Three-dimensional atom probe studies of phase transformations in Fe-Al-Ni-Cr alloys with B2-ordered NiAl precipitates

Full text: In the search for new high-temperature materials iron aluminium based alloys show promising properties. The main advantage of Fe-Al alloys is their resistance to high-temperature oxidation and sulphidation, which strongly depends on the Al content and is not yet tested for multicomponent systems. In order to improve strength and creep resistance of Fe-Al based alloys at high temperatures, novel alloys with intermetallic NiAl precipitates with B2 structure in a ferritic, disordered (A2) Fe(Al,Ni) matrix are being tested. The appropriate combination of phases and optimisation of phase distribution leads to a selection of alloy compositions with relatively low amounts of Al. Consequently, the oxidation resistance becomes questionable. Further alloying with Cr improves the oxidation resistance without affecting the NiAl precipitation. The resulting microstructure with coherent NiAl precipitates in the Fe(Al,Ni,Cr) matrix is analogous to those of the Ni-base superalloys. Thus such Fe-AI-Ni-Cr alloys are considered as the basis for novel ferritic superalloys. The formation of the NiAI phase in three different alloys of composition Fe-6.7Al-10.6Ni10.0Cr, Fe-9.5Al-15.9Ni-10.0Cr, and Fe-12.5Al-21.5Ni-10.0Cr (wt.%) during cooling after solution treatment at 1200oC has been investigated using field ion microscopy, 3-dimensional atom probe (3DAP) analysis, differential thermal analysis (DTA), scanning electron microscopy and transmission electron microscopy (TEM). NiAI precipitates mainly of two different sizes were identified in the samples by TEM analysis. The composition of the NiAI phase of the three samples was determined by means of 3DAP analysis. For the Fe-9.5Al15.9Ni-10.0Cr alloy very small Fe(Al,Ni,Cr) precipitates in the B2-NiAI phase were identified by 3DAP. The possible phase transformations are discussed on the basis of the combined 3DAP, TEM and DTA measurements. For instance, the DTA measurement of the Fe-6.7Al-10.6Ni-10.0Cr alloy shows an exothermic peak at about 900oC, which we believe to be correlated with the formation of the fine Fe(A1,Ni,Cr) precipitates, as seen in the 3DAP, during cooling. (author)