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.     

Electron microscopy studies of butterfly martensite

Crystallographic and substructural features of butterfly martensite formed in Fe-Ni-Cr-C alloys were investigated using transmission electron microscopy.{112}_b twins, lengthy dislocations and two types of {101}_bplanar defects were observed as substructures. The {112}_btwins were not distributed uniformly and the twin edges inside a martensite plate were irregular in contrast to those in Fe-Ni lenticular martensitcs. The length dislocations were parallel to the 〈 111〉_b and the dominant set was almost parallel to the expected shape strain direction. In the surrounding of martensite plate austenite slips on (111)_f plane which may be associated with the accommodation of the transformation shape strain were observed. The austenite-martensite orientation relationship was (111)ƒ/(011) and [101]y 2.0 deg from [TT1]_b. In general, the complex features of the substructures of butterfly martensite and the characteristics of {112} twins and {101} planar defects were found to be quite similar to those observed in nonpaired {225} plate martensites.     

X-ray diffraction study of the structural evolution in a grade 1469 (Al-Cu-Li) alloy during the production of hot-rolled sheets

X-ray diffraction is used to study the structural changes in a grade 1469 (Al-Cu-Li) alloy with a high lithium content that occur during the production of hot-rolled sheets according to the ingot → pressed strip → hot-rolled sheet schedule. In the pressed strip, a multicomponent Bs {110}〈112〉, Cu {211}〈111〉, and S {123}〈111〉 texture forms, which is typical of articles pressed from such alloys, and an unusual intense single-component texture of the Ex₁ {011}〈111〉 type forms in the hot-rolled sheets. Its formation is stimulated by cross rolling of the sheets. The low strength characteristics of the hot-rolled sheets after heat treatment are related to an elevated heating temperature used for quenching and to the oriented precipitation of the lamellar particles of the hardening T ₁ phase.     

Simulation de la texture de deformation en traction uniaxiale et calcul du coefficient de lankford R(α) de toles d'aciers extra doux—application au cas non orthotrope

Two calculations to simulate the plastic behaviour of b.c.c. materials are presented. The first one allows to calculate the plastic deformation textures and so the O.D.F. F(g) in Euler space G (gϵG). The second one allows to calculate the Lankford factor R(α) of a polycrystalline sheet in all cases of symmetry of the texture. Previous works were principally devoted to orthotropic symmetry. Both calculations use the two slip systems {110}〈111〉 and {112}〈111〉 in the Taylor approximation with variable ration of critical resolved shear stresses for the two slip planes {110} and {112}.     

Study of \{ 11\bar{2} 1\} Twinning in α-Ti by EBSD and Laue Microdiffraction

Activity of the $ \{ 11\bar{2} 1\} \langle \bar{1} \bar{1} 26 \rangle $ extension twinning (T2) mode was analyzed in a commercial purity Ti sample after 2 pct tensile strain imposed by four-point bending. The sample had a moderate c-axis fiber texture parallel to the tensile axis. Compared with the many $ \{ 10\bar{1} 2\} \langle \bar{1} 011 \rangle $ extension (T1) twins that formed in 6 pct of the grains, T2 twins were identified in 0.25 pct of the grains by scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD) maps. Most of the T2 twins exhibited irregular twin boundaries (TBs) on one side of the twin. High-resolution EBSD revealed both intermediate orientations at some matrix/twin interfaces and substantial lattice rotation within some T2 twins. Interactions between matrix 〈c + a〉 dislocations $ \frac{1}{3} \langle 1\bar{2} 13 \rangle $ and a $ \{ 11\bar{2} 1\} $ T2 twin were investigated by combining SEM/EBSD slip trace characterization and Laue microdiffraction peak streak analysis. 〈c + a〉 dislocations that originally glided on a pyramidal plane in the matrix were found on other planes in both the matrix and the twin, which was attributed to extensive cross-slip of the screw component, whose Burgers vector was parallel to the twinning plane. On the other hand, thickening of the twin could engulf some pile-up edge components in front of the TB. During this process, these 〈c + a〉 dislocations transmuted from a pyramidal plane $ (0\bar{1} 11) $ in the matrix to a prismatic plane $ (\bar{1} 010)_{\text{T}} $ in the twin lattice. Finally, possible mechanisms for the nucleation and growth of T2 twins will be discussed.     

