Hierarchical microstructure and properties of ferritic alloys strengthened by two-phase intermetallic precipitates
Final Report Abstract
In the first part, the research project aims to investigate the phase transformation pathways in model ferritic Fe-Al-Cr-Ni alloys with different Ti levels leading to a hierarchical microstructure. The formation of two-phase B2-NiAl / L21-Ni2TiAl precipitates is studied using advanced transmission electron microscopy (TEM) techniques in combination with first-principles thermodynamic calculations. The hierarchical microstructure is exhibiting three levels of hierarchy due to (i) chemical ordering, (ii) the dimensionality of the phases with different degrees of ordering and (iii) the spatial distribution of the ordered phases. The early stages of phase separation are studied in a model alloy with composition Fe-8.1Al-12.2Cr-1.9Mo-18.2Ni (in wt.%) and 2 wt.% Ti obtained by melt-spinning. Darkfield TEM imaging reveals that B2-NiAl precipitates are embedded in the bcc-Fe matrix in the asquenched state. The nucleation and growth of L21-Ni2TiAl within the B2-precipitates is established by TEM and atom probe tomography (APT) in the course of the aging heat treatment at 700 ºC from 1 to 10 hours. After 10 hours of aging at 700 ºC the hierarchical microstructure is fully established. The primary B2-NiAl precipitate, with an edge length of 60-200 nm, is coherently embedded in the bcc-Fe matrix. A precipitate substructure of plate shaped L21-Ni2TiAl formed within the B2-precipitates with a size range of 15-20 nm. Aberration corrected high angle annular dark-field (HAADF) scanning TEM (STEM) proves the coherency between the B2- and L21-phases. It is also found that the B2/L21- interface shows strong fluctuations in position with an interface width of ~4 nm. At the same time, order-disorder fluctuations are observed within the B2- and L21-ordered regions. Cluster-expansion based Monte-Carlo simulations of the underlying bulk and interfacial thermodynamic properties support the experimental observations. The calculated phase boundaries between B2-NiAl and L21- Ni2TiAl, a pseudo binary system, are in good agreement with experimental data verifying the validity of the cluster-expansion approach. The most important feature of the Monte-Carlo simulations is that the interfacial energies of B2 and L21 reduce significantly with increasing temperature from values at 0 K of ~50 mJ/m2 to values on the order of 11 mJ/m2 at 973 K, corresponding to the aging temperature. The pre-wetting transition of initial ½[100] antiphase domain boundaries in L21-Ni2TiAl precipitates is investigated in an alloy with composition Fe-8.1Al-12.2Cr-1.9Mo-18.2Ni (in wt.%) and 4 wt.% Ti obtained by melt-spinning and subsequent aging at 700 ºC for 3 hours. Aberration corrected HAADF- STEM reveals the formation of disordered B2-NiAl zones within the L21-Ni2TiAl primary precipitates. From atomic column peak intensities the conservation of the antiphase domains is confirmed. The pre-wetting of a perfect ½[100] antiphase domain boundary in off-stoichiometric L21-Ni2TiAl is confirmed through cluster-expansion based Monte-Carlo simulations. Equilibrating the simulation cell at 973 K leads to the formation of a wetting layer of B2-NiAl with a width of ~2 nm at the initial APB. The second part is focusing on the formation of B2-NiAl / L21-Ni2TiAl based precipitates in conventionally cast ferritic Fe-Al-Cr-Ni-Ti alloys exhibiting superior high temperature mechanical properties. The precipitate structure of conventionally cast ferritic alloys with different levels of Ti is studied using dark-field TEM imaging techniques. The alloys are obtained by vacuum-induction-casting followed by hot isotactic pressing, homogenization at 1473 K for 30 min and subsequent aging at 700 ºC for 100 h. Coherent two-phase B2-NiAl / L21-Ni2TiAl precipitates with cuboidal shape and cube edge length of 50-100 nm are observed in an alloy with 2 wt.% Ti with a similar microstructure to that observed in the model alloys. Increasing the Ti level to 4 and 6 wt.% leads to the formation spherical or elliptical L21- Ni2TiAl precipitates. The size of the precipitates in the 4 wt.% Ti alloy is 200-500 nm and 100-200 nm in the 6 wt.% Ti alloy. The matrix-precipitate interface is semi-coherent for both alloy systems (4 and 6 wt.%) and Fe-matrix inclusions are observed within the precipitates.
Publications
- Configuration of superdislocations in the '-Pt3Al phase of a Pt-based superalloy. Intermetallics 48 (2014), pp. 71-78
C.H. Liebscher, U. Glatzel
(See online at https://doi.org/10.1016/j.intermet.2013.12.006) - Creep properties and microstructure of a precipitation-strengthened ferritic Fe–Al–Ni–Cr alloy. Acta Materialia 71 (2014), pp. 89-99
N.Q. Vo, C.H. Liebscher, M.J.S. Rawlings, M. Asta, D.C. Dunand
(See online at https://doi.org/10.1016/j.actamat.2014.02.020)
