Deformation modes in NbAl3

The DO₂₂ lattice of the NbAl₃ intermetallic compound shows very limited ductility at room temperature. In this study the slip and twinning systems that are active during the deformation process were investigated. Evaluation of the possible deformation modes was performed and contrast analysis in the transmission electron microscope revealed both expected and unexpected deformation modes. Two types of dislocations were found in the deformed structure, namely thea 〈110〉 superdislocation on the {112} plane and loops of unidentified dislocations on the {010} plane. No evidence of 〈201〉 superdislocations was found, probably due to the fact that this type of dislocation is expected to move in groups of four. Twins of the {112} type were found to play an important role in the deformation process since they supply a component of shear perpendicular to the (001) plane.     

Deformation and fracture of NiAl single crystals tested in torsion

Binary NiAl single crystals were tested in torsion. [001] oriented samples exhibited lower shear strength and higher shear strain to failure compared to [110] oriented samples. Transmission electron microscopy revealed an increasing dislocation density from the center of the sample to the outer surface. Most dislocations that were analyzed were of the b= <100= type, however, some 〈110〉 type dislocations were observed in the [110] oriented samples. Electron backscatter patterns and tilting experiments were used to determine the crystallography of two types of facets on failed samples: those near {112} and those ∼10° away from {001}.     

The Mode of Deformation in a Cold-Swaged Multifunctional Ti-Nb-Ta-Zr-O Alloy

Multifunctional titanium alloys, termed Gum Metal?, are β-phase Ti alloys first developed in 2003. These alloys exhibit many interesting properties including, for example, low rate of work-hardening and superplasticity during cold deformation. The original report described a new plastic deformation mechanism not involving major dislocation activity to explain such deformation behavior. In the current study, a comparable Ti-36.8Nb-2.7Zr-2.0Ta-0.44O (wt pct) alloy to the original investigators was produced by powder sintering, hot forging, solution treatment, and cold swaging with the aim at investigating the microstructural development during swaging. XRD and TEM showed that the forged/solution-treated alloy was β-phase with a small amount of ω-phase. After cold swaging by up to 96 pct area reduction, TEM/HRTEM revealed the existence of dislocations, deformation twins, ω-phase, nanodisturbances, and lattice bending, with EBSD showing the grains to be highly elongated in the swaging direction, fragmented, and distorted. Most notably, swaging also generated a strong 〈110〉 fiber texture, even after moderate strains. The foregoing structural analysis provides substantial evidence that dislocations are present in the alloy after cold swaging. The major support of dislocation glide processes acting as the dominant plastic deformation mode in the swaged alloy is the strong 〈110〉 fiber texture that develops, which is a characteristic feature of all cold-drawn/swaged body centered cubic metals and alloys.     

High- temperature deformation properties of nial single crystals

The high-temperature deformation properties of stoichiometric NiAl single crystals have been studied in the temperature range from 850 °C and 1200 °C. We have established a basic data set for and have explored the high-temperature deformation characteristics of this intermetallic compound. The results provide a basis for determining the controlling mechanisms of high-temperature deformation. Constant stress tension creep and constant stress or constant strain rate compression experiments were conducted on crystals oriented with loading axes along the “hard,” [001] orientation, where no driving force exists for glide ofb = (001) dislocations, and along various “soft” orientations, [223], [111], and [110], where deformation can occur by the glide of these dislocations. In addition to these monotonie tests, high-temperature deformation transients were studied using stress relaxation, strain rate change, and stress change experiments. These transient deformation experiments were conducted in an effort to further elucidate the mechanisms that control high-temperature deformation of this material. The steady-state deformation properties of these differently oriented single crystals can be characterized by creep activation energies that all coincide, within experimental error, with the activation energy for diffusion of Ni in NiAl, 308 ± 10 kJ/mol. The stress dependence of steady-state deformation can be characterized with stress exponents that range from about 9 at 850 °C to about 4 at 1200 °C. At all temperatures and stresses, the soft oriented crystals creep about two orders of magnitude faster than the hard oriented crystals at the same stresses and temperatures. Soft oriented crystals loaded along [223] and [111] axes tested in both tension creep and constant stress or constant strain rate compression are found to deform at the steady-state rate from the very beginning of the deformation experiment. Crystals with these orientations exhibit virtually no evidence of strain hard-ening. Transients associated with stress changes suggest that deformation is limited primarily by the mobility of dislocations and not by dislocation interactions. These characteristics of deformation are consistent with the operation of easyb = (001) glide processes in these crystals. Crystals loaded along [110] exhibit small deformation transients which indicate both sluggish dislocation motion and some substructure formation. We speculate that cross-slip of dislocations from {110} to {010} planes is responsible for this effect. Deformation in hard oriented crystals provides evidence for both mo-bility and substructure controlled deformation. Creep in hard oriented crystals is characterized by a dramatic sigmoidal transient suggesting very low dislocation mobility. However, the strain hardening observed in monotonic tests and the transient responses suggest that deformation is also limited by a dislocation substructure that forms during deformation. These findings support the conclusion, explored fully in a forthcoming article, that creep deformation in the hard orientation is controlled by the motion and interaction ofb = (101) dislocations.     

