Effect Of Lamellar Structure Research Articles

Mechanical Properties of Lamellar Shale Considering the Effect of Rock Structure and Hydration from Macroscopic and Microscopic Points of View

It is well known that the effect of shale hydration causes wellbore instability due to water phase invasion of drilling fluid to lamellar shale rich formations. This is because of that the mechanical properties (compressive strength, elastic modulus, etc.) of lamellar shale surrounding the borehole, which is rich in clay minerals, will decrease significantly after hydration. In this study, using the lamellar shale in the continental stratum of the southern Ordos Basin, the mechanical properties of lamellar shale were studied by compression tests considering the effect of lamellar structure and hydration from a macroscopic point of view. In addition, the mechanical mechanism was discussed combined with the CT scanning tests results from a microscopic point of view. The results demonstrate the following points. Lamellar shale has stronger anisotropy than bedding shale, the compressive strength (deviatoric stress) and elastic modulus of lamellar shale are both lower than those of bedding shale, and it is more prone to tension fracture. With the increase in the angle (β) between the lamina and the axial direction from 0° to 90°, the compressive strength of lamellar shale decreases when β < 30° and then increases, the elastic modulus of lamellar shale decreases greatly when β < 30° and then tends to flatten. With the increase in hydration time, the compressive strength and elastic modulus of lamellar shale both gradually decrease, and the rates of their decrements reduce. The mechanical properties of lamellar shale are more affected by hydration than those of bedding shale. The hydration of lamellar shale leads to the formation of new fractures and the expansion of existing fractures in the junction area between the laminae and rock matrix, resulting in easy tension fracture along the laminae of shale.

Understanding the atomistic deformation mechanisms of polycrystalline γ-TiAl under nanoindentation: Effect of lamellar structure

Molecular dynamics (MD) simulations have been performed to study the deformation behavior of polycrystalline γ-TiAl under nanoindentation. Effect of three different lamellar structure patterns is investigated respectively. Simulation results reveal that the lamellar samples are soft due to a dislocation-nucleation-controlled softening mechanism. Dislocations propagate along lamellar interface and experience no obstacle. Moreover, not only grain boundary (GB) but also lamellar interfaces can act as sliding interfaces, the additional sliding interface can also assist softening of lamellar samples. In addition, the different evolution processes of the three γ/γ interface variants are also investigated. Lamellar coarsening can be observed in the true-twin (TT) sample due to the direct migration and annihilation of TT interfaces. While the pseudo-twin (PT) and rotational-boundary (RB) interfaces cannot migrate directly but transform to each other at the origin interface planes, thus the origin lamellar spacing remains.

Effects of lamellar structure on tensile properties and resistance to hydrogen embrittlement of pearlitic steel

The hydrogen embrittlement (HE) and H trapping sites of pearlitic steel specimens with various lamellar spacings (λ) were evaluated through slow strain rate tensile testing and thermal desorption analysis. When λ decreases, both tensile strength and resistance to HE were unusually improved. This is because tearing, which is the initiation of H cracking, was delayed in the specimen with fine λ and short cementite (θ) platelets. Undeformed H-charged specimens showed a peak (peak 1), which is separable into two sub-peaks (peak 1-1 and peak 1–2) in their H desorption rate curves, regardless of λ. Peak 1-1 and peak 1–2 were generated by H atoms detrapped from FP/θ interfaces and from dislocations inside FP, respectively. The Ea values of H desorption for peak 1-1 and peak 1–2 were 23.2 kJ/mol, and 26.1 kJ/mol, respectively. Meanwhile, deformed H-charged specimens exhibited the second peak (peak 2) with peak temperature (TP) of ∼600 K, as well as peak 1 with TP of ∼375 K. When tensile strain increased, peak 2 increased at the expense of peak 1. Primary H trapping sites for peak 2 are strained FP/θ interfaces with interfacial dislocations.

The effect of lamellar structure in Mo 5Si 3–MoSi 2 alloy refined by annealing on the Vickers hardness at room temperature

Mo 5Si 3–MoSi 2 eutectic alloy, hypoeutectic alloy and hypereutectic alloy were prepared by arc-melting pure raw materials of Mo and Si. Annealing at 1200 °C for different times was conducted on all of the alloys to investigate the effect of annealing on the microstructure and mechanical property including Vickers hardness and indentation toughness at room temperature. Lamellar structure consisted of Mo 5Si 3 (D8m) phase and MoSi 2 (Cllb) phase was observed in all the alloys. For Mo 5Si 3–MoSi 2 eutectic alloy, a typical eutectic microstructure with primary Mo 5Si 3 phase and Mo 5Si 3/MoSi 2 lamellar structure was observed, and the primary Mo 5Si 3 phase was eliminated when annealing at 1200 °C for over 24 h. For Mo 5Si 3–MoSi 2 hypoeutectic alloy, the lamellar structure was found only after annealing. A full lamellar structure throughout the alloy developed well with fine spacing on the order of 100 nm after annealing at 1200 °C for 48 h. For Mo 5Si 3–MoSi 2 hypereutectic alloy, the lamellar structure was found both before and after annealing. Primary MoSi 2 was observed in the alloy even after annealing at 1200 °C for 96 h, although the volume fraction of lamellar structure was increased with prolonging annealing time. The effect of the formation, development and destruction of lamellar structure on Vickers hardness and indentation toughness of alloys was investigated. The Vickers hardness and indentation toughness of Mo 5Si 3–MoSi 2 hypoeutectic alloy show a same regularity, that is, they increase with prolonging annealing time and development of lamellar structure. The highest Vickers hardness and indentation toughness are about 1470 HVN and 4.8 MPa m 1/2, respectively. The Vickers hardness and indentation of Mo 5Si 3–MoSi 2 eutectic alloy and Mo 5Si 3–MoSi 2 hypereutectic alloy are all roughly constant with prolonging annealing time, that is, about 1400 and 1270 HVN for Vickers hardness and 4 and 3.2 MPa m 1/2 for indentation toughness.

