Most research about creep has focused on minimum creep rate or stress rupture properties, but primary creep is important for practical applications. In this paper, we compare how interstitial content and heat treatments of shorter duration than used in past studies affect the microstructure and primary creep resistance of a particular alloy containing TiB2 particles. More time and temperature in heat treatment homogenizes the microstructure and reduces the scatter of creep times to 0.5% strain. Increasing interstitial content increases the time to 0.5% creep, and it stabilizes the a2 phase. Preliminary evidence for large mechanical twinning strains parallel to lamellar interfaces shortly following loading is provided.

A complex two phase γ-TiAl alloy, Ti-47Al-lCr-1Mn-2Ta-0.2Si (at.%) in a fully lamellar condition, has been creep tested at a stress of 200MPa and a temperature of 700°C. This simulates the in-service operating conditions for several potential gas turbine aero engine applications where creep resistance is a design limiting material property. The results have indicate that reduction in lamellae thickness and avoidance of feathery type microstructures contribute to improved creep resistance.

The creep behaviour of a binary Ti-48%A1 intermetallic is presented as a function of stress for two fully lamellar microstructures. The two lamellar conditions differ in terms of the lamellar interface spacing and grain boundary morphology. Air cooling (AC) from the single phase a region causes planar grain boundaries and lamellar spacing of 90 to 130 nm, while furnace cooling (FC) causes interlocked lamellae along grain boundaries and 350 to 550 nm lamellar spacing. Monotonie and stress increment creep tests at 760°C indicate that the AC condition exhibits a lower mimimum creep strain rate at stresses between 105 MPa and 290 MPa. The stress exponent increases from ∼ 1 at low stress to ∼ 10 at high stress. Consecutive stress reduction creep tests indicate that the internal stress required for dislocation glide is higher for the AC condition. The results suggest that at low stress the creep rate is controlled by recovery mechanisms, while at high stress the creep rate is predominantly controlled by dislocation glide. It is postulated that at high stresses the lower creep rate of the AC condition, compared to the FC condition, results from the reduced lamellar interface spacing, which increases the internal stress required for dislocation glide.

The iron aluminides based on Fe3Al or FeAl being developed for intermediate temperature applications suffer from mediocre room temperature strength and ductility and poor high temperature tensile and creep strength. Attempts to overcome these problems have been restricted by the limited possibilities of structure modification by, for example, precipitation of stable strengthening particles. The present study examines two approaches to obtaining two-phase mixtures for improved strength and ductility: by adjusting chemical compositions such that two-phase order-disorder (α-α″) mixtures are obtained, and by mechanical alloying. Two-phase α-α″ mixtures are obtained by heat treatment of Fe-Al alloys with Al content near 20–24% and in ternary Fe-Al-Si alloys with suitably adjusted Al and Si contents. Microstructures of such alloys can be modified during heat treatments by ordering, precipitation or decomposition, and two-phase mixtures similar to those in the γ-γ superalloys obtained. Such two-phase alloys show good high temperature tensile and creep strength with some indication of reasonable ductility and reduced environmental sensitivity. Mechanical alloying can easily produce FeAl alloys of fine grain size reinforced with stable oxide particles. These structures lead to high room temperature strength with some ductility: controlled recrystallization can significantly modify both strength and ductility.

Our studies on yield stress anomaly of B2 FeAI single crystals are reviewed in this paper. A positive temperature dependence of yield stress, so-called “yield stress anomaly”, is observed in B2 FeAI in which excess vacancies are fully annealed out. Associated with the anomaly, characteristic asymmetry is found between tension and compression. While the strain-rate sensitivity is almost zero in the temperature range of the yield stress anomaly, the stress relaxation becomes significant with increasing temperature, indicating that a recovery process is thermally activated. It is ascertained by the two-surface trace analysis that slip transition from <111> direction at intermediate temperature to <100> at high temperature occurs around the peak temperature. Even at the peak temperature, in addition, operative slip vector for yielding is confirmed to be predominantly <111> by TEM. Also, it is observed that <111>-type superdislocations are frequently climb-dissociated in the temperature range of the anomaly. APB formation on {111} plane is energetically favorable, which is in agreement with the Flinn's calculation for the B2 superlattice that APB energy on {111} plane is lower than that on {110} plane. Such an anisotropy of APB energy would offer specific driving force for the climb dissociation on <111> superdislocations. On the basis of the observed results, the anomalous strengthening behavior of B2 FeAI single crystals is discussed.

Microstructural studies were performed on an alloy of composition Fe45Cr5Al50 heat treated at 950°C and oil-quenched and then given isothermal annealing treatments for times up to 200 hours at either 400°C or 500°C. The observed microstructures were correlated with variations in hardness during isothermal annealing. It was deduced that the thermal vacancies retained following the initial heat treatment are removed relatively rapidly from the lattice, which leads to an initial drop in hardness. However, during prolonged annealing, the coarsening of bothFeAl2 particles and a disordered a(Fe,Cr) phase leads to further softening. It was also deduced that the chromium atoms, which remain in solution, are effective solute strengtheners. The a(Fe,Cr) phase, which is coherent with the B2 matrix, appears to coarsen by a ledge growth mechanism.

