Timothy Hanlon

Developing Mechanistic Understanding of the Effect of Temperature and Environment on the Hold Time Fatigue Behavior of Haynes 282 Alloy

Advanced ultra-supercritical coal-powered steam plants achieve significant efficiency gains via higher operating temperatures and pressures than existing plants. A significant challenge is developing materials for these more aggressive operating conditions and environment. This study was undertaken to develop an understanding of the hold time fatigue mechanism of the Ni-based superalloy Haynes 282 and to separate out the environmental and creep effects of the fatigue behavior. Crack growth rate experiments on samples of Haynes 282 were conducted at temperatures of 1200, 1400, and 1600F in air, steam, and vacuum environments (compact tension sample, 25 ksi in1/2, R = 0.1). Foils from tested samples were then prepared from the bulk free surface oxide and from the fracture surface adjacent to and far from the crack tip using focused ion beam scanning electron microscopy (FIB-SEM). Transmission electron microscopy (TEM) was used to identify various phases present in the resulting oxides, with an attempt to correlate structure to crack growth rate.

3D Investigation of Cracking Behavior in a Ni Superalloy

The high temperature fatigue performance of Ni-base superalloys is critical to gas turbine applications and as such, requires a more fundamental understanding when designing and producing turbine components. To investigate the relationship to local microstructure, a fatigue specimen was cycled under conditions designed specifically to result in intergranular propagation. Prior to failure, the test was interrupted and a 3D data set was reconstructed destructively from optical and EBSD slices taken from around the tip of the growing crack. The data set was investigated to understand the character of grain boundary planes along the crack front with respect those of the bulk material.

Handling Misalignment and Drift in 3D EBSD Data Sets

GE Global Research is exploring the 3D reconstruction of high temperature materials to understand material behavior in lifing applications. Large volumes of material are required to generate the appropriate statistical understanding of grain morphologies, grain boundary types and distributions, and residual plastic strain. Consequently, GE has adopted mechanical sectioning coupled with EBSD analysis as the methodology for investigating regions of interest. Among the major factors that affect the accuracy of such a reconstruction are thermal or mechanical drift during EBSD measurements and the precision and accuracy of sample alignment. To correct for these unavoidable issues when reconstructing the final volume, an algorithm based on grain center-of-mass and various shape factors has been developed and applied to a data set. Results show a significant improvement in slice-to-slice registration.