Deformation-induced Dislocations Research Articles

Metal Working and Dislocation Structures

Microstructural observations are presented for different metals deformed from low to high strain by both traditional and new metal working processes. It is shown that deformation induced dislocation structures can be interpreted and analyzed within a common framework of grain subdivision on a finer and finer scale down to the nanometer dimension, which can be reached at ultrahigh strains. It is demonstrated that classical materials science and engineering principles apply from the largest to the smallest structural scale but also that new and unexpected structures and properties characterize metals with structures on the scale from about 10 nm to 1 μm.

The microstructure of ball milled nanocrystalline vanadium; variation of the crystal imperfection and the lattice parameter

Abstract Nanocrystalline vanadium powders have been produced by ball milling in a planetary mill. The morphology of the powder particles has been investigated by scanning electron microscopy. Crystallite size (size of coherently diffracting domains) and lattice-strain variation (microstrain) have been determined from the analysis of the X-ray diffraction-line broadening using the established integral breadth Williamson – Hall and Fourier Warren – Averbach methods. Results obtained from transmission electron microscopy analysis have been compared with the X-ray diffraction results. Ball milling causes an increase in the particle size and a decrease in the grain (crystallite) size with increasing milling time, a lattice-strain variation, due to deformation-induced dislocations, that increases with milling time and deformation-induced stacking faults of density increasing with milling time. The lattice parameter of the vanadium powders, as deduced from the diffraction-peak positions, decreases upon milling linearly with the inverse of the grain size, which has been attributed to grain (crystallite)-boundary stress.

Deformation-induced martensite formation and dislocation channeling in neutron-irradiated 316 stainless steel

The deformed microstructure in 316 stainless steel (316SS) after neutron irradiation in the range of 65–100 °C to 0.78 dpa was investigated by transmission electron microscopy (TEM). Deformation-induced martensite transformation and dislocation channeling were observed at irradiation dose higher than 0.1 dpa. Estimation of the resolved shear stress (RSS) associated with each dislocation channel indicated a tendency for the RSS and channel width to be greatest when the angle between tensile axis and slip plane normal is around 45°. Furthermore, channel width increased with increasing RSS, indicating that the most extensive localized channel deformation tends to occur at a high RSS level. Deformation-induced martensite phase was found at various strain levels even at room temperature and tends to be exhibited mainly at intersections of channels. This suggests that a very high stress could lead to the γ → α martensite formation by the spreading of a Shockley partial dislocation over successive 〈1 1 1〉 fcc planes.

Vacancy-type defects in cold-worked iron studied using positron annihilation techniques

Positron annihilation lifetime (PAL) and coincidence Doppler broadening of the annihilation radiation (CDBAR) measurements have been investigated on cold-worked iron with different percentages of a deformation up to 40%. The PAL spectra were analyzed into two lifetime components. The shorter lifetimes were explicitly smaller than the monovacancy lifetimes, which were possibly caused by deformation-induced dislocations and vacancy-impurity complexes. The longer lifetimes showed that, the deformation introduces also larger vacancy clusters (consisting of five to ten vacancies) as a result of a cold-working. The behavior of the S-parameter (extracted from CDBAR) with the rolling deformation was analogous to the mean positron lifetime. It increased at up to 5% rolling deformation and leveled off above this percentage of the rolling deformation. The CDBAR ratio curve with respect to pure iron showed that, above the 5% rolling deformation the nature of the defects trapping the positron changed.

Spatial characterisation of the orientation distributions in a stable plane strain-compressed Cu crystal: A statistical analysis

Single copper crystals of the stable Goss orientation {0 1 1}〈1 0 0〉 were deformed in plane strain compression and the deformation-induced dislocation structures were investigated by high-resolution electron backscattered diffraction. Although the orientation maps exhibited an anisotropic dislocation boundary structure it was shown that the mean disorientation angle between point pairs saturated and became isotropic if their spacing was large enough (typically >30 μm). This saturation behaviour was interpreted as being a consequence of the anti-correlations between nearby dislocation boundaries and is discussed in terms of recent stochastic models of boundary formation. It was found that the disorientation boundaries, which were considered as being formed at relatively low strains, underwent rigid body-like rotations during deformation.

Close up on crystal plasticity

A novel X-ray diffraction technique opens the way to investigate deformation-induced dislocation microstructures with submicrometre resolution.

