Axial load controlled high cycle fatigue experiments were conducted on the γ/α2 alloy, Ti-48A1-1V-0.2C (at%), at 23 and 815°C. Four different microstructures, produced through thermomechanical processing, were evaluated to examine the influence of grain size and α2 content on fatigue behavior. The load controlled fatigue life was significantly reduced by increasing grain size and unaffected by α2 content at both 23 and 815°C. Although, α2 content did not greatly influence high cycle fatigue life, the room temperature crack initiation and fast fracture was changed from transgranular to partially intergranular as the volume fraction of α2 was reduced in the fine grain size material. The fatigue strength at 107 cycles (FS) to ultimate tensile strength (UTS) ratio was 0.8 to 0.9 at 23°C and 0.5 to 0.6 at 815°C for all microstructures examined. Low tensile ductility, high work hardening rate and the difficulty in forming strain local-izations all aided the high FS/UTS ratio. The dislocation microstructures produced by fatigue at room temperature were examined in the fine grained high α2 (ductile) microstructure. They consisted of loop patches of all <110] regular dislocations without any <101] or <011] super dislocations because of the large difference in CRSS for these dislocation. The inability to nucleate and move superdislocations inhibited the formation of persistent slip bands as is often found in high and intermediate stacking fault FCC metals.

Polysynthetically twinned (PST) crystals of TiAl have been grown and deformed in tension and compression at room temperature. A room temperature tensile elongation of 12.8% which is far larger than any other reported values of room temperature ductility was obtained for a specimen whose tensile axis is inclined at 51° to the lamellar boundaries. Deformation mechanisms of PST crystals of TiAl are discussed on the basis of the results of electron microscope observations of deformation structures.

A study has been undertaken to understand the cyclic deformation behavior of singlephase B2 aluminides. The alloys chosen for this study were rapidly solidified powder extrusions of Fe60 Al40 (at.%) and Ni50 A130 Fe20 (at.%). These alloys were chosen to compare the cyclic deformation behavior of a material that deforms by <111> slip and thus meets the von Mises criterion for slip system compatibility (Fe60Al40) with one that deforms by <100> slip and does not meet the von Mises criterion (Ni50A130Fe20). Fully reversed tension/compression tests were run in plastic strain control at room temperature. Rapid cyclic hardening was observed for the Fe60Al40 alloy. However, intergranular failure occurred before saturation was reached at all plastic strain amplitudes. The Ni50A130Fe20 alloy displays no tensile ductility , with failure occurring before completion of the first cycle. For the Ni50A130Fe20 alloy, a lower extrusion temperature during materials processing, and compressive prestrain both increase the cyclic accumulated plastic strain to failure, as well as the monotonic tensile ductility. This made cyclic deformation of the Ni50Al30Fe20 alloy possible. In this alloy, limited cyclic hardening is followed by saturation. For both the Fe60Al40 and Ni50 Al30Fe20 alloys, significant ductility enhancement was achieved by the application of thin nickel films. All cyclic deformation results are compared to similar monotonic tension results and are discussed in terms of the dislocation substructures observed.

Recently it was proposed [S.C. Jha, et al., Mater. Sci. & Eng. A119 (1989) 103 & J.D. Whittenberger, et al., J. Mat. Sci. 25, (1990) 2771] on the basis of constant velocity testing at 1300 K that dispersion strengthened NiAl composites containing about 4 wt pct HfC possess threshold stresses for creep. Further 1300 K compression testing has been conducted on NiAl+4HfC,and diametrically opposite behavior has been found: for constant load creep tests normal power law behavior was observed (strain rate proportional to the stress raised to the power n); however additional constant velocity testing still indicates that the flow stress is essentially independent of strain rate below 10−6 s−1. Examination of NiAl+4.3HfC specimens after being deformed under constant velocity conditions revealed that the original hot extruded small grain structure could be converted to large, elongated grains during testing. Such a transformation appears to be responsible for the apparent threshold stress behavior in HfC dispersed NiAl.

