Crystallographic analysis of extra reflections in the electron diffraction patterns of the CuAu alloy ordered at <Emphasis Type="Italic">T</Emphasis> < 350°C

The electron-microscopic examination of the CuAu alloy subjected to rapid quenching from temperatures below T _C revealed in its structure the presence of a phase, whose structure is close (in terms of the lattice parameters) to the L1₀ structure, but is characterized by the presence of uncommon extra reflections in the electron diffraction patterns at distances 1/3〈201〉*, 1/5〈203〉*, 1/7〈304〉*, etc., in the coordinates of the L1₀ structure, and by the simultaneous presence of superlattice reflections of two types in their electron diffraction patterns. A crystallographic analysis showed that the extra reflections observed cannot correspond to the points of the reciprocal lattice of the long-period Z phase [4] or any other phase. The arrangement of these reflections is explained well by the punctures of the plane of diffraction by a set of [$ \bar 1 $ \bar 1 01]* parallel streaks passing through fundamental and superlattice reflections. These streaks are parallel to the (101) plane and to the plates of the phase revealed.     

Texture of primary recrystallization on nonoriented electrical steel sheet with phase transformation

The magnetic properties of nonoriented (NO) electrical steel sheet are commonly improved if the texture of their products possesses “cube texture” (e.g., {100 〈0vw〉, “goss texture”) (i.e., 110〈011〉, and less 222 texture). Industrially “cube type” has not been obtained, but “goss texture” has been. In a greater or lesser degree, {222} texture exists. To improve “goss texture” and reduce 222 texture, the grain size of the material prior to cold rolling should be larger. When the grain size before cold rolling is larger, during primary recrystallization, “goss texture” is enriched, 222 texture is decreased, and the grain grows so easily that higher induction and lower core loss can be obtained. This does not depend on the presence of phase transformation. In case of NO steel with phase transformation, heat treatment before cold rolling has been done below the austenite transition temperature (Acin1) in order to prevent the fine grain size caused by α → γ(+α) → α transformation. By using material that was heated over Acin1 and cooled with changing cooling rates, this study describes (a) the relationship between textures before cold rolling and the texture of the final product, and (b) the development of the magnetic properties.     

Study of the texture of aluminum alloys after cold rolling,annealing, and irradiation by Ar<Superscript>+</Superscript> ions

The crystallographic texture of samples (1–3 mm thick) of AMg6, VD1, and 1441 aluminum alloys has been studied after cold rolling, post-rolling annealing, and irradiation by accelerated Ar⁺ ions (E = 20–40 keV). A feature in common for the {200} and {111} pole figures of samples rolled to a medium degree of reduction (35–72%) is the absence of the pole density at the center of these figures. When the alloys under study undergo post-rolling technological furnace annealing at high temperatures, a texture is formed with a scattered main component of the {001}〈100〉 type, which also occasionally contains orientations from scatter regions of the rolling-texture components. An exposure of the cold-deformed samples to a beam of accelerated Ar⁺ ions generally leads to analogous textural changes (in the whole volume of the samples irradiated from one side) in a much shorter time and, most frequently, at temperatures 100–200 K lower than in the case of the furnace annealing. In the 1441 alloy, the formation of an unusual two-component cube {001}〈210〉 texture has been noted.     

<Superscript>63</Superscript>Cu NMR spectra,magnetic susceptibility,and transmission electron microscopy of the rapidly quenched alloy Ti<Subscript>50</Subscript>Ni<Subscript>25</Subscript>Cu<Subscript>25</Subscript>

Methods of transmission and scanning electron microscopy and nuclear magnetic resonance (NMR) at ⁶³Cu nuclei, as well as measurements of the static magnetic susceptibility χ(T) have been used to study a shape-memory alloy (SMA) Ti₅₀Ni₂₅Cu₂₅, which experiences a thermoelastic martensite transformation. The alloy was obtained from an amorphous ribbon in a bimodal nano- and submicrocrystalline state via a crystallization annealing for 1 h at 770 K with a subsequent quenching to room-temperature water. The resultant B2 austenite is characterized by a fine structure of the ⁶³Cu NMR spectra, which is connected with the different distribution of ⁶³Cu atoms on the second coordination shell. The evolution of the shape of the spectra with decreasing temperature reveals a structural transition B2 → B19. In addition, the ⁶³Cu NMR spectra, just as the transmission electron microscopy, indicate the presence of phase separation in the alloy, with the precipitation of a TiCu (B11) phase. The temperature dependence of the static magnetic susceptibility χ(T) also indicates the occurrence of a structural transition and has a hysteretic nature of “stepped” type. The discovered stepped nature of the χ(T) dependence is explained by the bimodal size distribution of grains of the B2 phase due to the size effect of the martensitic transformation.     

