Title

Analysis of elevated temperature fatigue crack growth mechanisms in alloy 718

Document Type

Conference Proceeding

Date of Original Version

12-1-1991

Abstract

In this paper observations concerning effects of mechanical, environmental, material variables on the crack growth process in Alloy 718 have been analyzed on the basis of the related deformation characteristics in the crack tip region. These analyses have suggested that the governing mechanism of the crack tip response is the degree of homogeneity of plastic deformation and associated slip density. For conditions promoting homogeneous plastic deformation with a high degree of slip density the environmental damage contribution is shown to be limited thus permitting the dominance of the cyclic damage effects which is characterized by a transgranular crack growth mode and lower crack growth rate. Under conditions leading to inhomogeneous plastic deformation and lower slip density the crack tip damage is described in terms of grain boundary oxidation and subsequent fracture. On the basis of this view that the variation in crack growth damage mechanism in Alloy 718 from fully cyclic-dependent to fully environment-dependent is governed by the slip character in the crack tip region, conflicting experimental observations under different operating conditions are examined. Approaches to increase the alloy resistance to environmental effects through microstructure control as well as sensitizing by mechanical conditioning are suggested.

Publication Title

American Society of Mechanical Engineers, Aerospace Division (Publication) AD

Volume

21

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