Effect of lamellar structure on microhardness and yield stress of directionally solidified intermetallic Ti-46Al-2W-0.5Si alloy

In recent years ordered TiAl based intermetallic alloys have been extensively studied as potential hightemperature materials in the gas turbine industry [1]. Of the numerous microstructures that can be developed in TiAl-based alloys, the fully lamellar or nearly lamellar microstructures consisting of TiAl (γ -phase and Ti3Al (α2-phase) are very promising for structural applications. Mechanical properties of fully lamellar polycrystalline TiAl alloys depend on the grain size, lamellar thickness, interlamellar spacing and volume fraction of coexisting α2and γ -lamellae [2]. In the case of directionally solidified (DS) TiAl alloys, mechanical properties such as yield stress, tensile elongation, fracture toughness and creep behavior were found to depend strongly on lamellar orientation to loading axis and interlamellar spacing [3–5]. A good combination of room temperature strength, ductility, toughness and creep strength can be achieved when the lamellar orientation is aligned parallel to the tensile direction. The aim of the present work is to study the effect of lamellar structure on room-temperature yield stress and microhardness of directionally solidified Ti46Al-2W-0.5Si (at.%) alloy. The alloy was developed by Nazmy and Staubli [6] for investment cast turbine blades with improved creep properties. Unlike previous studies on mechanical properties of DS TiAl based alloys, this study is focused on fully lamellar alloy reinforced with a constant volume fraction of hard Al2O3 particles. The alloy with the chemical composition Ti-46Al2W-0.5Si (at.%) was provided by Alstom Ltd in the form of cast cylindrical ingots with a diameter of 21 mm and length of 210 mm. Directional solidification was performed in Al2O3 molds (mean grain size of 9.3 μm and purity of 99.5%) of 11/15 mm diameter (inside/outside diameter) under argon atmosphere in a modified Bridgman-type apparatus [7]. All ingots were prepared at a constant growth rate of 1.18 × 10−4 ms−1 using experimental procedure described elsewhere [7, 8]. After directional solidification the ingots were heated to solution annealing temperature of 1633 K at constant heating rate of 0.28 K s−1, stabilized at this temperature for 1 h and then cooled to 973 K at constant cooling rates varying in the range from 0.078 to 0.61 K s−1. Further cooling to room temperature was accomplished by gas fan cooling. All annealing experiments were performed in dynamic argon atmosphere. After heat treatment Vickers microhardness measurements with applied load of 0.64 N were performed on longitudinal sections of specimens. Rectangular compression specimens with dimensions of 4 × 4 × 8 mm were cut from the ingots by electric discharge machining. Statistical evaluation of all compression specimens (8–10 specimens tested for each regime) revealed that the mean angle between lamellar orientation and the compression axis varied in the range 8 ◦ to 15 ◦ and 56 ◦ to 67 ◦ for the specimens with the compression axis perpendicular and parallel to the growth direction, respectively. Roomtemperature compression tests were performed at an initial strain rate of 1.39 × 10−3 s−1 in air using a screw driven universal testing machine Instron. The compression offset yield stress was measured at 0.2% plastic strain. Microstructural analysis was performed by optical microscopy (OM) and scanning electron microscopy (SEM). Quantitative metallographic analysis was performed on digitalized micrographs using a computerized image analyser. The microstructure of DS ingots consisted of columnar grains (3–5) aligned parallel to the growth direction. The microstructure within the columnar grains was dendritic, as shown in Fig. 1. The as-grown ingots contained regular well-aligned α2 (Ti3Al) and γ (TiAl) lamellae. A small volume fraction (about 1 vol%) of elongated B2-particles (ordered β-phase) was formed at the columnar grain boundaries and in the interdendritic region. Fine Ti5Si3-precipitates were observed at the lamellar interfaces and along the grain boundaries [9]. In addition, the as-grown ingots contained constant volume fraction of Al2O3 particles (1.5 vol%), which were formed in-situ due to a reaction between the alumina mold and the melt [8]. The ingots contained tree types of Al2O3 particles (dark color phase in Fig. 1): nearly spherical, needle-like and clusters of particles. The size of particles characterized by a mean value of minor axis calculated from statistical log-normal distribution function was 6 μm. Heat treatment of DS ingots consisting of solution annealing and subsequent cooling at constant rates ranging from 0.078 to 0.61 K s−1 resulted in formation of regular lamellar structure. Fig. 2 shows an example of typical regular lamellar microstructure formed in all heat-treated ingots. In addition, as in the case of the as-grown ingots, the heat-treated ingots contained elongated B2-particles, fine Ti5Si3-precipitates and constant volume fraction of Al2O3 particles (1.5 vol%).