The structure and composition of a previously unreported second phase were investigated in both Fe-43A1–0.12B and Fe-48Al-0.12B alloys. Energy dispersive x-ray and electron energy loss spectroscopy showed that the precipitates contained boron and were enriched in iron. This new boride phase had a tetragonal symmetry, with at = 4aB2 and ct = aB2, where aB2 is the matrix lattice parameter. The effect of iron content and heat treatments on the microstructure of those two boron-doped FeAl alloys are discussed.

The influence of Cr additions on the positive temperature dependence of the critical resolved shear stress of Fe3Al is investigated. Single crystals of binary Fe-28 at.% Al and ternary Fe-28 at.% Al-6 at.% Cr with different orientations are deformed in uniaxial compression between room temperature and 1273 K. The dislocation arrangement and the dissociation of superdislocations are studied by transmission electron microscopy using the weak-beam technique. The operative slip systems are discussed on the basis of the direct measurements of the antiphase boundary energies and of the activation volume.

A series of experiments were conducted to investigate the impact fracture behavior of Fe3Al and Fe3Al-20 Mn alloys. The results of this study indicated that: (i) The addition of Mn introduces an ordered Ll2-type phase in the Fe3Al-based alloys. On the other hand, the addition of Mn decreases the order parameter of the DO3 a phase, (ii) The total- impact energy of an Fe3Al alloy increases with the temperature at the low-temperature range (<600°C), then drops around 700°C, and finally increases again as the temperature further elevates, (iii) The trend of the variation of the impact energy of Fe3Al-20 at % Mn alloy with temperature is the same as that of the Fe3Al alloy, (iv) And the addition of Mn significantly improves the impact energy of the Fe3Al-based alloy, and changes the variation of the crack-growth energy with the testing temperature when the temperature is above 700°C.

The influence of Ti addition on the high temperature oxidation behaviors of FeAl intermetallic alloys in air at 1000°C and 1100°C have been investigated. The oxidation kinetics of FeAl alloys were examined by the weight gain method and oxide products were examined by XRI), SEM, EDS and EPMA. The results showed that the oxidation kinetic curves of both Ti-doped and binary Fe-36.5A1 alloys were described as different parabolas followed the formula: (ΔW/S)2 =Kpt. The parabolic rate constant, Kp values are about 2.4 and 3.3 mg2cm-4h-1 for Fe-36.5A1 alloy and about 1.3 and 2.0 mg2cm-4V-1 for Fe-36.5Al-2Ti alloy when oxidizing at 1000°C and 1100°C respectively. The difference between Fe-36.5Al and Fe-36.5Al-2Ti alloy is not only in the surface morphology but also in the phase components. In the surface there is only α-Al2O3 oxide for the Fe-36.5Al alloy while there are α-Al2O3 and TiO oxide for the Fe-36.5Al-2Ti alloy. The effects of Ti addition on the oxidation resistance of FeAl alloy were discussed based on the micro structural evidence.

The evolution of the B2-AlFe phase during mechanical grinding in Ar has been examined as a function of milling time by X-Ray diffraction, transmission Mössbauer spectroscopy and differential scanning calorimetry. Short and long range disorder was observed to increase with the mechanical treatment up to the attainment of a steady state. The evolution of the long range order parameter and of the local atomic configurations at Fe sites were analyzed in terms of possible mechanisms for milling induced disordering. The kinetics of the thermal reordering was studied under continuous heating and isothermal calorimetrie regimes. Modeling of the reordering processes by diffusion controlled growth of pre-existing ordered grains is presented as well as the estimated values of both the enthalpy and the activation energy of the reordering process. The results are consistent with a non uniform distribution of disorder throughout the sample and will be compared with preceding information on related systems.

The monotonie mechanical behavior in tension and compression of FeAl has been well documented. However, very little work has been done on the cyclic deformation behavior of this material. In this work, the behavior of FeAl (42 at. % Al) under low cycle fatigue was studied, including the effects of test environments and surface coatings. It was found that the fatigue life of this alloy is limited by environmental embrittlement. This embrittlement process can be equally well prevented by deformation in an oxygen environment or by coating the alloy with a protective film. The type of film applied appears to have little effect. Similar results were seen in monotonie testing.

Single crystals of Fe-40A1 were cold-rolled to plastic strains in the range 5% to 48%. Discs cut from the rolled crystals at different rolling strains were heated at 10 K/min in a differential scanning calorimeter from room temperature to 973 K. Three exothermic peaks were observed in the temperature ranges of 440–550 K, 610–650 K, and 860–930 K, all the peaks shifting to lower temperatures with increasing strain. The origins of these peaks are discussed in terms of the disorder and vacancies introduced during rolling.

