Fatigue Failure Modes of the Grain Size Transition Zone in a Dual Microstructure Disk

Abstract

Mechanical property requirements vary with location in nickelbased superalloy disks. In order to maximize the associated mechanical properties, heat treatment methods have been developed for producing tailored grain microstructures. In this study, fatigue failure modes of a grain size transition zone in a dual microstructure disk were evaluated. A specialized heat treatment method was applied to produce varying grain microstructure in the bore to rim portions of a powder metallurgy processed nickel-based superalloy disk. The transition in grain size was concentrated in a zone of the disk web, between the bore and rim. Specimens were extracted parallel and transversely across this transition zone, and multiple fatigue tests were performed at 427 °C and 704 °C. Grain size distributions were characterized in the specimens, and related to operative failure initiation modes. Mean fatigue life decreased with increasing maximum grain size, going out through the transition zone. The scatter in limited tests of replicates was comparable for failures of uniform gage specimens in all transition zone locations examined.

Strain Induced Abnormal Grain Growth in Nickel Base Superalloys

“…Controlling grain size is necessary for nickel base superalloys, designed for high temperature applications. Under certain circumstances abnormal grain growth (AGG) occurs during the forming process [1], leading to bimodal grain size distributions that are detrimental for the in-use properties [1][2][3]. In the present paper, two examples of such a heterogeneous microstructural evolution are depicted and analyzed in order to identify the underlying mechanisms.…”