Fracture of Fe 3 Al
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Fig. 8--Example of void formation in the interface region. The sample was tested at eo = 8.33 • 10-3 s ~and 825 ~ The local true strain in the region of the laminate shown above is e = 2.4. Overall this sample exhibited 675 pct elongation to fracture.
The very discrete interface produced in the as-bonded condition can yield optimal corrosion resistance with a minimum dependence on strategic resources. Furthermore, the mechanical properties of this composite are unique not only at intermediate temperature but also at room temperature. The composite exhibits an ultimate tensile strength of 1200 MPa (174 ksi) with approximately 20 pct elongation at room temperature. Future work is aimed in the direction of developing suitable diffusion barriers to prevent undesirable interface reactions. In this manner superplasticity can be greatly enhanced and the improved stability will allow thinner corrosion resistant claddings to be employed.
This work was supported by the Army Research Office under program DAAG-29-83-K-0153. The authors gratefully acknowledge the guidance and helpful comments of Dr. George Mayer.
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Stoichiometric, ordered Fe3A1 is reputed to be inherently brittle. This reputation is based on a study of fracture surfaces by Marcinkowski, Taylor, and Kayser ~ which shows "fully embrittled 25.5 at. pct A1 alloy exhibiting intergranular fracture." This fracture surface was obtained on a tensile specimen slowly cooled, presumably from the B2 phase, to 204 ~ (400 ~ and tested at that temperature. The brittle intergranular failure is considered to be inherent in the deformation behavior of polycrystalline DO3 ordered Fe3A1 of stoichiometric and higher aluminum concentrations.~ The material used was obtained from a previous study of mechanical properties 2 in which material from the same heat exhibited a room temperature plastic elongation of approximately 2 pct (estimated from graphical data). Recently, iron aluminum alloys containing 25, 35, and 40 at. pct A1, produced by extrusion consolidation of rapidly solidified powder, exhibited room temperature elongations of 8, 6, and 2.5 pct when tested at the AFWAL/ Materials Laboratory.3'4 A 31 at. pct
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