Measured Dislocation Densities Research Articles

Creep deformation study of squeeze cast AZ91 magnesium alloy

The AZ91 magnesium alloy has excellent mechanical properties; however, its application is limited by its poor creep resistance. In this study, AZ91 magnesium alloy prepared by squeeze casting technique has been tested for its creep performance at varying stresses and temperatures. The governing creep mechanism was examined in the wake of deformation. The microstructure for as-cast AZ91 alloy confirmed the massive dendritic β-Mg17Al12 phase. The creep study indicated that the governing creep mechanisms were dislocation creep and dislocation climb governed by lattice diffusion in the test regime of 50 MPa stress at 150oC to 175oC. The measured dislocation density values showed good agreement with the expected dislocation density in the power law creep mechanism.

Investigating Nanoscale Contact Using AFM-Based Indentation and Molecular Dynamics Simulations

In this work we study nanocontact plasticity in Au thin films using an atomic force microscope based indentation method with the goal of relating the changes in surface morphology to the dislocations created by deformation. This provides a rigorous test of our understanding of deformation and dislocation mechanisms in small volumes. A series of indentation experiments with increasing maximum load was performed. Distinct elastic and plastic regimes were identified in the force-displacement curves, and the corresponding residual imprints were measured. Transmission electron microscope based measured dislocation densities appear to be smaller than the densities expected from the measured residual indents. With the help of molecular dynamics simulations we show that dislocation nucleation and glide alone fail to explain the low dislocation density. Increasing the temperature of the simulations accelerates the rate of thermally activated processes and promotes motion and annihilation of dislocations under the indent while transferring material to the upper surface; dislocation density decreases in the plastic zone and material piles up around the indent. Finally, we discuss why a significant number of cross-slip events is expected beneath the indent under experimental conditions and the implications of this for work hardening during wear.

Strengthening contributions of dislocations and twins in warm-rolled TWIP steels

A twinning-induced plasticity (TWIP) steel was processed by warm rolling with different reduction ratios to elevate yield strength while maintaining considerable ductility. To investigate the effect of prior warm rolling on the deformation mechanisms during subsequent tensile straining, the contributions of individual strengthening mechanisms to the tensile flow stress of the as-received (AR) and warm-rolled (WR) steels were decoupled, based on the measured dislocation density evolutions determined by synchrotron X-ray diffraction (XRD) experiments. Whereas dislocation multiplication invariably governs the maximum flow stress of the AR and WR steels, deformation twinning becomes increasingly important for the strain hardening of steels with larger warm rolling reductions. In addition, twinning kinetics is enhanced by the pre-existing high dislocation densities caused by warm rolling, leading to an enhancement of twinning-induced hardening. The present work provides further insight into the influence of warm rolling on the deformation mechanisms of TWIP steels.

Effects of constrained groove pressing on mechanical properties of a TWIP steel

The evolution of mechanical properties and microstructure of a twinning induced plasticity (TWIP) steel sheet during constrained groove pressing (CGP) was investigated. It was found that by imposing CGP, the volume fraction of twins, hardness and yield strength increase significantly, while ductility decreases. The results of the pin-on-disk wear test showed that the CGP process improves the wear resistance of the TWIP steel, where oxidation is the main wear mechanism. The effect of annealing treatment at various temperatures on the hardness evolution of the CGP-treated samples was also investigated. Moreover, a modified work-hardening model was applied to predict the flow behaviour of TWIP steel during the severe plastic deformation. The model successfully predicts the flow curve and measured dislocation density and twin evolution.

Design, Theoretical, and Experimental Investigation of Tensile-Strained Germanium Quantum-Well Laser Structure

