Crack Paths 2012

finding: the fracture surface of a coated specimen cycled by a high strain amplitude

(a = 0.45 %, Nf= 397). The large carbides are indicated by white arrows.

(a)

(b)

(c)

uncoated

Al coating

Figure 5. (a) Fatigue life curves for coated and uncoated specimens at 800 °C; (b)

fatigue cracks in the coating (section parallel to the loading axis); (c) fracture surface in

the bulk of a coated specimen with carbides marked by white arrows

DISCUSSION

In the case of LCFsymmetric bending, the high maximal normal stress

a > 500 M P a

was sufficient for decohesion of the incoherent particle/matrix interface which

shortened the fatigue crack initiation phase and accelerated the crack propagation in the

early phase of experiment. This explains that the LCFbending performance of coated

specimens was worse than that of uncoated ones. On the other hand, in the case of

torsional loading (both LCFand H C Fdomains), the maximal possible normal stress a,

at the particle/matrix interface was about two times lower than that in the bending case.

This was not high enough for the particle/matrix decohesion and, therefore,

a smaller concentration of secondary phase particles was observed at initiation sites. In

this scenario, the higher hardness of the coating came into play leading to the higher

torsional fatigue strength of coated specimens. In the case of combined bending-torsion,

the bending component seems to be dominant, thus causing a higher fatigue life of

uncoated specimens. This corresponds to the fractographical observations revealing

a similar particle/matrix decohesion micromechanism of the fatigue crack initiation at

two opposite specimen sites that are subjected to the maximumbending loading.

In the case of the total-strain amplitude controlled push-pull tests at 800 °C, the

fatigue behaviour of coated and uncoated specimens in the LCFregion was qualitatively

identical to that observed at the room-temperature bending. At higher a, the

comparable fatigue life of coated and uncoated specimens can be understood in terms of

the plastic strain localization. Indeed, a higher number of cracks in the coated specimens

(in comparison to those uncoated) means a less localized plastic strain. Since the plastic

a, the Manson-Coffin curve for coated

part of

a becomes more important for higher

specimens is shifted towards a higher fatigue life in this domain [5].

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