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[en] Creep rupture properties and microstructures of an as-cast Mg-4Al-2Sn alloy with and without Sr contents (1 wt% and 2 wt%) were investigated. Creep rupture tests were performed at 150 ℃ and 200 ℃ at 50 MPa and 70 MPa. The microstructure of the Mg-4Al-2Sn alloy initially had Mg_17Al_12 and Mg_2Sn phases. The addition of Sr resulted in the formation of a ternary MgSnSr phase. Creep strength increased with an increasing Sr due to thermally stable MgSnSr precipitates, which suppressed coarsening of the Mg_17Al_12 precipitates during creep. Scanning electron microscopy showed that Mg_17Al_12 phases at the grain boundaries are the starting point of void initiation, the cracks propagated along the grain boundaries. However, the MgSnSr phase interrupted crack propagation
[en] This study analyzed the working area and procedure, legal bases, and organization of KINS for the improvement of nuclear safety review and inspection system in Korea and suggests policy issues needing improvement. And this study also analyzed the regulatory activities through the stage of design, construction, operation, and decommissioning and suggests the points of issues and improvements of regulatory activities at each stage. This study also contains the needing improvement of organization, manpower, and budget of KINS. For the establishment of efficient nuclear safety review and inspection system for ensuring nuclear safety of NPPs, KINS should improve its ability to have the confidence of the government and the nation, the government should authorize KINS to perform regulatory activities with responsibility, and all the related items should be prepared to put in practice as soon as possible
[en] Investment casting for the thin (4 mm thick) rectangular tube (40 mm wide, 80 mm high and 200 mm long) was carried out numerically and experimentally for Alloy 738, which is a precipitation-hardened Ni-base superalloy. Two types of rectangular tubes, one with a regular array (10 mm by 10 mm square array) of protruded rods (3 mm in diameter and 3mm in height) embedded on the outer surface and the other with just smooth surface, were investment-cast at the same time through the side feeding mold design. The investment casting simulation predicted the presence of cavities, particularly in the area away from the gate for both types of rectangular tubes. In particular, for the rectangular tube with embedded protruded rods cavities were found mainly in the areas between the protruded rods. This simulation result was qualitatively consistent with the experimental observation from the X-ray analysis. Also, both prediction and experiment showed that the dimensional shrinkage (particularly in the longitudinal direction) of the investment-cast rectangular tube is reduced by having protruded rods embedded on the outer surface. Additional numerical attempts were made to check how the amount of cavities and dimensional shrinkage change by varying the preheating temperature and the thickness of the mold. The results predicted that the amount of cavities and the dimensional shrinkage are significantly reduced by increasing the preheating temperature of the mold by 200 ℃. However, an increase in mold thickness from 10 mm to 12 mm showed almost no difference in cavity population and a slight decrease in dimensional shrinkage.
[en] Highlights: ► The volume of pores with diameters ranging from 50 nm to 500 nm is increased. ► The Pores larger than 200 nm in size increase in number during the initial time. ► The residual strength of the leached part with OPC ranges from 35% to 60%. ► The residual strength of the mineral admixture replacement ranges from 23% to 50%. ► The chloride diffusion coefficient of leached concrete is increased 2–5 times. -- Abstract: In radioactive waste repositories constructed underground and on shorelines, concrete members can remain in contact with groundwater for a long period. However, even pure water creates concentration gradients which lead to the diffusion of Ca ions from the pore water and the degradation of the underground concrete. Therefore, the purposes of this study are to investigate not only the alteration of the pore structure and the loss of compressive strength associated with dissolution but also the characteristics of chloride penetration after leaching-related degradation. The results show that as the leaching period increases, the volume of pores with diameters ranging from 50 nm to 500 nm is greatly increased. Also, pores larger than 200 nm in size rapidly increase in number during the initial leaching time, while those smaller than 200 nm increase in number only gradually. Furthermore, the residual strength of the leached part with OPC ranges from 35% to 60%. In addition, those of the mineral admixture replacement ranged from 23% to 50%. The chloride diffusion coefficient measured by the chloride profile increased two-to-five-times with the leaching duration
[en] An in-situ transmission electron microscopy study was conducted at room temperature in order to understand an underlying mechanism on room temperature ductility of TiAl alloys. From in-situ straining transmission electron microscopy experiments, it was revealed that the crack path is different between the TiAl alloys with/without room temperature ductility. The crack in TiAl alloys having room temperature ductility interacted with lamellae by forming bridging ligaments between the two α2 lamellae and the γ lamellae. In contrast, the cracks in TiAl alloys without room temperature ductility propagated along grain (colony) boundaries showing brittle intergranular fracture. Finally, we proposed the important microstructural factors to have room temperature ductility of TiAl alloys
[en] Highlights: • CRT waste glass was recycled as fine aggregate for heavyweight filling material. • The durability of the filling material is better than in normal mortar. • The linear attenuation coefficient of the filling material increased by 2.56 times. • The filling material have good potential for use in radioactive disposal facilities. - Abstract: Environmental problems caused by municipal and industrial waste are serious worldwide. In particular, landfill of recyclable industrial waste due to a lack of technology is a serious loss of resources. Heavyweight waste glass is an industrial waste that has been studied by many researchers for use as a fine aggregate. Until now, the heavy metal is removed through complex pretreatment. However, in this study, the waste glass was recycled through a simple crushing process without pretreatment. We investigated the fundamental properties, durability, and shielding property of filling material containing waste glass. The test results show that the durability and shielding property were all better than in ordinary mortar. Also, the reduction in strengths, and the expansion due to ASR, can be solved by using a mineral admixture. In conclusion, mortar containing a mineral admixture and waste glass is considered to be viable as filling material in radioactive waste disposal facilities.