Strain and band gap engineered epitaxial germanium (ε-Ge) quantum-well (QW) laser structures were investigated on GaAs substrates theoretically and experimentally for the first time. In this design, we exploit the ability of an InGaAs layer to simultaneously provide tensile strain in Ge (0.7–1.96%) and sufficient optical and carrier confinement. The direct band-to-band gain, threshold current density (Jth), and loss mechanisms that dominate in the ε-Ge QW laser structure were calculated using first-principles-based 30-band k·p electronic structure theory, at injected carrier concentrations from 3 × 1018 to 9 × 1019 cm–3. The higher strain in the ε-Ge QW increases the gain at higher wavelengths; however, a decreasing thickness is required by higher strain due to critical layer thickness for avoiding strain relaxation. In addition, we predict that a Jth of 300 A/cm2 can be reduced to <10 A/cm2 by increasing strain from 0.2% to 1.96% in ε-Ge lasing media. The measured room-temperature photoluminescence spectroscopy demonstrated direct band gap optical emission, from the conduction band at the Γ-valley to heavy-hole (0.6609 eV) from 1.6% tensile-strained Ge/In0.24Ga0.76As heterostructure grown by molecular beam epitaxy, is in agreement with the value calculated using 30-band k·p theory. The detailed plan-view transmission electron microscopic (TEM) analysis of 0.7% and 1.2% tensile-strained ε-Ge/InGaAs structures exhibited well-controlled dislocations within each ε-Ge layer. The measured dislocation density is below 4 × 106 cm–2 for the 1.2% ε-Ge layer, which is an upper bound, suggesting the superior ε-Ge material quality. Structural analysis of the experimentally realistic 1.95% biaxially strained In0.28Ga0.72As/13 nm ε-Ge/In0.28Ga0.72As QW structure demonstrated a strained Ge/In0.28Ga0.72As heterointerface with minimal relaxation using X-ray and cross-sectional TEM analysis. Therefore, our monolithic integration of a strained Ge QW laser structure on GaAs and ultimately the transfer of the process to the Si substrate via an InGa(Al)As/III–V buffer architecture would provide a significant step toward photonic technology based on strained Ge on a Si platform.

Mapping the microstructure evolution of nickel deformed by orthogonal cutting

Severe plastic deformation prompted by orthogonal cutting is used to create deformed microstructures in commercially pure nickel machined under multiple cutting conditions. The resulting thermomechanical conditions, strain, strain rate, and temperature rise are calculated resulting in the final microstructure of the chip. The microstructure response is quantified for the generated defect during deformation, i.e. dislocation densities besides the microhardness. The dislocation density is measured using the X-ray diffraction peak broadening implementing the Williamson–Hall, and Williamson–Smallman methods. The microstructural consequences are examined through creating the rate-strain-microstructure (RSM) mappings using the Zener-Hollomon (Z) parameter. A 2D visualization of the microstructure response is acquired, and the effects of the strain and Ln(Z) on the measured dislocation densities and microhardness are discussed. The microstructural consequences are validated with the microstructure results reported in the literature over a wide range of Ln(Z) involving temperatures in the range from room temperature to 1420K.

A quantitative evaluation of microalloy precipitation strengthening in martensite and bainite

The influence of V content on the strengthening mechanisms in martensite and bainite tempered between 500 and 650 °C was investigated with scanning electron microscopy, electron backscatter diffraction, transmission electron microscopy, and X-ray diffraction. A quantitative methodology to measure the volume fraction of microalloy (MX) precipitates was employed to directly calculate the contribution of precipitation strengthening after tempering at 500 and 650 °C. Increased V contents result in larger increases in hardness at higher tempering temperatures compared to lower tempering temperatures for both martensite and bainite. The increase in hardness from increased V content is not significantly different between martensite and bainite. Greater volume fractions of MX are observed with higher V contents. The increased MX volume fractions inhibit recovery during tempering as indicated by higher measured dislocation densities in the high V conditions. The majority of the strengthening imparted by increased V content is calculated to be a result of higher dislocation densities rather than greater MX precipitation strengthening after tempering at 500 °C. However, greater V contents are calculated to primarily raise the hardness by increased MX precipitation strengthening after tempering at 650 °C. In general, higher dislocation densities as well as increased MX precipitation strengthening are both important strengthening mechanisms influenced by V content during tempering in the secondary hardening regime.

Anomalous Annealing Response of Directed Energy Deposited Type 304L Austenitic Stainless Steel

Directed energy deposited (DED) and forged austenitic stainless steels possess dissimilar microstructures but can exhibit similar mechanical properties. In this study, annealing was used to evolve the microstructure of both conventional wrought and DED type 304L austenitic stainless steels, and significant differences were observed. In particular, the density of geometrically necessary dislocations and hardness were used to probe the evolution of the microstructure and properties. Forged type 304L exhibited the expected decrease in measured dislocation density and hardness as a function of annealing temperature. The more complex microstructure–property relationship observed in the DED type 304L material is attributed to compositional heterogeneities in the solidification microstructure.