Microstructural influence on the fatigue crack propagation of a γ-TiAl alloy

Considering the potential use of {gamma}-TiAl alloys for high temperature turbine materials, it is important to investigate the effect of microstructural variation on the fatigue crack propagation (FCP) behavior at room and high temperatures. Recent investigations on the FCP of {gamma}-TiAl alloys have shown that the fully lamellar microstructure represents superior crack growth resistance as compared to the other microstructures, such as duplex and near {gamma} microstructures, owing to the crack-tip shielding effects of lamellar structure. However, many fundamental questions on the FCP of {gamma}-TiAl alloys remain unsolved yet. In this investigation, the authors examined the fatigue crack growth resistance of Ti-47Al-1Cr-4Nb-0.2W-0.25Si-0.1B(at.%) alloy associated with the variation of the equiaxed {gamma}/lamellar volume fraction. A particular interest was given to study the role of colony/grain boundaries and lamellar interfaces on the FCP and to investigate the relative intrinsic fatigue properties of the above three microstructures.

The effects of lamellar structure in the TiAl intermetallic compound on its acoustic emission behavior during tensile deformation

Abstract The dynamic behavior of TiAl intermetallic compounds during tensile deformation has been determined by the different appearance of the (α2 + γ) lamellar structure using the acoustic emission (AE) method. The AE events of these materials are obviously different. There is, however, no effectively direct evidence to prove the relationship between the microstructure of these TiAl alloys and the AE behavior. In this paper, the effect of the interfaces on the transmitted wave are calculated quantitatively. According to these results, the effects of the microstructure of TiAl alloys on the AE behavior have been clarified.

Effect of lamellar structure and plastic anisotropy on the cyclic deformation and fatigue behaviour of TiAl PST crystals

Abstract Cyclic deformation and fatigue properties of TiAl polysynthetically twinned (PST) crystals were investigated by focusing on the effect of lamellar structure and plastic anisotropy on the cyclic hardening and fatigue life. Cyclic tension/compression testing at room temperature at total strain-controlled amplitude was carried out on Ti-49.1 at.% Al and Ti-50.8 at.% Al PST crystals containing fine and coarse lamellae, respectively. Strong anisotropic behaviour was observed, depending on the angle (φ) between the loading axis and the lamellar planes. As the strain amplitude was increased, the stress amplitude rose with increasing number of cycles and the cyclic hardening became large at φ = 0, while at φ = 45° specimens exhibited weak cyclic hardening and broke without significant hardening. Although the lamellar spacing in TiAl PST crystals did not lead to much difference in the cyclic hardening rate, refinement of lamellae prolonged the fatigue life in both orientations. The lamellar boundaries act as an effective barrier to the propagation of microcracks crossing the boundaries at φ= 0. The cyclic hardening and fatigue life of specimens with φ = 0 are sensitive to the deformation substructure, particularly with regard to the number of domains in which ½<110]-type ordinary dislocations form a densely tangled and piled-up substructure. The cyclic hardening and fatigue life of crystals with φ= 0 also depends upon the rotation angle (χ) between the loading axis and the ⟨112⟩ direction on (111) lamellar planes in the γ matrix.

Effect of lamellar structure on fracture behavior of TiAl polysynthetically twinned crystals

The fracture behaviors of Ti-49.1at.%Al and Ti-50.8at.%Al polysynthetically twinned crystals with fine and coarse lamellae, respectively, have been investigated, focusing on the effect of the lamellar structure and loading axis. Type A and B specimens, in which the notch directions were parallel and perpendicular to lamellar planes, respectively, were prepared and compact tension tests were performed at room temperature. The fracture behavior exhibited strong anisotropy, depending on the crack paths in the lamellae. For type A specimens, the crack propagated very easily parallel to the lamellae, exhibiting low K Q values, and a cleavage-like fracture on the (0001) plane in the α 2 phase was observed. In type B specimens, the crack propagated across the lamellar boundaries, exhibiting high K Q values, and the boundaries acted as an effective barrier to further crack propagation. Fine lamellae were beneficial in improving the fracture toughness.

Effect of Lamellar Structure on Ferroelectric Switching in Poly (Vinylidene Fluoride)

The solitary-wave model of ferroelectric switching (poling) in $\ensuremath{\beta}$-phase poly(vinylidene fluoride) is developed to allow comparison with recent measurements of the detailed time dependence of this process. This model is found to be consistent with these measurements.