The Experts below are selected from a list of 300 Experts worldwide ranked by ideXlab platform
Wuyang Chu - One of the best experts on this subject based on the ideXlab platform.
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In-situ TEM study of fracture mechanisms of polysynthetically twinned (PST) crystals of TiAl alloys
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2000Co-Authors: Y.g Zhang, L.j. Qiao, Y. B. Wang, C.q Chen, Wuyang ChuAbstract:Abstract The fracture mechanisms of polysynthetically twinned (PST) crystals of Ti–49at.% Al in different lamellar orientations have been investigated by using in-situ transmission electron microscopy (TEM). The results indicated that the fracture mechanisms and Crack propagation behavior depended strongly on the lamellar orientation of PST crystals. When the tensile axis was nearly parallel to the lamellae, the Main Crack propagated discontinuously by nucleation, growth and linkage of microCracks formed ahead of Crack Tip. Slip band decohesion, fracture along prism plane of α2 plates and twinning-induced microCracks are three modes of nucleation of microCracks. When the loading axis was perpendicular to the lamellae, the Crack parallel to the lamellae propagated by tearing and shear of shear ligament. Interfacial microCrack usually occurred ahead of the Main Crack Tip. Sometimes, the Main Crack propagated along (111) cleavage plane within γ lamellae.
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Hydrogen-enhanced dislocation emission, motion and nucleation of hydrogen-induced Cracking for steel
Science in China Series E: Technological Sciences, 1997Co-Authors: Tiancheng Zhang, Wuyang ChuAbstract:The change in dislocation configuration ahead of a loaded Crack Tip before and after charging with hydrogen wasin situ investigated in TEM using a special constant deflection loading device. The results showed that hydrogen could facilitate dislocation emission, mulTiplication and motion. The change in displacement field ahead of a loaded notch Tip for a bulk specimen before and after charging with hydrogen wasin situ measured by the laser moire interferometer technique. The results showed that hydrogen could enlarge the plastic zone and increase the plastic strain. Thein situ observation in TEM showed that when hydrogen-enhanced dislocation emission and motion reached a critical condition, a nanoCrack of hydrogen-induced Cracking (HIC) would nucleate in the dislocation-free zone (DFZ) or at the Main Crack Tip. The reasons for hydrogen-enhanced dislocation emission, mulTiplication and motion, and the mechanisms of nucleation of HIC have been discussed.
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The in-situ TEM observation of microCrack nucleation in titanium aluminide
Scripta Metallurgica et Materialia, 1994Co-Authors: Yue Zhang, Wuyang Chu, Yanbin Wang, Chi-mei HsiaoAbstract:Rice and Thomson assumed that the emissions of dislocation from a Crack Tip would lead to plastic blunting of the Crack Tip, and the materials were categorized as intrinsically cleavage if a condition for decohesion of the crystal plane ahead of the Crack was reached before those for dislocation emission from the Crack. For the ductile material, the TEM in-situ observation showed that a Main Crack Tip might still be sharp after the dislocations had been emitted from the Crack Tip and the dislocation free zone (DFZ) had been formed; thus, the local stress near the Crack Tip in the DFZ was high enough to equal the cohesive strength, and then a microCrack with size of nanometers nucleated in the DFZ. The microCracks initiated in the DFZ would blunt into voids through the movement of the existing dislocation and/or those emanated from the Crack Tip and the nearby dislocation source. The discontinuous void resulting from the blunting of the microCracks in the DFZ would link with each other and with the Main Crack, resulting in ductile fracture. For brittle materials, the authors wonder whether the DFZ also exist and whether a microCrack with the size of nanometers nucleated preferably inmore » the DFZ. If such occurred, the microCrack initiated in the DFZ should propagate into a cleavage Crack rather than blunt into void. The goal of the present work was to check the possibility of dislocation emissions, DFZ formation, and initiation and propagation of a microCrack in the DFZ for the brittle material. For the titanium aluminides, e.g., Ti-24Al-11Nb or TiAl, a brittle fracture occurred by slow stable Crack growth characterized by a well-defined resistance curve, i.e., the resistance of the cleavage Crack propagation increased with Crack growth.« less
Ting-chuan Lei - One of the best experts on this subject based on the ideXlab platform.
