Determination of Crystal Orientation by Ω-Scan Method in Nickel-Based Single-Crystal Turbine Blades

Abstract

The article presents an assessment of the crystal perfection of single-crystal turbine blades based on the crystal orientation and lattice parameter distribution on their surface. Crystal orientation analysis was conducted by the X-ray diffraction method X-scan and the X-ray diffractometer provided by the EFG Company. The X-scan method was successfully used for evaluation of the crystal orientation and lattice parameters in semiconductors. A description of the X-scan method and an example of measurement of crystal orientation compared to the Laue and EBSD methods are presented. High-pressure turbine blades (HPT) in aircraft and land-based gas turbines operate under severe conditions. These include heavy mechanical loads, high temperature and oxidizing gas environments. Turbine blades are manufactured from nickel-based superalloys in an investment casting process, which usually involves application of the lost-wax casting method. A columnar grain as well as single-crystal structure increases their high-temperature creep resistance. This kind of structure is obtained as a result of directional solidification processes. [1][2][3] Single-crystal casting is a highly advanced process in terms of maintaining proper solidification conditions, preparation of wax models and ceramic shell molds. The wax model of the turbine blade consists of a starter block and grain selector. Their specific shape makes it possible to obtain a single-crystal structure of the cast. The spiral shape of the grain selector ensures growth of only one grain at its end.[4] It has been established that during the initial stage of the solidification process, nuclei grow in a columnar manner in liquid metal toward the spiral-shaped selector. During the second stage of solidification, differences in the withdrawal rate value and the influence of the selector geometry result in elimination of the majority of disoriented columnar grains on the walls. This process enables the growth of one grain and makes obtaining a structure with a homogeneous crystallographic orientation possible. In most cases, single-crystal turbine blades are obtained with [001] direction parallel to or slightly deviating from the blade load axis, usually designated as 'z.' High deviation leads to a decrease in creep resistance due to the high anisotropy of the mechanical properties. Worldwide aircraft engine manufacturers' quality criteria require the value of the deviation angle (a) to be lower than 15 deg. [5,6] The microstructure of single-crystal nickel-based superalloys consists of two main phase components: the c phase, a solid solution of alloying elements in nickel, and c¢, an Ni 3 Al intermetallic phase. c¢ has an ordered structure-L1 2 , with a face-centered lattice-and is a main strengthening phase. The lattice parameters of the matrix (c phase) and precipitates (c¢ phase) have almost the same value: a 0 = 0.356 nm. High coherence of these phases allows classifying the structure of castings as single crystals, however with relatively low quality.

The Effect of Withdrawal Rate on Crystal Structure Perfection, Microstructure and Creep Resistance of Single Crystal Castings Made of CMSX-4 Nickel-Based Superalloy

“…Determination of the crystal orientation was conducted by an X-ray Ω-scan method using OD-EFG diffractometer (EFG, Berlin, Germany) [35]. The value of the deviation angle α z between the direction of the mold withdrawal and [001] crystal directions was measured (Figure 1).…”

On the Formation of Eutectics in Variations of the Al10Co25Cr8Fe15Ni36Ti6 Compositionally Complex Alloy

“…Single crystal CMSX-4 may grow up to 15 deg astray from a perfect [100] growth direction. [19] Observations in a directionally solidified CMSX-4 showed a bending of 1.7 deg Fig. 5-Calculations of the phase diagrams of the base alloy and its variants using ThermoCalc [6] and the TTNI7 database.…”

The Effect of Re Content on Microstructure and Creep Resistance of Single Crystal Castings Made of Nickel-Based Superalloys

“…The elemental parameter used to define the degree of crystal structure perfection is the value of the deviation angle α z between the crystallographic direction [001] and the withdrawal direction of the blade. The high value of α z (>15°) can significantly reduce the creep resistance of SX turbine blades [6][7][8][9][10][11] (Fig. 1).…”