Fatigue Crack Paths 2003

concentrated on improving room temperature (RT) ductility and high temperature (HT)

creep strength [2]. It has been shown that poor values of R T elongation commonly

observed in air tests are caused by an extrinsic effect - hydrogen atoms produced by the

reaction of water vapour with aluminum at the surface of the specimen [3,4]. The most

effective solute to combat this environmental embrittlement is chromium [5], with a

usual amount ranging between 2 to 6 at.%. Microalloying by cerium is beneficial both

to H Tstrength and R T ductility [6]. A more detailed description of the state of the art in

the research of iron aluminides is given in the review by Stoloff [2]. Fatigue behaviour

of some Fe3Al based alloys is described in [7-9].

Fracture and mechanical properties of two Fe-28Al-4Cr (at.%) based alloys with

cerium addition were studied as a function of heat treatment, testing temperature and

strain rate [10,11]. In comparison to conventional (disordered) alloys a variety of

fractographic features (intergranular decohesion, transgranular cleavage, intergranular

microcracks, serpentine glide and ductile dimple fracture) were often found on one

fracture surface. This paper reports an introductory study of fatigue behaviour of these

materials.

E X P E R I M E N TD EATLA I L S

Alloy Composition and Heat Treatment

Two alloys with nominal composition of Fe-28Al-3Cr-Ce (at.%) were prepared by

vacuum induction melting and casting. The first ingot was hot extruded at 1140 °C at

the area-ratio of 12:1 (reduction 92%) to the shape of a tube of 52 m min diameter with

7 m mthick wall and slowly cooled in air. The second ingot was hot rolled at 1100°C to

a plate 6 m mthick (reduction 75%)and subsequently quenched in a mineral oil. Results

of the chemical analysis of both alloys are shown in Table 1. The grain structure of the

extruded tube was revealed as consisting of equiaxed recrystallized grains of average

size about 75 m. Moreover, some large (recovered) grains elongated in the extrusion

direction were found. For more details about the characterization of the microstructure

of this material see [12]. The plate was only recovered (not recrystallized),

with

elongated grains having about 300 m in the direction of rolling and about 100 m in

the transverse direction.

Table 1. Chemical composition of studied materials.

Cr

Ce

Material

Al

C M n Si

Fe

0.08

(at.%)

28.9

3.6

0.16

0.20

0.07 Balance

Tube

0.23

(wt.%) 16.5

3.9

0.04

0.23

0.04 Balance

(wt.%) 28.4

2.6

0.02

0.12

0.40

-

Balance

Plate

(at.%)

16.1

2.8

0.06

0.03

0.46

-

Balance