Effect of Al-Si Pack Cementation Diffusion Coating on High-Temperature Low-Cycle Fatigue Behavior of Inconel 713LC
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INTRODUCTION
POLYCRYSTALLINE cast Inconel 713LC is a precipitation-strengthened nickel-base superalloy that is used for the manufacturing of blades and disks of gas turbine engines. During gas turbine service, blades are both thermally and mechanically highly loaded.[1] Thermally induced mechanical loadings result from transient and inhomogeneous temperature zones within the component. Additional mechanical loading is caused by centrifugal forces.[2] Furthermore, the components are typically subjected to a high-frequency mechanical loading resulting from vibrations as well as pressure fluctuations due to unsteady combustion. In such environments, superalloys have limited oxidation and hot corrosion resistance and therefore need a surface protection with some kind of coatings.[3] Widely used diffusion coatings are based on the saturation of the substrate surface with elements such as Al. Diffusion aluminide coatings are routinely used on Ni-base superalloys to protect them from high corrosive environment particularly at high temperatures.[4] Although this type of coating provides good oxidation resistance, further resistance to hot corrosion is maintained by addition of elements such as Cr or Si.[5] In addition, modification of aluminide coating by Si is reported[6] to improve the resistance to type II acidic fluxing that is said to be due to insolubility of oxide scales in acidic corrodents. Silicon-modified aluminizing process is one of the so-called high activity processes in MOHAMMADREZA MANSURI and ESMAIL ZARE, Masters, and SEYED MOHAMMAD MEHDI HADAVI, Doctoral, are with the Materials and Energy Research Center, Karaj, Iran. Contact e-mails: [email protected]; [email protected] Manuscript submitted January 2, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
which the growth of coating layers is usually controlled by inward Al diffusion.[7,8] At the initial stage of the coating, a single coating layer of Ni2Al3 having a small amount of Si forms on the surface of the substrate.[9,10] At this stage, no intermediate layer between the base material and the coated layer is reported. It is only after additional heat treatment that a layer of NiAl forms at the original coating layer and the base metal interface. The formation of this layer is mainly due to the decomposition of the original coating layer Ni2Al3 and the interdiffusion of Ni and Al. Ideally, the application of coatings should not have any negative affect on the mechanical properties of the substrate. One of the most important factors affecting the fatigue life of the coated turbine blades is the magnitude of the applied tensile strain, which should be lower than the fracture strain of the coating. In the other words, in the conceptual and detail design, maximum allowed strain in the turbine vanes and blades should be lowered than the fracture strain of the coating on the vane and blade surfaces. The fracture strain of the coatings could be obtained from temperature verses fracture strain graphs and interrupted tensile test results. However, because of differen
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