Abstract
Recent experiments show that the chemical composition of body-centered cubic (bcc) refractory high entropy alloys (HEAs) can be tuned to enable transformation-induced plasticity (TRIP), which significantly improves the ductility of these alloys. This calls for an accurate and efficient method to map the structural stability as a function of composition. A key challenge for atomistic simulations is to separate the structural transformation between the bcc and the ω phases from the intrinsic local lattice distortions in such chemically disordered alloys. To solve this issue, we develop a method that utilizes a symmetry analysis to detect differences in the crystal structures. Utilizing this method in combination with ab initio calculations, we demonstrate that local lattice distortions largely affect the phase stability of Ti–Zr–Hf–Ta and Ti–Zr–Nb–Hf–Ta HEAs. If relaxation effects are properly taken into account, the predicted compositions near the bcc–hcp energetic equilibrium are close to the experimental compositions, for which good strength and ductility due to the TRIP effect are observed.
Highlights
High entropy alloys (HEAs), or compositionally complex alloys (CCAs), have inspired a new era in alloy design[1,2]
The unique mechanical strength of HEAs is ascribed to local lattice distortions (LLDs) that are driven by chemical disorder
In fcc HEAs, interatomic distances were shown to be strongly affected by local chemical environments[13], and this clearly reveals the importance of local chemistry for determining LLDs
Summary
High entropy alloys (HEAs), or compositionally complex alloys (CCAs), have inspired a new era in alloy design[1,2]. For refractory HEAs, a critical drawback is their low ductility, as more generally known from bcc metals and alloys To overcome this deficiency, it has recently been suggested[5,6] to make use of transition-induced plasticity (TRIP) that relies on the structural transformation from the bcc to the hexagonal close-packed (hcp) phase. For bcc refractory HEAs containing a significant fraction of group 4 elements (Ti, Zr, Hf) the situation is, seriously complicated because of the imminent dynamical instability of these alloys at low temperatures. Complications can be expected by considering the intrinsic low-temperature dynamical instability of the individual group 4 elements in the bcc structure, which is reflected by imaginary phonon modes in ab initio calculations at 0 K18–21. Control over the ω phase is of high importance, because ω precipitates can cause embrittlement[12,31,32]
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