Mechanism of formation of shear bands upon cold deformation of a commercial Fe-3% Si alloy

A model of the formation of shear bands upon cold deformation of {111}〈112〉 crystals of a commercial Fe-3%Si alloy is proposed. The model suggests a two-stage mechanism for the formation of a shear band and its fine structure. At the first stage, there occurs an anomalous twinning in the {114}〈221〉 system; at the second stage, an almost complete secondary twinning of the band in two standard {112}〈111〉 systems occurs. As a result of these transformations, the shear band consists of regions with an almost cube-on-edge orientation {11 11 1}〈1 1 22〉 and an “octahedral” {111}〈112〉 orientation, which is symmetrical with respect to the initial {111}〈112〉 orientation. A crystallographic mechanism responsible for an anomalous twinning in the {114}〈221〉 system by the slip of (8a/18)〈221〉 partial dislocations is proposed. It is supposed that the shear bands observed consist of groups of different numbers of shear microbands.     

Structure and mechanical properties of an Ni<Subscript>3</Subscript>Al single crystal upon high-temperature deformation

Structure and strength properties of single-crystal 〈001〉 samples of Ni₃Al have been studied in the as-grown and homogenized state during tensile tests in the temperature range of 1150–1250°C. At the strain rate of 1.32 mm/min (2 × 10⁻⁵ m/s), the samples are in the state of superplasticity. The basic mechanism of relaxation is dynamic recovery; in some regions of the sample, recrystallized grains are formed. At 1250°C, coarse twins are observed in the zone of fracture, which indicates the “switching on” of additional slip systems necessary to guarantee the relaxation process.     

Orientation dependence of superelasticity in ferromagnetic single crystals Co<Subscript>49</Subscript>Ni<Subscript>21</Subscript>Ga<Subscript>30</Subscript>

Dependence of the amount of reversible deformation on the orientation of the crystal axis and testing temperature has been studied using [001] and [$ \bar 1 $ \bar 1 24] single crystals of the Co₄₉Ni₂₁Ga₃₀ (at %) alloy upon compression. It has been shown that in the [001] crystals with T ≤ M _s (M _s is the temperature of the onset of the forward martensitic transformation upon cooling) the reversible deformation is equal to 5.5–6.5% and consists of the deformation connected with the shape-memory effect (SME) equal to 4–4.2%, and of “ferroelastic” deformation equal to 1.5–2.2%, which is reversible upon unloading. The total reversible deformation exceeds the lattice deformation ɛ₀ observed upon the B2-L1₀ martensitic transformation, which is equal to 4.5%. At T > A _f (A _f is the finish temperature of the reverse martensitic transformation upon heating), the reversible deformation in [001] crystals is equal to 6.5%. It has been shown electron-microscopically that the reversible deformation equal to 1.5–2.2% in the temperatures range of T = 77−300 K is connected with the development of mechanical twinning in the L1₀ martensite on (110) _L10 planes, which proves to be reversible in the [001] crystals and can be partly irreversible in the [$ \bar 1 $ \bar 1 24] crystals. Upon heating, the (110) _L10 twins of the stabilized L1₀ martensite pass into the ($ \bar 1 $ \bar 1 12) _B2 twins of the B2 phase.     

Characterization of the orientation structure and distribution in rolled polypropylene

The polymer orientation structure in rolled polypropylene sheets was studied by polarized-light microscopy, scanning electron microscopy, and wide-angle x-ray diffraction. The initial spherulites became deformed to a pancake shape as rolling deformation proceeded and became more perfectly aligned as the rolling procedure continued. Most polymer crystallites were oriented with theirb- axis nearly perpendicular to the rolling direction, as shown by x-ray pole figures. Even-order 〈P_n(cosχ)〉_c coefficients of the Legendre polynomial series (n = 2 to 10) describing thec-axis or orientation distribution of the molecular chains were determined from measurements of polar angle scan using the transmission technique. The 〈P _n 〉 _c (n = 2 to 10) curves calculated from the pseudoaffine deformation theory agreed with the experimental data.     

