A closer look at the transgranular stress corrosion cracking of Cu-30Zn in cuprous ammonia
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Fig. 3--Scanning electron mlcrographs (SEM's) of the fracture surfaces of the single crystal ~amples tested m (at air and (b) the cuprous ammonia solution. The large holes result from solidification shrinkage and are not associated w~th the fracture process.
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Fig. 4--SEM's of the fracture surfaces of single crystal samples tested in the cupric ammonia solunon (a) low magnification showing cleavage-hke appearance and (b) higher magmficatlon displaying the crystallographic features
cleavage-like (Figure 6(a)), somewhat similar to those obtained in the cupric solution (Figure 4). in addition. the presence of etch pits in this region indicates that anodic dissolution of Cu was possible during the cracking process. Even so, it is clear that the primary features characteristic of typical T G S C C fractures in this alloy 1540--VOLUME 18A, AUGUST 1987
tFigure 4 ) - - f i a t facets and crystallographic s t e p s - - a p p e a r to have been significantly affected by the difference in solution chemistry. These differences became even more pronounced when the fracture surface farther away from the region of crack initiation was examined. For example, for the regions near the center of the fracture surface the METALLURGICAL TRANSACTIONS A
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Fig 5 - - S E M ' s of the fracture surfaces of the polycrystalhne samples tested m (a) mr and (b~ the cuplous ammoma solution.
features, though still somewhat cleavage-like in appearance (Figure 6(b)), appeared substantially more distorted than those in Figure 6(a). Furthermore, for the region opposite initiation (Figure 6(c)), the fracture was no longer cleavagelike but rather displayed a very complex appearance. METALLURGICAL TRANSACTIONSA
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Fig 6 - - S E M ' s from different regions of the fracture surface of the unplated, stogie crystal sample: (a) near initiation, (b) near the middle, and (el near completion. VOLUME 18A, AUGUST 1987-- 1541
The most probable explanation for the variation in the fractography in Figure 6 simply involves the fact that, as the crack lengthens during propagation, the transport of oxygen to the crack tip decreases, resulting in decreasing concentrations of cupric ions and lower rates of anodic dissolution. This is anticipated based on the slow rates of propagation ( - 0 . 1 /zm/s in cupric solutions) and the large diameter ( - 7 ram) of the single crystal sample used in the test. Thus, there is strong evidence that the differences in fracture relate to the variation of the cupric-ion concentration in the solution during testing. As a result, there appears to be some critical concentration of cupric ions necessary to cause TGSCC at a given strain rate and, for concentrations below this value, the failure occurs by conventional ductile fracture processes. Conversely, for a given cupric-ion concentration, above some minimum value, there will be a "window" of strain rates where TGSCC will occur; ~ for strain rates above the maximum defined by this window, the material undergoes general deformation because ther
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