Czasopismo | INŻYNIERIA MATERIAŁOWA | Rocznik 2013 - zeszyt 3

Application of EBSD technique to investigation of modern materials for aero engines turbine blades

nr katalogowy: 78020

Turbine blades, vanes, and other parts of aero engines exposed to high temperature are produced of nickel-based superalloys via investment casting. Directional solidification (DS) allows obtaining excellent mechanical properties of blades [1]. Because turbine blades are flight safety parts, they must be free of any defects. However, directional solidification is very complex and many casting defects may appear during this process [1]. The commonly defects in DS and single crystal (SX) castings are freckles [1÷6] and stray grains [7÷9]. Freckles are casting defects that appear on the surface of DS or SX castings in the form of long chains of equiaxed grains aligned parallel to gravity [1÷6]. Freckle dimensions depend on casting dimensions: the length is usually the same as the casting, but the width is from one to several millimeters. Freckles are enriched with elements segregated to the liquid phase during solidification [2÷4]. Compared with the freckle-free part of a casting, freckled areas are characterized by an increased carbide content, γ + γ′ eutectics, and porosity [4]. Freckle formation is attributed to thermosolutal convection and buoyancy forces in the mushy zone, which are in turn caused by density inversion in the interdendritic liquid [5]. The tendency for various superalloys to freckle is characterized by the freckling index F, which depends on the chemical composition [1]: F C C C C C C = + + - + Ta Hf Mo Ti W 1 5 0 5 0 5 1 2 . . . . Re (1) where CTa, CHf, CMo, CTi, CW, and CRe are the concentration (wt %) of tantalum, hafnium, molybdenum, titanium, tungsten, and rhenium, respectively. In case of F > 1, the freckling tendency is low. The density inversion of interdendritic liquid in the mushy zone is caused by strong segregation of some elements (i.e., W, Re) into the solid phase. Therefore, during directional solidification, these elements are depleted in the liquid alloy in interdendr[...]

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