A rationalization of tensile ductility and fracture in Alpha-Beta Ti-Mn alloys
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I.
INTRODUCTION
V O I D nucleation, void growth, and tensile fracture in c~-/3 Ti alloys have previously been studied in the Ti-5.25A15.5V-0.9Fe-0.5Cu alloy (Ti-5-5) ~ and in the Ti-6A1-4V (Ti-6-4) alloy in the aged 2'3 and unaged conditions. 3 Work in this area was recently reviewed4 and only a brief discussion relating to fracture is given here. The longest void, Lv,* at a particular true strain, e, was *See Appendix for glossary of abbreviations and symbols.
obtained 1'2'3 by serial sectioning. When Lv was plotted as a function of e, a linear plot was obtained for Ti-5-51 and Ti-6-4. 2'3 This plot passed through the origin ~'2'3 and when extrapolated to the fracture strain, ei, the longest void at onset of catastrophic fracture, Li, was obtained. The axial stress at fracture, obtained by applying the Bridgman correction factor s to the nominal fracture stress was identified as o-it. O-lcis the stress acting on Lf at fracture. A plot of o-ic vs Li v2 was linear in all three cases. 1'2'3 In the case of the Ti-5-5 alloy the data for both equiaxed (E) a and Widmanst~itten plus grain boundary (W + GB) a fell on a single line. The yield strength for both structures was the same. However, for the Ti-6-4 alloy separate O'fc-Lf 1/2 relationships were obtained for Ea and W + GB a structures in the aged 2 and as-quenched conditions) For the Ti-5-5 alloy, the o-s~-L71/2curve extrapolated to a stress close to the yield stress at Ly 1/2 = 0. As noted earlier the Lv-e curves passed through the origin. This behavior indicated that void formation began very close to the onset of plastic deformation. These two observations, taken together, suggested that fracture in the Ti-5-5 alloy could not occur below the yield strength because no void or crack existed below this stress. Although the Lv-e curves for Ti-6-4 also passed through the origin, as noted above, the Cr~c-Lv~a curves extrapolated HAROLD MARGOLIN is Professor and T.V. VIJAYARAGHAVAN is Graduate Student in the Department of Physical and Engineering Metallurgy, Polytechnic Institute of New York, 333 Jay Street, Brooklyn, NY 11201. Manuscript submitted December 23, 1982. METALLURGICALTRANSACTIONSA
to stress values considerably above the yield stress. It was suspected that these larger stress values were the result of void tip blunting. Measurement of the void tip radii of curvature of the longest voids, as a function of void length, for as-quenched and for aged Ti-6-4 3 indicated that these radii did, indeed, increase continuously with void length. Since the stress intensity at the void tip would decrease with increasing void tip radius, it is not surprising that the extrapolated stresses for Ti-6-4 structures considerably exceeded the yield strength. The O-;c-L7v2 curves for both Ti-5-5 and for Ti-6-4 alloys supported the idea that, even for ductile fracture, a Griffith criterion existed, i.e., that catastrophic failure would occur, when the energy released during ductile fracture was greater than the energy absorbed. The suggestio n for a Griffith type fracture relati
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