TEM investigation of the microstructure in a single crystal nickelbase superalloy creep tested in [011] orientation

Quantitative TEM investigations are performed on [011] oriented single crystals of Nickelbase superalloys creep tested at 1033 K under a load of 680 MPa. Multiple slip in the matrix occurs on four 1/2 (110){111} slip systems from beginning on. Octahedral cross slip is not activated. Cube slip does not occur, whereas during secondary creep screw interfacial dislocation cross slip in the cube γ/γ'-interface. In the matrix phase, deformation is concentrated in the "roof" channels. Calculations show that this is the result of the superposition of coherency and external stresses in small matrix channels. In the stage of secondary creep, γ' precipitates are sheared by Shockley superpartials. Common shearing of matrix and γ' particles by Shockley superpartials has been analysed. Continued common shear on adjacent {111} planes finally causes damage by mechanical twinning. The high creep rate in the [011] orientation is considered to be caused by the stress concentration in roof matrix channels and the few activated slip systems

Influence of the Structure and Orientation of the Parent Phase on the Hysteresis of Single-Crystal Shape Memory Alloys

The results of high-resolution electron microscopy, calorimetry and tension-compression tests on the Cu-Zn-Al single-crystals, of calorimetry on the Mn-Cu, NiTi single-crystal and on the NiTi polycrystals are presented. The features of the pseudo-plastic (ferroelastic) hysteresis by the deformation of the single-crystals in the martensitic state are investigated in detail. The similarity of ferroelastic and ferromagnetic hysteresis, the strongly orientation dependence of the ferroelastic hysteresis and its independence on the temperature are shown experimentally, which are well known peculiarities of the ferroelastic hysteresis. The dissipated energy (luring a one cycle of the ferroelastic deformation has parabolic dependence on the strain with an proportionality coefficient which is determined as an effective modulus of the martensitic polydomain. The hysteresis of the thermal-induced, stress-tree transformation of all samples is also investigated. Two equilibrium lines bracketing the permanent hysteresis inside the major hysteresis loop are found during the partial cycling. The dissipated energy involves two terms: the one, describing the permanent hysteresis, depends linear on the martensite phase traction and the other, parabolic. The influence of the tramforrnation type, polycristallinity and scanning rate on the thermal hysteresis are discussed