| Superalloys have been widely applied in aerospace field attributing to their excellent mechanical properties at high temperature. According to fundamentals of materials science, the mechanical performances of a material are greatly influenced by its microstructure, chemical composition, processing and so on.To design a material with better mechanical properties, it is necessary to understand its deformation mechanisms in detail. In this paper, two kinds of poly crystalline superalloys which are designed for turbine disk materials were chosen as the materials to be investigated. With transmission electron microscopy, the influence of γ’ precipitate size and temperature on the deformation mechanism of the alloys, respectively.1. A cobalt-rich nickel based superalloy with approximately 15% γ’(volume fraction) were studied. By changing the aging time, a series of specimen with different γ’ precipitate sizes were obtained. The specimen were deformed to fractured with a tensile strain rate of 3*10-4s-1 at 500℃. By investigating the microstructures after tensile deformation by transmission electron microscopy, we made the following conclusions:(1) Along with the increase of aging time, the mean size of γ’ precipitates increased as well, with a decreasing increasing rate. The chemical composition of the γ’ precipitate doesn’t change much. (2) With the increase of precipitate size, the dominant deformation mechanism firstly changes from anti-phase boundary shearing to stacking fault shearing of γ’ precipitates, and then with further increase of precipitate size, the Orowan bypassing mechanism becomes the main deformation mode. The mean sizes corresponding to these two transition are 15nm and 25nm, respectively. (3) In specimen which has high density of stacking faults, deformation microtwins were also observed. According to their distribution features, it is judged that they are derived from stacking faults.2. A nickel-cobalt based superalloy with approximately 45% γ’(volume fraction) were studied. The alloy were deformed at a tensile strain rate of 3*10-4s-1 to fracture at different temperatures. The temperature range was from room temperature to 760℃, which is close to the highest temperature the turbine discs withstand. Through detailed observation of the deformed specimen with transmission electron microscopy, the following conclusions were made:(1) With the increase of deformation temperature, the defect configuration in primary γ’ changes accordingly. When deformed at room temperature, there are nearly no defects in primary γ’. And then stacking faults restricted in primary γ’ and deformation microtwins appear successively with the increase of deformation temperature. (2) When deformed at 400℃ and 650℃, stacking faults belong to different slip systems interacted with each other and numerous Lomer-Cottrell locks were formed. These locks obstructed the extension of stacking faults to some extent and the material is strengthened as a result (3) When deformed at 725℃ and 760℃, the stacking faults and high-density deformation microtwins interact greatly, which has similar effects to Lomer-Cottrell locks. On the one hand, the extension of stacking faults is obstructed. On the other hand, the motion of twinning partials is also hindered, which suppresses the thickening of microtwins. (4) With respect to the formation mechanism of deformation twins in y’, it’s deduced that this process is greatly influence by temperature. At high temperature when the thermal motion of atoms is active, this process happens easily. The nucleation process of twin in y’can be delineate by "glide----diffusion and re-ordering----glide on the neighbor {111} plane". After nucleation, the twins can be thickened in similar way, which finally leads to microtwins several-layers thick. |
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