Twin Boundary Length Research Articles

Microstructure Heredity of Inconel 718 Nickel-Based Superalloy during Preheating and Following Deformation

Preheating and compression tests of Inconel 718 superalloy double cone specimens were carried out to investigate the microstructure heredity during hot working. Optical microscopy, scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM) were used to characterize the microstructure evolution. The results show that intense microstructure heredity can be found at the temperature 960~990 °C. During the preheating process, δ phase precipitation or grain growth could increase the fraction of high angle grain boundary (HAGBs) and Σ3n boundaries. Otherwise, the generation or spread of annealing twin could increase the fraction of LAGBs, Volume fraction of recrystallized grains was evaluated at the whole hot working process. At the temperature of 960~990 °C, the volume fraction of recrystallized grains increases with effective strain increasing. At the super solution temperature of δ phase, the volume fraction of recrystallized grains decreases and then increases with the increase of the effective strain. The unimodal grain size distribution and fully recrystallized grains can be obtained at low strains at 960~990 °C. The twin boundary length fraction of deformed specimens is always lower than that of preheated ones. Discontinuous dynamic recrystallization (DDRX) was considered as the dominant nucleation mechanism, and continuous dynamic recrystallization (CDRX) was strengthened with the increasing grain size. Twin introduced deformation will be the main deformation mode for alloy 718 with larger grain.

Effect of hafnium on annealing twin formation in as-hot isostatically pressed nickel-based powder metallurgy superalloy

The effect of hafnium on the microstructure of as-hot isostatically pressed nickel-based powder metallurgy superalloy FGH4097 has been investigated. It is shown that when adding 0.3 wt.% hafnium, the statistical annealing twin boundary length fraction of the hafnium-free superalloy decreases from 0.35 ± 0.01 to 0.30 ± 0.01 and its interaction events of annealing twins are considerably reduced. The main reason is found to be the increase of MC (where M = metal atom) carbide density caused by micro-addition of hafnium, which could effectively lower the annealing twin nucleation probability and impede its growth into grains by retarding the movement of Shockley dislocation in the end of non-coherent twin boundary.

Microstructural statistics for fatigue crack initiation in polycrystalline nickel-base superalloys

In advanced engineering alloys where inclusions and pores are minimized during processing, the initiation of cracks due to cyclic loading shifts to intrinsic microstructural features. Criteria for the identification of crack initiation sites, defined using elastic-plastic loading parameters and twin boundary length, have been developed and applied to experimental datasets following cyclic loading. The criteria successfully quantify the incidence of experimentally observed cracks. Statistical microstructural volume elements are defined using a convergence approach for two nickel-base superalloys, IN100 and René 88DT. The material element that captures the fatigue crack-initiating features in René 88DT is smaller than IN100 due to a combination of smaller grain size and higher twin density.

Influence of processing parameters on dynamic recrystallization and the associated annealing twin boundary evolution in a nickel base superalloy

Dynamic recrystallization (DRX) kinetics and its association with annealing twins in alloy 617 has been studied in a range of strain rates (0.001s−1 to 10s−1) and temperatures (1173–1473K). In both low (< 0.1s−1) and high (≥ 1s−1) strain rate domains, DRX fraction is high as compared to intermediate strain rate (~ 0.1s−1). The lower critical strain for DRX initiation with higher growth of DRX grains is responsible for large DRX fraction with coarse grains microstructure in the low strain rate domain. In contrast, DRX at higher strain rates is mainly controlled by the large stored energy along with adiabatic temperature rise which leads to higher DRX nucleation. It is also noticed that the evolution of twins is greatly influenced by deformation temperature at all imposed strain rates. Here, the evolution of twins is quantified by measuring twin boundary length fraction and length density. The twin-length density increases with increase in DRX fraction at low temperatures (< 1423K) but decreases with increasing DRX fraction at high temperatures (≥ 1423K). An exponential rise in recrystallized grain size with DRX fraction is observed at a temperature ≥ 1423K. This suggests that rapid migration of high-angle boundaries happened at ≥ 1423K which may not be favorable for nucleation of twins during DRX.

