Structural changes accompanying fatigue

6.11.3 Structural changes accompanying fatigue

Observations of the structural details underlying fatigue hardening show that in polycrystals large variations in slip-band distributions and the amount of lattice misorientation exist from one grain to another. Because of such variations it is difficult to typify structural changes, so that in recent years this structural work has been carried out more and more on single crystals; in particular, copper has received considerable attention as being representative of a typical metal. Such studies have now established that fatigue occurs as a result of slip, the direction of which changes with the stress cycle, and that the process continues throughout the whole of the test (shown, for example, by interrupting

a test and removing the slip bands by polishing; the bands reappear on subsequent testing). Moreover, four stages in the fatigue life of a specimen are distinguishable; these may be summarized

as follows. In the early stages of the test, the whole of the specimen hardens. After about 5% of the life, slip becomes localized and persistent slip bands appear; they are termed persistent because they reappear and are not permanently removed by electropolishing. Thus, reverse slip does not continue throughout the whole test in the bulk of the metal (the matrix). Electron microscope observations show that metal is extruded from the slip bands and that fine crevices called intrusions are formed within the band. During the third stage of the fatigue life the slip bands grow laterally and become wider, and at the same time cracks develop in them. These cracks spread initially along slip bands, but in the later stages of fracture the propagation of the crack is often not confined to certain crystallographic directions and catastrophic rupture occurs. These two important crack growth stages, i.e. stage I in the slip band and stage II roughly perpendicular to the principal stress, are shown in Figure 6.69 and are influenced by the formation of localized (persistent) slip bands (i.e. PSBs). However, PSBs are not clearly defined in low stacking-fault energy, solid solution alloys.

Cyclic stressing therefore produces plastic deformation which is not fully reversible and the build-up of dislocation density within grains gives rise to fatigue hardening with an associated struc- ture which is characteristic of the strain amplitude and the ability of the dislocations to cross-slip, i.e. temperature and SFE. The non-reversible flow at the surface leads to intrusions, extrusions and crack formation in PSBs. These two aspects will now be considered separately and in greater detail.

6.11.3.1 Fatigue hardening

If a single or polycrystalline specimen is subjected to many cycles of alternating stress, it becomes harder than a similar specimen extended unidirectionally by the same stress applied only once.

376 Physical Metallurgy and Advanced Materials

Figure 6.69 Persistent slip band (PSB) formation in fatigue, and stage I and stage II crack growth.

This may be demonstrated by stopping the fatigue test and performing a static tensile test on the specimen when, as shown in Figure 6.70, the yield stress is increased. During the process, persistent slip bands appear on the surface of the specimen and it is in such bands that cracks eventually form.

The behavior of a fatigue-hardened specimen has two unusual features when compared with an ordi- nary work-hardened material. The fatigue-hardened material, having been stressed symmetrically, has the same yield stress in compression as in tension, whereas the work-hardened specimen (e.g. prestrained in tension) exhibits a Bauschinger effect, i.e. weaker in compression than tension. It arises from the fact that the obstacles behind the dislocation are weaker than those resisting further dislocation motion, and the pile-up stress causes it to slip back under a reduced load in the reverse direction. The other important feature is that the temperature dependence of the hardening produced by fatigue is significantly greater than that of work hardening and, because of the similarity with the behavior of metals hardened by quenching and by irradiation, it has been attributed to the effect of vacancies and dislocation loops created during fatigue.

At the start of cyclic deformation the initial slip bands (Figure 6.71a) consist largely of primary dislocations in the form of dipole and multipole arrays; the number of loops is relatively small because

Mechanical properties I 377 2 )

20 w.h. 10 4 5 ⫻ 10 4 15 ⫻ 10 4 Shear stress (MN m

0 10 20 Glide strain (%)

Figure 6.70 Stress–strain curves for copper after increasing amounts of fatigue testing (after Broom and Ham, 1959).

Initial slip bands

Figure 6.71 Formation of persistent slip bands (PSBs) during fatigue.

the frequency of cross-slip is low. As the specimen work-hardens slip takes place between the initial slip bands, and the new slip bands contain successively more secondary dislocations because of the internal stress arising from nearby slip bands (Figure 6.71b). When the specimen is completely filled with slip bands, the specimen has work hardened and the softest regions are now those where slip occurred originally, since these bands contain the lowest density of secondary dislocations. Further slip and the development of PSBs takes place within these original slip bands, as shown schematically in Figure 6.71c.

As illustrated schematically in Figure 6.72, TEM of copper crystals shows that the main difference between the matrix and the PSBs is that in the matrix the dense arrays of edge dislocation (di- and multipoles) are in the form of large veins occupying about 50% of the volume, whereas they form

a ‘ladder’-type structure within walls occupying about 10% of the volume in PSBs. The PSBs are the active regions in the fatigue process while the matrix is associated with the inactive parts of the specimen between the PSBs. Steady-state deformation then takes place by the to-and-fro glide of the same dislocations in the matrix, whereas an equilibrium between dislocation multiplication and annihilation exists in the PSBs. Multiplication occurs by bowing-out of the walls and annihilation

378 Physical Metallurgy and Advanced Materials

(a)

(b)

Figure 6.72 Schematic diagram showing vein structure of matrix (a) and ladder structure of PSBs (b).

takes place by interaction with edge dislocations of opposite sign ( ≈75b apart) on glide planes in the walls and of screw dislocations ( ≈200b apart) on glide planes in the low-dislocation channels, the exact distance depending on the ease of cross-slip.