The Mechanical Properties of FeAl
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0 A 0 U
Air-cooled Furnace-cooled, 100K/hr Furnace-cooled, 50K/hr 673K, ll8hr
Figure 1. Microhardness versus Aluminum concentration in FeAl alloys after 1000 K anneals and various subsequent heat treatments. After reference 23.
2 'R 3A
42 at.% Al
Room-Temperature Strength Iron-rich deviations from the stoichiometric composition are accommodated by Fe anti-site atoms. In addition, triple defects - two vacancies on the Fe sublattice site with an antisite (Fe) atom on the other (Al) sublattice - are present, especially in near-stoichiometric compositions [ 16]. Chang et al.  calculated the room-temperature equilibrium vacancy concentrations in FeA1, arising from triple defects, and showed that the vacancy concentration and hardness had a similar dependence on Fe:AI ratio, suggesting that the vacancies controlled the room-temperature yield strength. Later, measurements by Xiao and Baker  of the vacancy concentrations at room temperature as a function of Fe:AI ratio also demonstrated a close similarity with the dependence of the yield strength on Fe:Al ratio [15, 18], see Figure 2. Thus, the room temperature yield strength appears to be controlled by vacancies.
Figure 2. Yield stress and percentage vacancy concen-
versus aluminum concentration for large-grained lowtemperature-annealed FeAI. After reference 29.
vacancy concentration concentration
tration on iron sites, Ye,
- 600 iYield 400-
45 at. % Al
Chang et al.  also showed that the hardness had a square-root dependence on the quenchedin vacancy concentration, see Figure 3. This behavior might be expected if there is no change in slip behavior with increasing vacancy concentration, Cv, and the vacancies do not cluster. If vacancies pin the dislocations causing them to bow out, then the strength increase due to vacancies is proportional to the reciprocal of their spacing on the slip plane, 2A. Since Ais proportional to C'-112, the experimentallyobserved parabolic relationship between hardness and Cv is obtained. Unfortunately, measurements show that the yield strength and strain-hardening rate are not described by such a simple dependence on the quench-in vacancy concentration , see Figure 4.
A Fe-50Ai Fe-48AI oD Fe-45A!I
r 2 .2--
Figure 3. Graph of microhardness versus (vacancy concentration)12 for FeAI polycrystals quenched from various temperatures. After reference 28.
600Figure 4. Graph of yield stress and strain hardening rate as a function of vacancy concentration for single-slip oriented single crystals of Fe-40AI. After reference 30.
• : "
S30 0-• "
Vacancy Concentration, x 103
Slip in FeAI, at least at room temperature [20-30], is by the glide of pairs of a/2 antiphase domain (APB) - coupled dislocations. It is possible that the strengthening effect of vacancies arises not only from clas