Varying Dislocation Density Research Articles

Effects of scan rotation angle and build orientation on mechanical anisotropy in additive manufacturing 316L stainless steel

The scan rotation angle (SRA) and build orientation (BO) in the laser powder bed fusion (LPBF) process create unique microstructural features that induce mechanical anisotropy in 316L-stainless steel (SS). To thoroughly investigate their individual and collaborative effects on anisotropy, 316L-SS samples were fabricated with SRAs between adjacent layers (abbreviated as R) of R0 (0°), R45 (45°), R67 (67°), and R90 (90°), and BOs in the XY plane ranging from XY0 to XY90 at intervals of 15°. Tensile testing results revealed that XY0–R67 samples exhibited the highest yield strength (YS), ultimate tensile strength (UTS), and ductility among all samples, while the lowest YS, UTS, and ductility were observed for XY0–R0 samples. Characterization at different length scales was performed to investigate the underlying reasons contributing to mechanical anisotropy. X-ray diffraction (XRD) results indicated that all samples possessed single-phase austenitic structures with varying dislocation densities. The dislocation density had the highest contribution to the YS of LPBF-built 316L-SS in the sequence of XY0–R67 > XY0–R90 > XY0–R45 > XY0–R0. The higher dislocation density in XY0–R67 samples stemmed from the larger residual stresses associated with the higher lattice strains due to the more complex thermal histories and higher cooling rates compared to other cases. A similar phenomenon was also observed for the XY45 BO, which exhibited higher YS due to higher dislocation densities compared to other orientations, regardless of SRAs. Additionally, SRAs significantly influenced the evolution of crystallographic texture, which also affected the YS of LPBF-built 316L-SS.

Microstructures and mechanical properties of additively manufactured Fe–21Mn-0.6C TWIP steel using laser powder bed fusion

Twinning induced plasticity (TWIP) high manganese steel exhibits high ultimate tensile strength (UTS) and ductility, but its low yield strength restricts its applications. This research presents a Fe–21Mn-0.6C TWIP steel with enhanced mechanical properties additively manufactured using laser powder bed fusion (LPBF). The average grain size of the LPBF-fabricated Fe–21Mn-0.6C was 17.1 μm in the vertical direction, which was a quarter of that of a wrought one. The tensile yield strength was 657 MPa and the UTS was 1089 MPa with an elongation of 47.9% for the vertical direction. Compared to the wrought Fe–21Mn-0.6C, the yield strength increased by 110%. The high strength of LPBF-fabricated Fe–21Mn-0.6C is primarily attributed to solution, grain boundary and dislocation strengthening. Serrations were observed in the stress-strain curves at the initial stage of deformation, showing large stress drops. In the annealed sample, serrations appeared at a later stage of deformation with little stress drops. This difference in serration phenomenon is attributed to the varying dislocation density in the two samples.

Phase-field modeling of stored-energy-driven grain growth with intra-granular variation in dislocation density

We present a phase-field (PF) model to simulate the microstructure evolution occurring in polycrystalline materials with a variation in the intra-granular dislocation density. The model accounts for two mechanisms that lead to the grain boundary migration: the driving force due to capillarity and that due to the stored energy arising from a spatially varying dislocation density. In addition to the order parameters that distinguish regions occupied by different grains, we introduce dislocation density fields that describe spatial variation of the dislocation density. We assume that the dislocation density decays as a function of the distance the grain boundary has migrated. To demonstrate and parameterize the model, we simulate microstructure evolution in two dimensions, for which the initial microstructure is based on real-time experimental data. Additionally, we applied the model to study the effect of a cyclic heat treatment (CHT) on the microstructure evolution. Specifically, we simulated stored-energy-driven grain growth during three thermal cycles, as well as grain growth without stored energy that serves as a baseline for comparison. We showed that the microstructure evolution proceeded much faster when the stored energy was considered. A non-self-similar evolution was observed in this case, while a nearly self-similar evolution was found when the microstructure evolution is driven solely by capillarity. These results suggest a possible mechanism for the initiation of abnormal grain growth during CHT. Finally, we demonstrate an integrated experimental-computational workflow that utilizes the experimental measurements to inform the PF model and its parameterization, which provides a foundation for the development of future simulation tools capable of quantitative prediction of microstructure evolution during non-isothermal heat treatment.

