The deformation mechanisms have been determined of two compounds based on Nb3Al containing additions of Ti (Nb-15Al-10Ti and Nb-15Al-40Ti, in at.%) such that they exhibited the B2 crystal structure. The compounds are strong over a range of temperatures, at least up to 900°C, and are inherently ductile. The compounds deform by activation of one or more of the following slip systems, namely <111>{110}, <111>{112} and <111>{ 123}. The dislocations are present in the form of dissociated superpartial pairs, each with Burgers vector, b, given by b=1/2<111>, which bound a ribbon of antiphase boundary(APB). From the separation of the partial dislocations, the APB energy decreases with increasing amounts of Ti.

The effects of strain rate and prestraining on tensile behavior of two-phase (γ+α2) titanium aluminides at 20 and 730°C have been investigated. At 20°C the elongation remains at about 1.4% as the strain rate increases from 5ȕ10−5 to 5ȕ10−2 s−1 and it drops to nearly zero at 5ȕ10−1s−1. At 730°C ( i.e.above DBTT) the plastic strain is about 13% when tested at 5ȕ10−5s−1, while it reduces significantly to less than 3% at 5ȕ10−4 and 5ȕ10−2 s−1. Again, the elongation is about zero at the highest strain rate tested, 5ȕ10−1 s−1. Regardless of the strain rate, fracture by an intergranular mode of the primary equiaxed γ appears to increasingly dominate as temperature changes from 20 to 730°C. Introduction of prior plastic deformation by prestraining beyond yielding at 945°C obviously increases the 20°C yield stress, however, with little influence on ductility. Transmission electron microscopy reveals that a number of dislocation loops are produced during prestraining. These loops are generally immobile resulting in the observed increase of flow stress and unchanged ductility.

Much effort has been devoted to the study of ordered materials at modest plastic strains and the problem of premature failure. However by utilizing stress states other than simple tension it is possible to study the deformation of intermetallic compounds up to large plastic strains and to consider the behavior of these materials in the regime where stresses approach the theoretical stress. The current work outlines studies of the work hardening rate of a number of titanium and nickel-based intermetallic compounds deformed in compression. Attention is given to the structural basis of the sustained work hardening. The large strain plasticity of these materials is summarized in a series of diagrams. Fracture in these materials in compression occurs via catastrophic shear at stresses of the order of E/80 (where E is the elastic modulus).

The three Laves phases consist of alternating single layers and triple layers of atoms. Shear of the structure within the triple layers may be achieved by moving synchrodislocations. The dislocation with the smallest Burgers vector is the synchroshockley a/6<l12> which has a core split over two planes. If a synchroshockley sweeps every triple layer of the cubic C15 it is twinned, if it sweeps every other triple layer, C15 is transformed into hexagonal C14. If the synchroshockley sweeps two triple layers, leaves out two, sweeps two etc. C15 is transformed into C36. Synchroshockleys travelling in pairs in any of the structures form dissociated perfect dislocations capable of giving slip.

Surface film softening, which is associated with the generation of mobile dislocations at a film-substrate interface and their motion into the substrate, has been demonstrated previously for bcc metals and B2 ordered alloy substrates. MoSi2 exhibits features of limited dislocation mobility and plasticity similar to these materials and would also be expected to exhibit film-enhanced plasticity, but at much higher temperatures. In this investigation, 150 nm thick films of SiO2, MoSi2 and ZrO2 were deposited on MoSi2 substrates by electron beam deposition at ∼50°C in an attempt to observe film softening. In initial experiments, hot hardness testing was utilized primarily to maintain deformation conditions characteristic of dislocation glide. ZrO2-coated MoSi2 demonstrates a large film-induced softening in hardness tests carried out at 0.5 - 1 kgf loads over the temperature range 25 - 1300°C.

The Al3Ti-based L12 alloy Al67Mn8Ti25 shows different temperature dependence of the yield stress and ductility from that of the model L12 alloy Ni3Al. The dislocation structures and dissociation modes of the alloy deformed at room temperature, 673 and 873K were investigated by TEM. It is determined that at room-temperature and 673K, a<110> dislocation dissociated into two a/3<112> superpartials with SISF between them on {111} plane, while at 873K, the dissociation is a/2<110> pairs on {111} planes separated by APB. The temperature dependence of yield stress and ductility may be related to the dislocation structures at various temperatures.