Residual stress and dislocation density development in single track deep rolled AISI 4140H steel

Plastic deformations induced by the rolling contact in the deep rolling process are the origin of the formation of residual stress fields and increased dislocation density in the material. The near surface material modification achieved through the process, like a compressive residual stress field, improved hardness and reduced surface roughness, give rise to improved component performance during operation. For a general understanding of the generated surface-near properties, an analysis of single tracks contributing to surface integrity is required. In the present study, a detailed investigation of the achieved residual stress state distribution as well as the local dislocation density of single processed tracks was performed through high spatial resolution X-ray diffraction analyses. Using two spherical deep rolling tools of 6 mm and 13 mm diameter, single tracks were processed with different parameters on samples of steel AISI 4140H. It could be observed that residual stress fields at the surface reach much larger lateral expansion than the plastically deformed track which can be determined by optical measurements, while a correlation with the dislocation density distribution was found. For the maximum dislocation density in the tracks, a correlation with the theoretical maximum equivalent stress was determined. Furthermore, a method to connect the measured dislocation density to the localized plastic strains during processing was applied to exemplary distributions.

Indentation size effect and dislocation structure evolution in (001) oriented SrTiO3 Berkovich indentations: HR-EBSD and etch-pit analysis

In the present work, the dislocation structure evolution and indentation size effect in (001) oriented STO have been studied by using sequential polishing, chemical etch-pit technique and high-resolution electron backscattered diffraction (HR-EBSD) analysis via Berkovich nanoindentation experiments. Nanoindentation load-displacement curves show multiple pop-in events, which relate to nucleation and extension of dislocation pile-ups around the residual impression. Sequential polishing and etching revealed the three-dimensional dislocation etch-pit structure at various sub-surface depths. The dislocation densities are determined for a 5 mN and 10 mN indentation below the surface via HR-EBSD and etch-pit analysis. With HR-EBSD, the lattice rotation and thereby GND densities are determined, while the etch-pit technique revealed the total dislocation density. Based on the independently measured dislocation densities, we clearly show a depth dependent dislocation density, where both total and GND densities increase with decreasing indentation depths. The higher dislocation densities below smaller indents explain the observed size dependency of the hardness in STO, which also sheds light on the extraordinary combination of high indentation hardness and low yield strength in STO.

Effect of Mo addition on the microstructure and hardness of ultrafine-grained Ni alloys processed by a combination of cryorolling and high-pressure torsion

An investigation was conducted to examine the effect of molybdenum (Mo) content on the grain size, lattice defect structure and hardness of nickel (Ni) processed by severe plastic deformation (SPD). The SPD processing was applied to Ni samples with low (~0.3at%) and high (~5at%) Mo concentrations by a consecutive application of cryorolling and high-pressure torsion (HPT). The grain size and the dislocation density were determined by scanning electron microscopy and X-ray line profile analysis, respectively. In addition, the hardness values in the centers, half-radius and peripheries of the HPT-processed disks were determined after ½, 5 and 20 turns. The results show the higher Mo content yields a dislocation density about two times larger and a grain size about 30% smaller. The smallest value of the grain size was ~125nm and the highest measured dislocation density was ~60×1014m−2 for Ni-5% Mo. For the higher Mo concentration, the dislocation arrangement parameter was larger indicating a less clustered dislocation structure due to the hindering effect of Mo on the rearrangement of dislocations into low energy configurations. The results show there is a good correlation between the dislocation density and the yield strength using the Taylor equation. The α parameter in this equation is slightly lower for the higher Mo concentration in accordance with the less clustered dislocation structure.

