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The Peierls barrier represents the inherent lattice resistance to dislocation glide, controlling dislocation movement and dictating the resulting mechanical properties. The rise of multi-principal element alloys, with their vast compositional space and local chemical fluctuations, introduces notable challenges in efficiently computing the Peierls barriers. Here, we propose a neural network model that captures the chemistry and structure of screw dislocations. By incorporating two crucial inputs, the local atomic type and displacement, our model enables accurate and efficient prediction of the Peierls barriers in multi-component space. Using this neural network, we construct a barrier diagram across the entire ternary space of refractory Nb-Mo-Ta alloys, from which the compositions with high or low barriers can be quickly identified. We discover that the NbMo binary alloy can have a higher barrier than NbMoTa ternary system, suggesting chemical complexity may not be the predominant factor governing dislocation barrier. Subsequently, three screened compositions with different barriers are selected for studying their effects on dislocation motion. Atomistic simulations reveal that a higher mean barrier slows down dislocation mobility, while a broader distribution of barriers facilitates kink pair nucleation, altering the rate-limiting process from kink pair nucleation to kink glide and cross kinking. This study presents a general neural network model that enables rapid screening composition with the desired barrier spectrum and will impact the exploration and discovery of alloys with improved dislocation-controlled properties. [Display omitted] [ABSTRACT FROM AUTHOR]

Refractory metal high entropy superalloys (RSAs) have been heralded as potential new high temperature structural materials. They have nanoscale cuboidal bcc+B2 microstructures that are thought to form on quenching through a spinodal decomposition process driven by the Ta-Zr or Nb-Zr miscibility gaps, followed by ordering of one of the bcc phases. However, it is difficult to isolate the role of different elemental interactions within compositionally complex RSAs. Therefore, in this work the microstructures produced by the Nb-Zr miscibility gap within the compositionally simpler Ti-Nb-Zr constituent system were investigated. A systematic series of alloys with compositions of Ti5NbxZr95−x (x = 25–85 at.%) was studied following quenching from solution heat treatment and long duration thermal exposures at 1000, 900 and 700 °C for 1000 h. During exposures at 900 °C and above the alloys resided in a single bcc phase field. At 700 °C, alloys with 40–75 at.% Nb resided within a three phase bcc + bcc + hcp phase field and a large misfit, 4.7–5%, was present between the two bcc phases. Evidence of nanoscale cuboidal microstructures was not observed, even in slow cooled samples. Whilst it was not possible to conclusively determine whether a spinodal decomposition occurs within this ternary system, these insights suggest that Nb-Zr interactions may not play a significant role in the formation of the nanoscale cuboidal RSA microstructures during cooling.

Thermodynamic and mechanical properties of 15 single-phase and 11 multi-phase refractory complex concentrated alloys (RCCAs) are reported. Using the CALPHAD approach, phase diagrams for these alloys are calculated to identify the solidus (melting, T m) temperatures and volume fractions of secondary phases. Correlations were identified between the strength drops at 1000 °C and 1200 °C and the alloy compositions, room temperature properties, melting temperatures and volume fractions of secondary phases. The influence of alloy density on the temperature dependence of specific yield strength was also explored. The conducted analysis suggests that the loss of high-temperature strength of single-phase BCC RCCAs is related to the activation of diffusion-controlled deformation mechanisms, which occurs at T ≥ 0.6 T m , so that the alloys with higher T m retain their strength to higher temperatures. On the other hand, a rapid decrease in strength of multi-phase RCCAs with increasing temperature above 1000 °C is probably due to dissolution of secondary phases. Image 1 [ABSTRACT FROM AUTHOR]

