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

Abstract

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, λ, 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, σ _y, obeyed a Hall–Petch-type relationship with λ, i.e., $\sigma_{y } = \sigma^{\prime}_{0} + k^{\prime}\lambda^{ - 1}$, where $\sigma^{\prime}_{0}$ 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.