Magnetoresistance of the La<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> single crystal

The dependence of the resistance ρ of the La_0.7Ca_0.3MnO₃ single crystal on the temperature (in a range of 77 < T < 410 K) and magnetic field H is studied. The dependence of the magnetoresistance Δρ/ρ of the ferromagnetic phase on the field is shown to be determined by the competition of two mechanisms. In low magnetic fields, the magnetoresistance is positive Δρ/ρ > 0 and is determined by changes in the resistance with changing magnetization orientation with respect to the crystallographic axes; in high magnetic fields, the magnetoresistance is negative Δρ/ρ < 0, since it is the suppression of spin fluctuations in the magnetic field that plays the principal role. The phase transition from the ferromagnetic to paramagnetic state is a first-order transition close to the second-order one. In the transition range, the magnetoresistance is determined by the resistivity in the zero field ρ(T) and by the shift of the transition temperature T _C(H) in the magnetic field. In the paramagnetic state, the resistivity ρ(T) has an activation character; similarly to the magnetoresistance of other lanthanum manganites, the magnetoresistance of this single crystal is controlled by a change in the activation energy in the magnetic field.     

RETRACTED ARTICLE: Dislocation-particle interaction in precipitation strengthened Ll<Subscript>2</Subscript>-ordered Ni<Subscript>3</Subscript>Al

Transmission electron microscope investigation has been performed on the particle-dislocation interactions in Ni₃Al-based intermetallics containing various types of fine precipitates. In an Ll₂-ordered Ni₃Al alloy with 4 mol.% of chromium and 0.2–0.5 mol.% of carbon, fine octahedral precipitates of M₂₃C₆ type carbide, which has a cube-cube orientation relationship with the matrix, appear during aging. Typical Orowan loops are formed in Ni₃Al containing fine dispersions of M₂₃C₆ particles. In the alloys with appropriate titanium content, fine precipitates of coherent disordered γ are formed during aging. The γ precipitates are initially spherical or rounded cubic in shape and grow into platelets as aging proceeds. Loss of coherency is initiated by the introduction of dislocations at the γ/γ′ interface and results in step formation at the dislocations. The γ precipitates become globular after the loss of coherency. In the γ′ phase hardened by the precipitation of the disordered γ phase, dislocations are attracted into the disordered γ phase and cut through the particles during deformation at any stage of aging. In Ni₃Al containing a fine dispersion of disordered γ, superdislocations are strongly attracted to the disordered particles and dissociate on the (111) plane in the γ particles, while they dissociate on the (010) plane in the matrix. It is shown by comparison that the strengthening due to attractive interaction is more effective than that due to repulsive interaction. The roles of the variation of the interaction modes and of the dissociation of superdislocations in the matrix and particles are discussed in connection with the optimum microstructures of Ll₂-ordered intermetallics as high temperature structural materials.     

Microstructures of Ti<Subscript>50</Subscript>Al<Subscript>45</Subscript>Mo<Subscript>5</Subscript> alloy powders produced by plasma rotating electrode process

Microstructures of Ti₅₀Al₄₅Mo₅ (at.%) alloy powders produced by the plasma rotating electrode process (PREP) were investigated. The powders have inhomogeneous structures, which consist of dendrites and rounded grains. The dendrites, which show a “rosettelike” morphology, are formed on the powder surface and around the rounded grains. The rosettelike dendrites are of hexagonal α ₂ (D0₁₉) phase even though the dendrites have an equiaxial morphology, and a small amount of β ₂ (B2) phase is also contained inside. It is suggested that the solidification to α (hcp-A3) phase occurred by the peritectic reaction between the primary β (bcc-A2) dendrites and the liquid: L+β→L+β+α. The rounded grains, on the other hand, are of β ₂ phase in which acicular α or α ₂ laths are precipitated with the Burgers orientation relationship. Antiphase domain boundaries in the β ₂ matrix are intersected by α(α ₂) laths. It is interpreted that the α(α ₂) laths were formed by the solid-state transformations: β ₂→β ₂+α→β ₂+α ₂. The formation of the two different microstructures in the powder particles is rationalized in terms of the changes in local composition of the liquid phase during the rapid solidification process.     

