary Pt–Al–Cr alloy, it was found that Cr atoms occupy Al sites. The influence of Ni as a fourth alloying element in a Pt–Al–Cr–Ni alloy on the site partitioning was also investigated. The detected results give evidence that in the quaternary alloy Cr and Ni atoms prefer the Pt sublattice. First principles calculations were used to support the experimental data.     

The isothermal and cyclic high temperature oxidation behaviour of Ti–48Al–2Mn–2Nb compared with Ti–48Al–2Cr–2Nb and Ti–48Al–2Cr

The isothermal and cyclic oxidation behaviour of Ti–48Al–2Mn–2Nb (at%) were studied at high temperatures in air in comparison with the intermetallic alloys Ti–48Al–2Cr–2Nb and Ti–48Al–2Cr. Tests were performed in air between 800 and 900°C. At 800°C Ti–48Al–2Mn–2Nb showed an excellent oxidation resistance under isothermal and cyclic conditions, comparable with Ti–48Al–2Cr–2Nb, and superior to Ti–48Al–2Cr. At 900°C the isothermal oxidation rate of Ti–48Al–2Mn–2Nb was similar as found for Ti–48Al–2Cr–2Nb, but much lower as that of Ti–48Al–2Cr. Upon cooling the oxide scale formed on Ti–48Al–2Mn–2Nb was prone to spallation. During the cyclic oxidation at 900°C, a steady state condition is reached for both niobium bearing materials, with a net linear mass loss rate, due to spallation and (re-)growth of the oxide scale. The linear mass loss rate for the Ti–48Al–2Mn–2Nb was higher than that of Ti–48Al–2Cr–2Nb, indicative of a higher susceptibility for spallation. During the initial stage of oxidation of all tested materials a complex multi-phased and multi-layered scale was formed consisting of α-Al₂O₃, TiO₂ (rutile), TiN and Ti₂AlN. After longer exposure times the outer scale was dominated by TiO₂. In case of the niobium containing materials no loss of protectivity of the oxide scale was found during the growth of the outer TiO₂ layer (under isothermal conditions). Two-stage oxidation experiments with isotope tracers were performed to study the oxidation mechanism in more detail.     

The site occupancies of alloying elements in TiAl and Ti3Al alloys

The site occupancies of V, Cr, Mn, Fe, Ni, Zr, Nb, Mo, Ta, Ga and Sn (1–5 at.%) in TiAl alloys with different compositions, and in Ti₃Al with the compositions of Ti–26 at.%Al–(1–2 at.%)X, were measured by the atom location channelling enhanced microanalysis (ALCHEMI) method. For TiAl alloys, the results show that Zr, Nb and Ta atoms invariably occupy Ti sites, while Fe, Ni, Ga and Sn atoms occupy Al sites, the alloy composition having no significant influence on their site preference. By contrast, the site preference of V, Cr, and Mn changes considerably with alloy composition (the Ti/Al ratio in particular), the probability of these elements substituting for Ti decreasing in the above order. For quaternary Ti–Al–V–Cr alloys, the site occupancies of V and Cr do not show much mutual influence. In general, with increasing atomic number, elements in the same period show increasing tendency to substitute for Al, as is the tendency to substitute for Ti for elements in the same group of the periodic table. For Ti₃Al alloys, Ga and Sn atoms occupy Al sites, while V, Cr, Mn, Zr, Nb, Mo and Ta atoms occupy Ti sites, the site preference of V, Cr, Mn and Mo in TiAl alloys being different from that in Ti₃Al. The experimental results are interpreted in terms of a Bragg–Williams-type model and bond-order data obtained from electronic structure calculation. Qualitative agreement between the model and measurements is reached.     

Effects of alloying elements on creep of TiAl alloys with a fine lamellar structure

Creep experiments were conducted on five powder-metallurgy TiAl alloys with fine grains (65–80 μm), fine lamellar spacings (0.1–0.16 μm), and different compositions [Ti–47Al (+Cr, Nb, Ta, W, Si)] at temperatures of 760°C and 815°C and stresses from 35 to 723 MPa. Results show that at a given lamellar spacing, replacing 1% Nb (atomic percent) with 1% Ta and replacing 0.2% Ta with 0.2% W induced little effect, but addition of 0.3% Si decreased the creep resistance by a factor of 3–4 under otherwise identical conditions. Field emission TEM was used to characterize the changes of microstructure and alloy element distribution before and after creep. It was found that thinning and dissolution of α₂ lamellae and continuous coarsening of γ lamellae were the main creep processes and the microalloying elements tended to segregate at lamellar interfaces, especially at ledges during creep. The effects of different alloying elements are interpreted in terms of the interaction of alloy segregants with misfit and/or misorientation dislocations at the lamellar interface. That is, the interaction retards the climb of interfacial dislocations and thus the creep process in the case of large segregants (Nb, Ta, W), but facilitates the climb and creep in the case of small segregants (Si).     

Microstructures,mechanical properties and oxidation behaviour of single-phase Fe3Al (D03) and two-phase α-Fe,Al (A2) + Fe3Al (D03) FeAlV alloys

Microstructures, mechanical properties and oxidation behaviour of single-phase Fe₃Al (D0₃) and two-phase α-Fe,Al (A2) + Fe₃Al (D0₃) FeAlV alloys have been studied. Alloying with vanadium substantially increases the strength of the single-phase alloys through solid solution hardening. In addition, vanadium markedly increases the D0₃/B2 transition temperature. The change from B2 to D0₃ at a given temperature yields an additional increase in strength. The two-phase alloys have a fine-scaled coherent microstructure with D0₃ precipitates ranging between 10 and 30 nm in the as-cast state. However, marked coarsening of D0₃ is observed after annealing at 700 °C for 1000 h. Up to this temperature the alloys show an appreciable flow stress and creep strength, comparable to those of other iron aluminide based alloys with coherent microstructures. Oxidation tests at 700–900 °C revealed formation of thick and porous Fe₂O₃ + Al₂O₃ scales above 700 °C. Therefore, above 700 °C the oxidation behaviour of these alloys is inferior compared to that of other iron aluminide alloys with about the same Al content. This poor oxidation behaviour does not improve when 4–8 at.% Cr are added.