The influence of oxygen on the structure, fracture, and fatigue crack propagation behavior of Ti-8.6 Wt Pct Al

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I.

INTRODUCTION

INTERSTITIAL oxygen has long been known to have a pronounced influence on the structure and mechanical properties of titanium alloys, particularly those containing the ordered Ti3A1 (hereafter, 0/2) phase, tt-51 Systematic studies on binary Ti-A1 alloys have suggested that a major portion of this influence is related to the fact that oxygen acts to stabilize 0/2 in addition to 0/ by raising the 0//0/ + /3 and 0//0/ + 0/2 phase boundariesJ z,61 The raising of the 0//0/ + 0/2 solvus in alloys which contain enough AI to form 0/2 has also been qualitatively linked to an increased tendency for inhomogeneous planar slip in alloys containing ->6 wt pct A1, probably due to higher volume fractions of 0/2 .H'6] In alloys containing < 6 wt pct AI, increased oxygen has also been suggested to increase the tendency for planar slip due to the enhanced degree of short-range order. The restriction of plastic flow to narrow slipbands, causing high local stresses, favors crack nucleation and fracture at low macroscopic plastic strains3 TM The onset of intense planar slip with increasing oxygen I61 correlates with decreased tensile ductility, fracture toughness, stress corrosion cracking resistance, and fatigue strength in titanium alloys. 19 ~41This decrease in fracture resistance is frequently accompanied by a transition from ductile rupture to cleavage-like fracture similar to that seen in bcc metals. Earlier studies have shown that coarse-grained Ti-AI alloys can fracture by cleavage and that the cleavage fracture plane is near to but not coincident with the basal plane. I~41 The brittle fracture facets observed in the present work are similar in appearance to those identified as cleavage. It41 Thus, we will refer to this fracture mode as cleavage or cleavagelike fracture throughout this paper. The transition from ductile rupture to cleavage-like G.T. GRAY Ill, Staff Member, is with the Los Alamos National Laboratory, Los Alamos, NM 87545. G. LUETJERING, Professor, is with Technische Universitaet Hamburg-Harburg, Harburger Schlosstrasse 20, 2100 Hamburg 90, Federal Republic of Germany. J.C. WILLIAMS, General Manager--EMTL, is with General Electric Aircraft Engines, One Neumann Way, Cincinnati, OH 45215-6301. Manuscript submitted April 1, 1988. METALLURGICAL TRANSACTIONS A

fracture is thought to be related to the combined effects of two factors. First, increasing the oxygen content increases the volume fraction of ordered Ti3A1 precipitates, leading to slip intensification in planar slipbands. Second, the effect of oxygen is additive to that of A1 in solid solution in promoting planar slip in the matrixJ 81 Therefore, oxygen intrinsically promotes slip localization by inhibiting dislocation pileup relaxation due to slipband broadening by double cross slip (between the 0/~ particles)J 81 Examination of the literature on 0/ and 0/ + /3 titanium alloys suggests that their fracture-related properties have a strong dependence on the deformation characteristics of the 0/phase, particularly slip character and slip lengt