Papers by Author: Wen Ru Sun

Abstract: The precipitating behavior of δ phase in GH4169G alloy at 910°C was investigated and a novel process for achieving fine-grained GH4169G sheet with average grain size of ASTM13 was proposed. Globular δ phase precipitation precedes that of the plate-like δ phase, forming at grain boundary first and later in grain. After globular δ phase reaches a peak volume, it begins to transform into plate-like in grain boundary, and then the plate-like δ phase forms in grain in certain direction. The grain-fined process includes the following steps:(1) solution treated at a proper temperature, then air cooled; (2) soaked for δ phase precipitation; (3) cold rolled; (4) annealed for recrystallization. Microstructure analysis from optical microscopy (OM) and scanning electron microscopy (SEM) revealed that both plate-like and globular δ phase remarkably restrained the grain growth during annealing recrystallization. The optimum parameters for the grain refinement were determined finally.

Abstract: Thermo-Span alloy is an oxidation resistant, low thermal expansion superalloy with good mechanical properties at the service temperature. This paper investigated the hot working behavior of casting Thermo-Span alloy deformed at 900~1150°C, with strains of 20%, 40% and 60% at strain rates of 1 and 10 s^-1. Dynamic recrystallization (DRX) grains were formed at 1110°C with a strain of 20%, at 1050°C with a strain of 40%. Increasing the deformation rate and strain can promote the DRX. However, when the strain exceeded 60% or the deformation temperature was above 1150°C with 40% strain, the surface cracks occurred, indicating that the alloy should be deformed in one heat no larger than 60%. By forging and rolling at 1050°C, Thermo-Span alloy with good surface quality and homogeneous grains was produced, and the tensile properties were still acceptable.

Abstract: The microstructure of as-cast 718 alloy after modified with Mo has been investigated by means of optical microscope (OM), scanning electron microscope (SEM) and X-ray diffraction (XRD). It was found that the dendrite arm space was widened and the interdendritic area was reduced by increasing Mo addition. And no new phase was precipitated in the modified alloys by Mo addition in the range of the present test. The addition of Mo promoted the precipitated of Laves phase, restrained the formation of δ phase, and elevated melting point of the Laves phase.

Abstract: In this paper, uniaxial compression testsfor GH4145 superalloywere conducted on Gleeble-3800 at room temperature and 60°C with strain rates ranging from 0.5s^-1 to 10s^-1. No cracks were observed in all the specimens deformed from 10% to 70%, indicating that the GH4145 superalloy possesses great ductility under room temperature. The flow stress of GH4145 superalloywas not sensitive to strain rate deformed around room temperature and it decreased slightly with the temperature increasing. Dynamic recovery during deformation will start to balance the effect of work hardening when the true strain was arisen up to 0.4. Good agreement between the calculated and simulated temperature and strain shows that calculation method is accurate enough for the prediction.

Abstract: By means of creep properties measurement and microstructure observation, the influence of the traces P, B on creep behaviors and fracture mechanism of GH4169 alloy was investigated. Results showed that during creep the twinning was thought to be the main deformation mechanism of GH4169 alloy, however the deformation mechanisms of alloy containing P, B were the twinning and slipping of dislocations activated within the grains. And the fact that the slipping of dislocations activated within the grain can delay the stress concentration in the local regions to restrain the initiation of the cracks to improve the creep resistance of the alloy. Compared to GH4169 alloy, the fracture of GH4169G alloy displayed the non-smooth surface. The adding traces P, B were segregated in the region near the boundary, which may promote the particle-like δ phase precipitated along the boundary to restrain the boundaries slipping and the cracks propagation. This was thought to be the main reason for enhancing the strength of the boundary and prolonging the creep life.

Abstract: The hot deformation behavior and microstructure of rolled Inconel 625 alloy have been studied from 930°C to 1180°C, and at strain rate from 10 s-1 to 80 s-1, respectively. The results indicate that, as deformation temperature rises, both peak flow stress (PFS) and recrystallization critical strain (RCS) decrease; as the strain rate increases, the PFS is enhanced, but the RCS drops. When the deformation temperature is within 1100°C and 1180°C, the grain size coarsens markedly with the temperature increasing. When the deformation temperature is lower than 1100°C，a higher strain rate is helpful for grain refinement. However, when the temperature is beyond 1100°C,the effect of strain rate on the grain size is reduced.

Abstract: The effects of P on element segregation, Laves phase dissolution and elemental diffusion in GH706 alloy have been investigated. The results show that P promotes the segregation of Nb and Ti. Phosphorus was enriched in Laves phase, which enhanced the precipitation of blocky Laves phase, while constrained the formation of eutectic Laves phase. Moreover, more needle-like phase was formed around blocky Laves phase in high P-doped alloy. It also demonstrated that P slightly lowered the melting-point of Laves phase. During the homogenization process, P hindered the diffusion of Nb which blocks the diffusion of Nb and dissolution of Laves phase, and complicated the homogenization treatment.

Abstract: The effect of phosphorus in some typical wrought superalloys including IN718, GH761 and GH4133 has been studied. Phosphorus was not dissolved in the γ matrix of superalloys and was segregated in the residual liquids seriously during solidification, and the solidification of the alloys added with phosphorus was always finished by the precipitation of a phosphorus bearing phase. Two factors controlling the segregation tendency of a minor element were determined, one is the solubility of the element in the matrix, another is the precipitation temperature of a phase in which the element is absorbed. The detrimental effect of phosphorus on the solidification could be eliminated by a proper homogenization treatment. Phosphorus improved the stress rupture properties of the alloys with a fine grain structure. By combining the grain refinement with phosphorus addition, the properties of GH761 alloy was improved efficiently.