Papers by Author: Jin Guo Li

Abstract: This paper studies the coupled effects of alloy elements for precipitation strengthening to rupture life of single crystal Ni-based superalloys. Through setting up the Radial Basis Function model to predict rupture life, the generalization ability test indicated that this model has better precision and practicability. According to this model, utilizing the theory of parameter sensitivity analysis to quantitative analysis the interactive effects of different alloy elements and/or groups to rupture life, providing the theory basis and reference for the alloy design and productivity of single crystal Ni-based superalloys.

Abstract: A single crystal Ni-base superalloy was prepared with five withdrawal rates: 2, 4, 6, 7 and 10 mm/min. Microstructures including dendrite arm spacing, element segregation, and porosity of the as-cast superalloy were investigated. The results showed that as the withdrawal rate increasing, the primary and secondary dendrite arm spacing decreased markedly, the γ/γ′ eutectic became smaller and more dispersive. Meanwhile, when withdrawal rate was higher, W, Ti, Ta and Al segregated in comparatively larger extent. Furthermore, as the withdrawal rate increasing, the amount of alloy microporosity increased, though the size of which decreased gradually. It can be concluded that the withdrawal rate of 4 mm/min and 6 mm/min are optimal for the experimental alloy.

Abstract: SUS 304 austenitic stainless steel was processed by HPT at room temperature with different rotation speed. It was found that the microstructure evolution and composed phases along the progress of HPT were sensitive to the strain rate (rotation speed). During deforming with the low strain rate, the deformation-induced dynamic phase transformation (DPT) from austenite (γ) to martensite (α’) occurred and the microstructure is characterized by elongated submicron α’ grains after 10 revolutions. While the euqiaxed nanocrystalline α’ grains were produced after HPT at the continuously alternative low and high strain rate. XRD analyses showed that multiple DPT of γ→α’→γ→α’ took place during HPT at the continuously alternative low and high rotation speed. Based on the experimental results, it was proposed that the euqiaxed ultrafine grained structure were produced by multiple DPT under the high strain and strain gradient.

Abstract: Formation of nanocrystalline structure by severe plastic deformation has studied extensively. Although ultra fine grained structure (grain size larger than 100 nm) had been obtained in many processes such as heavy cold rolling, equal channel angular pressing (ECAP) or accumulative roll bonding (ARB), the formation of nano grained structure (< 100 nm) is limited to processes such as ball milling, shot peening or drilling. In the present study, high pressure torsion (HPT) deformation and drilling were carried out to understand the conditions necessary to obtain nano grained structure in steels. The results of HPT experiments in pure Fe showed that HPT has superior ability of strengthening and grain refinement probably due to a strain gradient but the saturation of grain refinement occurs before reaching nano grained structure. Drilling experiments in high carbon martensitic steel revelaed that nano grained ferrite forms at the drilled hole surface only when the transformation from ferrite to austenite takes place during drilling. Considering various other processes by which nano grained ferrite was produced, it is proposed that heavy strains with large strain gradients together with dynamic transformation are necessary to reach nano grained ferrite structure.

Abstract: The formation of nanocrystalline structure in steels by ball milling, shot peening and drilling were studied. In ball milling and shot peening, nanocrystalline layers form with sharp boundaries from deformed structure regions. Nanocrystalline layer showed extremely high hardness. By annealing, nanocrystalline layer showed substantially slow grain growth without recrystallization. The temperature of the specimen during deformation is low and deformation is done in ferrite state. In drilling, several μm thick nanocrystalline layers form at the top surface of a drill hole. Nanocrystalline layers showed high hardness and good thermal stability. The fresh martensite and retained austenite near a drill hole indicate that the temperature reached above Ac3 and nanocrystalline layers are produced in austenite condition. It is recognized that nanocrystalline layers produced in the processes studied in the present investigation has similar characteristics irrespective of the temperature it produced. It is proposed that deformation with a large strain gradient is an important condition to produce nanocrystalline structure.