NiAl has only three independent slip systems operating at low and intermediate temperatures whereas five independent deformation mechanisms are required to satisfy the von Mises criterion for general plasticity in polycrystalline materials. Yet, it is generally recognized that polycrystalline NiAl can be deformed extensively in compression at room temperature and that limited tensile ductility can be obtained in extruded materials. In order to determine whether these results are in conflict with the von Mises criterion, tension and compression tests were conducted on powder-extruded, binary NiAl between 300 and 1300 K. The results indicate that below the brittle-to-ductile transition temperature (BDTT) the failure mechanism in NiAl involves the initiation and propagation of cracks at the grain boundaries which is consistent with the von Mises analysis. Furthermore, evaluation of the flow behavior of NiAl indicates that the transition from brittle to ductile behavior with increasing temperature coincides with the onset of recovery mechanisms such as dislocation climb. The increase in ductility above the BDTT is therefore attributed to the climb of <001> type dislocations which in combination with dislocation glide enable grain boundary compatibility to be maintained at the higher temperatures.

Single crystals of B2 aluminides, including FeAl, NiAl, and CoAl, were tested to investigate their fracture behavior. Specimens were aligned along specific orientations of the crystal lattice. Both uniaxial tension and notched bend tests revealed a cleavage fracture in all crystals. The cleavage occurred along a specific crystalline plane: {100} for FeAl; {110} for NiAl and CoAl. A model based on the ordering energy of unlike atoms is proposed to explain the preferred cleavage plane in these B2 structures. Fracture on transient planes around {115} are observed in NiAl before the crack growth continues on a cleavage plane.

In this study, the strain rate sensitivity of single crystal NiAl has been investigated by performing tensile tests as a function of temperature and two strain rates. Three crystallographic orientations, [100], [110], and [111] were studied. The tensile test results investigated include yield strength, work hardening rate and plastic strain to failure. The data are discussed in terms of deformation mechanisms in NiAl.

This paper deals with the role of charging conditions and strain rate on the embrittlement of several Fe.Al and Fe3Al alloys. Fracture paths are transgranular or mixed mode in both air and after precharging of the iron aluminides, unlike the intergranular cracking typical of Ll2 superlattices exposed to hydrogen. The fractographic observations are consistent with a decohesion mechanism. The role of long range order in transport of hydrogen will be discussed.

Compressive creep behavior is investigated In polycrystalline Ni3Al intermetallic compounds in which the focus is put on the effect of offstoichiometry and ternary additions of Ti, Ta, V and Co on the creep resistance of the compounds. In all cases, the steady-state creep rate has the power-law type dependence on the applied stress. The stress exponent thereby obtained leads us to regard the creep behavior of the compound to be of the Class I type. It is shown that the creep resistance Increases with increasing Ni concentration on both sides of the stoichiometric Ni3Al composition and a discontinuity exists in the variation at stoichiometry. The gap at the discontinuity becomes clearer by the addition of ternary element in most cases as compared to the binary Ni3Al. Although the exact reason for the phenomenon is not clearly understood at present, it is shown that it could be interpreted by the concentration dependence of activation energy in the power-law type creep equation.

Compressive creep behavior is investigated in ternary Ni3Al single crystals containing Ti, Si, Hf and Cr with stress axes parallel to the crystallographic orientation near [001]. Then a comparison is made with the results of high temperature compression tests under a constant strain rate for the same orientation where plastic behavior is characterized by a distinct yield drop followed by steadystate deformation. It is found that the deformation mechanism for the two cases is identical, namely octahedral viscous flow being expressed by the state equation of the power-law type with a stress exponent of about 3 to 4. The effect of offstoichiometry on the creep resistance is then examined in Ni3(Al,5 at%Ti) alloys with different Ni concentrations. The results support the observation in the polycrystalline compound where the creep resistance increases with Ni concentration on both sides of stoichiometry exhibiting a discontinuity at stoichiometry. Finally, the apparent activation energy in the power-law type state equation for the steady state creep deformation is estimated for all the ternary alloys examined. They are in general in good agreement with that for diffusion of ternary elements in Ni3Al. However, the relative magnitude of the value can not simply be compared since the activation energy depends on deviations from stoichiometry.

