![]() BO SUNDMAN, in The SGTE Casebook (Second Edition), 2008 IV.6.5 ConclusionsĪ five-component model alloy representative for single-crystal superalloy application, Ni–13 at.% Al–10 at.% Cr–2.7 at.% Ta–3 at/% W was produced and experiments were performed regarding the measurement of data relevant to thermodynamic equilibrium, such as the liquidus and solidus temperatures, and the partition coefficients. The tests includes a multiaxial test of nonstationary stress as well as those of stationary stress. Thin-walled tubular specimens with the crystal direction oriented in the axial direction are used. In the present work, combined tension-torsion creep tests of a nickel-base single crystal superalloy CMSX-2 are performed at 900☌ to observe anisotropic effects, and applicability of the crystal theory is examined by simulating the anisotropic effects observed in steady-state creep. The crystal theory was applied to viscoplasticity, in which the problems mentioned above may not arise. It is especially of interest to examine applicability of the theory to multiaxial creep so that the noncoaxiality between stress and creep rate can be discussed. ![]() Such studies however have been done only in a few works so far. It is thus worthwhile to study how accurately the crystal theory describes creep of nickel-base single crystal superalloys. It is obvious that the crystal theory is not appropriate if the climb induced deformation is dominant. Secondly, the climb of γ/γ' interfacial dislocations may occur for matrix dislocations to overcome the γ' precipitates. Firstly, creep deformation is not homogeneous microscopically dislocations glide easily in the γ phase matrix, while they shear occasionally the γ' precipitates. Such anisotropy in single crystals may be described straightforwardly using the crystal theory, because it is based on slip-systems, which characterize the anisotropy in creep and plasticity.įor creep in nickel-base single crystal superalloys, however, there would be problems in application of the crystal theory. They in general exhibit strong anisotropy in creep and plasticity resulting from the crystal structure. Nickel-base single crystal superalloys, which have high temperature strength due to the γ' precipitation in the γ phase matrix, are used as materials for gas turbine blades. Kobayashi, in Advances in Engineering Plasticity and its Applications, 1993 1 INTRODUCTION Parameter optimization studies have shown that cracking resistance is enhanced by high power density and low heat input ( 44). Preheating has been shown to mitigate cracking through a reduction in thermal stresses. Elemental partitioning and the presence of low-melting-point eutectic phases during nonequilibrium solidification are contributing factors to cracking susceptibility. #Singlecrystal 2.3 crackCracking typically occurs along the stray grain boundaries, particularly along high-angle boundaries that provide easy paths for crack propagation. Solidification cracking is another issue that is encountered during weld repair of single crystal superalloys ( 38). The LBW process allowed for a larger processing window, presumably due to the steeper temperature gradient in the weld pool. Studies on CMSX-4 alloy using both GTAW and LBW demonstrated that both reduced power and increased travel speed were beneficial toward preserving the single crystal structure ( 43). However, due to the high degree of undercooling at the weld centerline, equiaxed growth and loss of the single crystal structure is difficult to avoid. High energy density processes (e.g., EBW, LBW) that offer steeper temperature gradients to minimize undercooling of the liquid are favorable for avoiding the formation of stray grains ( 40–42). One of the major difficulties in achieving successful weld repair of single crystal superalloys is the formation of equiaxed ‘stray’ grains in the weld, which is attributed to constitutional supercooling ( 38, 39). The chemical compositions of several single crystal superalloys are provided in Ref. ![]() For these reasons, they are used in gas turbine engines as blades and vanes. Another advantage of these alloys is an increased incipient melting temperature due to the absence of secondary elements such as B and Zr, which are employed for grain boundary strengthening in wrought Ni-base alloys. The absence of grain boundaries in single crystal superalloys provides for superior creep and thermal fatigue resistance compared to polycrystalline alloys. Single crystal superalloys are produced through directional solidification techniques whereby the final component comprises only a single grain. Sowards, in Comprehensive Materials Processing, 2014 6.09.5.3.3 Single Crystal Superalloys ![]()
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