Effect of Zr Addition on Dynamic Recrystallization during Hot Extrusion in Al Alloys

The presence of Al 3 Zr precipitates promotes continuous dynamic recrystallization in Al-Zn-Mg alloys during hot extrusion, resulting in a fine-grained structure. The mechanism for this phenomenon was investigated by observing the change in microstructure under hot extrusion using high resolution EBSP. In the rear of the extrusion die mouth, grain boundary mobility is low since the flow stress is comparatively low and grain boundary migration is inhibited by Al 3 Zr particles. Thus, in order to reduce the deformation-induced high dislocation density, continuous dynamic recrystallization occurs, which does not involve long-range grain boundary migration, resulting in a finer-grained structure. The flow stress, and hence, the mobility of the grain boundary increase near the die mouth. This promotes long-range grain boundary migration, which reduces the dislocation density. Continuous dynamic recrystallization is not observed under these conditions. Since Al 3 Zr precipitates inhibit long-range grain boundary migration, increase of the Al 3 Zr precipitate content expands the region where continuous dynamic recrystallization occurs towards the die mouth, resulting in a finer-grained structure.

Dislocations in (Hg, Re)Ba2Ca2Cu3O8+δ high-temperature superconducting ceramics plastically deformed at room temperature

Dislocations in (Hg, Re)Ba2Ca2Cu3O8+ δ superconducting ceramics plastically deformed at room temperature have been characterized by transmission electron microscopy (TEM). Samples were submitted to strain levels up to 8, 18 and 63% by uniaxial compression under a confining pressure. Dislocation densities within grains were found to be in the range 109–1012 cm−2. The average grain size decreases from about 20 μm in the as-grown material to about 0.5 μm for the sample deformed up to 63%. Evidence for (001)⟨ 100⟩, (001)⟨ 110⟩ and glide systems was obtained by TEM. Dislocations are characterized by long straight screw segments and shorter edge parts. These features are interpreted as resulting from high Peierls valleys along the ⟨ 100⟩ and ⟨ 110⟩ screw directions. Cross-slip as well as climb dissociation of ⟨ 110⟩ dislocations occur. The shape of deformation-induced dislocations, their evolution during deformation and the interaction between dislocations in the (001) ⟨ 110⟩ and {}⟨ 110⟩ glide systems are discussed.

Analysis of high-speed-deformation-induced defect structures using heterogeneity parameter of dislocation distribution

An appropriate parameter, named the heterogeneity parameter, is introduced for quantitatively representing the nature of distribution of deformation-induced dislocations. The parameter assumes values ranging from 0 to 1, the extremes corresponding to homogeneous and heterogeneous distribution, respectively. The parameter can represent distribution of intermediate nature, typified by stochastically random distribution which has a heterogeneity parameter of about 0.35. Heterogeneity parameter is used to analyze variation in dislocation structure in many kinds of metals for a wide range of deformation speed from 10 −2 to 10 7 s −1 in strain rate. The parameter has large values at slow-speed-deformation, and decreases with increasing strain rate, reaching the level of random distribution at a deformation speed of around 10 4 s −1. Above this strain rate, formation of vacancy clusters increases remarkably. On the basis of these results of analysis, occurrence of dislocation-free plastic deformation during high-speed-deformation is proposed.

The kinetics of static globularization of Ti-6Al-4V

The kinetics of the evolution of the lamellar-colony microstructure to an equiaxed morphology during heat treatment of a hot-worked, two-phase titanium alloy were established. For this purpose, the alpha/beta alloy Ti-6Al-4V was isothermally upset forged at 900 °C or 955 °C and subsequently annealed for times ranging from 0.5 to 100 hours. The degree of the breakup of alpha-phase lamellae into lower-aspect-ratio grains during static annealing was measured and related to the imposed strain estimated using finite-element analysis (FEA). The kinetics of the static globularization of the alpha phase were found to depend on the amount of strain and the annealing temperature but were not affected by the specific deformation temperature in the 900 °C to 955 °C range. These results demonstrated that deformation-induced dislocation substructure has a small effect on the static-globularization process. In addition, the relative globularization kinetics at 900 °C and 955 °C were rationalized in terms of classical coarsening theory.

Deformation-induced dislocations in 15R-SiC grown by sublimation

Single-crystal 15R-SiC boules have been successfully grown by sublimation. The Vickers hardness of a Si-terminated (0001) face has been measured in the temperature range 25-1300C. As expected, the hardness decreases with increasing temperature from about 30GPa at room temperature to about 10GPa at 1300C. The fracture toughness is estimated to be about 1.0MPam 1/2 at room temperature. Transmission electron microscopy investigation of the dislocations introduced by indentation at 900 and 1300C shows that they are activated predominantly on the basal plane. Most of them consist of a single leading partial without the corresponding trailing partial.

