CRYSTALLOGRAPHIC FATIGUE CRACK PROPAGATION IN SINGLE CRYSTAL NICKEL-BASE SUPERALLOY

Date of Completion

January 1985

Keywords

Engineering, Metallurgy

Degree

Ph.D.

Abstract

A study of the fatigue crack propagation behavior of single crystal nickel-base superalloy MAR-M200 (Ni-9Cr-10Co-12W-1Nb-2Ti-5Al) was conducted in order to gain a detailed understanding of how fatigue cracks propagate along crystallographic planes. This included both the study of Stage I fatigue crack propagation using conventional compact-type (C-T) specimens and four-point loading specimens specially designed to achieve a pure cyclic shear condition. A two dimensional Boundary Integral Equation (BIE) method was used to calculate both the shear stress intensity factor (K(,l l)) and the tensile stress intensity factor (K(,l)) of C-T specimens with angled cracks as well as for pure shear specimens in four-point loading.^ It was observed that crystallographic fatigue crack propagation occurred exclusively along octahedral (111) planes and was independent of the specimen crystallographic orientations. For the first time, it has been demonstrated that fatigue cracks propagate along crystallographic planes subject to pure cyclic shear loading. A shear component is necessary to propagate a crack and the crystallographic crack propagation is solely governed by the shear stress intensity factor (K(,l l)). The tensile stress intensity factor (K(,l)) does not provide any driving force for the crack propagation.^ A basic crystallographic fatigue crack propagation mechanism, namely the cross-slip extrusion mechanism, has been identified in this study. It is observed that at least two slip systems receiving equal shear stress in the same crack plane are required in propagating the crystallographic fatigue crack in the absence of oxygen. Crack advance can be viewed as an intrusion process resulting from dislocation cross-slip using a primary slip system. The excess material accumulated as a result of the cross-slip process can either be extruded out toward the free surface or be transported using a secondary slip system. In addition to the above mechanism which operates in the absence of oxygen, a oxidation mechanism is responsible for crack advance using only one slip system when oxygen is present. The crack increment per loading cycle is the result of the oxidation of the freshly exposed surface ahead of the crack tip upon loading. The oxide break down at the crack tip due to dislocation pile-ups further enhances the irreversibility required in fatigue crack propagation. ^

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