Diffusion-controlled brittle intergranular fracture in cu-alloy bicrystals and polycrystals / Ranjani C. Muthiah.

Format:

Book

Manuscript

Microformat

Thesis/Dissertation

Author/Creator:

Muthiah, Ranjani C.

Contributor:

University of Pennsylvania.

Language:

English

Subjects (All):

Penn dissertations--Materials engineering.

Materials engineering--Penn dissertations.

Local Subjects:

Penn dissertations--Materials engineering.

Materials engineering--Penn dissertations.

Physical Description:

xx, 226 pages : illustrations ; 29 cm

Production:

1997.

Summary:

Dynamic embrittlement occurs by quasi-static decohesion, usually along grain boundaries, caused by the stress-driven diffusion of surface-active embrittling elements into a polycrystalline material. The Bika-McMahon kinetic model for dynamic embrittlement predicts that the cracking rate is proportional to the diffusivity of the embrittling species along the grain boundary.

The objective of this thesis is to study the phenomenon of dynamic embrittlement in bicrystals and polycrystals of copper-based alloys. Diffusion-bonded bicrystals of Cu-7%Sn with a $\Sigma$ 5 (031)/(100) symmetrical tilt boundary were used to test the Bika-McMahon model and to establish a mechanistic understanding of the cracking process. Polycrystals of a precipitation-hardened Cu-(0.26-0.4)%Be alloy tested in oxygen-containing atmospheres were used to test the hypothesis of a generic embrittlement phenomenon. This was achieved by crack-growth-rate studies using fixed-displacement tests in four-point-bending on both alloy systems.

The Cu-Sn bicrystals, in which surface-segregated tin was the embrittling species, were tested at 265$\sp\circ$C in vacuum parallel and perpendicular to the tilt axis. Cracking occurred parallel to the tilt axis, the fast diffusion direction, by the propagation of a sharp crack at a maximum rate of ${\sim}2\mu$m/sec and at a stress intensity of less than 3.5 MPa$\surd$m. Cracking appeared to be continuous, suggesting that the tin diffusion occurs in the core of the crack tip. No cracking occurred perpendicular to the tilt axis, i.e., the slow-diffusion direction. In the absence of cracking, plastic creep occurred with the formation of cavities at the grain boundaries. This supports the hypothesis of a grain-boundary-diffusion process leading to cracking. The susceptibility to this type of cracking appears to be extremely sensitive to grain boundary structure. Bicrystals that had no particles (polishing grit) at the boundary were resistant to cracking, even in the parallel direction.

The Cu-Be alloys were tested in oxygen-containing atmospheres in the temperature range 150 to 225$\sp\circ$C. Cracking was found to be brittle, intergranular and occurred at a few micrometers/sec in air and 1 atm. of oxygen, while it was three orders of magnitude slower in vacuum. The temperature dependence of the maximum cracking rate in oxygen was found to be 6.6kcal/mole, which is close to that for grain-boundary diffusion of oxygen in copper. Cracking occurred in a manner similar to that of S-induced cracking of steels and Sn-induced cracking of bronze, i.e., smooth intergranular facets and intermittent cracking. Thus, the embrittling species from the atmosphere, oxygen, behaves like those in solid solution, i.e., sulfur in steels and tin in bronze. This supports the hypothesis of a generic embrittlement phenomenon.

Notes:

Thesis (Ph.D. in Materials Engineering) -- University of Pennsylvania, 1997.

Includes bibliographical references.

Local Notes:

University Microfilms order no.: 98-00904.

OCLC:

244969745