The creep behavior of a directionally solidified multicomponent Ni3Al alloy was investigated in a temperature range from 923 to 1173 K. The dislocation structure during secondary stage creep has been examined by transmission electron microscopy. At lower temperatures from 923 to 1023K under a stress of 500 MPa, there exist a number of dense three – dimensional dislocation networks in the Ni3Al creep specimen, while at a higher temperature of 1173 K under a stress of 200 MPa the dislocation structure degenerates to regular two – dimensional dislocation networks. Climb of dislocations occurs in the overall test temperture range. The stress dependence and temperature dependence of creep rates for the Ni3Al alloy were also determined. It was found that power law creep is obeyed with the stress exponent equal to 4.7 and the activation energy equal to 326.6 kJ/mol. The climb of dislocations was suggested to be the rate controlling factor for the secondary stage creep rate.

An attempt to improve the ductility in Cr modified Ni3Al alloys is made by means of microstructural control. In order to select an alloy composition such that it locates in the γ ' single phase region at 1000 °C and in the γ + γ ' two-phase region at 1200 °C of the Ni-Al-Cr ternary phase diagram, we first calculated γ and γ ' phase boundaries, using the Cluster Variation Method. Single crystal specimens with desired composition of Ni-19.5Al-7.5Cr at% were grown and homogenized at the above two temperatures for 72 hours and the mechanical properties were tested at room temperature. It is found that the elongation of specimen homogenized at 1200 °C is almost twice as large as that of one homogenized at 1000 °C. The theoretically predicted microstructures and the γ precipitates are also confirmed by optical, SEM micrographs and EDS analysis. It is concluded that a small amount of disordered γ phase precipitates in the γ ' matrix can drastically enhance the tensile ductility of Ni3Al (Ll2)-base alloys

The development of Fe3Al aluminides has been restricted in the past by poor ductility at ambient temperatures, and it is only recently that possible solutions to this problem have been found. It was shown a few years ago (1) that the addition of 2-6% Cr to a Fe3Al base alloy led to good ductility and this improvement was explained by a reduction of the APB energy, increasing the separation of the superpartial dislocations and thereby allowing easier dissociation of these dislocations, easier cross slip and a reduced tendency to stress and strain concentrations. However, at a later stage, an alternative explanation was proposed (2,3) based on examinations of both FeAl and Fe3Al alloys under different environments, and the ductility change was explained in terms of chemical attack at the tip of a crack leading to local hydrogen embrittlement.

The present study re-examines the behaviour of a Fe3Al alloy both with and without the addition of Cr. Strength, work hardening behaviour and failure ductility are examined under conditions where environmental effects should not be important, and the mechanical behaviour is interpreted in terms of significant variations in the type of order, the ordered domain structure and the resulting dislocation structures. It is seen that the addition of Cr can lead to a better ordered material and the differences in ordered state between the two materials can significantly affect dislocation behaviour and mechanical properties.

Room temperature tensile properties of Ti-47.5Al-lV-0.2C (at%) and Ti-48.2Al-lV-.06C (at%) alloys were measured after thermal exposure at 775°C (in air and vacuum) to evaluate the influence of carbide precipitation and environment. Both alloys possess 蝶2% tensile ductility in the unexposed condition. Thermal exposure of fully machined tensile samples consistently reduces the ductility of the 0.2 carbon alloy to 蝶0.5%. Exposure of the low carbon (0.06) alloy in vacuum results in no ductility loss, while exposure in air reduces the ductility to 1.3%. Yield strengths are unaffected by thermal exposure and are 450 MPa and 300 MPa for the 0.2 and the low carbon alloys, respectively. The pre-exposure ductility is recovered for tests in which thermally exposed machined tensile samples had 蝶10µm of their surfaces removed by polishing. Thermal exposure prior to machining results in no change in tensile behavior. SEM and TEM examination of thermally exposed surfaces show that below the oxide surface layer, a layer close in composition to Ti2Al, having a simple cubic (aQ=6.85Å) structure is present. Beneath this layer, Ti3AlC carbides are observed in the 0.2 carbon alloy. The density of these carbides is observed to decrease away from the surface. No carbides were observed in the low carbon alloy. Although carbon significantly enhances the yield strength of this alloy it also makes it much more susceptible to embrittlement from thermal exposure.