Stacking Fault Energy of 6H-SiC and 4H-SiC Single Crystals

Abstract Single crystal 4H and 6H polytypes of SiC have been deformed in compression at 1300°C. All the deformation-induced dislocations were found to be dissociated into two partials bounding a ribbon of intrinsic stacking fault. Using two-beam bright-field and weak-beam dark-field techniques of transmission electron microscopy, the stacking fault energy of these two SiC polytypes has been determined from the separation width of the two partials of dissociated dislocations. The stacking fault energy of 4H-SiC is determined to be 14.7±2.5mJm−2, and that of 6H-SiC to be 2.9±0.6mJm−2. As a verification, the stacking fault energy of 4H-SiC has been determined also from the minimum radius of curvature of extended nodes. This latter method gave a value of 12.2±1.1mJm−2 which is within the range determined from measurement of partial dislocation separations. The experimental values of stacking fault energy for 4H- and 6H-SiC have been compared with estimates obtained from a generalized axial next-nearest-neighb...

Stacking fault energy of 6H-SiC and 4H-SiC single crystals

Abstract Single crystal 4H and 6H polytypes of SiC have been deformed in compression at 1300°C. All the deformation-induced dislocations were found to be dissociated into two partials bounding a ribbon of intrinsic stacking fault. Using two-beam bright-field and weak-beam dark-field techniques of transmission electron microscopy, the stacking fault energy of these two SiC polytypes has been determined from the separation width of the two partials of dissociated dislocations. The stacking fault energy of 4H-SiC is determined to be 14.7±2.5mJm−2, and that of 6H-SiC to be 2.9±0.6mJm−2. As a verification, the stacking fault energy of 4H-SiC has been determined also from the minimum radius of curvature of extended nodes. This latter method gave a value of 12.2±1.1mJm−2 which is within the range determined from measurement of partial dislocation separations. The experimental values of stacking fault energy for 4H- and 6H-SiC have been compared with estimates obtained from a generalized axial next-nearest-neighb...

Fractal analysis of deformation-induced dislocation patterns

The paper reports extensive analyses of the fractal geometry of cellular dislocation structures observed in Cu deformed in multiple-slip orientation. Several methods presented for the determination of fractal dimensions are shown to give consistent results. Criteria are formulated which allow the distinguishing of fractal from non-fractal patterns, and implications of fractal dislocation patterning for quantitative metallography are discussed in detail. For an interpretation of the findings a theoretical model is outlined according to which dislocation cell formation is associated to a noise-induced structural transition far from equilibrium. This allows relating the observed fractal dimensions to the stochastic properties of deformation by collective dislocation glide.

High-temperature creep of polycrystalline BaTiO3

Compressive creep of dense BaTiO3 having linear-intercept grain sizes of 19.3–52.4 μm was investigated at 1200–1300 °C by varying the oxygen partial pressure from 102 to 105 Pa in both constant-stress and constant-crosshead-velocity modes. Microstructures of the deformed materials were examined by scanning and transmission electron microscopy. The stress exponent was ≈1, the grain-size dependence was ≈1/d2, and the activation energy was ≈720 kJ/mole. These parameters, combined with the microstructural observations (particularly grain displacement and absence of deformation-induced dislocations), indicated that the dominant deformation mechanism was grain-boundary sliding accommodated by lattice cation diffusion. Because of the absence of an oxygen partial pressure dependence, diffusion was probably controlled extrinsically.

Deformation-Induced Dislocations in 4H-SiC and GaN

ABSTRACTBulk single crystals of 4H-SiC have been deformed in compression in the temperature range 550–1300°C, whereas a GaN thin film grown on a (0001) sapphire substrate was deformed by Vickers indentation in the temperature range 25–800°C. The TEM observations of the deformed crystals indicate that deformation-induced dislocations in 4H-SiC all lie on the (0001) basal plane but depending on the deformation temperature, are one of two types. The dislocations induced by deformation at temperatures above ∼1 100°C are complete, with a Burgers vector, b, of but are all dissociated into two partials bounding a ribbon of stacking fault. On the other 3 hand, the dislocations induced by deformation in the temperature range 550<T<∼ 1100°C were predominantly single leading partials each dragging a stacking fault behind them. From the width of dissociated dislocations in the high-temperature deformed crystals, the stacking fault energy of 4H-SiC has been estimated to be 14.7±2.5 mJ/m2. Vickers indentations of the [0001]-oriented GaN film produced a dense array of dislocations along the three 〈1120〉 directions at all temperatures. The dislocations were slightly curved with their curvature increasing as the deformation temperature increased. Most of these dislocations were found to have a screw nature with their b parallel to 〈1120〉. Also, within the resolution of the weak-beam method, they were not found to be dissociated. Tilting experiment show that these dislocations lie on the {1100} prism plane rather than the easier (0001) glide plane.