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Mechanical properties and toughening mechanisms of SiCw/ZrO2 (6% mol Y2O3) ceramic composites
Ceramics International, 1992Co-Authors: Yu Zhou, Wei-zhong Zhu, Ting-chuan LeiAbstract:Abstract Microstructure, mechanical properties and toughening mechanisms of hot pressed SiC w /ZrO 2 (6% mol Y 2 O 3 ) ceramic composites were investigated by means of XRD, SEM, TEM and EPMA. The results show that the high density (98%) of the composites was obtained and Vickers hardness and fracture toughness increased with increasing SiC w content. With 10% wt SiC w the hardness and fracture toughness increased from 14·6 GPa and 3·7 MPa √m of the ZrO 2 (Y 2 O 3 ) matrix to 17·2 GPa and 5·5 MPa √m of the composite, respectively. TEM observations show that a glassy phase containing Zr, Y and Si was formed at interfaces between some whiskers and the matrix. SEM observations of Crack propagation revealed the following toughening mechanisms: (1) some whiskers were pulled out, bridging the two sides of the Cracks which propagated straightly; (2) some Cracks were deflected on meeting with the whiskers; (3) the whiskers fractured with matrix together and the Cracks propagated straightly; (4) the Cracks were branched and microCracks were formed in front of the Main Crack Tip.
Yanbin Wang - One of the best experts on this subject based on the ideXlab platform.
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In situ TEM research of dislocation emission and microCrack nucleation for Ti after adsorption by Hg
Corrosion Science, 1999Co-Authors: Yanbin WangAbstract:Abstract Using a special constant deflection device, the change in dislocation configuration ahead of a loaded Crack Tip for a-Ti, before and after adsorption of Hg atoms, and the initiations of Hg-induced microCracks have been observed in TEM, as well as the in situ extension in TEM without Hg. The results showed that chemisorption of Hg atoms facilitates dislocation emission and motion. When the chemisorption-enhanced dislocation emission and motion develop into a critical situation, a microCrack in LME nucleates from the Main Crack Tip and/or in the dislocation free zone (DFZ), and propagates in cleavage mode. During in situ extension in TEM without liquid metal, when dislocation emission and motion induced by thermal activity and applied stress reach a certain critical condition, microCracks nucleate but do not blunt into a void.
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The in-situ TEM observation of microCrack nucleation in titanium aluminide
Scripta Metallurgica et Materialia, 1994Co-Authors: Yue Zhang, Wuyang Chu, Yanbin Wang, Chi-mei HsiaoAbstract:Rice and Thomson assumed that the emissions of dislocation from a Crack Tip would lead to plastic blunting of the Crack Tip, and the materials were categorized as intrinsically cleavage if a condition for decohesion of the crystal plane ahead of the Crack was reached before those for dislocation emission from the Crack. For the ductile material, the TEM in-situ observation showed that a Main Crack Tip might still be sharp after the dislocations had been emitted from the Crack Tip and the dislocation free zone (DFZ) had been formed; thus, the local stress near the Crack Tip in the DFZ was high enough to equal the cohesive strength, and then a microCrack with size of nanometers nucleated in the DFZ. The microCracks initiated in the DFZ would blunt into voids through the movement of the existing dislocation and/or those emanated from the Crack Tip and the nearby dislocation source. The discontinuous void resulting from the blunting of the microCracks in the DFZ would link with each other and with the Main Crack, resulting in ductile fracture. For brittle materials, the authors wonder whether the DFZ also exist and whether a microCrack with the size of nanometers nucleated preferably inmore » the DFZ. If such occurred, the microCrack initiated in the DFZ should propagate into a cleavage Crack rather than blunt into void. The goal of the present work was to check the possibility of dislocation emissions, DFZ formation, and initiation and propagation of a microCrack in the DFZ for the brittle material. For the titanium aluminides, e.g., Ti-24Al-11Nb or TiAl, a brittle fracture occurred by slow stable Crack growth characterized by a well-defined resistance curve, i.e., the resistance of the cleavage Crack propagation increased with Crack growth.« less
Claude Guillot - One of the best experts on this subject based on the ideXlab platform.