Structural characterization of Laves-phase MgZn<Subscript>2</Subscript> precipitated in Mg-Zn-Y alloy

Structural characterization of Laves-phase MgZn₂ precipitated in Mg-6 wt.%Zn-1 wt.%Y alloy was performed using a high-resolution transmission electron microscope (HRTEM). Plate- and rod-shaped precipitates with the Laves structure were observed in an aged sample at 200 °C. An individual rod of the β₂′ phase precipitated in the matrix contained several domains with various orientations. Numerous antiphase boundaries (APBs) and stacking faults (SFs) were observed in the β₂′ precipitates. The stacking sequence of the 14C structure partially changed to that of the C15 structure due to the presence of a stacking fault. The orientation relationship between the C14 structure and the C15 structure in the β₂′ precipitates was [11–20]_C14‖[1–10]_C15 and (0001)_C14‖(111)_C15. Modification of the stacking sequence based on the synchroshear mechanism was discussed.     

Mechanisms of plastic deformation in microcrystalline and nanocrystalline TiNi-based alloys

Mechanisms of plastic deformation of a high-temperature B2 phase that act upon tension, compression, and high-pressure torsion in TiNi-based single crystals have been studied depending on the crystal orientation. For the crystals with orientations located near the [$ \bar 1 $ \bar 1 11] and [$ \bar 1 $ \bar 1 12] poles in the standard stereographic triangle, multiple dislocation slip prevails upon both compression and tension. In “hard” crystals with the deformation axis close to the [001] direction, in which the Schmid factors for dislocation slip are close to zero, the main deformation mechanisms are the mechanical twinning in the B2 phase and the stress-assisted B2 → B19′ martensitic transformation. All the above listed mechanisms take part in the formation of the {111}〈hkl〉 texture. The mechanism of the change in the orientation of “hard” polycrystalline grains upon the formation of a nanocrystalline and amorphous-crystalline state has been demonstrated on the example of the evolution of the structure of [001] crystals upon severe plastic deformation in a Bridgman cell.     

The effect of precipitation on the evolution of recrystallization textures in an AA 8011 aluminum alloy sheet

The texture of an AA 8011 aluminum alloy sheet cold rolled by 95% showed a typical β-fiber, which runs from the copper orientation [C={112}〈111〉] over S [{123}〈634〉] to brass [B={011}〈112〉]. The development of annealing textures depended on annealing temperatures due to the interaction between precipitation and recrystallization. Upon annealing at a low temperature of 275°C, precipitation took place before recrystallization. This led to a weak recrystallization texture consisting of {011}〈122〉, {001∼〈100〉, and {hk0}〈001〉, among which the {011}〈122〉 orientation developed near large FeAl₃ particles as the main orientation and the cube [{001}〈100〉] orientation originating from the matrix was relatively weak. After annealing at 350 and 500°C, a strong cube texture developed along with a weak {011}〈122〉 orientation. When the cube orientation developed, the copper orientation disappeared most rapidly. These results were discussed based on the interaction between precipitation and recrystallization.     

Evaluation of the Peierls stress for boundary dislocations

The Peierls stress for 1/6〈111〉 twinning dislocations and 1/2〈111〉 perfect dislocations in a bcc structure has been evaluated. The calculations have been performed using the Peierls-Nabarro formalism. The Peierls stresses have been determined from the migration energy of a twin boundary γ_tbm and the energy of an unstable stacking fault γ_us. The values of γ_tbm and γ_us have been calculated using a set of generalized many-body interatomic potentials. The potentials were defined so as to ensure different stability of the bcc structure relative to other structures. It has been shown that this approach provides realistic values of the Peierls stress. The values of the Peierls stress for 1/6〈111〉 twinning dislocations are very sensitive to the model parameters, unlike those for 1/2〈111〉 perfect dislocations.     

Effect of the hot-rolling microstructure on texture and surface roughening of Al-Mg-Si series aluminum alloy sheets

This study investigates the effect of microstructural changes induced by hot rolling on the formation of the texture in high-strength aluminum alloy sheets used in automotive applications. Fully (S2) and partially (S1) re-crystallized samples were fabricated by controlling the final hot-rolling temperature. Optical microscopy (OM) was used to observe the microstructure of the transverse direction (TD)-plane of the hot-rolled strips, and the electron back-scattered diffraction (EBSD) technique was used to evaluate the texture of the normal direction (ND)-plane of the finished sheets that were subjected to hot rolling, followed by cold rolling and a solid-solution heat treatment (SSHT). The grains in S1 showed a low-angle grain boundary, and the concentrations of Cube {001}〈100〉 and Goss {011}〈100〉 orientations were detected at different sheet thicknesses. In contrast, a randomized texture was observed in S2. On the basis of the above results, this paper discusses how the microstructure and texture achieved after hot rolling can influence the final microstructure, texture and surface roughening behavior.     

