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I.

INTRODUCTION

F A T I G U E - c r a c k initiation in Ti-6A1-4V alloys has been studied by many investigators/~-15] and a variety of FCI mechanisms have been observed. Since different thermomechanical fabrication practices for this alloy system lead to a wide variety of microstmctures and textures, its fatigue performance would be expected to vary markedly with fabrication practice. The factors that have been found to influence the FCI resistance of Ti-6A1-4V are

(1) (2) (3) (4)

surface topography; [L2] residual s t r e s s e s ; [2-6] microstructure; [7-13j and crystallographic texture. [7,9.1~

Fatigue-crack initiation in Ti-6A1-4V has been observed to occur by several different mechanisms operating either at or beneath the surface. These mechanisms include a / / 3 interface cracking, [9,12,15] primary a cracking at slip bands (trans-a cracking) [8,1~ or slipless crackins, {161 and subsurface cracking by a cleavage-type mechanism. I~7,181 The particular operative mechanism is dependent upon the four factors cited above, as well as a variety of mechanical and environmental conditions such as frequency of loading, I]9] mean stress, [2~ and corrosive environment. [8.14] Surface topography can affect and even dominate the initiation of fatigue cracks. Small defects or scratches at the surface have been observed to facilitate fatigue initiation. I2] In commercial products such as medical devices, the surface layer is often in a mechanically polished condition. This polishing process can result in small (< 1.0/.tm) scratches on the surface of the material, which may be detrimental to FCI resistance. Shot peening of Ti-6AI-4V, while imparting compressive residual stresses, can also result in a very irregular and topographically rough surface that may negate the benefits of the conJEREMY L. GILBERT, Assistant Professor, is with the Biological Materials Division, Northwestern University, Chicago, IL 60611. HENRY R. PIEHLER, Professor, is with the Metallurgical Engineering and Materials Science Department, Carnegie Mellon University, Pittsburgh, PA 15213. Manuscript submitted September 25, 1987. METALLURGICAL TRANSACTIONS A

comitant compressive residual stresses. Takemoto et al. m observed that the radii of scratches or surface defects arising from machining or shot peening have a greater influence on fatigue strength than the depth of these defects. Residual stresses at the surface of the sample can arise from a variety of processing sources. Machining, forging, heat treating, shot peening, and polishing all result in a residual stress gradient at and near the surface of the sample. These residual stresses at the surface can be tensile or compressive in nature, but since the residual stresses must be in equilibrium across the entire crosssection, there must be a region below the surface where the residual stresses are of the opposite sense. In cases where a compressive surface residual stress results from processes such as shot peening, polishing, or machining, the corresponding tensile stresses required to balance