Study of deformation microstructure of nickel samples at very short milling times: effects of addition of \u03b1-Al2O3 particles

Deformation microstructure of pure Ni powder milled for short duration was studied. The microstructure, as obtained on the basis of Rietveld analysis and microstructural modeling, shows inhomogeneity and consists of ‘heavily deformed’ with twin faults probability and ‘slightly deformed’ components with varying dislocation densities. The modified Williamson–Hall analysis was performed for both the subcomponents with the relevant scaling parameter. Attempts were made to elucidate the nature of dislocation present in the milled samples. The correction due to extra broadening from stacking fault was calculated from the refined values of twin fault probability. In-depth microstructural modeling with restrictedly random distribution of dislocation and a lognormal distribution of spherical particles were done for the two subcomponents to explore the inhomogeneous microstructure. The effect of α-Al2O3 addition is found in reduction in the Ni particle size and yielding more uniform size distribution. It is shown that X-ray line profile analysis is capable of modeling such inhomogeneous microstructure.

Microstructural Characterization of Ferritic/Martensitic Steels by Positron Annihilation Spectroscopy

Positron annihilation (PA) studies are carried on two ferritc/martensitic steels (modified 9Cr-1Mo and EUROFER97) and Fe-9Cr binary alloy. Normalized modified 9Cr-1Mo steel is subjected to isochronal heat treatments between 300 K – 1273 K with PA studies at different intervals. Due to changes in the concentration of positron trapping open volume defects associated, different stages of the microstructural changes are identified. These results are corroborated with scanning electron microscopy studies. In EUROFER97 steel, the relative increase in precipitate number density due to the additional cold-work (after normalization) is brought out. In binary Fe-9Cr alloy, which is the model alloy of 9Cr ferritic/martensitic steels, the effect of dislocations on Cr segregation is brought out by studying alloys with two different initial conditions of varying dislocation densities.

Modeling effects of initial dislocation microstructure on the yield strength in FCC single crystal Cu with dislocation dynamics

Three dimensional dislocation dynamics (3D-DD) method was used to study how initial dislocation microstructure, with varying dislocation density, length and distribution, affects the yield strength in face-centered cubic (FCC) single crystal Cu. Initial dislocations are generated for three average lengths for each one of the seven dislocation densities ranging from 0.1–5×1012m/m3.They are randomly distributed on slip planes. The uniaxial tension simulation results for two crystallographic loading orientations, [100] and [111], all show that the yield strength has a well-defined linear relationship with the average length of these dislocations, except for very low dislocation densities where small number of dislocations lead to a stochastic behavior. This indicates that dislocation-density based crystal plasticity formulations, including the square-root relationship between flow stress and dislocation density, only work when dislocation densities/numbers are sufficiently large so that a statistically accurate description of the microstructure can be obtainable.

Structure and properties of dislocations in interfaces of bonded silicon wafers

The realization of defined dislocation networks by hydrophobic wafer bonding allows the characterization of electrical properties of individual dislocations. The present paper describes the fabrication and characterization of SOI MOFSETs with various dislocations densities in the Si channel. The aim was to investigate the electrical properties in samples containing only 6 dislocations. A drain current of ID > 1×10−2 A induced by a single dislocation was determined by data extrapolation from current measurements in combination with previously analyzed samples containing a varying dislocation density.

Dislocation Nucleation and Source Activation during Nanoindentation Yield Points

The onset of plasticity during nanoindentation of a tungsten single crystal was examined as a function of pre-existing dislocation density. Vickers indentations were used to generate a spatially varying dislocation density, and nanoindentation was then carried out at regions of high and low dislocation densities. Even with dislocation densities as high as 1.8 × 1013 m−2, a sharp elastic-plastic transition was observed during some indentations. At lower dislocation densities, 3.5 × 109 m−2, the shear stress at the elastic plastic transition increased and approached the theoretical shear stress of the crystal. A first-order model that predicts the load required for the onset of plasticity during nanoindentation from the activation of a dislocation source within a critical volume of material, rather than homogeneous dislocation nucleation, is developed. The model correlates well with experimentally measured loads at the onset of plasticity for dislocation densities of 1012 m−2 and higher for these nanoindentation conditions.

Determination of absolute defect concentrations for saturated positron trapping – deformed polycrystalline Ni as a case study

Positrons may be used in many cases to determine defect densities of vacancies and dislocations. In case of saturated positron trapping, i.e. all positrons are getting trapped, only a lower-limit estimation can be given. However, a combination of positron back-diffusion measurements using a monoenergetic positron beam in combination with conventional lifetime spectroscopy can be used to overcome the problem of saturated positron trapping. As a case study, this combination was used for the determination of dislocation densities in polycrystalline nickel samples of highly varying dislocation density. Saturated positron trapping into dislocations and small voids was observed. The total positron trapping rate was calculated from the positron diffusion length obtained by back-diffusion experiments. The trapping rates of the two defects were finally obtained using the decomposition of lifetime spectra. The results were found in good agreement with those determined by the analysis of synchrotron Bragg-diffraction profiles, measured on the same set of samples. From the comparison of both techniques, the positron trapping coefficient was found to be μdisl=3.9±0.3cm2/s for a high density of dislocations in Ni.

