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

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and R.D. 
Zipp, Ed., American Society for Testing and Materials, 1981, p 5-31 
90. H. Hänninen and T. Hakkarainen, Metall. Trans. A, Vol 10A, 1979, p 1196-1199 
91. A.W. Thompson and J.C. Chesnutt, Metall. Trans. A, Vol 10A, 1979, p 1193 
92. M.F. Stevens and I.M. Bernstein, Metall. Trans. A, Vol 16A, 1985, p 1879 
93. C. Chen, A.W. Thompson, and I.M. Bernstein, OROC. 5th Bolton Landing Conference, Claitor's, Baton 
Rouge, LA 
94. J.C. Chesnutt and R.A. Spurling, Metall. Trans. A, Vol 8A, 1977, p 216 
Note cited in this section 
* All fatigue crack growth rates in this article are given in millimeters per cycle (mm/cycle). To 
convert to inches per cycle (in./cycle), multiply by 0.03937. See also the Metric Conversion Guide 
in this Volume. 
Modes of Fracture 
Victor Kerlins, McDonnell Douglas Astronautics Company Austin Phillips, Metallurgical Consultant 
Effect of Environment 
The environment, which refers to all external conditions acting on the material before or during fracture, can significantly 
affect the fracture propagation rate and the fracture appearance. This section will present some of the principal effects of 
such environments as hydrogen, corrosive media, low-melting metals, state of stress, strain rate, and temperature. Where 
applicable, the effect of the environment on the fracture appearance will be illustrated. 
Effect of Environment on Dimple Rupture 
The Effect of Hydrogen. When certain body-centered cubic (bcc) and hcp metals or alloys of such elements as iron, 
nickel, titanium, vanadium, tantalum, niobium, zirconium, and hafnium are exposed to hydrogen, they are susceptible to a 
type of failure known as hydrogen embrittlement. Although the face-centered cubic (fcc) metals and alloys are generally 
considered to have good resistance to hydrogen embrittlement, it has been shown that the 300 series austenitic stainless 
steels (Ref 95, 96, 97, 98) and certain 2000 and 7000 series high-strength aluminum alloys are also embrittled by 
hydrogen (Ref 99, 100, 101, 102, 103, 104, 105, 106, 107). Although the result of hydrogen embrittlement is generally 
perceived to be a catastrophic fracture that occurs well below the ultimate strength of the material and exhibits no 
ductility, the effects of hydrogen can be quite varied. They can range from a slight decrease in the percent reduction of 
area at fracture to premature rupture that exhibits no ductility (plastic deformation) and occurs at a relatively low applied 
The source of hydrogen may be a processing operation, such as plating (Fig. 30) or acid cleaning, or the hydrogen may be 
acquired from the environment in which the part operates. If hydrogen absorption is suspected, prompt heating at an 
elevated temperature (usually about 200 °C, or 400 °F) will often restore the original properties of the material. 
The effect of hydrogen is strongly influenced by such variables as the strength level of the alloy, the microstructure, the 
amount of hydrogen absorbed (or adsorbed), the magnitude of the applied stress, the presence of a triaxial state of stress, 
the amount of prior cold work, and the degree of segregation of such contaminant elements as phosphorus, sulfur, 
nitrogen, tin, or antimony at the grain boundaries. In general, an increase in strength, higher absorption of hydrogen, an 
increase in the applied stress, the presence of a triaxial stress state, extensive prior cold working, and an increase in the 
concentration of contaminant elements at the grain boundaries all serve to intensify the embrittling effect of hydrogen. 
However, for an alloy exhibiting a specific strength level and microstructure, there is a stress intensity, KI, below which, 
for all practical purposes, hydrogen embrittlement cracking does not occur. This threshold crack tip stress intensity factor 
is determined experimentally and is designated as Kth. 
