A mechanism for transgranular stress-corrosion cracking
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I.
INTRODUCTION
T H E R E is ample evidence that the combination o f stress and corrosive environment can lead to transgranularstress corrosion cracking (T-SCC). When this does occur, the resulting cracks have been found to be crystallographic in nature, with matching fractographic features on opposing surfaces, m Depending on the specific combination o f prevailing conditions, w h i c h must fall within certain limits for T-SCC to occur, ~2] the c r a c k appearance can range from very brittle to very ductile; and f o r specific growth directions, the cracking can be discontinuous, manifested by crack-arrest markings w h i c h are correlated with the occurrence o f crack-blunting slip. [3] The presence o f ductile features is not surprising, but the fact that the c r a c k can appear to be very brittle suggests that cleavage is involved and this is surprising when the materials undergoing T-SCC are ductile in noncorrosive environments. There are many generally agreed upon mechanisms that can be invoked to explain the initial nucleation o f such cracks, e.g., pitting, t4t dissolution o f active slip planes, t51 the failure o f a brittle film t61 o r spongetT] on the surface, etc.; but the propagation o f the resulting c r a c k into the material and its continued renucleation and propagation a f t e r c r a c k arrest occurs are more difficult to explain. Time-dependent embrittlement by hydrogen tS~ and the growth o f surface sponge o r film at the c r a c k tip t6j have been proposed as mechanisms and these can lead to cracking, but the observed kinetics o f c r a c k growth and the absence o f hydrogen in many occurrences o f T-SCC requires that a different mechanism b e available as well. W . F . F L A N A G A N and B . D . L I C H T E R , Professors, and LIJUN ZHONG, Graduate Student, are with the Department of Materials Science and Engineering, Vanderbilt University, Nashville, T N 37235. This article is based o n a presentation made i n the symposium " Q u a s i Brittle Fracture" presented during the TMS fall meeting, Cincinnati, OH, October 2 1 - 2 4 , 1991, under the auspices of the TMS Mechanical Metallurgy C o m m i t t e e and the A S M / M S D F l o w and Fracture Committee. METALLURGICAL TRANSACTIONS A
W e have studied T-SCC in alpha-brass tSj and in goldcopper alloys t2,3,7,91 u n d e r conditions where just such restrictions apply, i.e., where hydrogen is precluded, where no films are formed, and where the kinetics o f c r a c k growth is such that sponge formation is insufficient to lead to continued cracking. However, it should be pointed out that in these studies, passivation-like behavior occurred because o f an "ennobled" surface being formed by dealloying. Based on the results o f these and other studies, t~°,l~l mechanisms for c r a c k nucleation and for the propagation and arrest o f a c r a c k once it has been (re-)nucleated will be discussed.
II.
DESCRIPTION OF M O D E L
A . Nucleation o f Crack It has been proposed E12j that Lomer-Cottrell locks can act to nucleate T - S C C cracks
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