ASM Metals HandBook Volume 12 - Fractography
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ASM Metals HandBook Volume 12 - Fractography


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tendency toward intergranular decohesion (Fig. 51). When the 300 type stainless steels are sensitized--a 
condition that results in the precipitation of chromium carbides at the grain boundaries, causing depletion of chromium in 
the adjacent material in the grains--the steel becomes susceptible to SCC, which occurs principally along grain 
boundaries. 
Figure 52 shows the effect of the electrochemical potential, E, on the fracture path in a cold-worked AISI C-1018 low-
carbon steel that stress corroded in a hot sodium hydroxide solution. At an electrochemical potential of E = -0.76VSHE, the 
fracture path is predominantly intergranular; at a freely corroding potential of E = -1.00 VSHE, the fracture path is 
transgranular (Ref 182). 
SCC Aluminum. Aluminum alloys, especially the 2000 and 7000 series, that have been aged to the high-strength T6 
temper or are in an underaged condition are susceptible to SCC in such environments as moist air, water, and solutions 
containing chlorides. The sensitivity to SCC depends strongly on the grain orientation with respect to the principal stress, 
the short-transverse direction being the most susceptible to cracking. Figure 53 shows examples of stress-corrosion 
fractures in a 7075-T6 (maximum tensile strength: 586, MPa, or 85 ksi) aluminum alloy that was tested in water. The 
fracture occured primarily by intergranular decohesion. 
SCC of brass in the presence of ammonia and moist has long been recognized. The term season cracking was used to 
describe the SCC of brass that appeared to coincide with the moist weather in the spring and fall. Environments 
containing nitrates, sulfates, chlorides, ammonia gas and solutions, and alkaline solutions are known to stress corrode 
brass. Even distilled water and water containing as little as 5 × 10-3% sulfur dioxide have been shown to attack brass (Ref 
176, 178). Depending on the arsenic content of the Cu-30Zn brass, SCC in distilled water occurs either by intergranular 
decohesion by a combination of cleavage and intergranular decohesion (Fig. 54). When brass containing 0.032% As in 
stress corrode in water containing minute amounts of sulfur dioxide, it exhibits a unique transgranular fracture containing 
relatively uniformly spaced; parallel markings (Fig. 55). These distinct periodic marks apparently represent the stepwise 
propagation of the stress-corrosion fracture. 
SCC titanium alloys has been observed in such environments as distilled water, seawater, aqueous 3.5% sodium 
chloride, chlorinated organic solvents, methanol, red fuming nitric acid, and molten salts. Susceptibility depends on such 
variables as the microstructure (Ref 183, 184, 185), the amount of internal hydrogen (Ref 186), the state of stress (Ref 
187, 188), and strength level (Ref 188). In general microstructures consisting of large-grain \u3b1 phase or containing 
substantial amounts of \u3b1 phase in relation to \u3b2, high phase, high levels of internal hydrogen, the presence of a triaxial state 
of stress, and high yield strengths all promote the susceptibility of an alloy to SCC. If hydrogen is present in the corrosive 
environment, SCC will probably occur by a hydrogen embrittlement mechanism. Depending on the environment, alloy, 
and heat treatment (microstructure), mild stress-corrosion attack can exhibit a fracture that cannot be readily distinguished 
from normal overload, while more severe attack results in cleavage or quasi-cleavage fracture. 
Figure 56 shows a stress-corrosion fracture in an annealed Ti-8Al-1Mo-1V alloy that was tested in aqueous 3.5% sodium 
chloride. The stress-corrosion fractures in titanium alloys exhibit both cleavage (along with fluting) and quasi-cleavage. 
Corrosion products are a natural by-product of corrosion, particularly on most steels and aluminum alloys. They not 
only obscure fracture detail but also cause permanent damage, because a portion of the fracture surface is chemically 
attacked in forming the corrosion products. Therefore, removing the corrosion products will not restore a fracture to its 
original condition. However, if the corrosion damage is moderate, enough surface detail remains to identify the mode of 
fracture. 
Depending on the alloy and the environment, corrosion products can appear as powdery residue, amorphous films, or 
crystalline deposits. Corrosion products may exhibit cleavage fracture and secondary cracking. Care must be exercised in 
determining whether these fractures are part of the corrosion product or the base alloy. Some of the corrosion products 
observed on an austenitic stainless steel and a niobium alloy are shown in Fig. 57 and 58, respectively. Detailed 
information on the cleaning of fracture surfaces is available in the article "Preparation and Preservation of Fracture 
Specimens" in this Volume. 
 
Fig. 57 Corrosion products observed on an austenitic stainless steel hip implant device. (a) View of the fracture 
surface showing a mud crack pattern (arrow) that obscures fracture details. (b) Surface after cleaning in 
acetone in an ultrasonic cleaner. Arrow points to region exhibiting striations and pitting. (C.R. Brooks and A. 
Choudhury, University of Tennessee). 
 
Fig. 58 Corrosion products on the intergranular fracture surface of an Nb-106 alloy. These corrosion products, 
which are residues from acid cleaning, contributed to failure by SCC. (L. Kashar, Scanning Electron Analysis 
Laboratories, Inc.) 
Effect of Exposure to Low-Melting Metals. When metals such as certain steels, titanium alloys, nickel-copper 
alloys, and aluminum alloys are stressed while in contact with low-melting metals, including lead, tin, cadmium, lithium, 
indium, gallium and mercury, they may be embrittled and fracture at a stress below the yield strength of the alloy. If the 
embrittling metal is in a liquid state during exposure, the failure is referred to as liquid-metal embrittlement (LME); when 
the metal is solid, it is known as solid-metal embrittlement (SME). Both failure processes are sometimes called stress 
alloying. 
Temperature has a significant effect on the rate of embrittlement. For a specific embrittling metal species, the higher the 
temperature, the more rapid the attack. In addition, LME is a faster process than SME. In fact, under certain conditions, 
LME can occur with dramatic speed. For liquid indium embrittlement of steel, the time to failure appears to be limited 
primarily by the diffusion-controlled period required to form a small propagating crack (Ref 189). Once the crack begins 
to propagate, failure can occur in a fraction of a second. For example, when an AISI 4140 steel that was heat treated to an 
ultimate tensile strength of 1500 MPa (218 ksi) was tested at an applied stress of 1109 MPa (161 ksi) (the approximate 
proportional limit of the material) while in contact with liquid indium at a temperature of 158 °C (316 °F) (indium melts 
at 156 °C, or 313 °F), crack formation required about 511 s. The crack then propagated and fractured the 5.84-mm (0.23-
in.) diam electropolished round bar specimen in only 0.1 s (Ref 189). In contrast, at 154 °C (309 °F), when the steel was 
in contact with solid indium, crack nucleation required 4.07 × 103 s (1.13 h), and failure required an additional 2.41 × 103 
s (0.67 h) (Ref 189). 
Although gallium and mercury rapidly embrittle aluminum alloys, all cases of LME and, especially, SME do not occur in 
such short time spans. The embrittlement of steels and titanium alloys by solid cadmium can occur over months of 
exposure; however, when long time spans are involved, the generation of hydrogen by the anodic dissolution of cadmium 
in a service environment can result in a hydrogen embrittlement assisted fracture. The magnitude of the applied stress, the 
strain rate, the amount of prior cold work, the grain size, and the grain-boundary composition can also influence the rate 
of embrittlement. In general,