Toughening mechanisms in ductile niobium-reinforced niobium aluminide (Nb/Nb 3 Al) in situ composites
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INTRODUCTION
DUCTILE-PHASE toughening through the incorporation of ductile reinforcements into a brittle matrix has proven to be an effective method of improving the ductility and toughness of many intermetallic and ceramic materials.tl,:,3[ The technique has been particularly successful in the development of T-TiA1 composites with small volume fractions of Nb, TiNb, and Ti-6A1-4V particles, where sharply rising resistance-curve (R-curve) behavior (increasing toughness with crack extension) due to crack bridging by uncracked ductile particles in the crack wake has been reported. E~71More recently, the approach has also been used to toughen Nb3A1 intermetallic alloys through reinforcement with Nb, both in the form of a microlaminated composite with -2-/xm thick Nb layers[8,9] and an in situ dual-phase composite containing discontinuous Nb particles.[1o-131
The prime objective of ductile-phase toughening is to enhance the fracture resistance of the material through crack-tip shielding arising from tractions provided by unbroken ductile ligaments that bridge the crack wake. t~,2,31 When the length of the bridging zone is small compared to the specimen and crack length dimensions, the fracture toughness increases with crack extension up to a maximum steady-state (plateau) level, KssB, associated with the de-
C.D. BENCHER, Staff Scientist, formerly Research Student, Department of Materials Science and Mineral Engineering, University of California-Berkeley, is Staff Scientist with Applied Materials Inc., Santa Clara, CA 95050. A. SAKAIDA is Assistant Professor with the Department of Mechanical Engineering, Akaski College of Technology, Kyoto 674, Japan. K.T. VENKATESWARA RAO, Research Scientist, and R.O. RITCHIE, Professor, are with the Department of Materials Science and Mineral Engineering, University of California-Berkeley, Berkeley, CA 94720-1760. Manuscript submitted September 23, 1994. METALLURGICAL AND MATERIALSTRANSACTIONS A
velopment of a steady-state bridging zone, LssB. At this small-scale bridging limit, KssB is given by:[71 KssB = ~//~2+ E ' f t
~ro X
[11
where K, is the crack-tip stress intensity for crack initiation (often taken as the fracture toughness of the unreinforced matrix), E' is the plane-strain elastic modulus for the composite [= El1 - v2), where E is Young's modulus and v is Poisson's ratio]; 0% f, and t refer to the yield strength, volume fraction, and characteristic microstructural dimension of the ductile phase, respectively. The nondimensional work of rupture, X, is determined from the area under the normalized-reinforcement stress [oiu)] - displacement [u] function, as [21
X = ,~o i..~-~o J \ t /
[2]
where u* is the critical crack-opening displacement at the point of rupture of the reinforcement phase. Values for X can vary between 0.5 and 8, depending upon the extent of interfacial debonding and constitutive properties of the ductile reinforcement;E4.5] in Nb/Nb3A1 composites, where the Nb/matrix interface is well bonded, X - 2.7.[~3] Providing that the crack intercepts the
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