B.A. Shollock

Directional and single-crystal solidification of ni-base superalloys : Part I. The role of curved isotherms on grain selection

The development of crystallographic texture during directional solidification has been quantitatively analyzed in columnar castings of the Ni-base superalloys, CMSX4 and CM186LC, produced with a range of cooling rates and liquidus front curvatures. It is proposed that the more diffuse crystallographic texture developed in CMSX4 relative to CM186LC results from a combination of the differing local orientation stability condition and the alloys’ solidification characteristics. The implications of these additional factors on the evolution of the axial grain texture, the grain orientations produced in singlecrystal processing, and the stability of spurious grains in processing CMSX4 are discussed. An experimental method is presented to quantitatively analyze the grain selection process in the case of curved liquidus isotherms by retaining the stereology of the primary 〈001〉 dendrite growth direction and the local thermal gradient vector. This can account for the stability of spuriously nucleated edge grains in a single-crystal matrix.

Morphological aspects of competitive grain growth during directional solidification of a nickel-base superalloy, CMSX4

Quenched directional solidification of specially oriented bi-crystals of the Ni-base superalloy CMSX4, was carried out in an attempt to understand the role of the dendritic morphology in the process of competitive grain growth. For the range of misorientations considered (primary 〈001〉 misoriented by up to 7° from the uniaxial thermal gradient), there was no evidence of overgrowth of the primary misoriented dendrite by the secondary arms on the leading aligned primary. In fact, it was observed that for this range of misorientations, the tip of the retarded primary suppresses the growth of secondaries on its leading neighbour. This subsequently simply restricts the growth of the mis-aligned crystal to its original boundary, rather than reducing its size and is suggested as a possible reason for the range of stable axial orientations encountered during directional solidification of CMSX4.

Formation of low angle boundaries in Ni-based superalloys

The evolution of misorientation accompanying branching and growth of dendrite stems advancing into an increasingly undercooled liquid at a geometrical discontinuity in cross section (platform of a turbine blade) during solidification of a Ni-base superalloy has been experimentally investigated. The misorientation generated is cumulative in nature with a systematic rotation of the principal � 001 � -crystal axes. It is proposed that the misorientation arises from plastic deformation of the dendrite stems during growth. Low angle boundaries are subsequently produced across impinging dendrite fronts.

Directional and single-crystal solidification of Ni-base superalloys: Part II. Coincidence site lattice character of grain boundaries

The development of grain boundary misorientations with an evolving axial texture during directional solidification has been examined using the electron backscattered diffraction (EBSD) technique on the Ni-base superalloys, CMSX4 and CM186LC. A preferred grain boundary misorientation distribution (GBMD) for a sharp 〈001〉 axial texture in CM186LC was associated with a clustering of misorientation axes (MAx) in the proximity of 〈001〉. This is accompanied by an enhanced distribution of coincidence site lattice (CSL) boundaries. The increased distribution of low angle boundaries, Σ1 and Σ5, can be attributed to the existence of a preferred MAx and accommodation by secondary intrinsic grain boundary dislocations. The more diffuse 〈001〉 axial texture in CMSX4 is associated with a significant proportion of MAx deviating from 〈001〉 and a dramatic reduction in the proportion of CSL boundaries.

Quantitative characterisation of last stage solidification in nickel base superalloy using enthalpy based method

Abstract An enthalpy based method was used to determine the solidification characteristics in the Ni base superalloy IN713LC with the emphasis on the late stages of solidification. Solidification commences with freezing of γ-solid, which is followed by precipitation of carbides (MC) and subsequent divorced growth of MC and γ until solidification terminates. During solidification enthalpy change was measured using differential thermal analysis and latent heat was calculated using a multicomponent thermodynamic software database. The measured enthalpy and calculated latent heat were then used to determine liquid fraction evolution and local freezing rate. A quantitative comparison of calculated fraction liquid evolution and local freezing rate with those determined using equilibrium and Scheil approximations was carried out. The comparison reveals that the present method offers a more accurate approach for characterising the late stages of solidification than the equilibrium and Scheil models.

Implications of dislocation micromechanisms for changes in orientation and shape of single crystal superalloys

The development and exploitation of gas-turbine blades cast from nickel-base superalloys in single crystal form has been one of the most successful industrial and commercial ventures relating to advanced structural materials over the last twenty years or so. The benefits of producing gas-turbine blades from single crystal superalloys has resulted from at least two factors. Firstly, the elimination of grain boundaries from the component has increased the creep ductility by removing sites where cavities develop and has allowed the chemistry of superalloys, designed specifically for use in the single crystal form, to be free of grain boundary strengthening elements thus increasing the melting temperature and potential operating temperature. Secondly, by controlling the orientation of the single crystal casting, normally having the low modulus direction parallel to the centrifugally stressed blade axis, thermal stresses are minimized and the thermal fatigue resistance is greatly enhanced. However, these advantages have been obtained at the expense of dealing with a highly anisotropic material. In order to exploit the full potential of single crystal superalloys, an effective anisotropic design strategy is required.

Characterisation of crystallographic evolution during creep deformation of a single crystal superalloy

In the present study, the electron back scattering diffraction (EBSD) technique has been used to monitor the changes in microcrystallinity that occurs in specimens of SRR99 with complex crystal orientation when deformed to various creep strains and to failure at a range of stresses and temperatures. The current version of the anisotropic model has been used to simulate these deformations and predict the crystal rotations that will occur as a function of creep strain. The investigation compared the experimental and predicted extents and nature of crystal rotation. It also examined the wider issues of sources in variation of experimental measurements and the influence of the microstructural heterogeneities on the uniformity and consistency of crystal rotation determinations.