The orientation dependence of deformation mode and structure in stoichiometric NiAl single crystals deformed by high temperature steady-state creep

Single crystals of stoichiometric NiAl having predetermined orientations have been deformed by steady-state creep in the temperature range 750° to 1055°C, the stresses being varied from 1.76 to 7.02 kg mm_-2. In crystals oriented 7.5 deg from [101], only (100) slip was observed; the primary slip plane was 100 although secondary slip occurred on 110. As predicted by elastic anisotropy considerations, dislocations on 100 with b = α(100) have a zig-zag shape with the segments aligned along (110). On 110 they tend to lie along (111) and (112). Cross-slip of short segments having screw orientations can occur between l00 and 110, giving rise to dipoles and prismatic loops. Crystals with tensile axes 14 deg from [1ll] slip in all three (100) directions on both cube and dodecahedral planes. Characteristic structures are dislocation entanglements, dipoles, loops, and networks containing nodes. Often two (100) type dislocations react and form a segment of (110) dislocation. Crystals approximately 19 deg from the cube orientation slip in both the (100) and (110) directions, and the contribution of (110) slip to the total glide strain increases at higher temperatures. The (110) dislocations are mostly pure screw as predicted by elastic anisotropy. Two sets of α(100) dislocations with mutually perpendicular Burgers vectors can form a network of twist or mixed character. If the contact plane is not one of the cube planes, the network is lozenge shaped, and small (110) segments form at the node points. Slip in the (111) direction was not observed with certainty; (100) slip occurred in all specimens, and (110) slip only in crystals reasonably close to the cube orientation.     

Stress asymmetry of stoichiometric NiAl single crystals

The yield stress properties of stoichiometric NiAl single crystals were investigated in terms of crystal orientation, temperature and the deformation mode. The calculated critical resolved shear stress (CRSS) was a strong function of crystal orientation, temperature and the deformation mode whether tension or compression. The CRSS was, in a wide range of experimental conditions, higher in the sequence of {110}〈100〉, {100}〈100〉 and {hk0}〈100〉 slips. The CRSS in compression was higher particularly at low temperatures than the CRSS in tension. The tension-compression asymmetry on the CRSS was understood qualitativelys being due to the effect of the normal stress on the core structure of a 〈001〉 dislocation and a 〈111〉 dislocation. It was suggested that a compressive normal stress makes the core configuration more sessile, resulting in the increased stress effectively at low temperatures.     

Deformation modes in NbAl3

The DO₂₂ lattice of the NbAl₃ intermetallic compound shows very limited ductility at room temperature. In this study the slip and twinning systems that are active during the deformation process were investigated. Evaluation of the possible deformation modes was performed and contrast analysis in the transmission electron microscope revealed both expected and unexpected deformation modes. Two types of dislocations were found in the deformed structure, namely thea 〈110〉 superdislocation on the {112} plane and loops of unidentified dislocations on the {010} plane. No evidence of 〈201〉 superdislocations was found, probably due to the fact that this type of dislocation is expected to move in groups of four. Twins of the {112} type were found to play an important role in the deformation process since they supply a component of shear perpendicular to the (001) plane.     

