New High-strength Spinodal Alloys

Dartmouth researchers have developed a new series of metallic iron-nickel-manganese-aluminum (Fe-Ni-Mn-Al) alloys. These alloys exploit a combination of spinodal-, order-, and precipitation-hardening mechanisms to achieve very high strength and hardness, low density, good oxidation resistance, and reasonable ductility.

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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