On twin density and resistivity of nanometric Cu thin films

Crystal orientation mapping in the transmission electron microscope was used to quantify the twin boundary length fraction per unit area for five Ta38Si14N48/SiO2 encapsulated Cu films with thicknesses in the range of 26–111 nm. The length fraction was found to be higher for a given twin-excluded grain size for these films compared with previously investigated SiO2 and Ta/SiO2 encapsulated films. The quantification of the twin length fraction per unit area allowed the contribution of the twin boundaries to the size effect resistivity to be assessed. It is shown that the increased resistivity of the Ta38Si14N48 encapsulated Cu films compared with the SiO2 and Ta/SiO2 encapsulated films is not a result of increased surface scattering, but it is a result of the increase in the density of twin boundaries. With twin boundaries included in the determination of grain size as a mean-intercept length, the resistivity data are well described by 2-parameter Matthiessen's rule summation of the Fuchs-Sondheimer and Mayadas Shatzkes models, with p and R parameters that are within experimental error equal to those in prior reports and are p = 0.48(+0.33/−0.31) and R = 0.27 ± 0.03.

Microstructure evolution of titanium after tensile test

The qualitative and quantitative behavior of titanium T40 during tensile loading with a special emphasis on the presence of deformation twins in the observed microstructures is described. The samples for tensile tests were cut out from the rolled titanium sheet along the rolling and transverse directions. Several microstructure maps were determined using Electron Backscatter Diffraction technique (EBSD). These data were used to obtain crystallographic textures, misorientation distributions, grain size, twin boundary length, grain orientation spread, low and high angle boundary fractions and Schmid and Taylor factors. The deformation mechanisms and microstructure characteristics are different in the samples stretched along rolling and transverse directions. A strong appearance of tensile twins was observed in the samples deformed along transverse direction. On the other hand, more frequent subgrain formation and higher orientation spread was observed in the sample deformed along rolling direction, which caused’‘orientation blurring’ leading to an increase of grain size with deformation, as determined from OIM analysis.

Twin boundary characters established during dynamic recrystallization in a nickel alloy

For the purpose of providing fundamentals for grain boundary engineering design for high-temperature alloys, twin boundary characters are studied quantitatively in a nickel alloy during DRX to improve the understanding of twin boundary evolution during hot deformation. The nickel alloy samples were hot compressed at temperatures of 1120–1180°C and strain rates of 0.1s−1 and 1s−1 to a fixed true strain of 1.1. Refined equiaxed grain structure is obtained with abundant twin boundaries. Most of the twin boundaries form inside the grains, being highly coherent and showing rather limited mutual interaction. An inversely proportional relationship is found between Σ3 twin boundary length density (BLDΣ3) and DRX grain size (DDRX). An analytical model is derived here, in good agreement with current experimental results, indicating that the stored strain energy difference across grain boundary is the main driving force for grain boundary migration and determines the annealing twin formation during DRX. Σ3 twin boundary length density (BLFΣ3) developed in DRX was found varying within a small range of 44%–48%, which is shown to result from the inverse proportionality between grain boundary length density (BLDGB) and DDRX as well as the inverse proportionality between BLDΣ3 and DDRX.

EFFECT OF TEMPERATURE ON MICROSTRUCTURE AND NANOINDENTATION MECHANICAL PROPERTIES OF ELECTRODEPOSITED NANO-TWINNED Ni

The nanocrystalline nickel thin films with high density nano-scale growth twins were synthesized by means of pulse electrodeposition technology. Three samples were deposited at different bath temperatures of 30, 50 and 80 , by keeping all the other parameters as constant, such as electrolyte, pH value, current density and on-time and off-time period. The effect of temperature on deposition rate, sample texture, grain size, and twin boundary length and twin lamella thickness were systematically analyzed by SEM, XRD and TEM techniques. The microscopical feature of twin boundary were investigated by HRTEM. The results show that the nano-twinned nickel films deposited at the rates ranging from 20 nm/s to 30 nm/s have a preferred growth plane of (220) when de- posited at 30 and 50 , but changes to (200) when the temperature increases to 80 . With increasing

Effect of Strain and Strain Path on Texture and Twin Development in Austenitic Steel with Twinning-Induced Plasticity