The Structure of Extruded NiAl

The deformation structure of Ni-rich NiAl extruded at 550°C has been characterized by transmission electron microscopy and by optical microscopy. Dislocations having a«100» Burgers vectors were found as complex networks, tangles, and prismatic loops. a«110» dislocations which were rare, were concluded to arise from reactions of a«100» dislocations. Evidence of recovery and recrystallization was obtained. Extrusion was deemed to have been possible by the operation ofhk0 «001» slip systems (often in plane strain flow) plus diffusion-assisted processes.     

Texture simulation for hot rolling of aluminium by use of a Taylor model considering grain interactions

The hot rolling textures of aluminium are simulated by means of a Taylor type model which takes into consideration dislocation slip on 111〈110〉 and 110〈110〉 glide systems and the interaction of grains. For the investigation of the stability of the cube component during hot rolling various ratios of the corresponding critical resolved shear stresses τ¹¹⁰/τ¹¹¹ are applied. Whereas the orientation densities and the positions of the β-fibre components 112〈111〉 and 123〈634〉 are not substantially influenced by slip on 110〈110〉 glide systems the cube component is developed at the expense of the 110〈112〉 orientation when the yield surface for 110 slip is within that for 111 slip.     

Dislocation behavior at tilt boundaries of infinite extent

A detailed theoretical analysis has been made of the stress fields which arise from the motion of glide dislocations across a symmetric tilt boundary. These stress fields are long range and result from the formation within the boundary of virtual grain boundary dislocations. A similar type of long range stress field is generated when two crystals of differing lattice size are joined together. In the former case the long range stress fields may be sufficient to generate dislocation glide loops while in the latter case these loops are of the prismatic type and move by climb. Portions of both types of loops move to the boundary and become incorporated into them as interface dislocations, in turn reducing the long range stress fields arising from the virtual dislocations.     

Electron-microscopic study of the slip systems in an Mg-4.5% Al-1% Zn alloy

Samples of a hot-rolled Mg-4.5% Al-1% Zn alloy plate cut normal to the rolling direction are studied to determine the density of dislocations with different Burgers vectors after warm rolling at low total strains. In the experiment, 22 different grains and their orientations are studied in a JEM-1000 high-voltage transmission electron microscope at an accelerated voltage of 750 kV using the WB DF technique (g, ng, where n = 2, 3, …) for the observation of dislocations. The main method used for the analysis of the Burgers vector of the dislocations is the invisibility criterion gb = 0. In the samples, dislocations with Burgers vectors 〈a〉, [c], and 〈a + c〉 are found. The dislocation dissociation reaction of the 〈a + c〉 dislocation into the 〈a〉 and [c] dislocations has been found for the first time, and an assumption is made that all of the found [c] dislocations result from this reaction. The density of the basal and nonbasal 〈a〉 dislocations does not depend on the orientation of the grains. Rarely found [c] and 〈a + c〉 dislocations are in grains whose orientations are near the texture maximum in the {0002} pole figure.     

Cold work hardening in stages IV and V of F.C.C. metals—I. Experiments and interpretation

The paper presents numerous measurements which confirm stages IV and V to be general ranges of cold work deformation. Analogous to stage II, stage IV exhibits a linear athermal hardening with constant strain rate sensitivity and activation enthalpy. In stage IV the dislocation cell size is constant, while the dislocation density growth rate is markedly reduced compared with stages II and III. Features of stage V are analogous to stage III, the increase of strain rate sensitivity (decrease of activation enthalpy) indicating the onset of thermally activated dislocation annihilation. In stage V, the mechanism is identified as dislocation climb from observing subgrain formation and saturation in density of deformation induced vacancies. Comparisons with recent investigations of stage IV and V at high temperatures suggest a common picture of low and high temperature deformation which only requires principles of storage and annihilation for both screw and edge dislocations.     

