Analysis of Cold Dwell Fatigue Crack Initiation Site in a β-Forged Ti-6242 Disk in Relation with Local Texture

Abstract

The α and prior β textures and microtextures of a lamellar Ti6242 forged disk were characterized by advanced electron back scattered diffraction (EBSD) and related to crystallographic features of faceted crack initiation sites of dwell fatigue specimens tested along the radial direction (RD). Large feather-like structures of α colonies with close orientations were observed at boundaries of elongated prior β grains. Their orientation belongs to the <11-20>α//z fiber, aligning the c-axis in RDs (z: axial direction of the disk). They are inherited from prior β grains belonging to the major <100>β and minor <111>β//z fibers. These feather-like structures are strong regions of the forging that act as a preferential crack initiation site. Adjacent to them, one can observe large colony with evidence of prismatic slip. Thus, the facet formation seems triggered by stress redistribution from “weak” to “strong” regions due to the elastic and plastic anisotropy. Finally, the occurrence of neighboring β grains able to share close oriented feather-like colonies is discussed considering the reconstructed β microtexture and texture. This study may be helpful for further texture control during the forging process.

Titanium Alloys: Processing and Properties

“…These publications are highly pertinent given the current interest around a recent in-service dwell-related failure of a Ti 6/4 fan disc [8]. The development of textured microstructures and the relationship to fatigue crack initiation via quasi-cleavage faceting is described by Germain et al [9]. The formation of inhomogeneous microstructures through highly localised variant selection during the forging process is highlighted as a subsequent driver for stress redistribution during loading between "weak" and "strong" regions of the material.…”