In order to obtain more complete information about the strain rate sensitivity of the flow stress of L12 alloys, Ni3Al,Ta) single crystals have been deformed in compression, over a range of temperatures (293–1273K), at two different strain rates and in stress relaxation experiments.

During the stress relaxation tests, at almost all the temperatures (except T~470K), a logarithmic decrease of the stress as a function of time has been observed. This is in fair agreement with the classical frame work of the thermal activation theory of dislocation glide. Thus, the corresponding apparent activation volumes have been determined and the nonmonotonic variation of this activation parameter with the temperature indicates that it is necessary to consider more than one plastic deformation process. These new results are discussed in terms of the previously published models which account for the plastic behaviours of the L12 compounds.

We present the new promising method to improve the ductility of intermetallic compound, Ni3Al. Stoichiometric and Alrich Ni3A1 were unidirectionally grown at the growth rate of 24 mm/h by a floating zone method (FZ-UDS). The stoichiometric Ni3Al had a columnar-grained and single phase structure, whereas the Al-rich Ni3Al had a Ni3Al matrix with martensite-like precipitates. These alloys exhibited a large tensile elongation along the growth direction at room temperature. Fracture mode is a complete transgranular fracture in the stoichiometric Ni 3Al alloy, and a mixture of transgranular fracture of the matrix and cleavage of the martensite-like precipitates with cracking at the phase boundary in the Al-rich Ni3Al alloys.

Fatigue behaviour of cast Ni3Al-based alloys with the addition of alloying elements, such as Hf, Zr and Ti etc., has been investigated. It was found that: (1) The alloying elements change remarkably mechanical property and fatigue property of cast Ni3Al-based alloys, in which, the fatigue strenghts of No.1 and No.2 alloys are 215.6MPa and 205.8MPa respectively, while that of No.3 alloy is only 78MPa, for the same cyclic life of 107; (2) Fatigue cracks initiate at the (γ'+γ) /γ interface and “matrix channel” in No.1 and No.2 alloys are at the grain-boundary or micropore, of No.3 alloy often fatigue cracks initiated from the edge of specimen under the cyclic stress; (3) Fbitigue crack propagation of cast Ni3Al-based alloys follows “zigzag” propagation path, and it proceeds through the big blocky (γ'+γ) eutectic and “matrix channel” and along the (γ';+γ)/γ interface in No.1 and No.2 alloys, while the fatigue crack propagation of No.3 alloy sometimes follows the “zigzag” propagation path. Finally, the effect of alloying elements on the fatigue behaviour of cast Ni3Al-based alloys has been discussed.

This paper reports the results of uniaxial and compression tests on IC-218LZr extruded bars at temperatures from 700 to 1000°C in air. Tensile results indicate reduction in ductility at temperatures between 750 to 900°C. However, the results show improvement in ductility above 950°C. Compression test results do not show a decrease in the ductility in the temperature range.

The microstructure and the mechanical properties at room and high temperatures of monocrystalline Ni3Al containing 0, 0.52, 1.37, 2.22 and 3.00 at%B have been studied. The results show that the ultimate and yield strengths at 20°C and 800° C of monocrystalline Ni3Al obviously increase with the increase of B content, and reach maxima when B content is 1.37 and 0.52 at% respectively. With further increasing the B content to 2.22 at%, they slowly drop. The tensile ductilities at 20°C and 800°C of monocrystalline Ni3Al when the B content is 0 and 0.52 at% respectively are the best in five kinds of Ni3Al studied, then they continuously decrease with the increase of B content due to the precipitation of boride eutectic. The rupture life and elongation at 850°C and 100 MPa markedly drop as the B content is raised in the monocrystalline Ni3Al, but they are still higher by 1 to 2 orders of magnitude as compared with those of polycrystalline Ni3Al with the same compositions. When B content exceeds 1.37 at%, the eutectic structure of Ni20Al3B6 and Ni3Al is formed, which is enhanced by the B content and is harmful to the strength and the ductility.