Photoluminescence intensity of GaN films with widely varying dislocation density

We investigated the impact of the presence of dislocations on room-temperature photoluminescence intensity in GaN films grown by molecular beam epitaxy. To determine both screw and edge dislocation densities, we employed x-ray diffraction in conjunction with a geometrical model, which relate the width of the respective reflections to the polar and azimuthal orientational spread. There is no direct dependence of the emission efficiency on the density of either type of dislocation in the samples under investigation. We conclude that dislocations are not the dominant nonradiative recombination centers for GaN grown by molecular beam epitaxy.

Identification of dislocations in n-type heterostructures by deep-level transient spectroscopy

Plastic relaxation of layers on (100) Si leads to formation of misfit dislocations at the heterointerface and threading dislocations through the heterostructure. We report here a deep-level transient spectroscopy (DLTS) investigation of dislocations in n-type heterostructures grown by selective epitaxy using low-pressure chemical vapour deposition (LPCVD). DLTS was used to detect deep states correlated with dislocations. Measurements were performed on large-area samples (relaxation degree of 63%) as well as on small areas (relaxation degree of 4%) grown selectively on the same wafer. The bias-dependent DLTS peak heights are consistent with a spatially varying dislocation density. This variation, and the combined interpretation of the corresponding DLTS peaks from large and small areas, allowed us to associate one of the deep levels with misfit dislocations at the heterointerface, a second one with threading dislocations and the third one with defects present in the SiGe layer near the interface.

Minority- and majority-carrier trapping in strain-relaxed Ge0.3Si0.7/Si heterostructure diodes grown by rapid thermal chemical-vapor deposition

Strain-relaxed Ge0.3Si0.7/Si, grown by rapid thermal chemical-vapor deposition, has been investigated with deep-level transient spectroscopy (DLTS) and bias-dependent electron-beam-induced current (EBIC). A single electron trap and several hole traps have been detected in these samples. The apparent electron capture cross section is found to be ∼2×10−13 cm2, which is several orders of magnitude larger than the apparent hole capture cross sections (∼10−17 cm2), and is responsible for the detection of the minority-carrier electron trap even under reverse-bias majority-carrier capture conditions. All observed traps which were investigated as a function of filling pulse time exhibit logarithmic capture kinetics, as expected for extended defects, and the bias-dependent DLTS peak height and EBIC relative defect contrast are consistent with the spatially varying dislocation density. Moreover, the trap concentration, as determined by DLTS, is correlated to the dislocation density, as determined by EBIC measurements. Based on a comparison of Arrhenius plots, the observed logarithmic capture kinetics, the correlation of trap density to dislocation density, and the observed bias dependence, the electron trap appears to be related to dislocation core states, while two of the hole traps appear to be related to either dislocation core states or Cottrell atmospheres.

Characterization of single-crystalline GaAs by imaging with ballistic phonons

Ballistic phonon propagation in single-crystalline [001]-oriented gallium arsenide has been studied using low-temperature scanning electron microscopy for imaging. Deviations in the phonon focusing pattern due to dispersion effects were found by comparing the phonon images to theoretical calculations of the long-wavelength limit. The phonon propagation behavior in, samples cut from differently prepared wafers has been investigated. For highly impure crystals we found a pronounced increase of the diffusive signal component at the expense of the ballistic one. Samples with varying dislocation densities also showed a sensitive dependence, of the ballistic phonon propagation on these crystal defects. For focusing calculations considering elastic scattering processes the diffusivity of the phonons could be determined as a function of the mean scattering length. We have found phonon mean free paths of 0.35 mm to 0.80 mm for the various GaAs crystals.

Growth of High Quality Lwir Films by Liquid Phase Epitaxy

ABSTRACTGrowth of long wavelength infra-red mercury cadmium telluride films by liquid phase epitaxy has usually yielded films of inferior electrical properties as evidenced by Hall mobilities lower than theoretical values by factors of 5 or more at 77°K. In addition the Hall behavior over the entire temperature range did not follow classical patterns. A systematic series of investigations was conducted to improve the electrical performance of these films by four methods: i) growth of HgCdZnTe films, where a portion of the cadmium was replaced by zinc, ii) growth at temperatures > 550°C, iii) growth at a slow rate on a CdZnTe substrate following ‘Cleaning’ in the melt of the substrate surface and iv) doping the film by controlled levels of indium. The first method did not lead to any improvements in the Hall behavior. In addition, the films grown displayed varying dislocation densities. The second method led to a small but definite increase in the yield of non-anomalous or classical films (5%). The third method yielded films with classical or non-anomalous Hall behavior about 20% of the time. The last method consistently led to films with classical Hall behavior. This was accomplished with indium doping levels at 1.5× 1014/cm3. Preliminary data indicate that it will be possible to go to still lower doping levels while maintaining classical Hall behavior. The improvement in electrical properties of these epifilms can be attributed to the reduction or elimination of type inhomogeneities known to degrade Hall mobilities in films grown by liquid phase epitaxy.