Tensile tests have been carried out on notched Ni3Al(B) specimens with different thickness/width ratios. The result indicate a transition in the fracture mode upon increasing the specimen thickness, from ductile transgranular to intergranular fracture. Experiments at high strain rates show that the transition cannot be explained in terms of the strain rate at the root of the notch. Rather, it is explained in terms of an increasing degree of triaxiality of the stress state.

Miniaturized disk-bend tests were conducted on stoichiometric monocrystalline NiAl alloys of two different nominal purities. Disks 3 mm in diameter and ∼250 mm thick were prepared with faces oriented parallel to either (100) or (110) and tested in biaxial bending. The specimens exhibited some ductility, even in the “hard” (100) orientation, prior to catastrophic failure. The yield strength of the specimens was higher in the (100) orientation than in the (110) orientation, as expected, but the specimens in (110) orientation were considerably more ductile. The higher purity alloy was considerably more ductile in both orientations. The estimated CRSS of the samples (110) in orientation is ∼80 to 85 MPa. which is somewhat lower than reported values for deformation on the ‹001›{010} slip system. For the most part, the fracture surfaces are similar in both alloys, with cleavage being the dominant mode of fracture. There is no visual evidence on the fracture surfaces that can account for the differences in ductility of the two alloys tested.

Constant load creep tests were conducted on fine-grained ( - 23 µm) polycrystalline Ni-50(at.%) Al in the temperature range 1000 - 1400 K. Power-law creep with an average stress exponent, n, of 6.6 and an average activation energy, Qc, of about 300 kJ mol-1 was observed above 25 MPa, while n ≈ 2 and Qc ≈ 95 kJ mol-1 for σ < 25 MPa. Primary creep was observed in both regions thereby signifying dislocation activity during the initial period of the test. Preliminary experiments with coarse-grained Ni-50A1 suggested that the rtechanism in the n = 2 region is dependent on grain size. Transmission electron microscopy observations of the deformed specimens revealed dislocation tangles, dipoles, loops and networks in the power-law creep regime. The Burgers vector was determined to be <100> with the dislocations lying on the {100} and {110} planes. Although well-defined subgrains were not always observed, there was a greater tendency towards subgrain formation at stresses above 25 MPa. The deformation microstructures were inhomogeneous in the n = 2 creep regime and many grains did not reveal any dislocation activity. The observed characteristics of the low stress region suggest the dominance of an acccanmodated grain boundary sliding mechanism.

Creep characteristics of polycrystalline single-phase intermetallics, α2-Ti3AI and γ-TiAl, at 1100 K have been summarized to evaluate their relative strength at elevated temperature at three strain-rate levels, 10-1 10−4 and 10−8s−1, and are compared with those of α Ti-Al terminal solid solution alloys. The strength of these intermetallics is obviously higher than that of α, random solid solutions, especially at low strain-rates. However, the difference in high-temperature long-term strength among intermetallic alloys is not significant at any strain-rate level.

Creep behavior was investigated along the pseudo-binary line between L12-type Ni3Si and Co3Ti system with constant Si content. Addition of Co to Ni3Si and addition of Ni to Co3Ti enhanced creep strength due to precipitation hardening, respectively. Maximum creep strength on the pseudo-binary line was found at the composition which Co/(Ni+Co)=0.3 with 11 at.% Si content. The peak of creep strength on the pseudo-binary line was shifted to higher Co/(Ni+Co) value with decreasing Si content. Variation of creep strength was discussed based on metallo-graphic observations.

Also, the creep strength of these pseudo-binary alloys was improved by further addition of other elements, such as Hf and Ta.

