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The microstructure of the
alloys consists, wholly or in part, of a regular array of ultrafine
(5-50 nm wide) coherent regions that differ in chemical composition and
degree of atomic ordering. This
structure is similar to that found in nickel-based superalloys,
in which an ordered phase provides excellent strength and creep resistance,
while the disordered matrix preserves ductility. However, in the present case, the
microstructure arises via a continuous chemical separation–– spinodal decomposition–– that occurs during
air cooling and proceeds further upon subsequent annealing. This continued decomposition increases
local variations in coherency strain in the material, making it more
resistant to deformation. The
decomposition is very rapid, resulting in a metastable
two-phase microstructure that can be hardened still further by precipitation
of a third phase. A high
aluminum content not only contributes to oxidation resistance, but makes
Fe-Ni-Mn-Al alloys lighter than any steels,
nickel-, cobalt-, or copper-based alloys.
For example, alloy B64550 has a density of 6.6 g/cm3 (0.24
lb/in3), a yield strength of 1500 MPa
(220 ksi), and a hardness of 500 VPN (50 RC) in the
as-cast state. After one hour at 550°C (1020°F), the alloy hardens to 590 VPN (55
RC). With longer heat treatments, a
yield strength of 2350 MPa (340 ksi)
and a hardness of 770 VPN (63 RC) have been achieved. These values are comparable to the
strongest maraged aircraft steels and hardest
bearing steels available on the market, but with a better strength-to-weight
ratio. It is anticipated that Fe-Ni-Mn-Al alloys will be useful as bearing or stamping
materials, as well as in aerospace or other applications where weight
minimization is an important issue. We
are in the process of patenting this technology and are seeking an industrial
partner interested in its commercialization. (Ref: J248) |
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«Technology Transfer Office : Sponsored Projects : Dartmouth College |
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11 Rope Ferry Road #6210 |
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Hanover, NH 03755-1404 |
Phone: (603) 646-3027 |
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Fax: (603) 646-3670 |
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