Textures and plastic anisotropy in γ-TiAl

A polycrystalline alloy of composition Ti-36 wt % Al consisting mainly (about 95 vol. %) of γ-TiAl has been deformed in compression at 450°C as well as in rolling at 1040°C. The textures of the deformed specimens were measured and analyzed in terms of orientation distribution functions (ODFs). The textures after hot rolling show a cube-like component (100) [010] with an alignment of the c-axis with the transverse direction. A comparison of measured compression textures with those simulated on the basis of the Taylor theory of polycrystal deformation leads to the following conclusions. Both the “easy” {111} 〈110〉 and “hard” {111} 〈101〉-slip modes of deformation occur in γ-TiAl at 450°C. The critical resolved shear stresses (CRSSs) for these two slip modes differ by a factor of less than 2, the CRSS for {111}〈110〉-slip being higher than that for {111}〈101〉-slip. The rolled specimens show a pronounced plastic anisotropy, which can only be explained on the basis of microstructural considerations.     

Stress generation and vacancy annihilation during scale growth limited by cation-vacancy diffusion

The growth of cation-diffusing scales on pure metals is described in terms of intrinsic dislocations for an epitaxial metal-scale interface. This model is consistent with the experimental observations of high local deformation and intimate contact, and epitaxial relations between the innermost grains of the scale and the underlying metal. The annihilation of metal vacancies at the metal-scale interface occurs by the climb into the metal of some fraction of the intrinsic misfit interface dislocations. Epitaxy is maintained by an adjustment of the spacing of the remaining interface dislocations, a process which generates tensile stress in the metal and compression in the scale. Above a critical interfacial strain, the glide of dislocations in the metal, in combination with dislocation glide in the scale, recreates or resupplies the interface dislocations. These processes provide plastic deformation in both phases near the interface and permit the retention of epitaxy during metal recession. The model may explain the origin of stresses arising during the growth of cation-diffusing scales on an extensive flat surface and the influence of surface orientation and surface preparation on the oxidation kinetics, etc. The epitaxial growth of NiO scale on pure Ni is described as a typical example.     

Self-diffusion in aluminum at low temperatures

Values of the self-diffusivity of pure aluminum in the temperature range 130∮ to 200° have been determined by measuring the rate of annealing of prismatic and faulted dislocation loops in thin foils of quenched 99.999 pct Al using a diffusion-controlled climb model due to Seidman and Balluffi modified to take into account elastic interaction between vacancies and dislocations. Changes in line energy of prismatic loops with orientation were evaluated and found to produce a maximum error in D of 17 pct. Other possible sources of error were evaluated. The results giveD = 0.19 ± 0.06 exp—(1.28 ± 0.04ev /KT sq cm per sec. A direct method of determining the activation energy for self-diffusion from prismatic loop annealing rates is presented which minimizes the effects of many sources of variation in individual climb rates. Formerly with University College     

Plastic deformation of symmetrical bicrystals having σ3 coincidence twin boundary

Three different orientations of Fe-5.8 at.% Si bicrystals with the twin high coincidence grain boundary on {112} were deformed in compression as well as in tension at a strain rate of 3.5 × 10⁻⁵s⁻¹ at room temperature. It is shown that the results of macroscopic measurements can be interpreted in terms of dislocation reactions at the boundary. In spite of the fact the component crystals are elastically and plastically compatible in all tested bicrystals, plastic deformation differs for the three orientations of bicrystals and depends on the magnitude of Burgers vector of residual grain boundary dislocations. The larger residual Burgers vector the stronger barrier to penetration of lattice dislocations trough the boundary. Relationship between the constraints imposed by the sample heads on the rotations of component crystals and the extent of the end or grip effect is discussed.     

The wetting transition in high and low energy grain boundaries in the Cu(In) system

Wetting of two symmetrical tilt grain boundaries, 77° 〈110〉 and 141° 〈110〉, in synthetic copper bicrystals with a Cu(In) melt was studied in the temperature range 690°990°C. The contact angle at the site of GB intersection with the solid-melt interface was measured. A wetting transition occurred at T_w = 960 ± 6°C for the 77° 〈110〉 grain boundary and at T_w = 930 ± 5°C for the 141° 〈110〉 grain boundary. The contact angle approached zero for this transition. The relative surface energies of the two boundaries were measured using the thermal grooving technique. The energy of the 77° 〈110〉 grain boundary is about 40% lower than that of the 141° 〈110〉 grain boundary. Therefore, it has been shown experimentally that the lower the grain boundary energy, the higher the wetting transition temperature. This is in agreement with the thermodynamic model of wetting transitions on grain boundaries.