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.     

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.     

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.     

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).     

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.     

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.     

Concepts derived from phase diagram studies for the strengthening of Fe–Al-based alloys

Fe–Al-based alloys, i.e. alloys which contain either disordered A2 -(Fe,Al), B2-ordered FeAl or D0₃-ordered Fe₃Al as majority phase, have a considerable potential for developing materials for structural applications, but insufficient strength and creep resistance have been identified as obstacles for the use of Fe–Al-based alloys at high temperatures. At the ‘Discussion Meeting on the Development of Innovative Iron Aluminium Alloys’ held at the Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf on March, 9th 2004 a couple of presentations were made with emphasis on improving these properties at high temperatures. In the current article those strengthening mechanisms which are provided by the phase diagram—solid-solution hardening, strengthening by precipitates, or ordering—are reviewed. Besides results obtained for the binary Fe–Al system special emphasis is put forward to those ternary systems for which results have been presented at the ‘Discussion Meeting’.     

Sigmoidal creep of Ni3(Al, Ta)

Incremental creep tests have been used to explore the time-dependent plastic behavior of single-slip oriented Ni₃(Al, Ta) at low temperatures in the anomalous flow regime. For selected incremental creep experiments at 20 and 100 °C, it was discovered that Ni₃Al exhibited sigmoidal creep, where there is a significant time delay before the plastic strain rate accelerates to a maximum value during a creep experiment. Several of the factors that affect the sigmoidal creep response have been identified. The origin of sigmoidal creep is accounted for using a simple model of work hardening in Ni₃Al, where the acceleration of the creep rate is a direct result of the annihilation of the existing dislocation substructure.     

Deformation behaviour of intermetallic NiAlTa alloys with strengthening Laves phase for high-temperature applications. II. Effects of alloying with Nb and other elements

Ta-containing NiAl-base alloys with the Laves phase TaNiAl with C14 structure, which were studied in Part I of this study, were modified by further alloying and were studied with respect to constitution and deformation behaviour at ambient and high temperatures—including elastic deformation and creep—as a function of alloy composition and microstructure. Advantageous effects on the mechanical behaviour with reduction of brittle-to-ductile transition temperature and flow stress were found only for alloying additions of Nb, which can replace Ta completely and which was studied extensively, as well as Fe, Cr and Si. The NiAl---Ta---Cr case was selected as most promising for further alloy development and is subject of the subsequent Part III.     

Quantitative representation of the anisotropy and tension/compression asymmetry of the critical resolved shear stress of an L12-ordered intermetallic

The anomalous anisotropy and tension (t)/compression (c)-asymmetry of the critical resolved shear stress τ_o of the L1₂-long-range ordered intermetallic γ′_NIM are analysed in the light of a published (Miner RV, Gabb TP, Gayda J, Hember, KJ. Met Trans A 1986; 17A: 507) analytical expression

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. Particles of γ′_NIM strengthen the commercial nickel-base superalloy NIMONIC 105. characterises the orientation of the specimen, and T_o≈300 K and T_peak≈ 1000 K mark the temperature (T) range in which τ_o is anomalous. Except for γ′_NIM-single crystals with the [011]-orientation, the formula describes the anisotropy and (t/c)-asymmetry of the critical resolved shear stress satisfactorily.     

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.     

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.     

Ir–Nb–Si ternary refractory superalloys with a three-phase fcc/L12/silicide structure for high-temperature applications

Ir–Nb binary alloys doped with silicon have been used in this work to attain a three-phase fcc/L1₂/silicide structure. Typical Ir–Nb binary alloys, including a hypoeutectic Ir–10Nb, an eutectic Ir–16Nb, and a hypereutectic Ir–25Nb, were used as alloy bases, and Ir was further replaced by 5 at% Si. With the addition of Si, the microstructures of the Ir–(10–25)Nb–5Si ternary alloys contained three phases: fcc, L1₂, and compounds of Ir and Si (referred to silicide hereafter). Compressive tests from room temperature to 1500 °C showed that the Ir–10Nb–5Si alloy, with a predominant fcc microstructure, always had the highest deformation hardening rate, strength, and ductility; on the other hand, the Ir–25Nb–5Si alloy showed the worst performance. With the silicide in the microstructures, the damage sustained by the Ir–Nb–Si alloys at both room and high temperatures was dominated by interface debonding, which occurred between the fcc and the silicide or the L1₂ and the silicide. It is believed that the interface debonding is an instinct failure mechanism of Ir-based alloys. Additionally, a strong solid-solution hardening effect of Si acting on the fcc phase was found to occur without loss of ductility. A principle in the composition and microstructure design is proposed in this paper for further development of Ir-based alloys with Si addition. This principle is to saturate the fcc phase with Si and other alloying elements so as to achieve maximum solid-solution hardening and tie-in fine silicides homogenously distributed within the fcc by elimination of the grain boundary concentration of silicides.     

