Search

The micro-scale heterogeneity of shale matrix may significantly impact the diffusivity of shale gas and further affect the shale gas production of a fractured horizontal well. However, the impact of shale matrix heterogeneity on the gas diffusivity is still unclear. This study investigates the impact of the heterogeneity of shale matrix microstructure on gas diffusivity through a reconstructed shale matrix model. Firstly, shale matrix is composed of organic and inorganic matrices, whose pores are distributed at nano- and micro-scales and form slit-like or tube-like channels for gas flow. The magnitude of pore sizes within the organic matrix is one or two order smaller than that within the inorganic matrix. The distribution of shale pore sizes is described by a fractal theory. Secondly, shale gas is only adsorbed on the pore surface of organic matter and the inorganic matrix provides storage space and transport paths of free shale gas. Particularly, the transport properties of slit-like and tube-like channels are discussed and their mass flow rates are validated with three sets of experimental data. Thirdly, a microstructure of shale matrix is reconstructed by a quartet structure generation set (QSGS) and the homogenized diffusivity of this microstructure of shale matrix is obtained by a homogenization method at different volume fractions. Finally, sensitivity analysis is conducted to explore the mechanisms of gas flow including viscous flow with slip boundary, molecular diffusion (i.e. molecular self-diffusion), Knudsen diffusion, and surface diffusion in organic matter and viscous flow with slippage boundary, molecular diffusion and Knudsen diffusion in inorganic matrix. It is found that higher total organic content corresponds to smaller gas diffusivity. Gas desorption can only increase the pore space in organic matter due to gas pressure depletion. The homogenized diffusivity is mainly controlled by the pore channel in inorganic matrix but organic matter provides gas source and changes gas transport mechanisms. These results show that the heterogeneity of shale matrix has significant impacts on the diffusivity and cannot be ignorable in the analysis of shale gas production. [ABSTRACT FROM AUTHOR]

Abstract: The effects of directional compaction, non-Darcy flow and anisotropic swelling on the extraction of natural gas in coal seam reservoirs are important but not well understood so far. This paper investigated these combined effects through a numerical model which fully couples the interactions of coal seam deformation, anisotropic gas flow in fractures, and anisotropic absorption/adsorption of natural gas in coal matrix. In the sorption process, the Langmuir isotherm is extended by including some microstructure information to describe sorption induced anisotropic swelling strain (called anisotropic swelling). The Forchheimer equation is reformulated to consider the non-Darcy effect in directional flows (called non-Darcy flow). A strain ratio of matrix to fractured element is introduced for the directional compaction induced evolution of permeability (called directional compaction). The effect of anisotropic swelling strain is also converted into anisotropic swelling body forces in the mechanical deformation process. This model is verified by the experimental results of single fracture flow under compaction. This verified model is used to quantify the relative importance of directional compaction and matrix swelling through a block model containing a single fracture. It is found that directional compaction of the single fracture can reduce the permeability by 70% and matrix swelling can reduce the permeability by over 30%. This example illustrates the important contribution of matrix swelling to the anisotropy of permeability. This fully-coupled model is applied to a production well in a coal seam gas reservoir to investigate the combined effects of directional compaction, non-Darcy flow and anisotropic swelling on well production rates. It is found that both directional compaction and anisotropic swelling can significantly induce the anisotropy of permeability, forming a skin damaged zone and largely affecting the production rate of natural gas extraction. Directional compaction and anisotropic swelling can significantly accentuate the non-Darcy effect near wellbore for unconventional coal seam gas reservoirs. Thus, the anisotropic evolution of permeability near wellbore cannot be ignorable for the evaluation of production data. [Copyright &y& Elsevier]

Abstract: This work presents a study of a novel Ti50Ni47Fe2.5Nd0.5 shape memory alloy by EPMA, E-R analysis and tensile tests. Structural characterization shows the alloy consists of TiNiFe matrix, Nd3Ni and Ti2Ni intermetallic compounds with Fe solute. The latter two phases are well distributed in the matrix. Compared with Ti50Ni47.5Fe2.5 alloys without rare earth Nd, the novel alloy does not change the two-stage martensitic transformation behavior, however, characteristic temperatures increase rapidly. The critical stress needed for inducing martensitic transformation is near linear relationship with the increasing of the testing temperature above M s, which is in good agreement with Clausius–Clapeyron equation. The novel alloy exhibits an improved shape memory effect as a consequence of precipitation strengthening and the maximum recoverable strain attains to 7.8%. [ABSTRACT FROM AUTHOR]

