Transverse Creep of Nickel-Base Superalloy Bicrystals
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ODUCTION
SINGLE-CRYSTAL nickel-based superalloys prevail as the material of choice for high-temperature gas turbine applications. The Bridgman[1,2] or Liquid Metal Cooled (LMC)[3,4] single-crystal growth techniques are commonly used to produce directionally solidified turbine blades. During solidification of complex turbine airfoil shapes, it is challenging to maintain uniform thermal gradients and solidification velocities to yield a monolithic single crystal through the seed and subsequently throughout the entire withdrawal process.[5–7] While low-angle boundaries may not degrade hightemperature mechanical properties, high-angle boundaries will have a strong detrimental effect, particularly on creep properties.[8–10] From an engineering point of view, the presence of low-angle boundaries must be tolerated to maintain casting as an economically viable production technique. It is, therefore, important to determine the level of grain boundary misorientation that can be tolerated without any significant loss in mechanical performance. In addition, a more fundamental understanding of the mechanisms of damage development and failure at the low-angle boundaries would provide directions for development of more failure-resistant alloy compositions. Minor element additions have been studied as a means of making boundaries more damage resistant in a range of commercial alloys.[8–12] Significant improvement of tolerance to high-angle grain boundaries with minor element additions of carbon and boron have been observed in these alloys at temperatures between 1033 K and 1373 K (760 C and 1100 C).[8–12] However, creep ductility remains limited compared to defect free single crystals. To optimize these minor alloying additions, it is J.C. STINVILLE, Post Doc, K. GALLUP, Graduate Student, and T.M. POLLOCK, Professor, are with the Materials Department, University of California, Santa Barbara, Santa Barbara, CA 931065050. Contact e-mail: [email protected] Manuscript submitted August 8, 2014. Article published online April 2, 2015 2516—VOLUME 46A, JUNE 2015
necessary to understand their influence on the mechanisms inducing the loss of mechanical properties. This paper investigates the effects of creep deformation at high temperature on two alloys with very similar nominal composition but with differences in minor elements. The first alloy, GTD444,[13] was developed as a columnar-grained alloy, while the second alloy, Rene´ N4, was designed as a single-crystal alloy.[8,14–17] Both alloys have been produced using the liquid metal cooling method[3,4,18] allowing production of bicrystals with varying misorientation. Creep properties and damage accumulation for loading transverse to the boundaries have been studied in detail.
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EXPERIMENTAL APPROACH
The composition of the two alloys studied is given in Table I. Bicrystalline slabs of each alloy were solidified by the LMC method.[3] The liquid metal cooling container held a 500 kg Sn bath maintained at 523 K (250 C) and was combined with a dynamic, floating ceramic baffle. The 1.5-c
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