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


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°F) did not (Fig. 74). The fracture at 
room temperature exhibited intergranular dimple rupture because the material adjacent to the grain boundaries is weaker 
due to the depletion of coarse \u3b3' precipitates. The absence of dimples at 816 °C (1500 °F) was the result of intense 
dislocation activity along the grain boundaries, producing decohesion at M23C6 carbide/matrix interfaces within the 
boundaries (Ref 214). 
 
Fig. 74 Effect of temperature on double-aged Inconel X-750 that was tested at a nominal strain rate of 3 × 10-
5 s-1. (a) and (b) The fracture at room temperature occurs by intergranular dimple rupture. Note the evidence of 
dimple rupture network on the intergranular walls. (c) and (d) At 816 °C (1500 °F), the fracture shows 
intergranular decohesion with no dimple rupture. However, the intergranular facets are roughened by the 
presence of M23C6 carbides. Source: Ref 214. 
A distinct change in fracture appearance was also noted during elevated-temperature tensile testing of Haynes 556, which 
had the following composition: 
 
Element Composition, % 
Iron 28.2 
Chromium 21.5 
Nickel 22.2 
Cobalt 19.0 
Tungsten 2.9 
Molybdenum 2.9 
Tantalum 0.8 
Manganese 1.4 
Silicon 0.5 
Copper 0.1 
Nitrogen 0.1 
Three specimens were tested at a strain rate of approximately 1 s-1 at increasing temperatures. At 1015 °C (1860 °F), 
specimen 1 underwent a 72% reduction of area and fractured by dimple rupture (Fig. 75a). At 1253 °C (2287 °F), 
specimen 2 exhibited an 8% reduction of area. Fracture occurred intergranularly by grain-boundary decohesion (Fig. 
75b). Specimen 3, which was tested at 1333 °C (2431 °F), fractured because of local eutectic melting of TaC + austenite, 
with 0% reduction of area (Fig. 75c). 
 
Fig. 75 Effect of test temperature on the fracture of Haynes 556, which was tensile tested at a strain rate of 1 
s-1 at increasing temperature. (a) Dimple rupture fracture at 1015 °C (1860 °F). At the bottom of many of the 
dimples are TaC inclusions, which initiated microvoid coalescence. (b) Intergranular decohesion at 1523 °C 
(2287 °F). Secondary intergranular cracks are also visible. (c) Local eutectic melting of TaC + austenite at 1333 
°C (2431 °F). (J.J. Stephens, M.J. Cieslak, R.J. Lujan, Sandia National Laboratories) 
A final example of the effect of temperature on the fracture process is the behavior of a titanium alloy when tested at 
room temperature and at 800 °C (1470 °F). At room temperature, the fracture in a Ti-6Al-2Nb-1Ta-0.8Mo alloy (heat 
treated to produce a basket-weave structure consisting of Widmanstätten \u3b1+ \u3b2+ grain-boundary \u3b1 with equiaxed prior-\u3b2 
grains) that was tested at an approximate strain rate of 3.3 × 10-4 s-1 occurred predominantly by transgranular dimple 
rupture. At 800 °C (1470 °F), however, the alloy exhibited low ductility, which is associated with an intergranular dimple 
rupture (Fig. 76). The low ductility can be explained by void formation and coalescence along prior-\u3b2 grain boundaries 
because of strain localization in the \u3b1 phase within the grain boundaries (Ref 215). 
 
