Microstructural Design for Improving Ductility of An Initially Brittle Refractory High Entropy Alloy Page: 2
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rapid strain localization and necking. It cannot be applied to inherently brittle RHEAs that fracture without strain
localization/necking and, often, without any macroscopic strain.
Recently, several Al-containing RHEAs were reported9,30,31 to have a characteristic superalloy-like micro-
structure, consisting of cuboidal BCC nano-scale precipitates within a coherent B2 matrix, resembling the
'(fcc) +' '(ordered L12 precipitates) microstructure exhibited by many currently used nickel base superalloys.
Although they showed exceptionally good strength at both room and elevated temperatures, substantially
exceeding those of single-phase BCC RHEAs, these novel two-phase RHEAs have very limited room tem-
perature compressive ductility, which can be explained by the inherent brittleness of the ordered B2 matrix
phase32. Unfortunately, the approaches for improving ductility discussed above cannot be applied to this class of
Al-containing RHEAs.
The present work is the first demonstration of enhancing the ductility of high-strength BCC + B2 two-phase
RHEAs by controlling their microstructure. For this, Al0 5NbTa0 8Ti 5V0.2Zr was selected, based on its low density
(7.4 g/cm3) and previous reports of excellent room and high temperature yield strength9. This alloy was cast, hot
isostatically pressed (HIPed) and then homogenized at 1200 C for 24hrs followed by slow cooling (10 C/min)
to room temperature. The resultant microstructure consists of two BCC phases (one of which is likely ordered,
but this was not previously proven9) with very similar lattice parameters that form a very fine, inter-woven
baskteweave-like, nano-phase structure9. Subsequently, this will be referred as Condition (1). The alloy has a
room-temperature yield strength of 2035 MPa but only 4.5% compression strain before fracture in Condition (1).
The present study focuses on improving the ductility of this alloy by tuning the microstructure, while maintaining
its high yield strength.
The alloy in Condition (1) was solutionized at 1400 C for 20 min followed by water quenching to achieve
a single-phase microstructure. This will be referred to as Condition (2). The alloy in Condition (2) was then
annealed at 600 C for 120 hrs and water-quenched (subsequently referred to as Condition (3)) to possibly
develop a two-phase BCC + B2 microstructure.
Results
Microstructure Characterization. Condition (1) was studied here in greater detail using scanning electron
microscopy (SEM), transmission electron microscopy (TEM) and atom probe tomography (APT). The Condition
(1) microstructure is summarized in Fig. 1(a-c). Annealing at 1200 C resulted in large, equiaxed grains (grain
size=100 pm9) as seen in the backscattered electron (BSE) SEM image in Fig. 1(a). A dark-field TEM image,
acquired using a {001} superlattice reflection of the B2 phase, is shown in Fig. 1(b). The [001] zone axis electron
diffraction pattern is shown as an inset in the same figure. This dark-field TEM image of Condition (1) revealed
a highly-refined microstructure consisting of a two-phase mixture. The highlighted brighter regions, forming
the continuous matrix, correspond to the ordered B2 phase while the darker discrete pockets correspond to the
disordered BCC phase. The edge-to-edge length of the precipitates (disordered BCC) is -20 nm and the thickness
of the channels (ordered B2) is -2 nm. The precipitates had a very narrow size distribution and were arranged in
regular rows along <001 > direction.
The three-dimensional (3D) distribution of the B2 and BCC phases, and the elemental partitioning across
the interface, was studied using atom probe tomography (APT). An example of the reconstructed APT dataset
is shown in Fig. 1(c), depicting the raw ion map using Al (red) and Ta (blue) ions. Clearly, there is strong com-
positional partitioning of the constituent elements within the B2 and BCC phases. The composition profiles for
the different elements were plotted using a proximity histogram approach". These profiles are constructed by
delineating the B2/BCC interface using an iso-concentration surface of Al= 10.5 at%. The B2 phase (highlighted
in red in the reconstruction map) is rich in Al and Zr whereas the BCC phase is rich in Nb and Ta (Fig. 1(c)). The
approximate compositions of the two phases are: BCC: 5A1-27Nb-18Ta-11Zr-33Ti-6V (at%) and B2: 20A1-lONb-
4Ta-31Zr-31Ti-4V (at%).
The microstructure of the alloy in Condition (2) was substantially different from Condition (1). The aver-
age grain size in this condition is -150 pm. Figure 2(a) shows a selected area electron diffraction pattern from
the alloy in Condition (2), which can be indexed as the <011>BCC zone axis. Additionally, careful analysis of
the <011 >BCC zone axis (Fig. 2(a)) revealed extremely weak {100} B2 superlattice reflections, indicating that
this BCC-based phase has weak B2-type ordering. A dark-field TEM image, recorded from one of these {100}
B2 reflections, shown in Fig. 2(b), clearly shows highly refined nanometer scale ordered B2 regions dispersed
within a BCC matrix. Atom probe tomography was used to further investigate this condition. A raw ion map
consisting of Al and Ta ions is shown in Fig. 2(c). Despite the absence of any sharp demarcation between compo-
sitionally distinct phases, there appears to be a small degree of inhomogeneity (clustering) in this compositional
map. This inhomogeneity has been captured by plotting composition profiles (proximity histogram analysis) for
the different constituent elements across an artificially created interface using an iso-concentration surface of
Nb =20at%, as shown in Fig. 2(c). This compositional analysis indicates an early stage of phase separation into
a co-continuous mixture of Al, Zr and Ti rich regions interspersed with Nb and Ta rich regions (Fig. 2(c)). This
weak partitioning suggests that the alloy was in the single BCC phase field at the solution treatment temperature
of 1400 C, and the early stages of composition partitioning, via fluctuations, occurred during the quench. The
partitioning also suggests the existence of a miscibility gap in this HEA composition, similar to previously stud-
ied AlMo0.5NbTa0.5TiZr32. The final microstructure in case of Condition (2) can be described as highly refined,
nanometer-scale mixture of ordered B2 regions within a BCC matrix.
Condition (3) exhibits a typical superalloy type microstructure as seen in the backscatter SEM image
in Fig. 3(a). The average grain size in this condition is -150 pm. The bright, continuous matrix phase has
homogeneously-distributed second phase precipitates (darker contrast) arranged in a checkerboard-like pattern.A TEM dark field image (Fig. 3(c)), obtained from the {100} B2 superlattice spot in the <001>BCC zone axis
(Fig. 3(b)), revealed that the discrete precipitates are the ordered B2 phase, while the continuous matrix phaseSC R EPORT C1 (2018) 8:8816 I1DOI:10.1038/s41598-018-27144-3
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Soni, V.; Senkov, O. N.; Gwalani, B.; Miracle, D. B. & Banerjee, Rajarshi. Microstructural Design for Improving Ductility of An Initially Brittle Refractory High Entropy Alloy, article, June 11, 2018; London, United Kingdom. (https://digital.library.unt.edu/ark:/67531/metadc1181167/m1/2/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT College of Engineering.