Effect of Al content on the microstructure and mechanical behavior of two-phase FeNiMnAl alloys

The effects of varying the Al content in the range of 11-15 at.% on the microstructure and room-temperature mechanical properties of lamellar-structured FeNiMnAl alloys with compositions close to Fe30Ni20Mn35Al15 have been studied, and the temperature dependence of the yield strength of one composition, i.e., Fe36Ni18Mn33Al13, has been investigated. All alloys consisted of B2 and fcc phases. Decreasing the Al content initially (13 and 14 at.% Al) led to marked increases in both the fcc phase fraction and fcc lamellar spacing, lambda, but, on further reducing the Al content (11 and 12 at.% Al), the lamellar structure was no longer present. The elongation to fracture of the FeNiMnAl alloys increased with the decreasing Al concentration from 6.5 % at 15 at.% Al to 31.1 % at 11 at.% Al with a concomitant decrease in the yield stress from 820 to 255 MPa. For the lamellar-structured alloys, the yield stress, sigma (y), obeyed a Hall-Petch-type relationship with lambda, i.e., , where is the lattice resistance, and k' is a constant. The compressive yield stress of Fe36Ni18Mn33Al13 was found to be independent of temperature up to 700 K, after which it decreased dramatically because of the softening of the B2 phase. All alloys showed ductile fracture modes.