ASM Metals HandBook Volume 12 - Fractography
857 pág.

ASM Metals HandBook Volume 12 - Fractography

DisciplinaMetalurgia Mecânica154 materiais1.997 seguidores
Pré-visualização50 páginas
crack propagation rate in the 
saline solution was up to 10 times greater than that in air. The fracture surfaces of specimens tested in the saline solution 
contained more cleavage facets than those tested in air. Because cleavage fractures within individual grains can propagate 
up to ten times faster than the other fracture modes, the greater number of cleavage facets on the specimens fatigued in 
brine could account for the greater corrosion fatigue crack growth rate. Although not conclusively established, the 
cleavage fracture probably resulted from hydrogen embrittlement by hydrogen picked up at the crack tip from the 
dissociation of water at the new metal surfaces formed during the fatigue crack cyclic advance. 
An examination of the air and brine fracture surfaces indicated that the fatigue cracks advanced by three distinct fracture 
types: cleavage, furrows, and striations (Ref 236). The air fatigue fractures exhibited all three fracture types, while brine 
corrosion fatigue fractures showed only cleavage and furrow fractures. Cleavage fractures and the fatigue striations 
resulting from a duplex slip process are two fracture modes that are fairly well understood and do not require further 
Furrow-type fatigue fractures (Fig. 87) are of interest because they occured in both environments and were the dominant 
fracture type in air above a \u2206K of 10 MPa m (9 ksi in ). As the name suggests, furrows are grooves or trenches in the 
fracture surface. The furrows, which were about 1 \u3bcm wide and 1 \u3bcm deep and exhibited a separation of about 5 to 10 m 
between the grooves, were always oriented parallel to the crack growth direction. The fracture between the furrows 
exhibited very fine lines that made an angle of 30 ° with the furrows. From electron channeling patterns and 
metallography, it was concluded that the furrow fracture was the result of shear on secondary slip systems in the titanium 
metal (Ref 236). Furrow-type fracture apparently occurs when the fatigue crack approaches a grain that is not favorably 
oriented for either cleavage or a striation mode of fracture. 
Fig. 87 Furrow-type fatigue fracture in a commercially pure titanium alloy IMI 155 tested at room temperature 
in laboratory air. \u2206K = 16 MPa m (14.5 ksi in ), da/dN = 10-8 m/cycle. Source: Ref 236 
Corrosion Fatigue of Austenitic Stainless Steels. Type 304 austenitic stainless steel and nickel-base Inconel 600 
are often used as heat-exchanger tubing materials in nuclear power plants, where thermal cycling and vibration can 
subject the alloys to fatigue in the presence of a caustic environment. To evaluate the effect of a caustic environment, an 
annealed type 304 stainless steel and a solution-annealed Inconel 600 were fatigue tested in 140- °C (285- °F) air and in a 
boiling 140- °C (285- °F) 17.5 M (46 wt%) sodium hydroxide solution (Ref 237, 238). Round bar type 304 stainless steel 
specimens were fatigue tested at a mean stress of 248 MPa (36 ksi) (the 0.2% offset yield strength of the material was 228 
MPa, or 33 ksi), a cyclic stress range of about 25 to 200 MPa (3 to 29 ksi), a cyclic frequency of 10 Hz, and with and 
without a protective anodic potential. The anodic potential had no effect on the fatigue properties. No fatigue endurance 
limit was observed in the boiling caustic solution, and the air fatigue life of greater than 106 cycles to failure at a cyclic 
stress of 172 MPa (25 ksi) was reduced to 4 × 104 cycles (Ref 237). However, when an annealed type 304 stainless steel 
was fatigue tested under identical-conditions with a precracked specimen instead of a round bar specimen, no significant 
effect of the boiling sodium hydroxide solution on the corrosion fatigue crack growth rate was observed (Ref 238). 