Ion implantation into gallium arsenide

Secondary-ion-mass-spectrometry studies of the implantation profiles of 20- to 400-keV Si, Se, and Be ions into GaAs are reported. The measured profiles are fit with Pearson-IV distributions whose moments are fit to functions of the ion energy to obtain simple, widely applicable analytical formulas. Also, profiles are measured for varying wafer tilt and rotation angles to the ion beam, and for varying dislocation densities and doses. For implantation through dielectric caps, the profiles in the GaAs can be simulated using shifted, bare-wafer Pearson-IV distributions for Be, or mixtures of shifted Pearson-IV and Gaussians for Si and Se. Also, knock-on distributions of Si and O atoms resulting from implanting through SiO2 caps were measured.

Random sink model for vacancy loss during quenching

Abstract Earlier models for random distributions of dislocation sinks are extended to cover the loss of vacancies during quenching. A model of random spherical-surface sinks is also developed. Results of experimental measurements on platinum wires of two purities and varying dislocation densities are best fitted by the random spherical-surface sink model.

Ferrite morphologies and carbide precipitation in a Cr-Mo-V creep-resisting steel

Ferrite morphologies and carbide precipitate dispersions in a ½Cr-Mo-V creep-resisting steel have been studied after isothermal transformation at a number of subcritical temperatures in the ferrite and bainite ranges, and also after rapid quenching to room temperature to produce martensite. It has been found that VC precipitates by the interphase mechanism during direct transformation to ferrite, and towards the latter stages of the transformation, some fibres of Mo2C also form. Bainite morphologies ranged from granular at the higher transformation temperatures to lath-like at lower temperatures. Tempering of bainite and martensite at 700°C resulted in VC precipitation on dislocations and the interparticle spacing tended to decrease as the initial transformation temperature in the bainite range was increased. However, the best resistance to overageing during tempering was provided by ferrite produced by direct transformation at 700°C. These differences in dispersion stability are explained in terms of the varying dislocation densities and the numbers of interfaces in the structures.

Low-Temperature Diffusion of Chromium in a Fine-Grained Austenitic Stainless Steel with Varying Dislocation Densities

AbstractChromium tracer diffusion in a fine-grain austenitic stainless steel with varying dislocation densities has been analysed using exact solutions to the grain-boundary and dislocation problem. Down to ∼700°C the results could be explained in terms of lattice and grain-boundary diffusion components. The concentration profiles of tracer below 700°C, however, also required a dislocation component. No systematic dependence of the penetration profile on dislocation density was detected. The dislocation component was dominant at short times and led to an apparent enhancement of the lattice diffusion coefficient. Enhanced lattice diffusion by dislocations according to the Hart model was not observed in the temperature range studied even at high dislocation densities. If the accepted value of the dislocation-pipe diameter is assumed then it can be postulated that the lack of enhancement is due to the ratio D p/D L not being sufficiently high (i.e. D p/D L < 103). This implies that the dislocation-pipe diffu...

The effect of dislocations on the capacitance vs. bias voltage characteristic of mos capppacitors

MOS capacitors have been fabricated on silicon material that has been selectively damaged. Varying dislocation densities have been produced and the resulting effects on carrier mobility has been related to the C-V characteristics of the MOS capacitors. Critical dislocation densities that change the C-V characteristics from high frequency to low frequency at 1 KHz are presented and discussed.

Carrier Transport in Thin Silicon Films

Films of p-type and n-type silicon 1–2 μ thick have been epitaxially grown on single-crystal sapphire. The Hall mobilities of films with carrier concentrations varying between 1013/cc to mid-1020/cc have been measured. For carrier concentrations above 1017/cc the mobility decreased with increasing carrier concentration similar to the drop in mobility observed in bulk silicon, indicating the dominance of impurity scattering at higher carrier concentrations. At lower carrier concentrations the scattering from crystalline defects limited the mobility to values approaching 30% of the mobility of bulk silicon. The presence of crystalline defects acting as electron traps was demonstrated by measuring the electron concentration in films of constant donor density but varying dislocation density and noting that the electron concentration dropped as the defect density rose. This has lead to an estimate of the electronically active defect density in thin silicon films grown epitaxially on sapphire of about 1015/cc-1017/cc. Optical absorption data have indicated that there is considerable absorption at energies below 1.1 eV.