Plane strain compression of silicon-iron single crystals

Six different orientations of Fe-3 wt% Si single crystals have been deformed in plane strain compression (using a channel die) up to true strains of 0.5. The finite strain behaviours, i.e. the shape changes, lattice rotations and stress-strain curves, are compared with the predictions of the generalized Taylor analysis of partially constrained crystal deformation. The influence of the relative critical resolved shear stresses on the {110} and {112} 〈111〉 glide systems has been systematically examined. It is shown that for most crystals under multiple slip conditions the shape changes and lattice rotations are consistent with the hypothesis of glide on {112} being somewhat easier than on {110}. Comparison with previous work on b.c.c. crystals undergoing large strains leads to the suggestion that: (i) under conditions of single or colinear slip, glide on {110} is easier than on {112}; (ii) under conditions of intersecting 〈111〉 slip directions, glide on {112} is easier than on {110}. For silicon-iron, the critical resolved shear stresses on the {112} 〈111〉 systems, relative to those on the {110}〈111〉 systems are found to be 0.93 and 0.96 for the twinning and anti-twinning senses, respectively.     

Texture Development in the Ni47Ti44Nb9 Shape Memory Alloy During Successive Thermomechanical Processing and Its Effect on Shape Memory and Mechanical Properties

For improving the shape memory performance and mechanical properties of shape memory alloys (SMAs), crystallographic texture and second phase are generally induced in SMAs by suitable thermomechanical processing. For this purpose, the development of texture in the Ni₄₇Ti₄₄Nb₉ SMA during successive processing (e.g., hot forging, hot rolling, cold rolling, and heat treatment) and the effects of texture, grain size, and β-Nb particle precipitation on recoverable strains and tensile properties were studied. In the hot-forged and hot-rolled Ni₄₇Ti₄₄Nb₉ alloy rods, intense 〈111〉 fibers are formed, and water quenching from 873 K and 1123 K (600 °C and 850 °C) leads to the decrease in intensity of 〈111〉 fiber in the hot-rolled rod. When the hot-forged rod is hot-rolled into sheet, intense {001} and weak {123} fibers appear, but grain growth leads to the disappearance of {001} fiber and {110}〈001〉 becomes the strongest component. Cold-rolling deformation of the hot-rolled sheet promotes the development of γ-fiber and the convergence of {332} and {123} fibers to {233}〈110〉 and {123}〈121〉 components, respectively, and the intense component is turned into {111}〈110〉; in this case, the recoverable strain (ε _SRS) and tensile yield strength (σ _YS ) exhibit an anisotropy. When the quenching temperature is 1123 K (850 °C), some weaker components appear, the anisotropy of ε _SRS disappears, and the difference level in σ _YS along the rolling direction (RD) and transverse direction (TD) becomes smaller. Therefore, an appropriate heat-treatment temperature should be selected to maintain the deformation texture and also to obtain fine grains for different thermomechanical processing.     

Deformation in NiAl bicrystals

The effects of a concentrated local stress arising from elastic and plastic incompatibility at internal NiAl grain boundaries have been investigated in oriented NiAl bicrystals. Observations of surface slip markings and TEM characterization of the dislocations responsible for bicrystal deformation have been made. The glide of a〈110〉 dislocations on {110} planes is the primary mode of deformation in the vicinity of the oriented bicrystal interfaces investigated. The dominant mode of a〈110〉 dislocation generation is the nucleation of loops away from the bicrystal interface. The macroscopic stress required to activate these dislocations, in the presence of a stress concentration such as that produced by a dislocation pile-up at a grain boundary, is the same as that required to nucleate a〈100〉 dislocations in “soft” single crystals of NiAl. Although the statistical relevance of this phenomenon as it pertains to the deformation of NiAl polycrystals has not been established, it is speculated that the glide of non-a〈100〉 dislocations may play an important role in the ductility of polycrystalline NiAl at the ductile-to-brittle transition temperature. The movement of a〈100〉 dislocations provides the primary mode of deformation for several bicrystals investigated far (> 3 mm) from the bicrystal interface. It is suggested that the vacancies required for the extensive climb of a[100] vacancy loops is supplied by the climb of a[100] and a[010] dislocations, and that the gradient in the concentration of vacancies provides the driving force for the motion of the a[100] and a[010] dislocations.     

Effect of Austenite Pancaking on the Microstructure,Texture, and Bendability of an Ultrahigh-Strength Strip Steel

The effect of austenite pancaking in the non-recrystallization regime on microstructure and texture evolution and thereby on bendability was investigated in an ultrahigh-strength strip steel with a martensitic-bainitic microstructure. The results indicate that an increase in rolling reduction (R _tot) below the non-recrystallization temperature, which improves the strength and toughness properties, increases the intensities of the ~{554}〈225〉 _α and ~{112}〈110〉 _α texture components along the strip centerline and of the ~{112}〈111〉 _α component at the surface region. Even with the highest R _tot of 79 pct, the bendability along the rolling direction was good, but the preferred alignment of rod-shaped MA constituents along the rolling direction led to a dramatic decrease in the bendability transverse to the rolling direction, with severe cracking occurring even at small bending angles. The early cracking is attributed to localization of the strain in narrow shear bands. It is concluded that the R_tot value has to be limited to guarantee successful bendability.     