The effect of cast microstructure and crystallography on rafting, dislocation plasticity and creep anisotropy of single crystal Ni-base superalloys

In the present work we investigate three mechanical and microstructural aspects of high temperature and low stress creep of the single crystal superalloy LEK 94. First, we compare the tensile creep behavior of specimens loaded in precise [001] and [110] directions and show that tensile creep specimens with precise [110] directions show significantly lower minimum creep rates. However, small deviations from precise [110] orientations result in a significant increase of creep rate. Second, we use a novel SEM technique to measure dislocation densities. We show that after short periods of creep, dislocation densities in dendritic regions are always higher than in interdendritic regions. This finding is probably associated with wider γ-channels, higher concentrations of W and Re and higher misfit stresses in the γ-channels of dendrites. Finally, we show that internal stresses associated with solidification can drive complex rafting processes during high temperature exposure, which differ between dendrite cores and interdendritic regions.

A Practical Meso-Scale Polycrystal Model to Predict Dislocation Densities and the Hall&amp;ndash;Petch Effect

A practical SuperDislocation Model (SDM) has been developed and implemented to predict dislocation density distributions in a plastically deforming polycrystal and thereby the Hall­Petch effect. The model is composed of two stepwise simulation scales; the first scale is a finite element model of a polycrystal using a novel single-crystal constitutive equation and the second scale redistributes the mobile part of the dislocation density within grains consistent with the plastic strain distribution, and enforces slip transmission criteria at grain boundaries that depend on local grain and boundary properties. In this work, deformation of Fe­3% Si tensile specimen is simulated using SDM to compare dislocation densities obtained from the highresolution electron backscatter diffraction (HR-EBSD). The model accurately predicts the measured dislocation density at 10% deformation. In addition, size-dependent simulations show that the model qualitatively predicts Hall­Petch slope as well as the grain boundary strength of Fe­ 3% Si. [doi:10.2320/matertrans.MA201305]

Laser compression of nanocrystalline tantalum

Nanocrystalline tantalum (grain size ∼70nm) prepared by severe plastic deformation (high-pressure torsion) from monocrystalline [100] stock was subjected to shock compression generated by high-energy laser (∼350–850J), creating pressure pulses with initial duration of ∼3ns and amplitudes of up to ∼145GPa. The laser beam, with a spot radius of ∼1mm, created a crater of significant depth (∼135μm). Transmission electron microscopy revealed few dislocations within the grains and an absence of twins at the highest shock pressure, in contrast with monocrystalline tantalum. Hardness measurements were conducted and show a rise as the energy deposition surface is approached, evidence of shock-induced defects. The grain size was found to increase at a distance of 100μm from the energy deposition surface as a result of thermally induced grain growth. The experimentally measured dislocation densities are compared with predictions using analyses based on physically based constitutive models, and the similarities and differences are discussed in terms of the mechanisms of defect generation. A constitutive model for the onset of twinning, based on a critical shear stress level, is applied to the shock compression configuration. The predicted threshold pressure at which the deviatoric component of stress for slip exceeds the one for twinning is calculated and it is shown that it is increased from ∼24GPa for the monocrystalline to ∼150GPa for the nanocrystalline tantalum (above the range of the present experiments). Calculations using the Hu–Rath analysis show that grain growth induced by the post shock-induced temperature rise is consistent with the experimental results: grains grow from 70 to 800nm within the post-shock cooling regime when subjected to a laser pulse with energy of 684J.

Hardening of pure metals by high-pressure torsion: A physically based model employing volume-averaged defect evolutions

A physically based model to predict the increment of hardness and grain refinement of pure metals due to severe plastic deformation by high-pressure torsion (HPT) is proposed. The model incorporates volume-averaged thermally activated dislocation annihilation and grain boundary formation. Strengthening is caused by dislocations in the grain and by grain boundaries. The model is tested against a database containing all available reliable data on HPT-processed pure metals. It is shown that the model accurately predicts hardening and grain size of the pure metals, irrespective of crystal structure (face-centred cubic, body-centred cubic and hexagonal close packed). Measured dislocation densities also show good correlation with predictions. The influence of stacking fault energy on hardening is very weak (of the order of −0.03GPa per 100Jmol−1).