Abstract: Foam material is a typical micro-scale crushable material due to its special pore microstructures. Its pore distribution is a key factor to its physical–mechanical properties. After the complete collapse of all collapsible pores, its mechanical behaviors are like those of the intact materials. Therefore, the deformation of a foam material has two components: pore collapse and elastoplastic deformation of its intact material. In this paper, a rod model is proposed to quantitatively express the relationship between stress and pore collapse. The rod length, which expresses the pore size in foam materials, is assumed to follow the Weibull distribution. The joint stiffness between rods, which resists bending moment, can change with linking substances and work environments. Hence, a constitutive relation for the crushable component of microstructures is derived. This constitutive relation is then applied to some experimental data of hydrostatic compression to observe its performance. [Copyright &y& Elsevier]

Abstract: The effects of different processing parameters, such as heating rate, isothermal temperature and isothermal holding time, on the semisolid microstructures of the AZ91D alloy during partial remelting had been investigated in the present paper. The experimental result showed that the high heating rate shortened the growth process of renascent recrystal grains in the deformed alloys, which ultimately attributed to refine the globular α-Mg grains in the evolved semisolid microstructure. The liquid volume fraction f L of the semisolid slurry gradually enlarged with the elevating of isothermal temperature during partial remelting. The isothermal temperature between 560°C and 580°C for 30min was adequate for the AZ91D to produce an ideal semisolid slurry with liquid fraction range between 32% and 52%. During the partial remelting, it would experience a relatively long holding time that the liquid and solid phases reached a state of dynamic equilibrium in the semisolid slurry. For the AZ91D alloy, the isothermal holding time of 30min was relatively adequate for the present experiment. During the partial remelting, the primary irregular grains gradually spherodized under a function of interface curvature. Oswald ripening mechanism, a dissolution and reprecipitation diffusion-controlled growth, could account for the coarsening of globular grains in the present experiment. The spheroidization and coarsening evolution processes of solid particles in the semisolid slurry were deduced and elucidated by the corresponding simulant formulae in the present paper. [Copyright &y& Elsevier]

Abstract: The effects of different as-cast microstructures which were initially cast in graphite, metal, sand and firebrick moulds, respectively on the semisolid microstructure of AZ91D alloy, have been investigated during the strain-induced melt activation (SIMA) process. The experimental results showed that the moulds with high cooling capacity could produce the fine-grained as-cast microstructure in which the fine α-Mg dendrites were surrounded by a narrow layer of eutectic mixtures. After compressive deformation, in the fine-grained as-cast microstructure, the more systemic strain energy would be gradually accumulated and abundantly stored due to uniform inner crystal lattice distortion, so the recrystallization was easily induced by the stored strain energy at the elevated temperature. As a channel for the diffusion of atoms, the subgrain boundary along which Al element was enriched, foremost melted above the eutectic temperature and resulted in the separation of neighboring subgrains from primary dendrites. Therefore, the refining role of recrystallization on the microstructural evolution from dendrite to globular particles in morphology was easier to play in the fine-grained as-cast microstructure, which was advantageous for the production of fine-grained semisolid microstructure. Additionally, in the fine-grained as-cast microstructure, the melting fracture of narrow secondary dendritic arms was easy to occur in their roots, which also attributed to the production of fine globular grains in semisolid microstructure from primary dendrites. The finer dendrites in the initial as-cast alloy could evolve into the finer globular grains with relatively small grain size distribution range in the semisolid microstructure during partial remelting; therefore, the finer the dendrites in the initial as-cast microstructure, the better were the tensile properties of the evolved semisolid microstructure. [Copyright &y& Elsevier]

Abstract: The effect of predeformation on the α-Mg grains in the AZ91D alloys was investigated in different stages during the semisolid processing. The result showed that, with the increase of compressive ratio, the α-Mg dendrites in the as-cast microstructures gradually oriented themselves in the direction that was vertical to the compressive direction, and exhibited an obvious directional character. During the sequent heat treatment at 310°C for 20min, the recrystallization was induced in the deformed alloys by the increasingly accumulated systemic strain energy. The metallographical observation showed that the average size of recrystal α-Mg grains gradually reduced with the increase of compressive ratio. The reason might be that the nucleation to growth ratio of the renascent recrystal α-Mg grains gradually increased in the recrystallization process. Due to the recrystallization mechanism, the predeformation made the α-Mg grains particularly rounder and finer after the semisolid isothermal heat treatment at 560°C for 20min. The average diameter of the globular α-Mg grains in the semisolid microstructures also reduced with the increase of compressive ratio and this descending trend was almost similar to that in the recrystal microstructures, which further demonstrated that the recrystallization had an inevitable role on the evolution process of globular grains during SIMA process. While the compressive ratio surpassed 20%, the above descending trend was not so evident as that below 20%. In addition, the effect of predeformation on the tensile strength and elongation of the semisolid AZ91D alloys was also discussed in the present paper. [Copyright &y& Elsevier]

