Micromechanics of primary creep in Ni base superalloys

Abstract

The initial stages of creep in nickel base superalloy deserve a special attention since they prevail in most of the service live of critical jet engine components. In nickel base superalloys, a very sharp minimum in creep rate is observed at strains less than 0.5% in a wide range of temperatures: the lower the applied stress the steeper the minimum, often referred to as incubation period. In this paper, the primary creep stage of Ni superalloy is investigated using 3D discrete dislocation dynamics modeling. The simulated volume corresponds to a 3D periodic cell of the cubic arrangement of γγ' Ni-Al loaded along the (001) direction. Misfit stresses induced by the lattice mismatch (= -3.10 -3 ) between the precipitate and the matrix are taken into account through a coupling of the dislocation dynamics code with a finite element code. Frank-Read sources are positioned in the horizontal channel. It is shown that most of the dislocations propagate in the horizontal channels by repeated cross-slip on activated {111} glide planes and a few invade the vertical channels. This behavior is rationalized by the model. In the γ-γ' interfaces, the zigzagging lines of dislocations straighten out into [100] and [010] directions leading to a dislocation microstructure similar to the ideal misfit dislocation network needed to compensate the lattice mismatch. Calculations of internal stresses in the channels show that this dislocation microstructure induces biaxial planar stresses in the horizontal γ channels leading to an elastic contraction of the sample along the loading direction. Three-dimensional DDD simulations show that the plastic strain induced, in the loading direction, by the movement and multiplication of dislocations is less important than the elastic relaxation it produces along that same direction. Hence, for very low applied tensile stresses, Ni superalloys display an abnormal behavior often referred to as negative creep that can be evidenced experimentally, even in polycrystalline materials. This last observation clearly implies that the first active batches of dislocations, during creep, or static high temperature ageing are mobilized to cancel out the numerous and pervading internal elastic strain fields generated within the crystals by the γ' precipitates due to their misfit with the γ matrix.

An efficient algorithm of dislocation-precipitate interactions for single crystal nickel-based superalloys within discrete dislocation dynamics and its application

“…Following Huang et al [39], the misfit stress is pre-calculated by offline FEM through applying an artificial heating of the whole model, setting different thermal expansion coefficients for two phases according to the misfit coefficient. Then this misfit stress field is tabulated for the following DDD simulations by superposition scheme, which is similar to the other researcher's treatment [49,57]. It should be mentioned that the stress by dislocations deposited on the γ/γ interfaces may counteract the misfit stress and release the misfit eigenstrain.…”

The Effect of Strain Rate on the Tensile Deformation Behavior of Single Crystal, Ni-Based Superalloys

“…The APB energy has the major effect on yield stress, [18] but the lattice misfit between the c and c¢ phases may also affect the yield point as it changes the stress in the c channels of the coherent microstructure, raising the effective shear stress in the horizontal channels and reducing it in the two vertical channels. [19,20] The stress strain curves from specimens tested at strain rates of 10 À2 s À1 to 10 À6 s À1 show linear deformation up to a yield point around 950 MPa. At the yield point, the strain is ~1 pct, much higher than would result from elastic deformation alone and this is explained by the activity confined to the c channels in the sample interrupted before yield, Figure 2.…”

Effects of Manganese and Zirconium Dispersoids on Strain Localization in Aluminum Alloys

“…DDD simulations are performed using the edge-screw model TRIDIS [24] with the material parameters corresponding to Aluminum recalled in Table 2. Note that this code is precisely the one that was previously used to address dislocation-precipitate interactions in Reference [22], and applied to the case of fatigue of Waspaloys [23] and creep of Ni Superalloys [25]. The DDD principle is detailed in Reference [24], but the main ingredient used in TRIDIS are recalled hereafter.…”