The role of inclusion and pore content on the fracture toughness of powder-processed blended elemental titanium alloys
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INTRODUCTION
TITANIUM alloy components made from blended elemental powders can be a cost-effective alternative to components made from ingot-processed alloys or powders processed from ingots. ~ Although the as-sintered densities of blended elemental powder alloys approach the theoretical density of ingot-processed alloys, inherent microstructural defects of pores and chloride inclusions reduce the mechanical properties and the crack growth resistance of these alloys when compared to corresponding ingot-processed alloy values. ]2-51 Two approaches have been used to obtain required properties from powder-processed titanium alloys for specific applications. They are (l) the substitution of powder-processed alloys with compositions that differ from those of ingot-processed alloys specified for the component and (2) hot isostatic pressing of powderprocessed alloys to reduce the pore content. The first approach was used by Smugeresky and Moody fSJto find an as-sintered, powder-processed alloy that had tensile properties and fracture toughness values comparable to ingot-processed Ti-6A1-4V. These criteria were met with powder-processed Ti-10V-2Fe-3A1. However, the tensile properties and crack growth resistance of this powderprocessed alloy were less than those of ingot-processed Ti- 10V-2Fe-3AI. The second and most commonly used approach to improve properties is hot isostatic pressing. This technique produces relative densities greater than 99 pct of the theoretical density with an increase in strength, ductility, N.R. MOODY, J.E. SMUGERESKY, and J.E. COSTA, Senior Members of the Technical Staff, are with the Sandia National Laboratories, Livermore, CA 94551-0969. W.M. GARRISON, JR., Professor, is with the Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213. Manuscript submitted November 15, 1991. METALLURGICAL TRANSACTIONS A
and crack growth resistance when compared to as-sintered material, t3] The strengths and ductilities are also comparable with ingot-processed alloys of the same composition. However, the fracture toughness values remain significantly lower than the ingot-processed alloy values at similar strength levels. The lower fracture toughness values are attributed to the presence of chloride inclusions which prematurely trigger ductile fracture.]SJ Ductile fracture is a mode of material failure that occurs when pores or voids nucleated during deformation coalesce to form a continuous fracture path. In many alloy systems, these voids nucleate at inclusions and secondphase particles by decohesion of the particle-matrix interface or by particle fracture. ]6] Most experimental evidence suggests that for a given particle type, void nucleation occurs first at larger particles and then at smaller panicles as deformation continues. ]7-10] In many engineering alloys, there exist several particle types. It has been observed that voids will nucleate first at particles of a particular type and that voids will nucleate much later in the fracture process or perhaps not at
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