Development of new Fe–Al–Nb(–B) alloys for structural applications at high temperatures

Abstract

It is known for Fe–Al–Ta alloys, that a homogeneous distribution of strengthening Laves phase precipitates in the matrix and aligned at the grain boundaries can be obtained when the formation of the stable Laves phase is preceded by the formation of the metastable Heusler phase. Several Fe–Al–Nb alloys with different Al and Nb contents and with or without boron doping are studied to elucidate whether comparable microstructures can be obtained in this system. It was found that the Heusler phase only occurs within a limited composition range. The time-dependent evolution of the microstructure shows that the transformation proceeds faster in Fe–Al–Nb alloys. Microhardness was measured in dependence on the microstructural evolution with increasing annealing time, and compressive yield stress was determined for alloys annealed 700 °C/1000 h to evaluate the influence of microstructure and composition. Graphic Abstract

Phase Equilibria in the Fe-Al-Nb(-B) System at 700 °C

“…This explains why in a preceding study the metastable Heusler phase has only been observed in an alloy with about 25 at.% Al. [37] Iron aluminide based alloys are well suited for casting parts. [3] The good casting properties of these materials are partly due to the small difference between liquidus and solidus temperatures, which helps to avoid residual coring during casting.…”

“…Fe-31Al-3Nb-0.06B [37] 65.6 ± 0.3 32.5 ± 0.3 1.9 ± 0.2 0.94 52.7 ± 1.0 22.1 ± 0.6 25.2 ± 0.7 0.06 Fe-33.6Al-3Nb [37] 64.2 ± 0.6 33.6 ± 0.4 2.2 ± 0.2 0.96 52.0 ± 0.9 23.1 ± 0.6 24.9 ± 0.7 0.04 Fe-33.6Al-3Nb-0.03B [37] 63.1 ± 0.4 34.4 ± 0.1 2.5 ± 0.3 0.97 51.5 ± 1.5 23.5 ± 0.6 25.0 ± 1.8 0.03 actual compositions of the alloys and their slope changes continuously, indicating that equilibrium has been attained in all samples and that there is no conflict between individual measurements. In Fig.…”

“…The microstructural development at 700 °C in dependence on annealing time was investigated and reported before. [37] After 1 h at 700 °C, the metastable Heusler phase formed coherent precipitates in Fe-24.4Al-2Nb-0.03B, but not in alloys with higher Al content, which completely dissolved after 100 h. [37] Figure 1 shows a typical microstructure after 1000 h annealing at 700 °C for two of the investigated alloys with different compositions. Laves phase precipitates are lining the grain boundaries and are distributed in the grains of the Fe-Al matrix in all investigated alloys.…”

“…[39] All tie-lines, connecting the two coexisting phases, run through the Table 1 Actual alloy compositions as determined through wet-chemical analysis and compositions and volume fractions of the coexisting phases after heat treatment 700 °C/1000 h as determined by EPMA and XRD. Data are partially taken from previous research [37] Alloy Fe Fe-24.4Al-2Nb-0.03B [37] 74. Fe-31.9Al-3Nb [37] 66.2 ± 0.2 32.0 ± 0.3 1.8 ± 0.2 0.95 52.5 ± 1.0 21.9 ± 1.2 25.6 ± 0.6 0.05…”

“…Data are partially taken from previous research [37] Alloy Fe Fe-24.4Al-2Nb-0.03B [37] 74. Fe-31.9Al-3Nb [37] 66.2 ± 0.2 32.0 ± 0.3 1.8 ± 0.2 0.95 52.5 ± 1.0 21.9 ± 1.2 25.6 ± 0.6 0.05…”