Guang Xie

Partial recrystallization in the nugget zone of friction stir welded dual-phase Cu–Zn alloy

For single and quasi-single phase metallic materials, complete dynamic recrystallization has been observed in the nugget zones (NZs) of friction stir welds (FSWs), producing fine and uniform equiaxed grains. In this study, partial dynamic recrystallization was observed in the NZ of 5 mm thick friction stir welded brass plates. Whereas the top and the advancing side of the NZ were characterized by fine completely-recrystallized grains, the remaining region consisted of coarse non-recrystallized deformed grains, annealed recrystallized grains and deformed recrystallized grains. The occurrence of partial recrystallization was attributed to the inhibiting effect of a high volume fraction of fine β′-phase particles. Increasing the FSW passes reduced the fraction and size of non-recrystallized deformed grains, but could not eliminate the partially recrystallized zone completely.

The Effect of Long-Term Thermal Exposure on the Microstructure and Stress Rupture Property of a Directionally Solidified Ni-Based Superalloy

Microstructural degradation and microstructure-property relationship during long-term thermal exposure in a directionally solidified Ni-based superalloy are systematically studied. The coarsening kinetics of γ′ precipitation conforms well to the LSW model during the long-term thermal exposure. The detailed time dependence of MC decomposition during the long-term thermal exposure is revealed. Grain boundary coarsening was mainly facilitated by γ′ and M23C6 precipitates coarsening in GBs region, and the GB coarsening kinetics conforms well to the JMAK theory. During different stages of the thermal exposure, dominant factors for the decrease of stress-rupture lifetime vary due to the evolution of multiple microstructures (γ′ coarsening, MC decomposition, and grain boundary coarsening).

Effect of cyclic recovery heat treatment on surface recrystallization of a directionally solidified superalloy

Microstructural evolution and micro-hardness of a directionally solidified Ni-base superalloy subjected to shot-peening during cyclic recovery heat treatment was studied. It was found that gamma dissolved and dislocation annihilation occurred during heating, while gamma re-precipitated when temperature dropped. Due to the formation of a stable dislocation network at gamma/gamma interface, full recovery and therefore complete elimination of the surface recrystallization are difficult by the present cyclic recovery heat treatment.

Precipitate Free Zones along Recrystallization Grain Boundaries during Creep in a Directionally Solidified Superalloy

Microstructural evolution of an experimental directionally solidified superalloy with local recrystallization during creep was studied. Precipitate free zones (PFZs) were formed along the recrystallization grain boundaries that are perpendicular to the applied stress. The formation mechanisms of PFZs and creep cracks associated with PFZs were discussed based on the experimental observation.

Microstructural Degradation and the Effects on Creep Properties of a Hot Corrosion-Resistant Single-Crystal Ni-Based Superalloy During Long-Term Thermal Exposure

In the present study, the kinetics of microstructural degradation during long-term thermal exposure (LTTE) and the effects on creep deformation mechanisms of a hot corrosion-resistant single-crystal Ni-based superalloy with a low γ′ volume fraction and γ/γ′ lattice misfit were investigated in detail. The kinetic of γ′ coarsening in the experimental alloy conforms well to the Lifshitz–Slyozov–Wagner theory during LTTE at 900xa0°C up to 10,000xa0hours. The evolution of γ/γ′ lattice misfit during the LTTE was also investigated by a first attempt. The focused research emphasized on the influences of γ/γ′ lattice misfit evolution after the LTTE on the microstructural degradation, dislocation motion, and different creep mechanisms during high-temperature low-stress creep and high-temperature high-stress creep. The results show that the decreasing of the absolute values of γ/γ′ lattice misfit and change of γ′ size and morphology after the LTTE contribute to the weakening of barrier to the dislocation cutting process into γ′ precipitates during creep and the sharp reduction of stress-rupture lifetime at 950xa0°C/280xa0MPa after 1000xa0hours exposure. As the applied stress decreased to 230xa0MPa at 950xa0°C, the creep mechanisms change from the dislocation cutting through γ′ precipitates at high applied stress to the dislocation glide and climb around γ′ precipitates. The dislocation glide and climb by-pass deformation mechanism were not significantly influenced by the change of γ′ precipitates morphology and magnitude of γ/γ′ mismatch within 1000xa0hours thermal exposure, and the minimum creep rate and creep lifetime after 1000xa0hours thermal exposure were similar to that of the original heat-treated samples.

EFFECT OF Re AND W ON RECRYSTALLIZATION OF AS-CAST Ni-BASED SINGLE CRYSTAL SUPERALLOYS

Ni-based single crystal (SX) superalloys have been used as blades in aero-space industry and landbased applications due to their excellent high-temperature properties. However, residual strain is introduced into ascast SX superalloy blades during the manufacturing process, such as casting, grinding or shot peening, and so on. Recrystallization (RX) occurs easily during subsequent high temperature heat treatment. In previous work, it is believed that RX has detrimental effect on the mechanical properties of SX superalloy. Furthermore, in order to improve the mechanical properties, more and more refractory elements, such as W, Re, Mo, Ta, are added into SX *国家自然科学基金项目50901079,国家重点基础研究发展计划项目2010CB631201及国家高技术研究发展计划项目2012AA03A513资助 收到初稿日期: 2015-08-26,收到修改稿日期: 2015-10-30 作者简介:濮 晟,男, 1981年生,工程师 DOI: 10.11900/0412.1961.2015.00456

Effect of Recrystallization on Crack Initiation and Propagation during Creep in a Directionally Solidified Ni-Based Superalloy

Crack initiation and propagation during creep test in a recrystallization (RX) grain containing directionally solidified (DS) nickel-base superalloy was studied by employing electronic backscattered diffraction (EBSD) technique. Creep samples were locally deformed and then heat treated to get local RX at the center of the gauge length. Creep test was carried out at 980 °C/235 MPa. It was found that during creep test, precipitate free zones (PFZ) were formed along the RX grain boundary and the cracks were initiated and propagated along the interface (a) between PFZ and RX grains, (b) between PFZ and the matrix or (c) within PFZ. It indicated that the crack initiation and the propagation were crystal orientation dependent.