Fig. 76 Intergranular dimple rupture in a Ti-6Al-2Nb-1Ta-0.8Ta alloy tested at 800 °F (1470 °F). The fracture 
path in this alloy, tensile tested at a strain rate of about 3.3 × 10-4 s-1, changes at 800 °C (1470 °F) from a 
predominantly transgranular dimple at room temperature (not shown) to intergranular dimple rupture. Source: 
Ref 215 
As has been shown, testing temperature can significantly affect fracture appearance. However, in addition to temperature, 
such factors as the strain rate and solid-state reactions must be considered when evaluating the effect of temperature on 
the fracture process. 
Effect of Oxidation. A natural consequence of high-temperature exposure is oxidation. Engineering alloys exposed to 
elevated temperatures in the presence of an oxidizing medium, such as oxygen (air), form oxides. The degree of oxidation 
depends on the material, the temperature, and the time at temperature. Oxidation, which consumes a part of the fracture 
surface in forming the oxide, can also obscure significant fracture detail. 
Figures 77, 78, and 79 show the effects of high-temperature air exposure on the overload fracture surfaces of two titanium 
alloys and a steel. The progressive deterioration of the fracture surface of an annealed Ti-6Al-2Sn-4Zr-6Mo alloy exposed 
for various times at 700 °C (1290 °F) in air is illustrated in Fig. 77. As seen in Fig. 77(b), the oxide formed after only a 3-
min exposure already obscured the fine ridges of the smaller dimples. An example of an extremely severe oxidation attack 
is shown in Fig. 78. The oxide cover is so complete that it is not possible to identify the fracture mode. A similar result 
was observed for a 300M high-strength steel fracture exposed for only 5 min at 700 °C (1290 °F) (Fig. 79). The relatively 
short exposure formed an oxide film that completely covered the fracture surface and rendered even the most prominent 
features unrecognizable. 
 
Fig. 77 Effect of a 700- °C (1290- °F) air exposure on an annealed Ti-6Al-2Sn-4Zr-6Mo alloy dimple rupture 
overload fracture. (a) As fractured. (b) The identical area as in (a) except exposed for 3 min (c) 10 min. (d) 30 
min. As time at temperature increases, the fracture surface becomes progressively more obscured by the oxide. 
(V. Kerlins, McDonnell Douglas Astronautics Company) 
 
Fig. 78 Effect of a 15-min 800- °C (1470- °F) air exposure on a dimple rupture fracture surface of an annealed 
Ti-6Al-6V-2Sn alloy. (a) Fracture appearance before exposure. (b) The identical fracture surface after exposure. 
The oxide buildup is so great that it is impossible to identify the fracture mode. (V. Kerlins, McDonnell Douglas 
Astronautics Company) 
 
Fig. 79 Effect of a 5-min 700- °C (1290- °F) air exposure on a 300M (2028 MPa, or 294 ksi) high-strength 
steel overload fracture. (a) Fracture appearance before exposure. (b) The same area after exposure. The entire 
fracture, which exhibited dimple rupture, was covered by an oxide that obscured all fracture detail. Some areas 
on the oxidized surface contained needlelike whiskers, which are probably an iron oxide. (V. Kerlins, McDonnell 
Douglas Astronautics Company) 
Effect of Environment on Fatigue 
Of the different fracture modes, fatigue is the most sensitive to environment. Because fatigue in service occurs in a variety 
of environments, it is important to understand the effects of these environments on the fracture process. Environments can 
include reactive gases, corrosive liquids, vacuum, the way the load is applied, and the temperature at which the part is 
cycled. 
Just as environments vary, the effects of the environments also vary, ranging from large increases in the fatigue crack 
propagation rates in embrittling or corrosive environments to substantial decreases in vacuum and at low temperatures. 
Because fatigue is basically a slip process, any environment that affects slip also affects the rate at which a fatigue crack 
propagates. In general, conditions that promote easy slip, such as elevated temperatures, or interfere with slip reversal 
(oxidation), enhance fatigue crack propagation and increase the fatigue striation spacing. Environments that suppress slip, 
such as low temperatures, or enhance slip reversal by retarding or preventing oxidation (vacuum) of newly formed slip 
surfaces decrease crack propagation rates, which decreases the fatigue striation spacing and in extreme cases obliterates 
striations. 
In some embrittling or corrosive environments, however, the fatigue crack propagation rate can be affected not only by 
interfering with the basic slip process but also by affecting the material ahead of the crack front. This can result in the 
formation of brittle striations and in the introduction of quasi-cleavage, cleavage,