The Experts below are selected from a list of 5940 Experts worldwide ranked by ideXlab platform
Masaaki Tabuchi - One of the best experts on this subject based on the ideXlab platform.
-
interfacial failure in dissimilar weld joint of high boron 9 chromium steel and nickel based alloy under high temperature Creep Condition
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2017Co-Authors: Tetsuya Matsunaga, Hiromichi Hongo, Masaaki TabuchiAbstract:Abstract The advanced ultra-supercritical (A-USC) power generation system is expected to become the next-generation base-load power station in Japan. Dissimilar weld joints between high-Cr heat-resistant steels and nickel-based alloys with a nickel-based filler metal (Alloy 82) will need to be adopted for this purpose. However, interfacial failure between the steels and weld metal has been observed under high-temperature Creep Conditions. Fractography and microstructure observations showed the failure initiated in a brittle manner by an oxide notch at the bottom of the U-groove. The fracture then proceeded along the bond line in a ductile manner with shallow dimples, where micro-Vickers hardness tests showed remarkable softening in the steel next to the bond line. In addition, the steel showed a much larger total elongation and reduction of area than the weld metal at low stresses under long-term Creep Conditions, leading to mismatch deformation at the interface. According to the results, it can be concluded that the interfacial failure between the 9Cr steels and Alloy 82 weld metal is initiated by an oxide notch and promoted by softening and the difference in the plasticity of the steels and weld metal.
-
strain rate sensitivity enhanced by grain boundary sliding in Creep Condition for az31 magnesium alloy at room temperature
Materials Science Forum, 2016Co-Authors: Tetsuya Matsunaga, Hidetoshi Somekawa, Hiromichi Hongo, Masaaki TabuchiAbstract:This study investigated strain-rate sensitivity (SRS) in an as-extruded AZ31 magnesium (Mg) alloy with grain size of about 10 mm. Although the alloy shows negligible SRS at strain rates of >10-5 s-1 at room temperature, the exponent increased by one order from 0.008 to 0.06 with decrease of the strain rate down to 10-8 s-1. The activation volume (V) was evaluated as approximately 100b3 at high strain rates and as about 15b3 at low strain rates (where b is the Burgers vector). In addition, deformation twin was observed only at high strain rates. Because the twin nucleates at the grain boundary, stress concentration is necessary to be accommodated by dislocation absorption into the grain boundary at low strain rates. Extrinsic grain boundary dislocations move and engender grain boundary sliding (GBS) with low thermal assistance. Therefore, GBS enhances and engenders SRS in AZ31 Mg alloy at room temperature.
Zhuangqi Hu - One of the best experts on this subject based on the ideXlab platform.
-
effect of mo concentration on Creep properties of a single crystal nickel base superalloy
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2010Co-Authors: Jinguo Li, Zhuangqi HuAbstract:The effect of Mo concentration on Creep properties of a single crystal nickel-base superalloy has been studied at 1010 degrees C and 800 degrees C by using three kinds of alloys with 1%, 2% and 4% of Mo (wt.). The Creep properties of alloys were measured and microstructure evolution was analyzed. The results showed that after heat treatment, all the three alloys were composed of gamma and gamma' phase without TCP phase precipitation. The increase of Mo concentration from 1 wt.% to 2 wt.% pronouncedly enhanced the Creep properties. Compared with 1 wt.% Mo content, 2 wt.% Mo addition can generate denser gamma/gamma' interfacial dislocation networks during high temperature Creep test, thus lower minimum Creep rate. At medium temperature Creep Condition, Mo addition enhanced Creep properties by decreasing the stacking fault energy. When the Mo content reached to 4%, the over-saturation of Mo content in gamma phase led to the formation of p. phase precipitation which degraded the Creep properties at both high and medium temperature Creep Conditions. (C) 2010 Elsevier B.V. All rights reserved.
Toshiyuki Koyama - One of the best experts on this subject based on the ideXlab platform.
