Fatigue Crack Paths 2003

Fatigue CrackPropagation in Ordered Alloys Based on Fe 3 A l

M.Karlík, I. Nedbal, J. Siegl, H. Lauschmann,J. Prahl and T. ýernoch

Czech Technical University in Prague, Faculty of Nuclear Sciences and Physical

Engineering, Department of Materials, Trojanova 13, 120 00 Praha 2, Czech Republic

E-mail: Miroslav.Karlik@fjfi.cvut.cz

ABSTRACT.A study of fatigue behaviour of two Fe 3Al based alloys with Cr and Ce

addition was performed on single edge notched (SEN) and compact tension (CT)

specimens. First attempts to measure the fatigue crack growth rate v(K)were carried

out. The measurement of the fatigue crack growth rate by the optical method was

difficult to perform, because the crack propagates in a discontinuous manner and often

the surface measurement is not representative for the bulk crack front advance. In

consequence, the crack growth rate was measured also by the potential method. Owing

to a very similar character in the micromorphology of fatigue and static fracture

surfaces, it is very difficult to recognize the boundary between fatigue crack and final

rupture. The fatigue fracture surface shows a varied micromorphology. Besides

striations, known from some commonalloys, another micromorphological features were

found. Fractographic analysis also proved that some fatigue damage originated ahead

of the crack front. However, an unambiguous interpretation of all these features in the

relation with crack growth rate is not clear and thus a further study of fatigue damage

micromechanisms in this type of material is necessary.

I N T R O D U C T I O N

Iron aluminides based on Fe3Al or FeAl are investigated as new high-temperature

structural materials because of their low cost, low density (about 6.7 gcm-3), good wear

resistance, ease of fabrication and superior resistance in oxidizing and sulphidizing

atmospheres. These alloys offer also the potential for reducing the use of Ni and Cr

indispensable in commonhigh temperature steels or nickel superalloys. Iron aluminide

based alloys are expected to be used in the coal gassification plants or furnaces for

burning garbages, in the chemical industry, automotive industry and other applications

as heating elements, furnace fixtures, heat-exchanger piping, sintered porous gas–metal

filters, automobile and other industrial valve components, catalytic converter substrates

and components for molten salt applications [1].

Mechanical properties of Fe3Al based intermetallics are significantly influenced by

the change of order - D03 ↔ B2 order-order transition - at the temperature about 540°C.

As in the B2 ordered region the tensile and creep strength of Fe3Al rapidly decrease, the

limiting temperature for its structural applications is around 600 °C. Research has been