Microstructural evolution of single Ni2TiAl or hierarchical NiAl/Ni2TiAl precipitates in Fe-Ni-Al-Cr-Ti ferritic alloys during thermal treatment for elevated-temperature applications

Abstract

Precipitate features, such as the size, morphology, and distribution, are important parameters determining the mechanical properties of semi-or fully-coherent precipitatehardened alloys at elevated temperatures. In this study, the microstructural formation and evolution of recently-developed Fe-Ni-Al-Cr-Ti alloys with superior creep resistance have been systematically investigated using transmission-electron microscopy (TEM), scanning-electron microscopy (SEM), and atom-probe tomography (APT). These alloys were designed by adding 2 or 4 weight percent (wt. %) Ti into a NiAl-hardened ferritic alloy with a nominal composition of Fe-6.5Al-10Cr-10Ni-3.4Mo-0.25Zr-0.005B in wt. %. These alloys were, then, subjected to a homogenization treatment at 1,473 K for 0.5 hour, followed by aging treatments at 973 K for 1 ~ 500 hours. In the homogenization-treated case, both alloys contain a primary L2 1-type Ni 2 TiAl precipitate, but with the distinct size and morphology of the precipitates and precipitate/matrix interface structures. In the subsequent aging treatments, the 2 wt. % Ti alloy establishes a hierarchical-precipitate structure consisting of a fine network of a B2-type NiAl phase within the parent L2 1-type Ni 2 TiAl precipitate, while the 4 wt. % Ti alloy retains the single Ni 2 TiAl precipitate. It was found that the hierarchical structure is more effective in remaining the coherent interface during the growth/coarsening of the precipitate. The formation of the different types of the precipitates, and their effects on the microstructural evolution are discussed, and the driving forces for these features are identified from the competition between the interface energy and elastic interactions due to the lattice misfit and misfit dislocations.

High Temperature Deformation Mechanism in Hierarchical and Single Precipitate Strengthened Ferritic Alloys by In Situ Neutron Diffraction Studies

“…Both alloys often contain a small Fe inclusion within the precipitates, which was previously determined by energy-dispersive X-ray spectroscopy1819. It was suggested that the hierarchical precipitate structure is effective to retain the coherent interface, whereas the single Ni 2 TiAl precipitate tends to form misfit dislocation at the interface in order to accommodate the large lattice mismatch1820.…”

“…The microstructures of the alloys, such as chemistry and morphology of the constitutive phases within the precipitates, have been systematically characterized using transmission-electron microscopy (TEM) and atom-probe tomography (APT)181920. The microstructural features of the 4-wt.% and 2-wt.%-Ti alloys aged at 973 K for 100 hours are displayed in Fig.…”

Effect of Ti substitution for Al on the cuboidal nanoprecipitates in Al0.7NiCoFeCr2 high-entropy alloys

“…It has been demonstrated that the coherent strengthening of cuboidal B2-AlNi or L2 1 -Ni 2 AlTi nanoprecipitates in the BCC matrix can improve mechanical property of alloys on a large extent. 23,35,38 Comparing the T1 with the T2 containing similar microstructure but different Ti contents, the latter T2 HEA exhibits a relatively higher strength (1826 6 42 MPa) than the former T1 (1791 6 18 MPa), which might be caused by the smaller particle size of L2 1 nanoprecipitates. Further increasing the Ti content up to 6.25 at.% in the T3 alloy with an Al/Ti ratio of 1/1, the alloy is brittle and the fracture happens before yielding, which is caused by the brittle r matrix.…”

“…32,33 Intriguingly, further addition of 2 wt% Ti into FBB8 renders the alloy with cuboidal L2 1 /B2 hierarchical nanoprecipitates, resulting in a significant improvement of the creep-resistant property at a HT of 700°C. [33][34][35][36] In addition, cuboidal L2 1 nanoparticles were also precipitated in a newly designed light-weight BCC-based Al 1.5 CrFeMnTi HEA, which exhibits a high microstructural stability. 37 In our recent work, we designed a series of HEAs 23,38 with a composition of Al 2 M 14 (M represents different combinations of Ni, Co, Fe, and Cr) with the guide of the cluster formula approach, 39 in which the cuboidal B2 nanoparticles are found to be precipitated coherently into the BCC matrix.…”

Primary and secondary precipitates in a hierarchical-precipitate-strengthened ferritic alloy

“…%)] [25][26][27][28]. More recently, the coarsening behavior of HPSFA has been systematically studied during long-term thermal treatment at 973 K up to 500 hours, which reveals the duplex precipitates, such as, the primary and secondary, and the morphological transition of the hierarchical precipitates, which is associated with the elastic coherency of the interface between the primary precipitate and matrix [29]. However, a detailed microstructural characterization of the precipitates is necessary to elucidate the creep mechanism(s) and to achieve the further optimization of the creep properties.…”