Strength can be controlled by edge dislocations in refractory high-entropy alloys

Energy efficiency is motivating the search for new high-temperature (high-T) metals. Some new body-centered-cubic (BCC) random multicomponent “high-entropy alloys (HEAs)” based on refractory elements (Cr-Mo-Nb-Ta-V-W-Hf-Ti-Zr) possess exceptional strengths at high temperatures but the physical origins of this outstanding behavior are not known. Here we show, using integrated in-situ neutron-diffraction (ND), high-resolution transmission electron microscopy (HRTEM), and recent theory, that the high strength and strength retention of a NbTaTiV alloy and a high-strength/low-density CrMoNbV alloy are attributable to edge dislocations. This finding is surprising because plastic flows in BCC elemental metals and dilute alloys are generally controlled by screw dislocations. We use the insight and theory to perform a computationally-guided search over 10(7) BCC HEAs and identify over 10(6) possible ultra-strong high-T alloy compositions for future exploration

Phase Separation in Lean-Grade Duplex Stainless Steel 2101

The use of duplex stainless steels (DSS) in nuclear power generation systems is limited by thermal instability that leads to embrittlement in the temperature range of 204°C to 538°C. New lean-grade alloys, such as 2101, offer the potential to mitigate these effects. Thermal embrittlement was quantified through impact toughness and hardness testing on samples of alloy 2101 after aging at 427°C for various durations (1–10,000 h). Additionally, atom probe tomography (APT) was utilized in order to observe the kinetics of α–α′ separation and G-phase formation. Mechanical testing and APT data for two other DSS alloys, 2003 and 2205, were used as a reference to 2101. The results show that alloy 2101 exhibits superior performance compared to the standard-grade DSS alloy 2205 but inferior to the lean-grade alloy 2003 in mechanical testing. APT data demonstrate that the degree of α–α′ separation found in alloy 2101 closely resembles that of 2205 and greatly exceeds 2003. Additionally, contrary to what was observed in 2003, 2101 demonstrated G-phase like precipitates after long aging times, although precipitates were not as abundant as was observed in 2205

An atom probe perspective on phase separation and precipitation in duplex stainless steels

Three-dimensional chemical imaging of Fe–Cr alloys showing Fe-rich (α)/Cr-rich (α') phase separation is reported using atom probe tomography techniques. The extent of phase separation, i.e., amplitude and wavelength, has been quantitatively assessed using the Langer-Bar-on-Miller, proximity histogram, and autocorrelation function methods for two separate Fe–Cr alloys, designated 2101 and 2205. Although the 2101 alloy possesses a larger wavelength and amplitude after annealing at 427 °C for 100–10 000 h, it exhibits a lower hardness than the 2205 alloy. In addition to this phase separation, ultra-fine Ni–Mn–Si–Cu-rich G-phase precipitates form at the α/α' interfaces in both alloys. For the 2101 alloy, Cu clusters act to form a nucleus, around which a Ni–Mn–Si shell develops during the precipitation process. For the 2205 alloy, the Ni and Cu atoms enrich simultaneously and no core–shell chemical distribution was found. This segregation phenomenon may arise from the exact Ni/Cu ratio inside the ferrite. After annealing for 10 000 h, the number density of the G-phase within the 2205 alloy was found to be roughly one order of magnitude higher than in the 2101 alloy. The G-phase precipitates have an additional deleterious effect on the thermal embrittlement, as evaluated by the Ashby–Orowan equation, which explains the discrepancy between the hardness and the rate of phase separation with respect to annealing time (Gladman T 1999 Mater. Sci. Tech. Ser. 15 30–36).Keywords: atom probe tomography, thermal embrittlement, phase separation, Cu cluster, Fe-Cr alloy, G-phaseKeywords: atom probe tomography, thermal embrittlement, phase separation, Cu cluster, Fe-Cr alloy, G-phas

High-throughput design of high-performance lightweight high-entropy alloys

Developing affordable and light high-temperature materials alternative to Ni-base superalloys has significantly increased the efforts in designing advanced ferritic superalloys. However, currently developed ferritic superalloys still exhibit low high-temperature strengths, which limits their usage. Here we use a CALPHAD-based high-throughput computational method to design light, strong, and low-cost high-entropy alloys for elevated-temperature applications. Through the high-throughput screening, precipitation-strengthened lightweight high-entropy alloys are discovered from thousands of initial compositions, which exhibit enhanced strengths compared to other counterparts at room and elevated temperatures. The experimental and theoretical understanding of both successful and failed cases in their strengthening mechanisms and order-disorder transitions further improves the accuracy of the thermodynamic database of the discovered alloy system. This study shows that integrating high-throughput screening, multiscale modeling, and experimental validation proves to be efficient and useful in accelerating the discovery of advanced precipitation-strengthened structural materials tuned by the high-entropy alloy concept