The results of these two tests are not contradictory, but indicated that because the fatigue life of smooth, round bar 
specimens is strongly influenced by fatigue initiation the boiling caustic solution apparently assisted in crack initiation by 
interfering with the repassivation of the anodic film (Ref 237). The lack of a significant effect of the boiling sodium 
hydroxide solution on fatigue crack propagation was confirmed by examining the fracture surfaces, which showed that the 
air and caustic fractures had a very similar appearance. 
Corrosion Fatigue of Nickel-Base Alloys. The round bar solution-annealed Inconel 600 specimens were fatigue 
tested at a mean stress of 248 MPa (36 ksi) (the 0.2% offset yield strength of the alloys was 225 MPa, or 33 ksi), a cyclic 
stress range of 140 to 220 MPa (20 to 32 ksi), a cyclic frequency of 0.1 to Hz, and both with and without an anodic 
passivation potential (Ref 238). Fatigue testing in boiling sodium hydroxide solution under conditions identical to those 
used to test the round bar type 304 stainless steel produced completely opposite results. Instead of a substantial decrease 
in the fatigue life observed for the stainless steel, the fatigue life of the Inconel 600 was actually higher in the boiling 
caustic solution, particularly at the lower cyclic stress levels, than in air (Ref 238). For example, when fatigued at a 180-
MPa (260-ksi) cyclic stress and a frequency of 1 Hz, the cycles to failure were greater than 2 × 106 in the boiling sodium 
hydroxide solution versus 6 × 105 in air, and anodic passivation had a positive effect. 
Because the air and the sodium hydroxide fatigue fracture surfaces exhibited identical fracture appearances (slip plane 
cracking during Stage I and well-defined striations in Stage II), it was concluded that for Inconel 600 the slip at the crack 
tip was controlled more by local strain and strain rates than the caustic environment. The fracture appearance and the 
greater increase in the fatigue life at the lower cyclic stresses indicated that, as in the case of the type 304 stainless steel, 
the boiling sodium hydroxide principally affected the process by which fatigue cracks initiated. Therefore, the increase in 
fatigue life was believed to be primarily due to a delay in crack initiation by the caustic solution dissolving or blunting the 
incipient cracks, although some crack tip blunting during fatigue crack propagation may also occur (Ref 238). 
Effect of Vacuum. The general effect of vacuum is to retard the rate at which fatigue cracks propagate. Because a hard 
vacuum (usually 10-6 torr or better) essentially excludes such aggressive environments as water (moisture) and oxygen 
that are present in laboratory air, embrittling reactions that accelerate crack growth in air are eliminated, and the oxidation 
of newly formed slip surfaces at the crack tip is significantly reduced. This permits more complete slip reversal during the 
unloading portion of the fatigue cycle, resulting in a decrease or even arrest in the propagation of the crack. Crack 
advance can also be retarded by the partial rewelding of the fracture surfaces at the crack tip. Whatever the retardation 
mechanism, the fracture surfaces of fatigue cracks propagating in vacuum usually exhibit poorly formed fatigue striations 
or no striations at all (Ref 19, 25, 239, 240, 241, 242). 
Effect of Vacuum on Aluminum Alloys. Compared to fatigue testing in air, some of the greatest improvements in 
the fatigue crack propagation resistance in vacuum have been exhibited by a 7475 aluminum alloy in an underaged 
condition (aged 6 h at 120 °C, or 250 °F) with a 02% offset yield strength of 451 MPa (65 ksi) and an average grain size 
of 80 \u3bcm (expressed as the diameter of a sphere having a volume that is approximately equivalent to that of an average 
grain) (Ref 239). The fatigue tests were conducted at approximately 22 °C (72 °F) in air with a relatively humidity of 50% 
and a vacuum at a pressure of 10-6 torr, using a stress intensity range of \u2206K = 3 to 15 MPa m (2.5 to 13.5 ksi in ), a load 
ratio of R = 0.1, and a cyclic frequency of 30 Hz.