-
origin of the morphological change from rafted structure to irregular shape of the γ phase in single crystal nickel based superalloys
Computational Materials Science, 2014Co-Authors: T Tanimoto, Md Moniruzzaman, Yoshinori Murata, Nobuhiro Miura, Yoshihiro Kondo, Yuhki Tsukada, Toshiyuki KoyamaAbstract:Abstract Microstructural evolution in single crystal Ni-based superalloys is investigated by the phase field simulation. During Creep of the alloys, the morphology of the γ ′ phase changes from the cuboidal shape to the rafted one and the rafted structure is collapsed in the later stage of Creep. It is understood that rafting is caused by the strain energy resulting from γ / γ ′ coherent interface. In this study, we focused on the decrease of interfacial coherency in Creep Condition and investigated the dependence of morphological evolution on it. Creep strain, misfit strain and gradient energy coefficient are changed with time in a two dimensional (2D) phase field simulation to express the γ / γ ′ interfacial transition from coherent to semi-coherent in Creep Condition. The semi-coherent interface model recreates the two types of microstructural evolution of the rafted structure to collapse observed in CMSX-4 and NKH71 superalloys. Furthermore, Creep strain rate, instantaneous strain and time of starting the interfacial transition from coherent state to semi-coherent state decide substantially the morphology in the later stage of Creep. These morphological evolutions can be explained by the relationship between the amount of Creep strain and degree of coherency.
Yongdian Han - One of the best experts on this subject based on the ideXlab platform.
-
theoretical and numerical analysis of the Creep crack initiation time considering the constraint effects for pressurized pipelines with axial surface cracks
International Journal of Mechanical Sciences, 2018Co-Authors: Yongdian HanAbstract:Abstract A theoretical approach and numerical simulation were conducted to investigate the Creep crack initiation (CCI) time and the effect of constraints induced by the geometrical sizes of pipelines with axial surface cracks. The theoretical enhancement model of the C*–Q* approach under the transient Creep Condition, which considers the load-independent constraint parameter Q*, was proposed to predict the CCI time around the crack front. The results revealed that the distribution regulation of Q* along the crack front for circumferential internal surface cracks and external surface cracks was similar. The maximum constraint level occurred near the deepest crack front part for cracks with small a/c (a/c 0.4). The constraint values at the same position (2Φ/π) increased with the increasing of the crack depth when a/c kept constant. In addition, the axial internal surface cracks of pipelines were proved more dangerous than the external surface cracks with the same geometrical size. Furthermore, the CCI times were decided by the peak values of constraint, or the CCI firstly occurred at the position where the constraint level was maximum. Additionally, the variation of hydrostatic stresses and triaxiality considering the constraint was discussed. The suitability of the analytical C*–Q* approach was verified to predict CCI. The comparison of CCI times between the analytical approach and the BS 7910 as well as the FE results demonstrated that the solutions under stress intensity factor—Riedel–Rice (K-RR) control (initially by K, then by transient Creep stress or Riedel–Rice Conditions) were more accurate when internal pressure P 15 MPa.
Xiaofeng Sun - One of the best experts on this subject based on the ideXlab platform.
-
tensile Creep behavior and microstructure evolution of an as cast ni based k417g polycrystalline superalloy
Journal of Materials Science & Technology, 2018Co-Authors: Liyuan Sheng, Chuanyong Cui, Jinxia Yang, Yufeng Zheng, Xiaofeng SunAbstract:Abstract The Ni-based K417G superalloy is extensively applied as aeroengine components for its low cost and good mid-temperature (600–900 °C) properties. Since used in as-cast state, the comprehensive understanding on its mechanical properties and microstructure evolution is necessary. In the present research, the tensile, Creep behavior and microstructure evolution of the as-cast K417G superalloy under different Conditions were investigated. The results exhibit that tensile cracks tend to initiate at MC carbide and γ/γ′ eutectic structure and then propagate along grain boundary. As the temperature for tensile tests increases from 21 °C to 700 °C, the yield strength and ultimate tensile strength of K417G superalloy decreases slightly, while the elongation to failure decreases greatly because of the intermediate temperature embrittlement. When the temperature rises to 900 °C, the yield strength and ultimate tensile strength would decrease significantly. The Creep deformation mechanism varies under different testing Conditions. At 760 °C/645 MPa, the Creep cracks initiate at MC carbides and γ/γ′ eutectic structures, and propagate transgranularly. While at 900 °C/315 MPa and 950 °C/235 MPa, the Creep cracks initiate at grain boundary and propagate intergranularly. As the Creep Condition changes from 760 °C/645 MPa to 900 °C/315 MPa and 950 °C/235 MPa, the γ′ phase starts to raft, which reduces the Creep deformation resistance and increases the steady-state deformation rate.