HVOF thermal spray deposited Y<Subscript>2</Subscript>O<Subscript>3</Subscript>-stabilized ZrO<Subscript>2</Subscript> coatings for thermal barrier applications

High velocity oxy-fuel (HVOF) thermal spray has been successfully used to deposit yttria-stabilized zirconia (YSZ) for thermal barrier coating (TBC) applications. Adherent coatings were obtained within a limited range of spray conditions using hydrogen as fuel gas. Spray parameters such as hydrogen-to-oxygen ratio, spray distance, and substrate cooling were investigated. Spray distance was found to have a pronounced effect on coating quality; adherent coatings were obtained for spray distances between 75 and 125 mm from the gun exit for the hydrogen-to-oxygen ratios explored. Compared to air plasma spray (APS) deposited YSZ coatings, the HVOF deposited coatings were more fully stabilized in the tetragonal phase, and of similar density, surface roughness, and cross-sectional microhardness. Notably, fracture surfaces of the HVOF coatings revealed a more homogeneous structure. Many theoretical models predict that it should not be possible to melt YSZ in an HVOF flame, and therefore it should not be possible to deposit viable YSZ coatings by this process. The experimental results in the present work clearly contradict those expectations. The present results can be explained by taking into account the effect of partial melting and sintering on particle cohesion, as follows. Combustion chamber pressures (P _o) of ∼3.9 bar (58.8 psi) realized during HVOF gun operation allows adiabatic flame temperature values that are above the zirconia melting temperature. Under these conditions, the Ranz-Marshall heat transfer model predicts HVOF sprayed particle surface temperatures T _p that are high enough for partial melting of small (∼10 μm) zirconia particles, T _p=(1.10−0.95)T _m. Further analysis shows that for larger particles (38 μm), adherent coatings are produced when the particle temperature, T _p=0.59−0.60 T _m, suggesting that sintering may have a role in zirconia particle deposition during HVOF spray. These results suggest two different bonding mechanisms for powders having a broad particle size distribution.     

Dissolution of Eutectic β-Mg<Subscript>17</Subscript>Al<Subscript>12</Subscript> Phase in Magnesium AZ91 Cast Alloy at Temperatures Close to Eutectic Temperature

Knowledge on the dissolution kinetics of β_-eut phase in cast Mg AZ91 alloy at temperatures close to the eutectic temperature is very useful for various processes of the alloy. In the present study, dissolution of β_-eut phase has been investigated experimentally and considered theoretically. Results have confirmed that the kinetics of β_-eut dissolution is basically diffusion controlled. Optimum times for dissolution heat treatment practice of different sizes of cast microstructure which are cooling rate dependant during casting could be suggested based on the present calculation. For fusion welding of the alloy, the present results indicate the difficulty of having a heating rate lower than the critical value (“critical heating rate”) for a significant reduction of the phase to avoid constitutional liquation.     

Formation of ferromagnetism in pseudobinary PrNi<Subscript>5 − <Emphasis Type="Italic">x</Emphasis></Subscript>Cu<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> (2.5 ≤ <Emphasis Type="Italic">x</Emphasis> ≤ 5) solid solutions

The structure and magnetic properties of the PrNi_{5 − x} Cu _x alloys have been studied in a composition range of 2.5 ≤ x ≤ 5. Single-phase solid solutions with a hexagonal structure of the CaCu₅ type have been shown to be realized within this composition range. It has been found that upon the introduction of nickel into the Van Vleck paramagnet PrCu₅ the ground state of the alloys with x ≤ 4.3 becomes ferromagnetic. All the compositions under study exhibit high magnetocrystalline anisotropy of the “easy-basal-plane” type. With allowance for the literature data available, a complete magnetic phase diagram of the PrNi_{5 − x} Cu _x system was constructed; it is characterized by two maxima in the compositional dependence of the Curie temperature. The earlier suggested model of the effect of local random crystal fields on the magnetic state of Pr³⁺ ions in alloys with low copper contents was shown to be applicable also for the explanation of magnetic properties of alloys with low nickel contents. The results of this study confirm the hypothesis about the determining role of local irregular crystal fields in the formation of the magnetic properties of the pseudobinary PrNi_{5 − x} Cu _x intermetallic compounds.