Transient Liquid Phase Bonding Single-Crystal Superalloys with Orientation Deviations: Creep Properties
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I.
INTRODUCTION
THE increasing requirement of thermal efficiency of the areoengines has promoted the development of single-crystal superalloy turbine blades[1,2] in the past decades. Higher thermal efficiency induces higher operating temperature, which may exceed the melting point of Ni-based superalloys[2] leading the failure of the alloys prior to the designed requirements. Therefore, several advances have been made to increase the heat resistance of the turbine blade, for example, coating technologies[3–7] and internal gas cooling systems.[8,9] In addition, the internal geometries of the single-crystal turbine blade become more and more complex so as to increase the cooling efficiency.[2] However, the production yield of the single-crystal turbine blades with such complex internal cooling channels is usually rather low. This is mainly because there are many sharp corners at the cross sections where the temperature gradient is not flat and stray grains may form as a result.[10,11] Mechanical properties will be decreased once the stray grains formed.[12] In order to produce the turbine blades with NAICHENG SHENG, formerly a Ph.D. candidate with the Superalloys Division, Institute of Metal Research, Chinese Academy of Sciences, No. 72, Wenhua Road, Shenyang, 110016, P. R. China, is now Postdoctoral Researcher with the Institute of Science and Technology of Metals (WTM), Department of Materials Science and Engineering, University of Erlangen-Nuremburg FAU, Martens Str. 5, 91058 Erlangen, Germany. Contact e-mails: [email protected], [email protected] JIDE LIU, Associate Professor, TAO JIN, Supervisor and Professor, XIAOFENG SUN, Professor, and ZHUANGQI HU, Professor and Academician, are with the Superalloys Division, Institute of Metal Research, Chinese Academy of Sciences. Manuscript submitted January 27, 2015. Article published online September 15, 2015 5772—VOLUME 46A, DECEMBER 2015
complex internal cooling channels, researchers have proposed a method of preparing an integral turbine blade through transient liquid phase (TLP) bonding single-crystal segments.[13] Therefore, TLP bonding is now used with the desire to increase the production yield and decrease the cost of the production of the turbine blades. Transient liquid phase (TLP) bonding was developed by Duvall[14] in order to bond heat-resistant steels. Materials which are sensitive to cracking in the heat-affected zone during welding can be bonded using TLP methods, for example, Ni-based superalloys,[15–17] stainless steels[18–20] and titanium alloys.[21] TLP methods allow bonding materials under low stresses in comparison to welding methods (linear friction welding, electron beam welding[22,23], etc) avoiding damage or phase transformations at bonding interface. The whole TLP bonding process is usually divided into four separate stages[14,24,25]: (i) interlayer melting and solid state diffusion, (ii) homogenization of the interlayer liquid and dissolution of the substrate, (iii) isothermal solidification stage and (iv) post-heat treatment and solid-state
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