Stress corrosion of Cu- Zn and Cu- Zn- Ni alloys: The role of dealloying
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I.
INTRODUCTION
STRESS corrosion cracking of copper alloys has been extensively investigated, but a clear understanding of the cracking mechanisms is still lacking. It is well known that cracking can occur intergranularly or transgranularly depending on various conditions, and many investigators believe that different mechanisms are responsible for cracking in the two cases ( e . g . , Reference I). Until recently, the most widely accepted mechanism for intergranular stress corrosion was the tarnish rupture model, which postulated that cracking in arnmoniacal solutions occurred due to preferential penetration of a macroscopic tarnish film at grain boundaries followed by rupture of the film, this process being repeated to final fracture. This model has been questioned because fractographic studies by the authors and others 2 have shown that the requisite tarnish film is not present on the fracture surface in the vicinity of the crack tip. In fact, the stress corrosion fracture surfaces have been shown to be resistant to tarnishing in the ammoniacal solutions): Other mechanisms that are currently widely accepted, while differing in detail, all suggest that propagation of a stress corrosion crack is due to enhanced anodic dissolution at the crack tip. 5-t~ The dissolution models leave several features of the cracking process unexplained. Evaluations of transgranular stress corrosion fracture surfaces have revealed that the fracture was crystallographic, the fracture surfaces being similar to cleavage crack surfaces. 9'H Beavers and Pugh have shown that transgranular stress corrosion in single crystals and polycrystals of admiralty brass occurs on parallel (II0) facets.~2 In addition, they found parallel markings on th~ fracture surface, normal to the crack propagation direction. Corresponding markings were found on the opposite fracture faces, and the features on the two fracture surfaces were found to be matching and interlocking. These features led to the conclusion that the parallel markings were due to crack arrest and that transgranular stress corrosion in admiralty brass occurred by a discontinuous brittle cracking mecha-
ARVIND PARTHASARATHI, Research Scientist, and NED W. POLAN, Corrosion Group Supervisor, are both with Olin Brass Metals Research Laboratory, New Haven, CT 06511. Manuscript submitted March 4, 1982.
METALLURGICAL TRANSACTIONS A
nism; this conclusion was further supported by acoustic emission studies. While the reason for the embrittlement was not clear, a dissolution mechanism was ruled out. That this apparent embrittlement of normally ductile copper alloys could be due to dealloying, as suggested by Edeleanu and Forty, 13 was considered by Beavers and Pugh. They estimated that the value of the bulk diffusivity of Zn in Cu at room temperature, extrapolated from high temperature data, is - 1 0 -34 cm 2 per second. This value was many orders of magnitude smaller than that required, in their estimation, to account for the observed cracking rates. Beavers and Pugh accordingly discarded the deal
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