Laser compression of monocrystalline tantalum

Monocrystalline tantalum with orientations [100] and [111] was subjected to laser-driven compression at energies of 350–684J, generating shock amplitudes varying from 10 to 110GPa. A stagnating reservoir driven by a laser beam with a spot radius of ∼800μm created a crater of significant depth (∼80 to ∼200μm) on the drive side of the Ta sample. The defects generated by the laser pulse were characterized by transmission and scanning electron microscopy, and are composed of dislocations at low pressures, and mechanical twins and a displacive phase transformation at higher pressures. The defect substructure is a function of distance from the energy deposition surface and correlates directly with the pressure. Directly under the bottom of the crater is an isentropic layer, approximately 40μm thick, which shows few deformation markings. Lattice rotation was observed immediately beneath this layer. Further below this regime, a high density of twins and dislocations was observed. As the shock amplitude decayed to below ∼40GPa, the incidence of twinning decreased dramatically, suggesting a critical threshold pressure. The twinning planes were primarily {112}, although some {123} twins were also observed. Body-centered cubic to hexagonal close-packed pressure induced-transformation was observed at high pressures (∼68GPa).The experimentally measured dislocation densities and threshold stress for twinning are compared with predictions using analyses based on the constitutive response, and the similarities and differences are discussed in terms of the mechanisms of defect generation.

Synchrotron radiation X‐ray topography and X‐ray diffraction of homoepitaxial GaN grown on ammonothermal GaN

AbstractIn this study, homoepitaxial GaN grown on ammonothermal GaN substrates is non‐destructively investigated by synchrotron radiation X‐ray topography (SR‐XRT) and X‐ray diffraction (XRD). The homoepitaxial GaN layer was found to be of excellent crystal quality with individual dislocations clearly visible in the SR‐XRT images. SR‐XRT images and XRD rocking curves suggest the dislocations are mainly of mixed type. To the best of our knowledge the measured dislocation density is the lowest reported in a homoepitaxial GaN film (© 2012 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

In-situ tensile testing of single-crystal molybdenum-alloy fibers with various dislocation densities in a scanning electron microscope

Abstract

New observations on high-temperature creep at very low stresses

Creep tests were conducted in compression to evaluate the flow behavior of aluminum at very high temperatures and low stresses. The experiments used two types of specimens: single crystals of 99.999% purity and oligocrystalline samples of 99.97% purity. Results obtained for the single crystals lie consistently between the conventional region of Harper–Dorn creep and the anticipated behavior based on an extrapolation of conventional 5-power creep. Using etch pit studies with the single crystals, it is shown there is no evidence for subgrain formation and the measured dislocation densities are consistent with data extrapolated from the conventional 5-power region. The creep results on single crystals suggest the stress exponent is close to ∼3 at low stresses. It is demonstrated these results are consistent with earlier data including the results of Harper and Dorn when their data are plotted in terms of the true applied stress without incorporating a threshold stress.

Anisotropic elastocreep in glacial ice: A mechanism for intergranular melt and recrystallization

[1] Bonded ice crystals under pressure are in a heterogeneous stress state because of the mechanical anisotropy of constituent grains. This condition plays a role in intergranular melt and recrystallization, which in turn influence properties such as permeability and biologic habitability. To examine this, we develop an anisotropic elastocreep model simulating subgrain-scale stresses in polycrystalline ice, choosing in particular the thermal and deformational regime of the Lake Vostok accretion ice. A critical assumption with regard to localized melting is the macroscopic deviatoric stress. Our calculations indicate that for a macroscopic deviatoric stress of 50 kPa, ice at 0.04°C below the bulk melting temperature would contain regions, at scales ∼1% the grain size, where the liquid phase is thermodynamically favored. This translates to scales of ∼5 mm in the lower few meters of the Vostok accretion under a deviatoric stress that is plausible near the lake perimeter. Less internal melting is indicated in analyses of ice relatively far from the lake margins due to the assumed presence of lower macroscopic deviatoric stress. Simulated strain energy jumps of 1–2% across grain boundaries of accretion ice near the lake boundaries are large relative to free surface energy, indicating the importance of the former in boundary migration recrystallization. Collectively, these findings are consistent with measured dislocation densities (>107–1010 m−2) and irregular grain morphologies found in core samples. The calculated melting point depressions associated with anisotropy fall short of experimental values by a factor of ∼2, while the calculated strain rates deviate by ∼20%.