Evolution of Texture from a Single Crystal Ti-6Al-4V Substrate During Electron Beam Directed Energy Deposition

Additive manufacturing of Ti-6Al-4V commonly produces 〈001〉 _β -fiber textures aligned with the build direction. We have performed wire-feed electron beam directed energy deposition on the {112} _β plane of a single prior β-grain. The build initially grew epitaxially from the substrate with the preferred 〈001〉 growth direction significantly angled away from the build direction. However, continued layer deposition drove the formation of a 〈001〉 _β -fiber texture aligned with the build direction and the direction of the strongest thermal gradient.     

Plate-shaped transformation products in zirconium-base alloys

Plate-shaped products resulting from martensitic, diffusional, and mixed mode transformations in zirconium-base alloys are compared in the present study. These alloys are particularly suitable for the comparison in view of the fact that the lattice correspondence between the parent β (bcc) and the product α (hcp) or γ-hydride (fct) phases are remarkably similar for different types of transformations. Crystallographic features such as orientation relations, habit planes, and interface structures associated with these transformations have been compared, with a view toward examining whether the transformation mechanisms have characteristic imprints on these experimental observables. Martensites exhibiting dislocated lath, internally twinned plate, and self-accommodating three-plate cluster morphologies have been encountered in Zr-2.5Nb alloy. Habit planes corresponding to all these morphologies have been found to be consistent with the predictions based on the invariant plane strain (IPS) criterion. Different morphologies have been found to reflect the manner in which the neighboring martensite variants are assembled. Lattice-invariant shears (LISs) for all these cases have been identified to be either {10 $\bar 1$ 1} _α 〈 $\bar 1$ 123〉 _α slip or twinning on {10 $\bar 1$ 1} _α planes. Widmanstätten α precipitates, forming in a step-quenching treatment, have been shown to have a lath morphology, the α/β interface being decorated with a periodic array of 〈c + a〉 dislocations at a spacing of 8 to 10 nm. The line vectors of these dislocations are nearly parallel to the invariant lines. The α precipitates, forming in the retained β phase on aging, exhibit an internally twinned structure with a zigzag habit plane. Average habit planes for the morphologies have been found to lie near the {103} _β — {113} _β poles, which are close to the specific variant of the {112} _β plane, which transforms into a prismatic plane of the type {1 $\bar 1$ 00} _α . The crystallography of the formation of the γ-hydride phase (fct) from both the α and β phases is seen to match the IPS predictions. While the β-γ transformation can be treated approximately as a simple shear on the basal plane involving a change in the stacking sequence, the α-γ transformation can be conceptually broken into a α → β transformation following the Burgers correspondence and the simple β-γ shear process. The active eutectoid decomposition in the Zr-Cu system, β → α + β′, has been described in terms of cooperative growth of the α phase from the β phase through the Burgers correspondence and of the partially ordered β′ (structurally similar to the equilibrium Zr₂Cu phase) through an ordering process. Similarities and differences in crystallographic features of these transformations have been discussed, and the importance of the invariant line vector in deciding the geometry of the corresponding habit planes has been pointed out.     

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.     

Solution hardening of zinc crystals with aluminium and copper additions during the tensile test

Zinc single crystals, containing varying amounts of copper or aluminium, have been subjected to plastic deformation in the uni-axial tensile test in the temperature range of 173–473 K. The effect of the alloying additions and temperature on: initial forest dislocations density ρ_f (0001), critical resolved shear stress (CRSS) value in the basal and pyramidal systems, twinning stress τ_T value in the {10–12} 〈1̄011〉 systems has been tested. Photographs of the etch pits before and after plastic deformation of the single crystals tested have been presented.     

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}.     

Competition among {110}, {112}, and {123} <111> slip modes in BCC metals

The conditions for the activation of various combinations of {ll0} <111>, {112} <1ll>, and {123} <111> slip modes have been examined by analyzing the yield loci. It is found that {ll0} slip will occur in preference to the other two modes if α > 1.155 and α₂ > 1.134, where α₁ and α₂ are respectively the critical resolved shear stress for slip on {112} and {123} systems relative to {ll0} slip. Slip occurs on {112} systems alone if α₁ < 0.866 and α₁ < 0.917α₂: and on {123} systems alone if α₂ < 0.945 and α₂ < 0.982α₁. The influence of {112} slip asymmetry and {112} <111> twinning on the choice of deformation modes has also been analyzed.     

