Peritectic reaction was studied by directional solidification of Cu-Ge alloys.A larger triple junction region of peritectic reaction was used to analyze the interface stability of the triple junction region during peritectic reaction.Under different growth conditions and compositions,different growth morphologies of triple junction region are presented.For the hypoperitectic Cu-13.5%Ge alloy,as the pulling velocity（v） increases from 2 to 5 μm/s,the morphological instability of the peritectic phase occurs during the peritectic reaction and the remelting interface of the primary phase is relatively stable.However,for the hyperperitectic Cu-15.6%Ge alloy wim v=5 μm/s,the nonplanar remelting interface near the trijunction is presented.The morphological stabilities of the solidifying peritectic phase and the remelting primary phase are analyzed in terms of the constitutional undercooling criterion.
Elevated temperature creep behaviors at 1100℃ over a wide stress regime of 120-174 MPa of a thirdgeneration Ni-based single crystal superalloy were studied. With a reduced stress from 174 to 120 MPa, the creep life increased by a factor of 10.5, from 87 h to 907 h, presenting a strong stress dependence. A splitting phenomenon of the close-(about 100 nm) and sparse-(above 120 nm) spaced dislocation networks became more obvious with increasing stress. Simultaneously, ao<010> superdislocations with low mobil让ies were frequently observed under a lower stress to pass through γ precipitates by a combined slip and climb of two ao<110> superpartials or pure climb. However, ao<110> superdislocations with higher mobility were widely found under a higher stress, which directly sheared into y precipitates. Based on the calculated critical resolved shear stresses for various creep mechanisms, the favorable creep mechanism was systematically analyzed. Furthermore, combined with the microstructural evolutions during different creep stages, the dominant creep mechanism changed from the dislocation climbing to Orowan looping and precipitates shearing under a stress regime of 137-174MPa, while the dislocation dim bing mechanism was operative throughout the whole creep stage un der a stress of 120 MPa, resulting a superior creep performanee.
Quanzhao YueLin LiuWenchao YangChuang HeDejian SunTaiwen HuangJun ZhangHengzhi Fu
To clarify the solidification behavior of Re- and Ru-containing Ni-based single-crystal superalloys, four experimental alloys with varied contents of Re and Ru were investigated by differential scanning calorimetry （DSC） and metallographic techniques. To obtain the - solvus temperatures, the stepwise solution and aging heat treatments were used. DSC analysis shows that Re leads to the increase in freezing range and γ-solvus temperature. On the contrast, Ru only has negligible influence on the freezing range, but leads to the lower γ-solvus temperature. In comparison with Ru, Re leads to more severe segregation and higher eutectic fractions in as-cast microstructures. Furthermore, the castability and phase stability of Ni-based superalloys were analyzed by the results of DSC and metallographic analysis, such as freezing range, critical nucleation temperature, γ-solvus temperature and eutectic fractions. It shows that Re leads to the wider freezing range and lower critical nucleation temperature, indicating the worse castability of Re-con- taining Ni-based single-crystal superalloys.
High temperature stress rupture anisotropies of a second generation Ni-base single crystal（SC） superalloy specimens with [001], [011] and [111] orientations under 900 ℃/445 MPa and 1100 ℃/100 MPa have been investigated in the present study, with attentions to the evolution of γ/γ′ microstructure observed by scanning electron microscopy and the dislocation configuration characterized by transmission electron microscopy in each oriented specimen. At 1100 ℃/100 MPa as well as 900 ℃/445 MPa, the single crystal superalloy exhibits obvious stress rupture anisotropic behavior. The [001] oriented specimen has the longest rupture lifetime at 900 ℃/445 MPa, and the [111] oriented sample shows the best rupture strength at 1100 ℃/100 MPa. While the [011] oriented specimen presents the worst rupture lifetime at each testing condition, its stress rupture property at 1100 ℃/100 MPa is clearly improved, compared with900 ℃/445 MPa. The evident stress rupture anisotropy at 900 ℃/445 MPa is mainly attributed to the distinctive movement way of dislocations in each oriented sample. Whereas, at 1100 ℃/100 MPa, together with the individual dislocation configuration, the evolution of γ/γ′ microstructure in each orientation also plays a key role in the apparent stress rupture anisotropy.
Guanglei WangJinlai LiuJide LiuTao JinXiaofeng SunXudong SunZhuangqi Hu
The microstructure, size, elemental composition and hardness of the surface eutectic layer formed during directional solidification of a Ni-based single crystal superalloy were studied. The formation mechanisms of the surface eutectic on the outer surface of the casting were also discussed. The metal/mould interactions did not play any role in the formation of the surface eutectic. The formation cause of surface eutectic layer was attributed to the interdendritic residual liquid for excretion caused by solidi？cation shrinkage.
The grain boundary microstructures of a heat-treated Ni-based cast superalloy IN792 were investigated. The results show that M5B3 boride precipitates at the grain boundary. A special orientation relationship between M5B3 phase and the matrix at one side of the grain boundary is found. At the same time, two M5B3 borides with different orientations could co-exist in a single M5B3 particle as an intergrowth besides existing alone, thus forming orientation relationship between the two M5B3 phases and matrix. This phenomenon could be attributed to the special orientation relationship between M5B3 phase and the matrix.