A number of theories have been advanced to explain the phenomenon of hydrogen embrittlement. These include the 
exertion of an internal gas pressure at inclusions, grain boundaries, surfaces of cracks, dislocations, or internal voids (Ref 
40, 108, 109); the reduction in atomic and free-surface cohesive strength (Ref 110, 111, 112, 113, 114, 115, 116); the 
attachment of hydrogen to dislocations, resulting in easier dislocation breakaway from the pinning effects of carbon and 
nitrogen (Ref 38, 112, 117, 118, 119, 120, 121, 122); enhanced nucleation of dislocations (Ref 112, 123); enhanced 
nucleation and growth of microvoids (Ref 109, 110, 113, 116, 122, 124, 125, 126); enhanced shear and decrease of strain 
for the onset of shear instability (Ref 112, 127, 128); the formation of methane gas bubbles at grain boundaries (Ref 129, 
130); and, especially for titanium alloys, the repeated formation and rupture of the brittle hydride phase at the crack tip 
(Ref 131, 132, 133, 134, 135, 136, 137). Probably no one mechanism is applicable to all metals, and several mechanisms 
may operate simultaneously to embrittle a material. Whatever the mechanism, the end result is an adverse effect on the 
mechanical properties of the material. 
If the effect of hydrogen is subtle, such as when there is a slight decrease in the reduction of area at fracture as a result of 
a tensile test, there is no perceivable change in the dimple rupture fracture appearance. However, the dimples become 
more numerous but are more shallow at a greater loss in ductility (Fig. 43). 
Fig. 43 Effect of hydrogen on fracture appearance in 13-8 PH stainless steel with a tensile strength of MPa (237 
ksi). Top row: SEM fractographs of a specimen not embrittled by hydrogen. Bottom row: SEM fractographs of a 
specimen charged with hydrogen by plating without subsequent baking. 
Hydrogen Embrittlement of Steels. At low strain rates or when embrittlement is more severe, the fracture mode in 
steels can change from dimple rupture to quasi-cleavage, cleavage, or intergranular decohesion. These changes in fracture 
mode or appearance may not occur over the entire fracture surface and are usually more evident in the region of the 
fracture origin. Figure 44 shows an example of a hydrogen-embrittled AISI 4340 steel that exhibits quasi-cleavage. 
Fig. 44 Quasi-cleavage fracture in a hydrogen-embrittled AISI 4340 steel heat treated to an ultimate tensile 
strength of 2082 MPa (302 ksi). Source: Ref 138 
When an annealed type 301 austenitic stainless steel is embrittled by hydrogen, the fracture occurs by cleavage (Fig. 45a). 
An example in which the mode of fracture changed to intergranular decohesion in a hydrogen-embrittled AISI 4130 steel 
is shown in Fig. 45b. 
Fig. 45 Examples of hydrogen-embrittled steels. (a) Cleavage fracture in a hydrogen-embrittled annealed type 
301 austenitic stainless steel. Source: Ref 98. (b) intergranular decohesive fracture in an AISI 4130 steel heat 
treated to an ultimate tensile strength of 1281 MPa (186 ksi) and stressed at 980 MPa (142 ksi) while being 
charged with hydrogen. Source: Ref 111 
When a hydrogen embrittlement fracture propagates along grain boundaries, the presence of such contaminant elements 
as sulfur, phosphorus, nickel, tin, and antimony at the boundaries can greatly enhance the effect of hydrogen (Ref 111, 
139). For example, the segregation of contaminant elements at the grain boundaries enhances the hydrogen embrittlement 
of high-strength low-allow steels tempered above 500 °C (930 °F) (Ref 92). The presence of sulfur at grain boundaries 
promotes hydrogen embrittlement of nickel, and for equivalent concentrations, the effect of sulfur is nearly 15 times 
greater than that of phosphorus (Ref 140). 
Hydrogen Embrittlement of Titanium. Although titanium and its alloys have a far greater tolerance for hydrogen 
than high-strength steels, titanium alloys are embrittled by hydrogen. The degree and the nature of the embrittlement