High-manganese (15 to 30 wt pct) austenitic steels exhibit extreme strain hardening because of twinning with increased strain. Twinning in these low stacking fault materials promotes retention of the austenitic microstructure and impedes dislocation motion. A dearth of information is available concerning the extent to which strain path influences twinning in so-called twinning-induced plasticity (TWIP) steels. The present study focuses on the influence of strain level and strain path on texture and twinning in a high-Mn content TWIP steel (Fe17.2Mn0.6C). Electron back-scatter diffraction was employed to measure the twin fraction, twin deviation, twin boundary length, grain misorientation, and volume fraction of different texture components as a function of both uniaxial and biaxial deformation. This information, which is part of the necessary first step toward linking crystallographic texture and twinning to mechanical properties, was used to quantitatively assess the extent to which these critical metallurgical features depend on the amount of straining and the strain path.

Effect of an Electric Field on the Texture and Microstructure Evolution during Annealing of High Reduction Cold-Rolled Ni

The influence of an electric field on the annealing of high purity (99.999%) cold rolled nickel has been investigated. Annealing was carried out for 2 hours at temperatures between 300oC and 800oC with and without an electric field of strength 2.0KVcm-1. The microstructure and fraction of cube texture resulting were characterized using electron backscattering pattern (EBSP) technique. Annealing in an electric field leads to somewhat smaller average values of the cube fraction and grain sizes compared to annealing without an electric field. The highest temperature (800oC) annealing in an electric field results in microstructures with a lower fractional twin boundary length.

The effect of a transverse magnetic field on the microstructure of directionally solidified CdTe

The purpose of this study was to examine several microstructural features in Bridgman-grown CdTe and determine whether the use of a tranverse, 4.6 kG magnetic field during solidification had any effect on these features. Any variation in these microstructural features due to the use of the magnetic field could be attributed to changes in the connective state of the melt and the resulting disruption or stabilization of the solid-liquid interface. In order to accomplish this objective, samples of CdTe were directionally solidified by the vertical Bridgman-Stockbarger method and a static magnetic field of 4.6 kG was applied transverse to the growth direction during the solidification of selected samples. The microstructural features examined included the preferred crystallographic growth direction of primary gains, grain and twin boundary length per unit area and etch-pit and subgrain boundary densities on a (111)-type plane. Measurements of these features were performed using the Laue X-ray backreflection technique and quantitative optical microscopy. No measurable difference in the microstructural features occurred as a result of using the 4.6 kG transverse magnetic field. The lack of any effect is attributed to either an insufficient magnetic field intensity or the possibility that the effect of a 4.6 kG transverse magnetic field on the fluid flow may not play a significant role in the mechanisms generating the microstructural features analyzed by the characterization techniques used in this study. Subgrain boundary densities were found to vary along the length of each boule and this variation was attributed to post-growth annealing.

Disclination recombination kinetics in water-surface monolayers of 22-tricosenoic acid

It has been predicted that the insulating properties of monodomain fatty acid Langmuir-Blodgett films should be much better than those fabricated by conventional techniques. Annealing of the monolayer on the water surface is a method which can in principle lead to oriented monolayers, and which has already been demonstrated to achieve modest reductions in disclination density in floating monolayers of 22-tricosenoic acid. In the present paper a previous report is extended by investigating the kinetics of disclination recombination in the same system, and determining how the rate constant correlates with annealing conditions. The phase behaviour of the system was found to be very similar to that of docosanoic acid, with superliquid, close-packed solid and L 2′ phases in addition to the better-known liquid-condensed and solid phases. The kinetics in the liquid-condensed phase were found to be first order. This is to be expected if the molecules are tilted away from the substrate normal, so that disclinations are connected in pairs by twin boundaries. No significant correlation of the recombination rate with temperature or pressure was observed. Its functional dependence on the initial disclination density was consistent with a square root dependence, which is the expected law of variation of the inverse twin boundary length. There was a strong, essentially inverse, correlation with the period between sampling of the disclination density. It is concluded, and has been separately checked, that most of the recombination takes place during the sampling process, while the monolayer is being compressed for deposition onto a substrate and subsequently re-expanded, rather than while floating undisturbed, as was previously assumed.