Effect of Cooling Rate on the Phase Formation of AlCoCrFeNi High-Entropy Alloy

Abstract

High-entropy alloys have received significant attention because of remarkable structural properties exhibited by certain alloy compositions. However, these properties are strongly correlated to the crystallographic phase transformations that are endured during the synthesis of these alloys. Using molecular dynamics simulations, we examine how the cooling rates exerted on the alloy melt during synthesis impact the crystallization (and glass formation) of equiatomic AlCoCrFeNi high-entropy alloy. An increased cooling rate contributes to severe undercooling of the alloy, reducing the crystallization temperatures and promotes phase transformations. We predict a critical cooling rate of 2.5 9 10 10 K/s beyond which the alloy tends to solidify into an amorphous phase. Our results reveal that higher cooling rates exert severe lattice distortion and significantly enhance the structural properties due to increase in dislocation density and deformation by twinning.

Phase and thermodynamics-informed predictive model for laser beam additive manufacturing of a multi-principal element alloy

“…Furthermore, the effects of post-solidification heating and cooling cycles on the microstructures are significant when processing a multi-layer deposit and are beyond the scope of this study. Evaluation of CRs under these conditions implies that the MPEA cools from melt to solid (~3067 K-1600 K) in 0.002 s with a CR of 7 × 10 5 K/s (Roy et al, 2021b;Sreeramagiri et al, 2021). A relatively lower order is noted for the CR from the simulations as compared to the actual CRs experienced in LBAM (~10 6 K/s) because of the preheating of the substrate at ~1000 K and the lower scan speed considered.…”

“…CALPHAD results corroborate that the AlCoCrFeNi MPEA induces a liquid phase upon heating (melting temperature) at 1673 K, which is close to the peak temperature at 1,000 mm/s. The contribution of superheating effects due to rapid heating (Sreeramagiri et al, 2021) in LBAM can result in an upward shift in the melting point, producing an improper melting of all the powder at this peak temperature. In consequence, the alloy fabricated at 1,000 mm/s using 200 W realizes defects, such as lack of fusion and un/under-melted powder, causing unanticipated failures in the fabricated part.…”

“…The cooling rates (CRs) that the MPEAs are subjected to during synthesis drive the lattice distortions, resulting in significant variations in the structural properties (Roy et al, 2021b;Sreeramagiri et al, 2021). Literature on AlCoCrFeNi MPEAs suggests that a relatively higher cooling rate stimulates severe lattice distortion, which improves the elastic modulus at the expense of ductility (Roy et al, 2021b;Sreeramagiri et al, 2021). Figure 4C reproduces the variation of temperature at the mid-point on the laser-irradiated surface, recorded immediately before melting and after solidification, as a function of time for 333 mm/s scan speed.…”

Challenges and Strategies for Synthesizing High Performance Micro and Nanoscale High Entropy Oxide Materials

“…In the field of metal heat treatment, there is a definition of the critical cooling rate, and materials with different crystal structures are obtained at different cooling rates. [143] Some researchers have already proven the role of the degree of supercooling. Su et al employed in situ electron microscopy to visualize the entire formation process from polymer precursor to (La 0.2 Er 0.2 Sm 0.2 Yb 0.2 Y 0.2 ) 2 Ce 2 O 7 .…”

Vacancy Formation Energies and Migration Barriers in Multi-Principal Element Alloys

“…More recently, density-functional theory (DFT) [10,14] or molecular dynamics (MD) [15][16][17] and combined with machine-learning [7,8,18,19] techniques played a key role in accelerating the discovery of promising MPEAs.…”