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


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predominantly intergranular with no distinct fatigue striations. Source: Ref 242 
Effect of Vacuum on Inconel X-750. A material that was essentially unaffected by vacuum exposure at 25 °C (75 
°F) but showed a five- to tenfold decrease in the fatigue crack growth rate at 650 °C (1200 °F) is Inconel X-750 (Ref 
245). The annealed Inconel X-750 was fatigue tested in air and vacuum (10-5 torr) at a stress intensity range of \u2206K = 20 to 
50 MPa m (18 to 45.5 ksi in ) with a load ratio of R = 0.05 and a triangular wave form having a cyclic frequency of 10 
Hz. 
The largest decrease in the fatigue crack growth rate in vacuum at 650 °C (1200 °F) occurred at the lower \u2206K testing 
levels. Figure 90 shows the air and vacuum fatigue fractures produced at a stress intensity range of \u2206K = 20 MPa m (18 
ksi in ). The fractures in both environments were predominantly transgranular. The air fatigue fractures exhibited a more 
crystallographic, faceted appearance than those in vacuum. The vacuum fractures contained periodic marking that were 
believed to be due to slip offsets resulting from stress gradients produced by crack front jumps (Ref 245, 246). The large 
decrease in the fatigue crack growth rate in vacuum at 650 °C (1200 °F) was attributed to the absence of oxygen, which 
can diffuse along slip bands to embrittle the material ahead of the crack tip and increase the rate at which a fatigue crack 
propagates. No reason was given for the identical fatigue propagation rates in air and vacuum at 25 °C (75 °F), although 
the high oxidation and corrosion resistance of this alloy at ambient temperatures rendered it immune to any adverse 
effects of air. 
 
Fig. 90 Fatigue fractures in Inconel X-750 tested at a stress intensity of \u2206K = 20 MPa m (18 ksi in ) in air 
and vacuum at 650 °C (1200 °F). The crack propagation direction is from bottom to top. The fracture in air (a) 
exhibited a faceted, crystallographic appearance; the vacuum fracture (b) was less faceted and contained 
distinct, periodic crack front markings. Source: Ref 245 
Effect of Vacuum on Commercially Pure Titanium. A reversal in the general effect of a reduction in the fatigue 
crack growth rate in vacuum was observed in an IMI 155 (British designation) commercially pure titanium containing 
0.34% O (Ref 236). The fatigue tests were conducted in laboratory air and vacuum (2 × 10-6 torr) at room temperature at a 
stress intensity range of \u2206K = 11 to 20 MPa m (10 to 18 ksi in ), a load ration of R = 0.35, and a cyclic frequency of 
130 Hz. Over the entire stress intensity range, the fatigue crack growth rate in vacuum was about three times greater than 
that in air. The difference in the crack propagation rates was attributed to the nature of the crack paths in air and vacuum. 
In air, the cleavage fractures are associated with crack branching and irregular crack fronts, which tend to result in lower 
fatigue crack growth rates as compare to vacuum fractures, which exhibited less branching of the cleavage facets and 
more uniform crack fronts (Ref 236). 
Effect of Temperature. As an environment, temperature is unique because it can substantially alter the basic 
mechanical properties of materials and can affect the activity of other environments that depend on oxidation or diffusion 
reactions to exert their influence. The principal mechanical properties or material characteristics that affect fatigue are the 
yield strength (more precisely, the elastic limit), the elastic modulus (E), and the cyclic work-hardening rate, which 
elevates the yield strength. As the yield strength, elastic limit, and cyclic work-hardening rate decrease, slip and plastic 
deformation can occur more readily, resulting in easier fatigue crack initiation and more rapid propagation. These are the 
primary reasons the fatigue properties are generally degraded at elevated and enhanced at cryogenic temperatures. 
Because the diffusion and oxidation rates increase with the temperature, the effect of elevated temperatures is to raise the 
effectiveness of environments that influence fatigue properties by oxidation or by the diffusion of an embrittling species. 
The opposite is true at cyrogenic temperatures. 
Because there is a large variation in the effect of temperature on materials, the effect of temperature on fracture 
appearance also varies. However, at constant testing conditions, the striation spacing generally increases, the striations 
become less distinct, and, depending on the environment, the fracture surfaces become more oxidized as the temperature 
increases. The effect of temperature on fatigue in several different materials is illustrated in the following examples. 
Effect of Temperature on Austenitic Stainless Steels. Increasing the temperature from 25 to 593 °C (75 to 1100 
°F) for an annealed type 316 austenitic stainless steel increased the fatigue crack growth rate by a factor of 3.5 when 
tested in vacuum (better than 10-6 torr) at a stress intensity range of \u2206K = 34 to 60 MPa m (31 to 54.5 ksi in ), a load 
ratio of R = 0.05, and a cyclic frequency of 0.17 Hz (Ref 242). The increase in temperature had no significant effect on 
the fracture appearance; both fractures, especially at the lower \u2206K values, were transgranular and contained 
crystallographic facets (Fig. 91). The absence of fatigue striations was probably the result of testing in a vacuum 
environment. The increase in the fatigue crack growth rate was attributed to the decrease in the elastic modulus at 
elevated temperature. 
 
Fig. 91 Typical fatigue fracture appearance of an annealed type 316 stainless steel tested in vacuum at 25 °C 
(75 °F) (a) and 593 °C (1100 °F) (b). The crack propagation direction is from left to right. Small arrows point 
to the interface between the precracked region and the propagating fatigue crack. Both fractures were 
transgranular and contained crystallographically oriented facets. Source: Ref 242. 
The way in which slight differences in composition can have a significant effect on how materials respond to changes in 
testing temperature is illustrated by annealed type 304, 316, 321, and 347 austenitic stainless steels. These stainless steels 
have a basically similar composition, except that type 321 and 347 stainless steels are stabilized by the addition of small 
amounts (less than 1%) of titanium and niobium, respectively, to prevent the depletion of chromium from material 
adjacent to the grain boundaries. Chromium depletion can occur when chromium carbides precipitate at elevated 
temperatures. 
The steels were fatigue tested in air at 420 to 800 °C (790 to 1470 °F) by using a triangular wave form, a zero mean 
strain, a total strain range of 1%, and strain rates of 6.7 × 10-3 s-1 and 6.7 × 10-5 s-1 (Ref 247). For materials having an 
ASTM grain size greater than 3, testing at a strain rate of 6.7 × 10-3 s-1 showed a continuously decreasing (by a factor of 
five to seven times) fatigue life with increasing temperature for all four stainless steels. However, when testing at a strain 
rate of 6.7 × 10-5 s-1, the fatigue life of the type 304 and 316 stainless steels decreased by a factor of three up to a 
temperature of 600 °C (1110 °F) and remained essentially unchanged to 800 °C (1470 °F). The type 321 and 347 stainless 
steels exhibited a continued decrease, by up to a factor of 15, in the fatigue life with increasing temperature in the range 
of 420 to 800 °C (790 to 1470 °F). The reason for this discrepancy in behavior became apparent when the dislocation 
structures of one of the stainless steels from each group were examined in the transmission electron microscope. 
The examination revealed that the dislocation substructure for the type 316 stainless steel was cell-type up to 600 °C 
(1110 °F) and changed to one of subboundary-type above 600 °C (1110 °F); for type 321 stainless steel, the dislocation 
substructure remained cell-type even above