[en] We compared deformation twinning activity and twin structure development of pure Ti at cryogenic temperature and room temperature by conducting unidirectional rolling at 77 K and 293 K. Twinning activity was significantly higher in rolling at 77 K than in rolling at 293 K. This was because lowering the deformation temperature increased the necessity for twinning operation to compensate for reduced accommodation of strain along the c-axis of the crystal structure, caused by inhibition of c+a> slips. Twin structure was also entirely different between the two rolling temperatures. Compared to the case of rolling at 293 K, rolling at 77 K generated thin and numerous individual twins in the twinned grains, thereby giving rise to the development of a totally different twin structure. The differences in twin structure were caused by the combined effect of significant local stress concentration at grain boundaries and a reduction in stacking fault energy by rolling at 77 K. The twin structure strongly contributed to severe twinning-induced grain refinement that occurs at cryogenic temperature.
[en] Highlights: • The properties of mortar used heavy weight waste glass as fine aggregate were compared. • Unit volume weight and shielding performance increased with the content of waste glass. • However, the strength decreased as the waste glass substitution increased. • The waste glass substitution affected on pores ranging from 10–100 nm. - Abstract: The quantities of heavy weight waste glass have increased over time due to rapid industrialization and changes in the quality of life. Moreover, most of this waste is not recycled. Concrete is the most widely used construction material, the huge amounts of natural resources are required to make concrete. Therefore, it is necessary to investigate the possibility of recycling of heavy weight waste glass as an ingredient in the manufacturing of concrete. In this study, the suitability of heavy weight waste glass as a fine aggregate material is considered. The results of flow test, unit volume weight, radiation shielding performance, compressive strength, flexural strength, and micropore and macropore distribution of mortar are compared and evaluated. It was found that when the heavy weight waste glass substitution ratio increases, the fluidity, unit volume weight and radiation shielding performance also increase. However, the compressive and flexural strength of mortar gradually decrease with an increase in the substitution ratio of heavy weight waste glass. Moreover, the micro pore size distribution is significantly affected by the substitution of heavy weight waste glass.
[en] Highlights: • Stress relaxation after aging 620 °C increased carbides and maintained γ′ fraction. • Aging temperature increase to 732 °C raised the γ′ increment after stress relaxation. • Small increase of carbides induced the large increase of γ′ after stress relaxation. • Loading for stress relaxation raised γ′ increment due to dislocation multiplication. - Abstract: Inconel X-750 is a Ni-based precipitation-hardened superalloy typically used in springs designed for high-temperature applications such as the hold-down springs in nuclear power plants. γ′ is a major precipitate in X-750 alloys which affects the strength, creep resistance, and stress relaxation properties of the spring. In this study, a solution-treated X-750 wire coiled into a spring was used that was aged at various temperatures and submitted to stress relaxation tests with and without loading. Small angle neutron scattering was employed to quantify the size and volume fraction of γ′ phase in the springs as a function of the aging temperature and the application of a load during stress relaxation. The volume fraction of γ′ precipitates increased in the specimen aged at 732 °C following stress relaxation at 500 °C for 300 h. However, the mean size of the precipitates in the samples was not affected by stress relaxation. The specimen aged at the lower temperature (620 °C) contained a smaller γ′ volume fraction and gained a smaller fraction of γ′ during stress relaxation compared with the sample aged at the higher temperature (732 °C). The smaller increase in the γ′ volume fraction for the sample aged at 620 °C was associated with a larger increase in the M23C6 secondary carbide content during relaxation. The Cr depletion zone around the secondary carbides raises the solubility of γ′ thereby decreasing the volume fraction of γ′ precipitates in Inconel X-750. In terms of stress relaxation, a larger increase in the γ′ volume fraction was measured with loading rather than without. This is probably associated with the dislocation accumulation generated under loading that facilitate the nucleation and growth of heterogeneous γ′ phase due to enhanced diffusion
[en] Inconel X-750 is a Ni-based precipitation-hardened superalloy having large tensile and fracture strengths. In the study, X-750 wires were cold drawn to different extents. Small angle neutron scattering was employed to quantitatively measure the size and volume fraction of the γ′ phase as a function of the cold drawing ratio (DR) and aging temperature. The presence and size of γ′ precipitates were confirmed by transmission electron microscopy. The drawing ratio had an important effect on the volume fraction of the γ′ precipitates. However, the size of the precipitates was independent on the drawing ratio. The specimen with the minimum drawing ratio (DR0) produced the largest volume fraction of γ′ as compared with large drawing ratio (DR) specimens such as DR17 and DR42. The small volume fraction of the γ′ phase for a sizeable drawing ratio was associated with the large amount of nucleation sites for secondary carbides, M23C6, and the fast diffusion path, i.e., dislocation, needed to form M23C6. A Cr depletion zone around the secondary carbides raised the solubility of γ′. Therefore, the significant drawing ratio contributing to the large volume fraction of the secondary carbides decreased the volume fraction of the γ′ precipitates in Inconel X-750. - Highlights: • The volume fraction of secondary carbides increased with the drawing ratio. • The volume fraction of γ′ decreased as the drawing ratio increased. • The drawing ratio affected the γ′ volume fraction with no variation of the γ' size. • The volume fraction of γ′ was affected by the secondary carbide volume fraction