Evidence of void nucleation and growth on planar slip bands in a Nb-Cr-Ti alloy
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I.
INTRODUCTION
VOID nucleation usually occurs at hard particles or inclusions in engineering alloys such as copper,[1] steels,[2] and Al alloys.[3] The process commences with interface decohesion or fracture of the hard particles, depending on the strength of the interface relative to the particle. Subsequent growth of the voids around the nucleation site invariably leaves the particle at the bottom of the voids. On the other hand, dimpled fracture also occurs in materials such as Ti[4] alloys, which exhibit planar slip behavior but do not normally contain hard particles. Dimpled fracture has also been reported for Nb-Cr-Ti,[5,6] Nb-Al-Ti,[7,8] and Nb-Si[9] solidsolution alloys. The Nb-Cr-Ti alloy was a bcc Nb solid solution that contained 13 at. pct Cr and 37 at. pct Ti and exhibited a large grain size ('1500 mm). The slip character was planar,[5] and the fracture toughness exceeded 100 MPa=m when measured on 5-mm-thick compact-tension specimens.[5,6] The voids in this Nb-Cr-Ti solid-solution alloy occurred on fracture surfaces that contained a combination of cleavage, slip-band decohesion, grain-boundary cracking, and dimpled fracture facets.[5,6] For this planar slip material, the void nucleation process responsible for the observed dimpled fracture remains elusive. The lack of understanding applies to other materials also. In Ti3Albased[10] and FeAl-based[11] intermetallic alloys, the fracture surfaces are also characterized by cleavage facets, but dimpled fracture faces containing voidlike features have been observed. Like the planar slip metals, the origin of these voidlike features in these intermetallics has not been identified. The formation of a cleavage crack by slip on two intersecting planes was proposed by Cottrell.[12] Nucleation of a microcrack by decohesion on a slip plane containing a dislocation pileup was proposed and analyzed by Stroh.[13] Recently, Chan combined intersecting slip and slip-band decohesion into one mechanism to explain the formation of dimples in planar slip materials.[14] Figure 1 shows a scheKWAI S. CHAN and DAVID L. DAVIDSON, Institute Scientists, are with the Southwest Research Institute, San Antonio, TX 78238. Manuscript submitted April 23, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
matic illustration of the proposed dimpled fracture mechanism. The process commences with planar slip on a set of planes impinging on another set of slip or cleavage planes (Figure 1(a)). The void nucleation process commences when impinging slip leads to microcrack formation at either the intersecting slip-band or cleavage plane (Figure 1(b)). Upon further deformation, the microcracks grow into voids as the result of continual slip on the slip planes (Figure 1(c)) and coalescence by linkage with voids on upper and lower planes that shear the common intersecting slip planes (Figure 1(d)). The proposed model has been used to explain the fractographic observations of parallelepiped, cuboidal, and spheroidal voids in Nb-Cr-Ti alloys.[14] Direct experimental observation of the dimpled fractur
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