Environmental Effects in FeAl
I. Baker, E.P. George (ORNL), D. Wu
Introduction
B2-structured FeAl alloys are being considered for applications requiring good oxidation resistance, low density and low cost. However, the low ductility and toughness at room temperature must first be improved. It has been shown that water vapor has a detrimental effect on the room-temperature ductility of iron-rich FeAl. The effect is due to the reaction of the aluminum with water vapor, which produces atomic hydrogen [1-3]. This diffuses into the metal at crack tips and results in hydrogen embrittlement. However, how hydrogen is transported into the lattice and how far ahead of the crack tip it needs to be in order to produce embrittlement is unclear.
In this study, we explore further the effects of the environment and strain rate on both the room temperature ductility and strength of iron-rich FeAl single crystals both in single-slip and duplex-slip orientations, and with and without boron.
Results
1. Environmental effects on elongation, yield strength.
As shown in table 1, the effect of the environment on both the strength and ductility is quite evident for the undoped FeAl. The elongation increases somewhat from 6% to 9% when the strain rate is increased from 4.4 x 10
-6 to 2.5 x 10
-3 s
-1, indicating a lesser influence of the environment at higher strain rates. It increases to ~20% when testing is performed at the higher strain rate under vacuum (in which some water vapor is still present) and to ~45% in oxygen. The higher elongation in oxygen probably reflects not only the low level of water vapor but also that the oxygen competes with any low partial pressure of water vapor to oxidize the aluminum in FeAl. The effect of water vapor on the strength is evident when the strain rate is increased from 4.4 x 10
-6 to 2.5 x 10
-3 s
-1 when the CRSS increases from 82 to 90 MPa. At the higher strain rate, testing under vacuum produced on additional benefit, but straining in oxygen increased the CRSS to 94 MPa, a 15% increase compared to the slow strain rate tests in air.
Table 1 Room-temperature CRSS and ductility of single-slip-oriented Fe-43Al single crystals strained at a variety of strain rates in different environments. Each value is the average of number of tests, which is indicated in parenthesis.
Specimen
(No. of tests) |
Environment |
Strain rate
(s-1) |
Elongation
(%) |
CRSS
(MPa) |
| Fe-43Al (4) |
Air |
4.4 x 10-6 |
6 |
82 |
| Fe-43Al (4) |
Air |
2.5 x 10-3 |
9 |
90 |
| Fe-43Al (4) |
Vacuum |
2.5 x 10-3 |
20 |
90 |
| Fe-43Al (2) |
Oxygen |
2.5 x 10-3 |
45 |
94 |
2. Environmental effects on fracture mode
When tested in air, transgranular cleavage fracture dominates in FeAl single crystal tensile specimens. Fracture initiated from the specimen surface. Fracture surfaces generally consisted of one part of smooth planar fracture with river marks (region A) and one part of non-planar fracture with a rougher topography (region B), a typical fracture surface is shown in Figure 1.
Figure 1. Fracture surface of a Fe-43Al single crystal tested at 1 s-1.
When tested in oxygen, no apparent secondary cracks were seen in the fracture surface. Ductile dimples, faceted holes are observed, see Figure 2.
Figure 2. Fracture surfaces of Fe-43Al single crystals tested in oxygen. (a) fracture initiated from both the surface and the inside of the specimen. (b) faceted cavities.
3. Boron effect on mechanical properties and fracture mode of FeAl single crystals
There are less secondary cracks in the fracture surfaces of Fe43AlB specimens than Fe43Al specimens, see Figure 3. Boron doping produces a slight improvement in elongation in air but increases the strength of FeAl in air at slow strain rates, compare tables 1 and 2.
Figure 3. Fracture surfaces of Fe-43Al single crystals with and without B. (a) secondary crack in a Fe-43Al specimen without B; (b) B-doped, no secondary cracks.
Table 2. The CRSS and ductility of B-doped Fe-43Al single crystals strained at room temperature in different environments at a variety of strain rates. Each value is the average of number of tests, which is indicated in parenthesis.
Specimen
(No. of tests) |
Environment |
Strain rate
(s-1) |
Elongation
(%) |
CRSS
(MPa) |
| Fe-43Al+B (4) |
Air |
4.4 x 10-6 |
8 |
98 |
| Fe-43Al+B (2) |
Air |
2.5 x 10-3 |
14 |
98 |
| Fe-43Al+B (2) |
Air |
1 |
30 |
100 |
| Fe-43Al+B (3) |
Vacuum |
2.5 x 10-3 |
24 |
98 |
| Fe-43Al+B (3) |
Oxygen |
2.5 x 10-3 |
33 |
99 |
| Fe-43Al+B (1) |
Air/vacuum |
4.4 x 10-6 |
3 |
95 |
At very low temperature, B does not have the strengthening effect. However, B does cause significant strengthening in the intermediate temperature and high temperature beyond yield peak, as shown in Figure 4.
Figure 4. Temperature dependence of the CRSS of B-free and B-doped Fe-43Al single crystals oriented for single slip.
References
1. C.T. Liu, E.H. Lee, and C.G. McKamey, Scripta Metall., 23 (1989) 875.
2. M. Nakamura and T. Kumagai, Metall. Mater. Trans. A, 30 (1999) 3089.
3. Y.F. Zhu, C.T. Liu, and C.H. Chen, Scritpa Mater., 35 (1996) 1435.