Optimization of X-Ray Techniques for Nondestructive Characterization of Single Crystal Turbine Blades
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ABSTRACT This paper describes nondestructive x-ray characterization techniques which detect
macroscopic and microscopic defects, determine the overall crystallographic perfection, and detect any unwanted secondary crystals both on the external surface as well as inthe interior of single crystal blades. The method of Asymmetric Crystal Topography for diffraction imaging the surfaces of single crystal turbine blades and the method of White Beam Transmission Topography for diffraction imaging through the thickness of single crystal turbine blades are both discussed and illustrated with representive diffraction images (topographs). It is clear that the images gained from these methods have a capability for providing information about the details of crystalline perfection (or lack thereof) in nickel-based alloy single crystal turbine blades. Such information can provide considerable leverage for the crystal grower to help in adjusting processing variables to enhance quality of a critical product. And the same methods of topography can conceiveably provide tools for evaluating the finished product ina way which has not been available to date. INTRODUCTION The efficiency of modern gas turbine engines, used both for airplane propulsion and electric power generation, increases with increasing combustion temperature. The physical requirements which limit the choice of turbine blade materials for high temperature operation are low density, thermal stability, toughness, and resistance to fatigue, high-temperature oxidation, and creep.
Creep caused by dislocation motion is resisted by addition of alloying elements in solid solution and formation of stable hard precipitates, which serve as dislocation pinning points. Diffusional creep is resisted most optimally by eliminating the grain boundaries, i.e. using single crystal blades. This use of metallic single crystals for structural engineering applications places new requirements on nondestructive techniques for process control. Of particular importance is the need for an improved inspection procedure after crystal growth for determination of the overall crystalline perfection of the final blades, both internally and on the external surfaces. Multicrystal and poor quality single crystal blades can result inloss of aircraft engines with the potential for disastrous consequences. For gas turbines used inthe electric power industry the failure of a blade may result in the loss of electric power with innumerable problems for the customers. This paper describes nondestructive x-ray characterization techniques for process control of single crystal turbine blade growth which detect macroscopic and microscopic defects, determine the overall crystallographic perfection, and detect any unwanted secondary crystals both on the external surface as well as in the interior of single crystal blades. THE EVOLUTION OF HIGH PERFORMANCE TURBINE BLADE TECHNOLOGY Since thermodynamic principles dictate that the efficiency of a gas turbine increases as the peak internal operating temperature can be i
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