A phenomenological model is used to explain the yield strength anomaly in FeAl. The model incorporates hardening by thermal vacancies at intermediate temperatures, and dislocation creep at elevated temperatures. Since the vacancy concentration increases exponentially with increasing temperature, the model predicts an exponential increase in strength with increasing temperature. This increasing strength is terminated by the onset of dislocation creep. The model captures the experimentally-observed strain rate dependency of the yield stress at high temperatures, and yields an activation enthalpy for vacancy formation which is in excellent agreement with a previously measured value [1].

Nickel-containing ternary iron aluminides with an aluminum concentration of 45 at. % were investigated with respect to room temperature strength, equilibrium vacancy concentration, and the kinetics of vacancy removal. As compared to binary iron aluminides with the same Al concentration, nickel additions reduce the thermal equilibrium vacancy concentration at 1273 K, whereas they increase this concentration at 973 K. Furthermore, at low temperatures such as 673 K, nickel additions increase dramatically the time needed to reach vacancy equilibrium. During prolonged annealing at 673 K, the density of <001> dislocations in Fe-45Al-3Ni (at. %) increased by an order of magnitude. This suggests that dislocations act as sinks for vacancies. At the same time, the number density of small (20–50 nm) voids decreased, indicating that they were not stable in the absence of substantial vacancy supersaturations. Our findings show also that the solid solution strengthening of iron aluminides due to Ni is much weaker than previously thought.

The structure and distribution of dislocations in the region of crack tips and crack edges have been examined in single crystal stoichiometric B2 NiAl using electron channeling contrast imaging (ECCI). ECCI, which allows examination of near surface substructural crystalline defects in bulk specimens, has been carried out for undeformed as well as in-situ deformed 4-point bend specimens. Images of dislocations at crack tips and along the crack fracture surfaces have been studied. Observations have been correlated with variations in thermal treatment corresponding with brittle and toughened conditions. A higher dislocation density was observed over a larger region in front of crack tips for the toughened materials as compared to brittle material.

The brittle-to-ductile transition (BDT) has been established for NiAl single crystals as evaluated by fracture toughness testing and also the effects of prestraining on the brittle-to-ductile transition temperature (BDTT) have been investigated. Specimens were prestrained to a 10% plastic strain level at 200°C under tension prior to toughness testing. The BDT of the prestrained specimens was compared to that of the as homogenized specimens. The results have revealed the occurrence of two competing effects upon prestraining: (1) an increase in dislocation sources causing a difficulty in micro-crack initiation and resulting in an increase in toughness at low temperatures, and (2) an increase in the flow stress resulting in an increase in BDT temperature. The crack initiation and propagation mechanisms were also analyzed and have been discussed.

We present atomistic simulations of the crack tip configuration near a high angle Σ= 5 [001](210) symmetrical tilt grain boundary in NiAl. The simulations were carried out using molecular statics and embedded atom (EAM) potentials. The cracks are stabilized near a Griffith condition involving the cohesive energy of the grain boundary. The atomistic configurations of the tip region are different in the presence of the high angle grain boundary than in the bulk. Three different configurations of the grain boundary were studied corresponding to different local compositions. It was found that in ordered NiAl, cracks along symmetrical tilt boundaries show a more brittle behavior for Al rich boundaries than for Ni-rich boundaries. Lattice trapping effects in grain boundary fracture were found to be more significant than in the bulk.

A modified Bridgman method involving vacuum treatment prior to the growth process was developed for preparation of high-accuracy stoichiometric NiAl crystals. Single crystals with stoichiometry deviations < 0.2 wt.% Ni characterized by residual resistance ratios R298K/R4.2K > 4 have been prepared. An improved gravimetric analysis ensured the best accuracy among the various wet chemical analysis methods tested and a total error less than 0.1 at.% for Ni determination in NiAl. The stoichiometric NiAl single crystals exhibit high compressive strain in <123> crystallographic axis of the order of 20 % in the as-grown state and 40 % after annealing 1 h at 1073 K. The critical resolved shear stress was reduced from 104 MPa to 45 MPa by annealing. In 3-point bend tests tensile strains to failure up to 10 % have been achieved. Microscopic oxide inclusions near the sample surface were identified as the cause of crack initiation in bent samples.

The mechanical grinding of prealloyed NiAl powder in liquid nitrogen (cryomilling) results in an intermetallic matrix composite where micron sized particle free aluminide cores (grains) are surrounded by thin mantles comprised of nanometer sized A1N particles and NiAl grains. Under high temperature, slow strain rate conditions both compressive and tensile creep testing have shown that the mechanical strength of hot extruded cryomilled NiAl approaches the levels exhibited by advanced NiAl-based single crystals and simple Ni-based superalloys. Transmission electron microscopy of cryomilled materials tested between 1100 and 1300 K revealed little, if any, dislocation structure within the mantle regions, while the NiAl cores contained subgrains and dislocation networks after testing at all strain rates between 10-4 and 10-8 s-1. These and other microstructural observations suggest that creep strength is the result of a fine NiAl grain/subgrain size, the inability of dislocations to move through the mantle and stabilization of the microstracture by the A1N particles.