Elsevier

Materials Characterization

Volume 87, January 2014, Pages 143-148
Materials Characterization

FIB–SEM tomography of 4th generation PWA 1497 superalloy

https://doi.org/10.1016/j.matchar.2013.11.003Get rights and content

Highlights

  • The microstructure of PWA1497 superalloy was examined using FIB–SEM tomography.

  • In case of modern single crystal superalloys, measurements of AA are adequate for VV.

  • During creep the γ channel width increases from 65 to 193 nm for ruptured specimen.

  • Tomography is a useful technique for quantitative studies of material microstructure.

Abstract

The effect of creep deformation on the microstructure of the PWA 1497 single crystal Ni-base superalloy developed for turbine blade applications was investigated. The aim of the present study was to characterize quantitatively a superalloy microstructure and subsequent development of rafted γ′ precipitates in the PWA 1497 during creep deformation at 982 °C and 248 MPa up to rupture. The PWA1497 microstructure was characterized by scanning electron microscopy and FIB–SEM electron tomography. The 3D reconstruction of the PWA1497 microstructure is presented and discussed.

Introduction

Single crystal (SC) nickel-base superalloys have been developed over the past 40 years especially for modern gas turbine applications. This group of alloys has superior mechanical properties such as creep resistance and high temperature strength. They are hardened by a high volume fraction of the ordered γ′ phase, which is coherently precipitated in the γ matrix. These materials are mainly applied as turbine blades and vanes in aero-engines and industrial gas turbines. Turbine blades operate at high temperature under a centrifugal force causing creep deformation of the material, which leads to the so-called rafting (i.e. directional coarsening). A characteristic microstructural property of SC superalloys is the ability of cubed γ′ phase particles to transform, under the influence of stress and temperature, into the plates (rafts). The rafts develop in the early stages of creep at high temperature (about 1000 °C) and low stress (about 100 MPa). Rafting appears to be an essential factor determining creep strength of nickel-base SC superalloys at high temperature influencing their applications [1], [2], [3].

The focused ion beam–scanning electron microscope (FIB–SEM) tomography has been recently widely applied in materials science for studying and modifying material systems at the micro- and nanometer levels. FIB tomography is based on a serial slicing technique employing a FIB–SEM dual beam workstation. Dual-beam FIB–SEM enables the acquisition of serial images with small and reproducible spacing between the single imaging planes because no mechanical stage tilting is necessary between the FIB milling and the electron beam SEM imaging steps. Serial FIB cross-sections are performed through the volume to be investigated and each exposed surface is imaged with an electron microscope. 3D mapping of particles with high Z-resolution by serial FIB slicing and SEM imaging was performed. Ga-ion beam was used to perform a precise in-situ milling. Repeated removal of layers as thin as several nm for some hundred times permits to investigate total volume of some μm3 with a voxel size of 10 nm × 10 nm × 10 nm. The SEM images at an accelerating voltage of 1.7 kV were taken utilizing a BSE (back scattered electron) detector.

Acquired two-dimensional data are processing with the help of computer algorithms and three-dimensional systems can easily be reconstructed and provide both qualitative and quantitative information [4], [5], [6]. To visualize the 3D reconstruction, the Amira 5.4.1 and ImageJA 1.45b software were used.

In this work, a quantitative microstructural characterization of the single crystal Ni-base superalloy PWA 1497 after heat treatment and after creep deformation is presented. The 3D reconstructions of a superalloy microstructure are generated to measure the volume fraction of the γ′ phase and the mean thickness of the γ phase in the direction parallel to the tensile axis.

Section snippets

Description of Materials

A fourth generation Ni-base PWA 1497 superalloy was solidified into single crystal bars and provided by Pratt & Whitney, US. The alloy chemical composition was as follows: Ni–2Cr–5.55Al–8.25Ta–6W–2Mo–5.95Re–3Ru–16.5Co–0.03C–0.15Hf (wt.%) and 40 ppm B. The solid bars were subjected to a standard heat treatment, involving solution annealing followed by aging.

The γ/γ′ Misfit Measurements by X-ray Diffractometry

The XRD measurements of γ/γ′ misfit were performed on selected samples including two samples from the as-received material and a sample after

Results and Discussion

SEM analysis of the baseline PWA 1497 superalloy sample, revealed regular cuboidal morphology of γ′ particles separated by γ channels, Fig. 2a. During high temperature creep deformation, the initially cubic-shaped microstructure transforms under the influence of stress and temperature into plates (rafts). The rafts develop in the early stages of creep. Simultaneously with the rafting process, a thickening of the γ matrix channels parallel to the rafts occurs. Further evolution leads to an

Summary

Quantitative characterization of the 4th generation Ni-base PWA 1497 superalloy microstructure (baseline and creep ruptured samples) was performed by the FIB–SEM tomography technique. The quantitative results of the γ′ area fraction (AA) and volume fraction (Vv) reveal that for baseline sample AA and Vv are not equal and reach respectively around 68 and 71%. For the ruptured specimen, AA and Vv are equal and reach around 71%. Those results clearly prove that in case of modern single crystal

Acknowledgments

This work was supported by the Polish Ministry of Science and Higher Education (project nr 11.11.110.148), and the AGH-UST Statutory Research. We would like to also acknowledge Pratt & Whitney, East Hartford, CT USA for providing the material used in this investigation.

References (10)

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    The density of rattan-shaped γ′-phase was defined as the length per unit area (L/A, where L is the length sum of rattan in a selected region and A is the area of the region). According to Maciej Ziętara's work [23], the stereoscopic structure of γ′-raft at a SC Ni-base superalloy PWA 1497 has been obtained via FIB–SEM where the specimen's surface was removed with a Ga-ion beam. This technology has also been applied for observing the γ′-precipitates [24] and carbide particles [25] in superalloys.

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