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

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"Visual Examination and Light Microscopy" in this 
Incomplete fusion or inadequate weld penetration can produce a material discontinuity similar to a crack. Subsequent 
loading can cause the discontinuity to grow, as in Fig. 103, which shows a fracture in a weld in commercially pure 
titanium that broke by fatigue from crack nuclei, on both surfaces, that resulted from incomplete fusion during welding. 
Fig. 103 Fracture in a weld in commercially pure titanium showing incomplete fusion. Unfused regions, on both 
surfaces (arrows), served as nuclei of fatigue cracks that developed later under cyclic loading. 
Inclusions. Discontinuities in the form of inclusions, such as oxides, sulfides, and silicates, can initiate fatigue fractures 
in parts subjected to cyclic loading (see, for example, Fig. 579 to 583 and Fig. 588 to 598 in the "Atlas of Fractographs" 
in this Volume, which illustrate the effect of inclusions on the fatigue crack propagation in ASTM A533B steel). In 
addition, such inclusions have been identified as initiation sites of ductile fractures in aluminum alloys and steels. At 
relatively low strains, microvoids form at inclusions, either by fracture of the inclusion or by decohesion of the 
matrix/inclusion interface. 
The very large inclusion shown in Fig. 104 was found in the fracture surface of a case-hardened AISI 9310 steel forging 
that broke in service. X-ray analysis of the inclusion led to the deduction that it was a fragment of the firebrick lining of 
the pouring ladle. 
Fig. 104 Inclusion in a surface of a service fracture in a case-hardened AISI 9310 steel forging. The diagonal 
view is a composite of several fractographs showing a very large inclusion, which was a fragment of the 
pouring-ladle firebrick lining. Fractographs 1 to 4 are higher-magnification views of areas indicated by arrows 1 
to 4, respectively, in the diagonal view. 
Figure 105 shows a large inclusion in a fracture surface of a cast aluminum alloy A357-T6 blade of a small, high-speed 
air turbine and two views of the fracture-surface features around this inclusion. 
Fig. 105 Fracture surface of a cast aluminum alloy A357-T6 air-turbine blade. (a) Overall view of the fracture 
surface showing a large inclusion (dark) near the tip of the blade. Approximately 0.4×. (b) and (c) Decohesion 
at the interfaces between the inclusion and the aluminum matrix 
Figure 106 shows the fracture features associated with inclusions in AISI 4340 steel with a tensile strength of 1790 to 
1930 MPa (260 to 280 ksi). Entrapped flux in a brazed joint can effectively reduce the strength of the brazement and also 
can create a long-term corrosion problem. A 6061 aluminum alloy attachment bracket was dip brazed to an actuator of the 
same alloy in a flux consisting of a mixture of sodium, potassium and lithium halides, then heat treated to the T6 temper 
after brazing. The flux inclusion, shown in Fig. 107, reduced the cross section of the joint, and a overload fracture 
occurred in the Al-12Si brazing alloy. 
Fig. 106 Fracture surface of an AISI 4340 steel that was heat treated to a tensile strength of 1790 to 1930 
MPa (260 to 280 ksi) showing deep dimples containing the inclusions that initiated them. (J. Kilpatrick, Delta Air 
Fig. 107 Halide-flux inclusion (rounded granules) in the joint between an actuator and an attachment bracket 
of aluminum alloy 6061 that were joined by dip brazing using an Al-12Si brazing alloy 
Stringers are elongated nonmetallic inclusions, or metallic or nonmetallic constituents, oriented in the direction of 
working. Nonmetallic stringers usually form from deoxidation products or slag, but may also result from the intentional 
addition of elements such as sulfur to enhance machinability. Figure 108 shows unidentified stringers on the fracture 
surface of an AISI 4340 steel forging. Overload cracking occurred during straightening after the forging had been heat 
treated to a tensile strength of 1380 to 1520 MPa (200 to 220 ksi). 
Fig. 108 Stringers on the surface of a fracture that occurred during straightening of an AISI 4340 steel forging 
that had been heat treated to a tensile strength of 1380 to 1520 MPa (200 to 220 ksi). Stringers are visible as 
parallel features inclined about 30° to right of vertical. They were not identified as to composition, but may be 
accidentally entrapped slag that was elongated in the major direction of flow during forging. 
Porosity is the name applied to a condition of fine holes or pores in a metal. It is most common in castings and welds, 
but residual porosity from the cast ingot sometimes still persists in forgings. In fractures that occur through regions of 
excessive porosity, numerous small depressions or voids (sometimes appearing as round-bottom pits) or areas with a 
dendritic appearance can be observed. At low magnification, fractures through regions of excessive porosity may appear 
dirty or sooty because of the large number of small voids, which look like black spots. 
Figure 109 shows random porosity (with pores surrounded by dimples) in a fracture of a cast aluminum alloy A357 air-
turbine blade. Fracture was caused by overload from an impact. 
Fig. 109 Porosity in a fracture of a cast aluminum alloy A357 blade from a small air turbine. The blade 
fractured by overload from an impact to its outer edge. 
Figure 110 shows a shrinkage void intersected by the fracture surface of a cast aluminum alloy A357-T6 gear housing. 
The dendrite nodules in the void indicate that the cavity was caused by unfavorable directional solidification during 
casting. The fracture was caused by overload. 
Fig. 110 Shrinkage void with dendrite nodules on a fracture surface of a cast aluminum alloy A357-T6 gear 
housing that broke by overload 
Figure 111 shows SEM fractographs of the surface of a fatigue fracture in a resistance spot weld that broke during bond 
testing of an aluminum alloy 7075-T6 specimen. The voids in the weld nugget are apparent in both fractographs. The 
fatigue path appears to favor the voids. 
Fig. 111 Fatigue fracture surface of a resistance spot weld that broke during bond testing of an aluminum alloy 
7075-T6 specimen. (a) Note voids (arrows) caused by molten-metal shrinkage in the weld nugget. (b) Both 
fatigue striations and shrinkage voids are evident, which indicates that the fracture path favored the porous 
Segregation. The portion of a fracture in a region of segregation may appear either more brittle or more ductile than the 
portion in the surrounding regions. Differences in fracture texture may be slight and therefore difficult to evaluate. 
Fractographic evidence of segregation should always be confirmed by comparing the microstructure and chemical 
composition of the material in the suspect region with those in other locations in the same part. 
Unfavorable Grain Flow. Grain flow in an unfavorable direction may be indicated by a woody fracture in some 
materials and by a flat, delaminated appearance in others. An area of woody fracture is indicated in region B in Fig. 112, 
which shows a fatigue fracture in a forged AISI 4340 steel aircraft landing-gear axle. The fatigue fracture occurred in an 
area where the resistance of the material to fatigue cracking was low because the fluctuating loads were applied nearly 
perpendicular to the direction of grain flow. In high-strength aluminum alloys extrusions and hot-rolled products, tension 
loads are occasionally applied perpendicular to the flow direction, which may cause splitting along flow lines. This 
somewhat resembles the pulling apart of laminated material. Splitting may also appear as secondary cracks perpendicular 
to the primary fracture when these materials are broken by bending loads. 
Fig. 112 Fatigue fracture through a region of unfavorable grain flow and large inclusions