Mechanical alloying (MA) has been used to produce NiAl powders from either elemental or prealloyed constituents. The powders were consolidated by hot extrusion resulting in material which was fully dense, with a grain size around 1 μm and a homogeneous distribution of oxide particles with sizes in the range 10 to 100 nm. TEM observation indicates the presence of a significant dislocation density after consolidation. Mechanical properties have been studied by compression testing from room temperature to 1300 K in air. Yield strengths ranged from 1453 MPa to 32 MPa depending on material and test temperature. Work hardening was observed at all test temperatures for both materials. Substantial ductility was observed even at room temperature where it exceeds 7.5 %. The effects of microstructure on the mechanical properties are discussed.

First-principles calculations of the elastic constants, shear fault energies, and cleavage strength of NiAl and FeAl are presented. For NiAl, we find that the dissociation of <111> superdislocations into partial dislocations is unlikely, because of a high antiphase boundary energy and a weak repulsive elastic force between partial dislocations. FeAl has a high ideal cleavage strength as a result of the directional d-bond formation at the Fe sites. The strong ordering behavior of NiAl is explained in terms of the Al-to-Ni charge transfer and the repulsive interaction between Al atoms. The spontaneous glide decomposition of the <111> superdislocations in NiAl is also discussed.

Tensile tests were conducted on an extruded Ni-30(at.%)Al-20Fe-0.05Zr intermetallic alloy in the temperature range 300–1200 K and the microstructures were characterized by optical, scanning and transmission electron microscopy. The alloy did not exhibit any room temperature ductility and failed by transgranular cleavage at a stress of about 710 MPa. This mode of fracture was observed at and below 873 K with the total ductility being less than 2.5%. However, the failure mode changed to dimpled fracture, triple point cracking and rupture above 873 K. Scanning electron microscopy of the fracture surfaces revealed that transgranular cleavage failure had always originated at pre-existing defects. In these cases, the fracture stress exhibited a good correlation with the defect size in accordance with linear elastic fracture mechanics, where the critical stress intensity factor was estimated to be about 4 MPa m1/2.

Mechanical alloying (MA) has been used to process the NiAl-based alloy Ni-35Al-2OFe such that a fine-grain (∼2 μm) microstructure is obtained through the addition of 2 v/o Y2O3 particles. When compared to a conventionally processed, coarse-grained (∼;28 μm) Ni-35–20 alloy without the Y2O3 particles, the MA alloy exhibits two to three times higher fracture toughness values, despite a 50% increase in yield strength. Room temperature KQ values as high as 34 √Pa m are observed accompanied by a yield strength in excess of 1100 MPa. Fractography confirms a change in fracture characteristics of the fine-grained MA alloy.

Rapidly solidified strips of Fe66.5-A128.5-Cr5 alloy with the addition of 0.5wt.% TiB2, were produced by planar flow strip casting. Correlations of the microstructure and room temperature mechanical properties were made for strips in the as-cast condition and after annealing at 1273K for periods up to 4hrs. The results showed that grain size, tensile strength, ductility, hardness and Young's modulus were very stable for the times investigated. Studies of tensile fracture surfaces revealed essentially 100% transgranular cleavage in the ascast strip with a greater tendency for intergranular failure after prolonged exposure to high temperatures. TEM studies of this alloy revealed diffraction patterns characteristic of DO3 ordering but the dislocations observed in both ascast and deformed specimens were those typical of the B2 structure without any extended APBs. This is attributed to the very fine DO3, domain size . At room temperature <111> slip is predominant.