Abstract: The semisolid microstructure evolution process of the predeformed AZ91D alloy during heat treatment was studied. After being compressed by a ratio of 20.6% in height at 220°C, all the dendrites oriented in the same way. With increasing heat-treatment temperature, the dissolution of eutectic β-Mg17Al12 first took place, resulting in the primary dendritic grains coarsening into interconnected non-dendritic grains. With heating continuously, a large amount of polygon recrystallization grains occur in the microstructure. Subsequently, the residual interdendritic β-Mg17Al12 at the edges of the primary grains and the new recrystallization grains melted in succession at higher heat-treatment temperature and the primary grains separated into small polygon grains. After being heat-treated at the semisolid temperature, the amount of liquid increased until the solid–liquid system reached its equilibrium state, and the grains spheroidized. [Copyright &y& Elsevier]

The effect of predeformation on the semisolid microstructure of Mg–9Al–0.6Zn (AZ91D) alloy is investigated. The results indicate that finer, more globular semisolid microstructure was attained in more heavily deformed alloys after heat treatment at 560 °C for 20 min. Furthermore, the liquid islands in the grains also become less with the increase in the compression ratio. It is mainly because the microstructural evolution mechanisms are different in alloys with various compression ratios. The heavy predeformation leads to the occurrence of the recrystallization, and the amount of recrystallization increases with the increase of the compression ratio. These recrystallization grains can easily give rise to the small initial grains that evolve towards a globular and small morphology rapidly during isothermal heat treatment at semisolid temperature of 560 °C. Moreover, there are also no liquid islands in the final grains after evolution. In addition to the recrystallization evolution mechanism, there are two other microstructural evolution mechanisms—melting and combining of dendritic arms, which are both discussed in this paper. [Copyright &y& Elsevier]

In this study, the microstructures of Rutelidae cuticle, a natural biocomposite, were observed with scanning electronic microscope (SEM). It was found that the insect cuticle has a laminated microstructure consisting of chitin fiber and protein matrix. There are particular spiry layups in the cuticle. The maximal pull-out force of the spiry layup is analyzed and compared with that of conventional 0°-layup based on their representative models. The result shows that the pull-out force of the spiry layup is markedly larger than that of the conventional 0°-layup. The corresponding experiment on the maximal pull-out forces of both spiry layup and the 0°-layup is conducted, and it shows that the maximal pull-out force of the spiry layup is markedly larger than that of the 0°-layup, which verifies and demonstrates the validity of the presented model. [Copyright &y& Elsevier]

Insect cuticle as a natural biocomposite includes many favored microstructures which have been refined over centuries and endow the cuticle eminent mechanical properties. This paper first studies the microstructures of chafer cuticle through SEM observations. Several peculiar fiber configurations and fiber-ply arrangements such as branched fiber, acanth-fiber and helicoid plies are observed. These microstructures are useful for man-made fiber-reinforced composites to improve their mechanical properties. Then, a special configuration of the branched fiber found in chafer cuticle is in details analyzed through a mechanical model and experimental verification. The pullout force of fibers as an index is firstly studied through parameter study. The factors, which can improve the pullout forces, are identified. Finally, the maximal pullout force of the branched fiber is experimentally tested and compared with that of plain straight fiber. It is proved that the maximal pullout force of branched fibers is obviously greater than that of the plain straight fibers. [Copyright &y& Elsevier]

The molluscan shell is a natural ceramic composite with excellent fracture strength and fracture toughness, which are attributed to their unique microstructures. SEM observation on Bivalva shell showed that the shell consists of laminated aragonites and organic layers. There are different shapes and arrangements of aragonites. The higher fracture roughness of this laminated microstructure is analyzed. The laminated microstructure of the shell is used for the biomimetic designs of the ceramic/polymer and ceramic/polymer/fiber composite. Comparison of test of the biomimetic ceramic composite with the original ceramic material shows that the fracture toughness of the biomimetically designed ceramic composite increases markedly than that of the original ceramic material. [Copyright &y& Elsevier]

Dynamics of grain boundary (GB) atoms in the bulk nanocrystalline aluminium is investigated by means of a large-scale molecular dynamics simulation. It is found that the GB atoms in the nanocrystalline aluminium display the glassy dynamics. Their dynamic features are, on one hand, similar to those in the glass systems and bicrystal GBs, in terms of the time-correlation functions of mean-square displacement (MSD) and non-Gaussian parameter (NGP). But on the other hand, these GB atoms also demonstrate some different dynamic behaviors due to the more complicated microstructure. In particular, the immobile GB atoms are localized into their equilibrium positions due to the strong cage effect, and they are mainly at the surfaces of grains, especially the large grains, while the mobile GB atoms are active and can easily hop away from the cage around them. These mobile GB atoms gather together at the surfaces of small grains and the triple junction regions of GBs, and they form some abnormally big clusters. The size distribution of these clusters significantly deviates from the power law. [ABSTRACT FROM AUTHOR]

