Crack Paths 2012

Figure 4. Fracture morphology in torsion specimens: (a) serrated crystallographic

facets; river-like patterns in (b) secondary electron image and (c) back scatter

electron image. (d) river-like patterns in the combined bending-torsion specimen

Push-pull fatigue tests

Fatigue life curve in terms of the total strain amplitude a

vs. Nf (see Fig. 5a)

can be expressed in the form

' c

ε σ =

ε

+

.

' f

E

b f f f (2N) (2N)

a

(1)

T

Parameters, the fatigue strength coefficient

f’, the fatigue strength exponent b, the

εf’ and the fatigue ductility exponent c have been evaluated

fatigue ductility coefficient

recently [5] in both coated and uncoated materials. Full and dashed lines in Fig. 5a

represent Eq. 1 for coated and uncoated materials, respectively. It can be seen that the

fatigue life is almost identical in the high amplitude domain (Nf < 103 cycles) while the

detrimental effect of the Al diffusion coating is apparent for low amplitudes.

A S E Mmicrograph of the section parallel to the loading axis of a coated specimen

cycled to fracture with low strain amplitude (a = 0.2 %, Nf=20184 cycles) at 800 °C is

shown in Fig. 5b. Several cracks can be seen in Fig. 2. They are located within D A C

(the cracks on the left), in the O L (the middle crack) or continue to the substrate (two

cracks on the right). Such cracks occurred in both coated and uncoated specimens for all

studied strain amplitudes. The density of surface cracks was several times higher in

coated specimens.

A thorough inspection of fracture surfaces revealed that percentage of carbides on

fracture surfaces is 6 times higher than that in a random section of the specimen gauge

region. It indicates that the fatigue fracture was mostly interdendritic both in coated and

uncoated materials. Moreover, the average carbide particle size in fracture surfaces was

6 times larger than that in the random sections. Fig. 5c shows an example of this

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