Deformation banding and copper-type rolling textures

It is shown by a crystallographic etching technique applicable to copper that deformation banding is an important deformation mode in f.c.c. metals and alloys. In a cold rolled coarse grain copper, deformation banding forms in a three dimensional manner dividing the grain on average into over 600 regions of different orientations. The influence of this important, but long ignored deformation mode, is studied by incorporating it into the Taylor model. The predicted textures from the new model are better than those from other existing models in mainly two respects. Firstly, the DB model predicts the co-existence of the three major f.c.c. rolling texture components, namely {123}〈634〉 or S component, >112>〈111〉 or C and >110>〈112〉 or {f}B{/f}. The existing models are deficient in that they predict either C and S or B, but not their co-existence. The second point is that textures predicted by the existing models are always too sharp compared to the experimental textures. The DB model predicts texture peaks with larger spread and hence more realistic texture sharpness. Another feature of the model is that only two independent slip systems, instead of five, are required to accomodate the imposed shape change, which agrees with experimental observation.     

Crystallographic preferred orientation induced by cyclic rolling contact loading

Fine focus X-ray diffraction methods have been applied to analyze the texture development of the ferrite phase during rolling contact fatigue of 6309 type deep groove ball bearing inner rings prepared from hardened and tempered SAE 52100 steel. Textures of the ferrite matrix as {100}〈110〉 and {111}〈211〉 (where {hkl} denotes the crystallographic plane that is preferably parallel with the contact surface and 〈uvw〉 denotes the crystallographic direction that is preferably parallel with and in the direction of over-rolling) have been identified in a small region below the rolling contact surface. These textures develop gradually with an increasing number of stress cycles and become noticeable in conjunction with changes in residual stress, microstrain, and volume fraction of retained austenite in the same region. Upon rolling contact loading, both textures can become very pronounced, while the shape of the subsurface volume, where plastic deformation takes place in particular, remains unchanged: material displacement in the subsurface volume is less than 5 μm in the three principal directions. Crack propagation in association with spalling fatigue failure has been shown to be related to the type of texture developed.     

高应变速率下钛-钢复合板界面组织特征及变形机制

在高应变速率下,钛-钢复合板不同材料以不同的变形机制协调变形,结合界面起到至关重要的作用.本文分析研究了高应变速率下钛-钢复合板的界面组织特征和变形机制.结果表明:在钢侧,随着应变速率的提高,小角度(3°~10°)晶界含量增多,织构组分{112}〈241〉逐渐演变为织构{665}〈386〉和{111}〈110〉.在钛侧,随着应变速率的提高,出现了明显的形变孪晶组织,三种形变孪晶如{1121}〈1100〉拉伸孪晶、{1122}〈1123〉压缩孪晶和{1012}〈1011〉拉伸孪晶产生的难易程度不一样,变形机制由常规的"孪生变形为主"转变为"位错滑移与孪生变形共存"的复合变形模式.在结合界面处,随着应变速率的提高,需要适应由两侧产生的不同变形抗力,才能够实现连续变形而不致使材料发生破坏,其主要的协调机制依靠结合界面及附近晶粒的滑移实现变形.      

Grain size effects on the texture evolution of α-Zr

The texture evolution during the plastic deformation at room temperature of Zr-2.5Nb round bars was studied in specimens with two different α-Zr grain sizes. It was found that during axisymmetric compression the strain producing mechanisms active during deformation depended on the grain size. In fine grained specimens there are two main phenomena characterizing the evolution of texture: (i) a rapid rotation of the grains about their 〈c〉 axis to form a <112¯0> fiber at strains below −0.20 and, (ii) a slow and progressive reorientation of the 〈c〉 axes of the grains towards the compression axis to form a [0001] fiber texture tilted approx. 20°. The latter process takes strains larger than −0.80. In coarse grained material, the texture evolution is characterized by a sudden rotation of the 〈c〉 axes to become aligned parallel to the compression axis at strains as low as −0.05. It is shown using a self-consistent viscoplastic model of texture evolution that the type of texture obtained depends on the mechanism controlling 〈c〉 axis deformation. In fine grained material prismatic slip, with basal and pyramidal 〈c + a〉 slip acting as complementary deformation modes, control the texture evolution process. In coarse grained material twinning is responsible for the final texture observed.     

Stress-Induced Twinning and Phase Transformations during the Compression of a Ti-10V-3Fe-3Al Alloy

A metastable β Ti-10V-3Al-3Fe (wt pct) alloy containing different α phase fractions after thermo-mechanical processing was compressed to 0.4 strain. Detailed microstructure evaluation was carried out using high-resolution scanning transmission electron microscopy and electron back-scattering diffraction. Stress-induced β → α′′ and β → ω transformation products together with {332}〈113〉β and {112}〈111〉β twinning systems were simultaneously detected. The effects of β phase stability and strain rate on the preferential activation of these reactions were analyzed. With an increase in β phase stability, stress-induced phase transformations were restricted and {112}〈111〉β twinning was dominant. Alternatively, less stable β conditions or higher strain rates resulted in the dominance of the {332}〈113〉β twinning system and formation of secondary α′′ martensite.