Strain-induced grain evolution in polycrystalline copper during warm deformation

The evolution mechanisms of dislocation microstructures and new grains at high strains of above 4 were studied by means of multiple compression of a polycrystalline copper (99.99 pct). Deformation was carried out by multipass compression with changing of the loading direction in 90 deg in each pass at temperatures of 473 K to 573 K (0.35 to 0.42 T m ) under a strain rate of 10−3 s−1. The flow stresses increase to a peak followed by a work softening accompanied mainly by dynamic recrystallization (DRX) at 523 K to 573 K. In contrast, the steady-state-like flow appears at 473K accompanied with the development of fine grains at strains as high as 4.2. The relationship of flow stress to the new grain size evolved can be expressed by a power law function with a grain size exponent of about −0.35, which is different from −0.75 for high-temperature DRX at above 0.5 T m . At 473 K, misorientations of deformation-induced dislocation subboundaries increase with increasing strain, finally leading to the evolution of new grains. It is concluded that the dynamic grain formation at 473 K cannot result from DRX, but from the evolution of deformation-induced dislocation subboundaries with high misorientations and, concurrently, the operation of dynamic recovery.

Defect analysis in high temperature deformed Al single crystals

Isochronal annealing studies of residual electrical resistivity were carried out on Al (111) single crystals which had been deformed in tension at T d = 0.5 T m and T d = 0.7 T m ( T m refers to the melting temperature in K). For all different plastic strains applied, two characteristic annealing stages in the resistivity isochrones were observed. While one annealing stage occurs at T d or above and corresponds to the annihilation of deformation induced dislocations, the other is situated at temperatures lower than T a. From comparative quenching experiments, the annealing of thermal defects can be ruled out. It is suggested that a second species of deformation-induced dislocations anneals here which gets mobile during the unloading of crystals subsequent to their deformation.

Mechanical properties and deformation-induced microstructure of L12-ordered Ni3 (Al,Cr) containing a fine dispersion of M23C6 carbide

Abstract The morphology of deformation-induced dislocations in polycrystalline Ni3 (Al,Cr) containing M23C6 precipitates has been investigated using transmission electron microscopy. Fine polyhedral precipitates of M23C6 were formed in the Ni3 (Al,Cr) matrix by aging at temperatures around 973 K after solution annealing at 1423 K. The aging behavior was investigated by microhardness measurements and the temperature dependence of the yield stress of Ni3 (Al,Cr) containing M23C6 precipitates was investigated by compression tests over the temperature range 298–1173 K. Typical Orowan loops were observed surrounding the M23C6 particles after deformation at temperatures below 973 K. At higher deformation temperatures, the Orowan loops disappeared and the morphology of dislocations at the particle-matrix interfaces suggested the existence of attractive interaction between dislocations and particles. The change in the modes of interaction between the dislocation and particles with increasing deformation temperature can be considered as a result of strain relaxation at the interface between the matrix and particles.

Positron annihilation lifetime study of irradiated and deformed Fe and Ni

In order to investigate the fundamental behaviors of radiation-induced defects, especially vacancy type defects and also deformation-induced dislocations and vacancies, positron annihilation lifetime measurements have been performed for Fe, FeCu and FeSi irradiated with electrons at low temperature (77 K) and also for Ni deformed at room temperature. From the isochronal annealing experiments it was found that vacancies become mobile above 200 K and form microvoids in Fe, but in FeCu and FeSi alloys the interaction between vacancies and solute atoms significantly suppresses the microvoid formation process. In FeCu alloy, instead of microvoid formation it was considered that vacancy-Cu complexes are formed by judging the value of positron lifetime. In the deformed Ni positron lifetime decreased gradually as the isochronal annealing temperature increased. From this result and positron lifetime calculation it was suggested that in deformed Ni positrons are trapped and annihilated at complexes of a dislocation and deformation-induced vacancies.