Microstructural control and mechanical properties of dual-phase TiAl alloys

This paper summarizes our recent work on the effects of microstructural features on the mechanical properties of TiAl alloys prepared by powder and ingot metallurgy. TiAl alloys based on Ti-47Al-2Cr-2Nb (at%) were alloyed with small amounts of Ta, W, and B additions for control of alloy phases and microstructure. The alloys were processed by hot extrusion above and below T, followed by short- and long-term heat treatments at temperatures to 1350 °C in vacuum. The microstructural features in the lamellar structures were characterized by metallography, SEM and TEM, and the mechanical properties were determined by tensile tests at temperatures to 1000 °C. The tensile elongation at room temperature is mainly controlled by the colony size, showing an increase in ductility with decreasing colony size. The yield strength, on the other hand, is sensitive to the interlamellar spacing. Hall-Petch relationships hold well for both yield strength and tensile elongation at room and elevated temperatures. TiAl alloys with refined colony size and ultrafine lamellar structures possess excellent mechanical properties for structural applications at elevated temperatures.     

Small punch creep in Fe28Al3Cr0.02Ce alloy

Small punch testing of Fe₃Al-based intermetallic alloy with Cr, Mn and Ce additions was performed in temperature range 773–873 K. It is shown that the testing procedure enables to study the creep properties of the alloy. The time dependence of the central deflection resembles creep curves observed in conventional uniaxial creep testing and the minimum deflection rate can be determined. Similarly as in conventional creep tests, the deflection process is then analyzed in terms of the activation energy and force exponent. In accordance with the phase structure of the alloy, two distinct areas can be observed. In the B2 range, the activation energy equals 181 kJ/mol; the force exponent is about 4.6. In the DO₃ range, the activation energy equals 82 kJ/mol and the stress exponent is about 44. The transformation temperature DO₃B2 is close to 812 K. The technique can be used for an estimation of creep resistance.     

The mechanical anomaly of L12 alloys: a review of recent experiments and models

The anomalous deformation behaviour of L1₂ alloys is reviewed with emphasis made on the contributions of the past few years. The present understanding of the microstructural organization is presented schematically and its principal components outlined. The competition between a potent exhaustion of mobile dislocation and the difficulty of multiplication is discussed based on dedicated mechanical tests aimed at emphasizing transients. The information brought about by computer simulations is analyzed. Recent findings on the lack of orientation dependence of the flow stress in binary alloys are presented.     

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.     

Microstructure and mechanical properties of Ti 45Al 5Nb + (0–0.5C) sheets

The possible application of gamma titanium aluminides in aerospace industry requires a detailed understanding of the microstructure–property relationship of sheets made from this material. This paper reports the mechanical properties of sheets up to 1000 °C, based on alloy concepts with high Nb concentrations and small additions of C. Sheets were manufactured by rolling powder metallurgical compacts with compositions Ti 45Al 5Nb and Ti 45Al 5Nb 0.5C. The microstructures of both sheets are “near gamma” and consist of γ-TiAl and ₂-Ti₃Al phases. The texture of both phases is very weak. The strengths levels are very high and that of the C-containing sheet exceeds that of the C-free material at RT by 200 MPa. The mechanical properties of Ti 45Al 5Nb are independent of the direction in the sheet, in the whole temperature range from RT to 1000 °C. However, for the C-containing sheet this is true only in the upper temperature range.     

Microstructure evolution and mechanical properties' improvement of NiAl–Cr(Mo)–Hf eutectic alloy during suction casting and subsequent HIP treatment

A NiAl–Cr(Mo)–Hf eutectic alloy was prepared by suction-casting technique and subsequently hot isostatic pressing treatment. Microstructure and mechanical tests were performed and the results revealed that the suction-cast alloy possessed fine NiAl/Cr(Mo) lamellar, large area fraction of eutectic cell and semi-continuously distributed Ni₂AlHf phase at the cell boundaries. After the HIP treatment, the Ni₂AlHf particles became fine and distributed evenly in the alloy. Moreover, some of the Ni₂AlHf particles along the eutectic cell boundaries were transformed into Hf solid solution phase. Compared with the conventionally cast alloy, the room-temperature compressive ductility and strength of the suction-cast alloy attained significant improvement. In addition, the room-temperature ductility and elevated temperature strength of suction-cast alloy were markedly enhanced by HIP treatment.     

The influence of composition and processing on the structure and properties of TiAl-based alloys

Alloys based on TiAl, with Al contents between 44 and 48 at %, have been further alloyed and processed to produce a range of microstructures and the influence of these different compositions and structures on the mechanical properties have been assessed. In addition, the microstructural stabilities of some of these alloys, against exposure at likely operating temperatures, have been determined and any changes in microstructure correlated with changes in properties. The observations are discussed in terms of the factors which influence the strength of these alloys and in terms of the balance between obtaining alloys and structures which have the desired combination of properties, but also have acceptable stability at temperatures at which they are likely to be used. Finally, in the light of the current knowledge, some questions are addressed concerning the development of commercially successful TiAl-based alloys.