High-temperature low-cycle fatigue behavior of K40S cobalt-base superalloy
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INTRODUCTION
K40S alloy, which is expected to replace the conventional X-40 cobalt-base alloy in its gas turbine usage, is a cobalt-base superalloy developed from X-40 alloy[1] by modifying the contents of Si, Mn, etc. In comparison with X-40 alloy, the alloy with 0.8Si and 0.8Mn (wt pct) shows superior high-temperature strength, elongation, and corrosion resistance.[2] Chemical composition of the two alloys is given in Table I. Cobalt-base superalloy is currently used in many military and commercial aircraft turbine engines for hightemperature structural components.[3,4] The choice of this material in gas turbine applications is primarily based on a good combination of superior tensile strength properties, excellent fabricability, weldability, and good hot corrosion resistance for prolonged exposure. In gas turbine engine applications, the components are subjected to repeated thermal stresses as a result of temperature gradients, which occur during startups, thrust changes, and shutdowns. Thermal stresses within a component are often highly localized and deformations are constrained by surrounding material, which generally results in straincontrolled deformations. Not surprising, therefore, is the fact that fatigue cracking is the major mode of failure in such parts and special attention has been directed to the strain-controlled low-cycle fatigue (LCF) behavior of superalloys. Superalloys used in aircraft engine components have been the subject of numerous publications mainly devoted to the LCF behavior of nickel-base and iron-nickel-base superalloys,[5–12] but for cobalt-base superalloy, very little information is available in the open literature.[13,14] Material investigated in the present study is a K40S cobalt-base F.M. YANG, Doctoral Candidate, X.F. SUN and H.R. GUAN, Professors, and Z.Q. HU, Professor and Member of Chinese Engineering Academy, are with the Institute of Metal Research, Chinese Academy of Sciences, Shenyang, 110016, People’s Republic of China. Contact e-mail: [email protected] Manuscript submitted January 3, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
superalloy, focusing on the LCF property and cyclic stress response of K40S alloy at elevated temperature. A detailed examination of the deformation substructure, microstructural change, and crack initiation and propagation modes has also been conducted with a view to understand the features that may influence cyclic stress response and fatigue life at elevated temperature. II. EXPERIMENTAL PROCEDURE The K40S alloy examined in this study was prepared in a conventional vacuum induction furnace. After investment casting to bars with a diameter of 16 mm and a length of about 140 mm, cylindrical test specimens with a diameter of 6 mm and a gage length of 10 mm were machined out. All test pieces were ground along the gage length in the longitudinal direction with SiC paper down to grit 1200 in order to minimize the influence of surface defects on LCF test results. Inspection of the test bars revealed the absence of major surface cracks. A servo
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