A single-crystal nickel-base superalloy was directionally solidified over a range of withdrawal rates to assess the benefits of using liquid-metal cooling (LMC) for small-scale castings. Cylindrical bars of 1.6-cm diameter were solidified at a rate of 3.4 mm/min using conventional (Bridgman) radiation cooling and at rates of 8.5, 12.7, and 21.2 mm/min using LMC. PRO-CAST modeling was used to predict dendrite arm spacings based on local thermal conditions. The LMC process exhibited higher thermal gradients and finer primary and secondary spacings of up to 50 and 70 pct, respectively, in comparison to the Bridgman process. The maximum refinement in dendritic spacings using the LMC process occurred at a withdrawal rate of 12.7 mm/min. Variability in axial and lateral dendrite spacings decreased with increasing withdrawal rate, up to the point of maximum refinement. Withdrawal rates exceeding 12.7 mm/ min increased the variability in spacings and produced lateral overgrowth of the primary dendrites by secondaries and promoted formation of high-angle grain boundaries.
Single crystals of a nickel-base superalloy were directionally solidified (DS) over a range of cooling rates to evaluate the benefits of a new high thermal gradient solidification process. Solidification experiments were conducted on cylindrical bars with a liquid-metal-enhanced cooling process. This higher gradient casting process was evaluated for the degree of structure refinement, microstructural variability, and porosity distributions. Cylindrical bars of 1.6-cm diameter were solidified at rates between 8.4 and 21.2 mm/min using a tin-based, liquid metal cooling (LMC) technique and at a rate of 3.4 mm/min with a conventional Bridgman process. The LMC process produced a refined microstructure with average primary dendrite arm spacing (PDAS) and secondary dendrite arm spacing (SDAS) values as low as 164 and 25 lm, respectively, for the bar geometry evaluated. An optimum intermediate withdrawal velocity of 12.7 mm/min produced up to a 50 and 60 pct refinement in PDAS and SDAS, respectively. Further increases in withdrawal velocity produced smaller SDAS and pore sizes, but undesirable grain boundaries and excessive secondary dendrite arm growth. Voronoi tessellation methods were used to examine the extremes of the dendrite arm spacings in comparison to the average measurements, the packing of dendrites, and the correlation of porosity size and location with the dendrite structure. A simple expression for prediction of the maximum pore size is developed.
The role of oxidation-induced layers in the failure process of aluminide-coated nickel base single crystals subject to high-temperature fatigue cycling has been investigated experimentally and via finite element analysis. Isothermal strain-controlled compressive fatigue experiments (R 5 ÀN) with 120 s holds in compression were conducted at 9821 and 10931C. Surfaceinitiated cracks containing a layer of alumina progressively grew through the coating layers into the superalloy substrate, ultimately causing failure. Growth stresses in the oxide provided a driving force for extension of the oxide into the softer coating and substrate layers. Finite element modeling shows the rate of growth of the oxide-filled cracks is sensitive to the strength of the constituent layers and the magnitude of the oxide growth strains. Implications for design of failure-resistant coating-substrate systems are discussed.
The fatigue property benefit for higher thermal gradient processing of single crystal superalloys has been evaluated experimentally with two different solidification processing approaches. Low cycle fatigue (LCF) testing was performed at 538 o C (R = 0, f = 0.5 Hz) for a René N5 and Ta-modified René N5 alloy solidified with a conventional radiation cooled and higher thermal gradient liquid-metal cooled (LMC) casting process to produce coarse and finer-scaled dendritic structures, respectively. In all specimens tested, cracks initiated at individual pores or large interconnected pores located within the internal volume and near to the surface. Based on statistical analyses of pore size distributions, a model has been developed that predicts the maximum pore size as a function of solidification conditions and the effect of these pores on fatigue life. The model is used to assess the potential property benefits of high thermal gradient processing approaches for a range of cooling rates and component scale sizes
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