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nano ductile Crack propagation in glasses under stress corrosion spatiotemporal evolution of damage in the vicinity of the Crack Tip
International Journal of Solids and Structures, 2005Co-Authors: Silke Prades, Daniel Bonamy, Davy Dalmas, Elisabeth Bouchaud, Claude GuillotAbstract:Recent experiments have evidenced the existence of a ductile fracture mode at the nanometer scale in Aluminosilicate glass. The present study is designed to check whether such a ductile mode is inherent to the amorphous nature of glass. Therefore, the slow Crack advance is observed in real time via an Atomic Force Microscope in a minimal glass, amorphous Silica, under stress corrosion. In this case, the Crack propagation proceeds by the nucleation, growth and coalescence of damage cavities as in the Aluminosilicate glass, but the cavity size is significantly larger. We focus here on the kinematics of Crack propagation by looking at the spatio-temporal evolution of both the Tip of the Main Crack and the cavity ahead. It is shown that the velocity of the Main Crack Tip is significantly lower than the one of the cavity edge toward the Main Crack Tip, like in metallic alloys. Moreover, the velocities of the different fronts (Main Crack, frontward and backward cavity Tips) at these nanometric scales is one order of magnitude smaller than the Crack Tip velocity at the continuum scale. This has important consequences for the modelling of stress corrosion, especially at ultra-slow Crack propagation.
Ke Yang - One of the best experts on this subject based on the ideXlab platform.
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understanding Main factors controlling high cycle fatigue Crack initiation and propagation of high strength maraging stainless steels with ti addition
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2021Co-Authors: Jialong Tian, Wei Wang, Ke Yang, Zhouhua JiangAbstract:Abstract The microstructure and high cycle fatigue (HCF) property of maraging stainless steels with Ti addition have been systematically studied. The two steels showed similar hardening responses in the aging process, which should be attributed to their identical microstructure characters, typical lath martensite immerged with dispersive precipitates Ni3Ti and R phase. It is proposed that the HCF Crack initiation mechanism depends on the value of normalized applied stress (NAS). At high NAS level (slip band control region), the yield strength dominated by nano-size precipitates is the Main factor controlling the HCF lifetime. At low NAS level (inclusion control region), the distance between TiN inclusion and the specimen surface is the Main factor controlling the HCF lifetime. TiN inclusion could sharply increase the fatigue Crack growth rate. It is found that TiN inclusion acted as the nucleation site of microCrack at front of Main Crack Tip, thus deteriorated the Crack propagation resistance of the maraging stainless steel.
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Role of microstructure on sulfide stress Cracking of oil and gas pipeline steels
Metallurgical and Materials Transactions A, 2003Co-Authors: Ming-chun Zhao, Bei Tang, Yi-yin Shan, Ke YangAbstract:Sulfide stress Cracking (SSC) behavior of three microstructures, i.e., ferritic-pearlitic microstructure, ultrafine ferrite microstructure, and acicular ferrite dominated microstructure, was investigated using the bent-beam test in aqueous hydrogen sulfide (H2S) environments. The critical stress (Sc) values of these three microstructures were determined experimentally to be 1008, 1190, and more than 1260 MPa, respectively. As a result, the acicular ferrite-dominated microstructure possessed the best SSC resistance, the ultrafine ferrite microstructure was in a second position, and the ferritic-pearlitic microstructure was relatively the worst. It was analyzed that hydrogen embrittlement (HE) was the Main failure mechanism in SSC Cracking for high-strength pipeline steels, and preferential hydrogen accumulation within the plastic zone of the Main Crack Tip accounted for the exhibited embrittlement. It was remarkable that the strength values of pipeline steels were not the only factor to determine their SSC susceptibilities. Microstructure played an important role in the SSC initiation and propagation of pipeline steels. In particular, both the fine dispersed precipitations of carbonitrides and the high-density tangled dislocations in acicular ferrite, which behaved as the hydrogen traps, should be attributed to the optimal SSC resistance of pipeline steels.