Abstract: Scanning electron microscope (SEM) observation shows that a shank bone is a kind of bioceramic composite consisting of inorganic hydroxyapatite and organic collagen protein. The hydroxyapatite possesses laminated microstructure and the each hydroxyapatite layer is composed of numerous long and thin hydroxyapatite sheets. The hydroxyapatite sheets in an arbitrary hydroxyapatite layer make an almost fixed angle with the hydroxyapatite sheets in their adjacent hydroxyapatite layers which compose a kind of particular screwy microstructure. The maximum pullout energy of the screwy microstructure, which is closely related to the fracture toughness of the bone, is theoretically investigated and compared with that of the parallel microstructure of the sheets. It shows that the maximum pullout energy of the screwy microstructure is markedly larger than that of the parallel microstructure, which is experimentally verified. [Copyright &y& Elsevier]

Abstract: The modification of in situ formed Mg2Si in Mg–Si alloys with yttrium (Y) was investigated. As-cast microstructure analysis of Mg–5wt.% Si alloys indicates that all the primary Mg2Si crystals are surrounded by a layer of α-Mg dendritic halos, which is again surrounded by the eutectic Mg2Si+Mg structures. When the Y content is 0.1 or 0.4wt.%, the sizes of primary and eutectic Mg2Si do not significantly reduce in Mg–5wt.% Si alloys. When the Y content is 0.8wt.%, the mean size of primary Mg2Si decreases from more than 100μm to about 30μm or less; furthermore, the Chinese script type Mg2Si particle also exhibits modified morphology as a fine fiber. When the Y content is further increased to 1.2wt.%, the primary Mg2Si becomes coarser again and even larger than that in the unmodified alloy; however, the eutectic Mg2Si still exhibits modified morphology as a fine size. The size and amount of Mg–Y point compounds are increased with the increasing of Y content in the alloys. In addition to distributing in α-Mg phase, the Mg–Y point compounds are also present in primary Mg2Si dendritic crystal. In the unmodified alloy, the coarse primary Mg2Si is formed by the preferred growth that occurs at the tips of branches; however, when the alloy is modified with 0.8wt.% Y, the preferred growth manner of the primary Mg2Si was depressed, while another type of growth, isotropic growth manner, is enhanced in the modified alloy. [Copyright &y& Elsevier]

The structure–property relationship of a spray formed Al–Y–Ni–Co alloy with two sets of processing conditions was investigated. Significant differences in tensile strength, yield strength, and high temperature ductility were observed with respect to the microstructural changes. Fracture toughness values were determined for both sets of specimens and found to be 7.5 and 5.8 MPa·m1/2, respectively. Three intermetallic phases were observed in the matrix and constitute a volume fraction of approximately 75%. It is believed that the specimens failed during fracture toughness testing by the mechanism of cleavage, observed in the Al3Y intermetallic particles. [Copyright &y& Elsevier]

[Display omitted] Dislocation is an important microstructural configuration in most crystalline materials, but it is still difficult for experiments to detect its local atomic structures and formation processes. Molecular dynamics simulation provides a powerful tool to investigate the dislocation by means of some geometrical structural analysis methods, but most of these methods cannot accurately distinguish the nuances of local configurations. Herein, we propose a new microstructural analysis method of cluster-type-index method (CTIM). The three-dimensional (3D) topological structures and formation processes of dislocations during tensioning Au nanowire are illuminated by means of CTIM. It is found that the favourite local atomic structures in dislocations are the defective FCC clusters represented by the CTIM indexes (13, 3/1421 2/1431 2/1441 4/1541 2/1661). The vector analysis method based on the accumulation of lattice distortions cannot identified the aggregation of disordered atoms in the initial nucleation of dislocation. In the formation processes of dislocation atoms, three trajectories of T1: FCC→(13, 3/1421 2/1431 2/1441 4/1541 2/1661), T2: FCC→(12, 2/1311 1/1411 9/1421) and T3: FCC→(11, 4/1311 7/1421) are favorite. The dislocation atoms following the T1 trajectory are mainly located at the core of dislocations, while that following the T2 and T3 trajectories is at the front of dislocations. CTIM would provide an effective tool to investigate the defective structures and their interactions in nanocrystalline structures. [ABSTRACT FROM AUTHOR]