On the nature and crystallographic orientation of subsurface cracks in high cycle fatigue of Ti-6Al-4V
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INTRODUCTION
SUBSURFACE cracking has been established as one possible initiation mechanism in high cycle fatigue o f Ti6A1-4V alloys, tl-6J This mechanism tends to increase in frequency as the number o f cycles to failure increases into the high cycle fatigue regime. Often the region o f initiation in Ti-6A1-4V does not contain any detectable defect that may have served as a stress concentration site for c r a c k initiation p r i o r to failure. However, the fracture surface o f subsurface-initiated fatigue samples contains a number o f flat, smooth features or facets at the point o f fatigue-crack initiation. In this case, c r a c k initiation has been proposed to arise as a result o f fracture o f the hexagonal t~ phase on particular crystallographic planes. 13-5,7] Neal and Blenkinsop t7] and Ruppen e t a l . t2,31 have proposed various dislocation-based mechanisms that account for an apparent defect-free initiation region. Neal and Blenkinsop,17~ based on transmission electron microscopy (TEM) observations, proposed that (a) dislocation pileups on p r i s m planes in the hexagonal lattice cause basal cleavage. Ruppen e t a / . I2] have proposed a dislocation mechanism where a pileup in the a phase causes cracking on the {100} o f the fl phase w h i c h , in t u r n , causes cracking in the t~ phase on a p r i s m plane. They also noted intense slip in the form o f dislocation pileups on the pyramidal planes. They show that pyramidal slip in the a grains can pass continuously through the adjacent fl regions on a {110} plane into the neighboring a grains. However, slip on p r i s m planes appears
JEREMY L. GILBERT, Assistant Professor, is with the Division of Biological Materials, Northwestern University, Evanston,IL 60611. HENRY R. PIEHLER, Professor, is with the Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, P A 15213. Manuscript submitted May 18, 1992. METALLURGICAL TRANSACTIONS A
to be blocked by the fl phase. It was suggested that the pyramidal slip system may aid in c r a c k initiation. Davidson e t al. t4,5] used electron channeling patterns (ECPs) to determine f a c e t orientations in subsurfaceinitiated fatigue failures o f fl annealed, near-a T i alloys with large prior/3 grain size. By using an electropolishing technique to remove the damaged l a y e r on the fracture surface b e l o w the fracture facet, a c l e a r ECP could be obtained. Their results indicate that facet orientation was between 4 and 32 deg o f the basal orientation. Davidson e t a l . had some difficulty in obtaining a pattern from these facets because o f the deformation present and the uncertainty in the orientation o f the facet to them beam axis, particularly a f t e r electropolishing. These factors introduce other unknowns into the determination o f the orientation o f the facet. In each o f these studies, it was determined that fatigue cracks initiated on crystallographic planes. Therefore, it is expected that the texture and, to an extent, the grain morphology o f the t~ phase
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