The room temperature cyclic deformation of a cast and extruded NiAl alloy has been investigated. Low-cycle fatigue tests were performed under plastic strain control at strain ranges from 0.0002 to 0.0016. Cyclic hardening behavior has been analyzed and compared to the monotonic tensile and compressive behavior of the same material. Electroless nickel films applied to NiAl samples have little effect on monotonic deformation behavior, but do have a significant effect on cyclic deformation behavior, particularly cyclic stress asymmetry. Fractographic analysis has suggested that cycling at low plastic strain ranges may promote stable microcracking, while step testing indicates that cycling at lower plastic strain ranges can improve the cyclic life and/or stress levels achieved at the higher plastic strain ranges. Low plastic strain range cyclic prestrain can also significantly increase the monotonic tensile yield and ultimate strength of the material while most of the tensile ductility is retained.

The room temperature tensile properties and fracture behavior of a Ti-47A1-1.5Cr-lV-2.3Nb gamma TiAl alloy, have been investigated for two duplex and a fully-lamellar microstructural conditions. These microstructures were controlled through forging and subsequent recrystallization/solution treatments followed by furnace cooling and aging treatment at 900°C. Tensile testing was conducted on either as-machined or electropolished round-bar specimens over a range of strain rates from 10−5 to 10−1 sec−1 in laboratory air. Experiments showed that both tensile ductility and strength levels vary not only with microstructure and surface condition, but also with applied strain rate in an unexpected way. The complex relationships between these variables are under continuing investigation. The results to date, which were analyzed using microstructural observations made on the deformed and fractured specimens, indicate that the deformation and fracture modes remain essentially the same within the strain rate range used. The possible involvement of environmental effects in the fracture process is suggested.

In this study, the room temperature tensile properties of a single crystal NiAl alloy were investigated as a function of orientation. Fifteen crystallographic orientations were tested, including the <001>, <110> and <111>. The tensile properties measured include yield strength, plastic strain to failure, and ultimate tensile strength. Room temperature ductility as high as 1.4% was measured as close as 10° from the <001> orientation.

Small additions of Hf to [001] oriented single crystal NiAl are shown to be effective in improving high temperature creep strength. The presence of Hf-rich second phases and/or solid solution strengthening are responsible for the improved behavior observed at high temperatures. In the as-cast condition, large Hf-rich interdendritic regions were found. Homogenization heat-treatments (1590 K for 50 hours) substantially reduced this interdendritic segregation. TEM observations of the as-homogenized microstructure revealed fine G-phase (Ni16Hf6Si7) precipitates, with plate or cuboidal shapes. Varying the cooling rates after homogenization resulted in the refinement or complete suppression of the G-phase. Aging the homogenized material at 1300 K resulted in the formation of Heusler precipitates (β'-Ni2AlHf), preferentially nucleating at the G-phase sites. These Heusler precipitates were more stable at this or higher temperatures and coarsened at the expense of the less stable G-phase. Post-test analyses of compression tested specimens, conducted at 1200, 1300 and 1400 K, revealed extensive changes in the distribution and size of the second phases. Deformation at 1300 K appears to occur by two distinct mechanisms: at high strain rates the stress exponent is  4 while at slower rates (< 10−6s−1) a much higher exponent (∼ 12) was found. Testing at 1300 K of specimens over-aged at 1400 K reduced the creep resistance of the alloy which suggests a contribution by precipitation strengthening to the overall strength of the alloy.

The formation mechanism of the columnar-grained stoichiometric Ni3Al by FZ-UDS (unidirectional solidification using a floating zone method) was investigated. The quenched solidification interface showed that Ni3Al and β-NiAl formed simultaneously in the form of lamellar structure at the interface. The β-NiAl dissolved into the Ni3Al matrix during cooling, resulting in the columnar-grained structure of single phase Ni3Al at room temperature. The columnar-grained Ni3Al had a large number of low angle boundaries and exhibited a large tensile ductility. The characteristics of the deformation behavior were also investigated. Slip lines easily transferred acros.s grain boundaries without crack initiation. Deformed bands with many stacking faults were produced by deformation.