High-strength High-entropy Alloys Research Articles

Connecting the mechanical properties of magnesium and high-entropy alloys

The deformation mechanisms in individual crystal structures have been extensively studied, such as the yielding phenomenon in face-centered-cubic (FCC) high-entropy alloys (HEAs) and the ductility in hexagonal close-packed (HCP) magnesium alloys. However, the physical connection of these mechanisms between the different crystal structures has rarely been investigated. For example, high-strength HEAs and ductile magnesium alloys both require low stacking fault energy (SFE), and the physical origin of this common feature is unclear. Here we provide a physical model for the SFE trend in FCC HEAs that connects the two phenomena. The physical model predicts the change of SFE based on elastic constants and geometrical parameters. This study paves the pathway to unifying the mechanics of close-packed materials.

High entropy alloy thin films on SS304 substrates: Evolution of microstructure and interface modulated by energetic condensation in nanoscale

High entropy alloys (HEAs), as a novel material in the 21st century, possess several advantages, such as excellent corrosion & oxidation resistance and high mechanical properties. HEA thin films show these favourable properties with lower material costs than their bulk counterparts. Studying the HEA film-substrate interface represents challenges but is of extreme importance for the understanding of growth mechanisms with important implications for film adhesion. However, most HEA films were deposited on monocrystalline silicon substrates with limited practical applicability. Further, where commercial stainless steel, aluminium or titanium alloy substrates were used, the microstructure and chemistry at the interface were neglected. Here, we deposited AlCrFeCoNiCu0.5 HEA thin films on stainless steel 304 (SS304) substrates using cathodic arc deposition with different substrate biases. The crystallography and microstructure were investigated using an X-ray and electron-microscopy based chatacterization. A transition of an incoherent to semi-coherent interface was observed from 0 V to -50 V of the substrate bias. Energy dispersive spectroscopy demonstrated a transition of Cr2O3 to aluminum oxide across the interface. The nanoindentation tests revealed the significant improvement of mechanical properties of SS304 with HEA coatings. High-strength HEA (8.0 ± 0.2 GPa) thin films with semi-coherent interfaces were manufactured on SS304.

Ultrastrong lightweight compositionally complex steels via dual-nanoprecipitation.

High-performance lightweight materials are urgently needed, given the pressing quest for weight reduction and the associated energy savings and emission reduction. Here, by incorporating the multi-principal element feature of compositionally complex alloys, we develop the concept of lightweight steels further and propose a new class of compositionally complex steels (CCSs). This approach allows us to use the high solid solution strengthening and shift the alloys' compositions into previously unattainable phase regions where both nanosized shearable κ-carbides and non-shearable B2 particles are simultaneously formed. The achievement of dual-nanoprecipitation in our CCSs leads to materials with ultrahigh specific tensile strength (up to 260 MPa·cm3 g-1) and excellent tensile elongation (13 to 38%), a combination outperforming all other high-strength high-entropy alloys and advanced lightweight steels. Our concept of CCSs is thus useful for guiding the design of ultrastrong lightweight metallic materials.

First-principles-based prediction of yield strength in the RhIrPdPtNiCu high-entropy alloy

High-entropy alloys are random alloys with five or more components, often near equi-composition, that often exhibit excellent mechanical properties. Guiding the design of new materials across the wide composition space requires an ability to compute necessary underlying material parameters via ab initio methods. Here, density functional theory is used to compute the elemental misfit volumes, alloy lattice constant, elastic constants, and stable stacking fault energy in the fcc noble metal RhIrPdPtNiCu. These properties are then used in a recent theory for the temperature and strain-rate dependent yield strength. The parameter-free prediction of 583 MPa is in excellent agreement with the measured value of 527 MPa. This quantitative connection between alloy composition and yield strength, without any experimental input, motivates this general density functional theory-based methodological path for exploring new potential high-strength high-entropy alloys, in this and other alloy classes, with the chemical accuracy of first-principles methods.

Cryogenic strength improvement by utilizing room-temperature deformation twinning in a partially recrystallized VCrMnFeCoNi high-entropy alloy

The excellent cryogenic tensile properties of the CrMnFeCoNi alloy are generally caused by deformation twinning, which is difficult to achieve at room temperature because of insufficient stress for twinning. Here, we induced twinning at room temperature to improve the cryogenic tensile properties of the CrMnFeCoNi alloy. Considering grain size effects on the critical stress for twinning, twins were readily formed in the coarse microstructure by cold rolling without grain refinement by hot rolling. These twins were retained by partial recrystallization and played an important role in improving strength, allowing yield strengths approaching 1 GPa. The persistent elongation up to 46% as well as the tensile strength of 1.3 GPa are attributed to additional twinning in both recrystallized and non-recrystallization regions. Our results demonstrate that non-recrystallized grains, which are generally avoided in conventional alloys because of their deleterious effect on ductility, can be useful in achieving high-strength high-entropy alloys.