Phase composition, microstructure and mechanical properties of six complex concentrated alloys, NbTiZr, NbTiZrV, NbTiZrVMo, NbTiZrVTa, NbTiZrVCr, and NbTiZrVAl 0.24 , are reported. The alloy density was in the range from 6.34 g/cm 3 to 8.43 g/cm 3 . After homogenization annealing at 1400 °C or 1200 °C, only the ternary NbTiZr alloy had a single-phase BCC crystal structure. NbTiZrV and NbTiZrVTa produced predominantly single-phase BCC structure with clusters of fine V-rich precipitates inside NbTiZrV grains and fine, Zr-rich grain boundary precipitates in NbTiZrVTa. The other three alloys contained BCC and Laves phases, and NbTiZrVMo also contained a second BCC phase. After compression deformation at 1000 °C, noticeable changes in the phase composition were found in NbTiZrV, NbTiZrVTa and NbTiZrVAl 0.24 , while the phase composition of other alloys retained unchanged. NbTiZr, NbTiZrV and NbTiZrVTa showed good ductility and noticeable work hardening rate at room temperature, while the ductility of other alloys rapidly decreased with increasing the amount of Laves phase. The RT yield stress varied from 975 MPa for NbTiZ to 1510 MPa for NbTiZrVMo. At 1000 °C, all the alloys showed excellent compression ductility; however only NbTiZrVCr and NbTiZrVMo were stronger than NbTiZr. The results indicate that increasing the composition complexity of refractory near-equiatomic alloys by increasing the number of the alloying elements does not necessary result in a simpler phase structure and/or better mechanical properties. The type of the alloying elements seems to be more important than their number. Additions of V, Al or Ta decrease while additions of Cr or Mo increase the 1000 °C strength of NbTiZr. [ABSTRACT FROM AUTHOR]

• TiZrHfV x Nb x Ta x BCC high-entropy alloys were designed by Bo-Md alloy design method. • TiZrHfV 0.1 Nb 0.1 Ta 0.1 exhibits strain-hardening with deformation-induced phase transformation from BCC to HCP. • TiZrHfV 0.1 Nb 0.1 Ta 0.1 reveals the stepwise transformation process, β to α″ at early strain and β to α′ or α″ to α′ at high strain. [Display omitted] Refractory high-entropy alloys (RHEAs) show a great potential as structural materials due to their remarkable properties, such as high strength and thermal stability. Despite these advantages, the RHEAs still suffer from the limited ductility at room temperature. To overcome the strength-ductility trade-off, the new RHEA design strategy, transformation-induced plasticity (TRIP) HEAs, have been developed by applying metastability-engineering approach. With the newly developed RHEAs design strategy, we designed metastable RHEAs, TiZrHfVxNbxTax (x = 1.0, 0.5 and 0.1) with decrease of the β-stabilizing elements (V, Nb, and Ta) to promote the transformation-induced ductility and work-hardening capability. The proper addition of the β-stabilizing elements (V, Nb, and Ta) into the ternary equi-molar TiZrHf alloy (TiZrHfV 0.1 Nb 0.1 Ta 0.1) leads to stress/strain-induced phase transformation from the body-centered cubic (BCC) to hexagonal-close-packed (HCP) α′ and/or orthorhombic α″ phases. Detailed studies on the phase transformation sequence during the deformation were carried out using X-ray diffraction (XRD), electron back-scattered diffraction (EBSD), and transmission-electron microscope (TEM), which revealed the stepwise phase transformation that is responsible for the strong strain hardening behavior. Furthermore, we further suggested the new transformation-induced-plasticity (TRIP) HEA design strategy, applying the Bo-Md approach. [ABSTRACT FROM AUTHOR]

The microstructure and mechanical properties of the equiatomic NbVZr alloy are reported. The laser-processed and as-cast samples both exhibit a dendritic BCC solid solution with hexagonal C14 and cubic C15 Laves phases in the interdendritic regions. The stability of the Laves phases is discussed based on first-principles total energy calculations. Nb is found to increase the stability of C14 and C15 phases. The Laves phases strengthen the material by acting as obstacles to dislocation motion, as evidenced by nanoindentation and TEM observations of deformed samples. A high concentration of stacking faults was observed in the Laves region, which confirms the low stacking fault energy predicted by first-principles calculations, and are expected to result in additional plasticity. This work serves as an example of a systematic study of the stability of intermetallic phases in refractory complex concentrated alloys using a high-throughput synthesis technique. Such phases play an important role in the overall mechanical properties of the alloy. [Display omitted] [ABSTRACT FROM AUTHOR]

Refractory high-entropy alloys (RHEAs), or multi-principal-element refractory alloys, have been intensively studied in recent years, due to their attractive potential for ultrahigh-temperature structural applications. One formidable challenge facing the materials development for RHEAs though, is to simultaneously achieve good oxidation resistance and good mechanical properties, which unfortunately often impose contradictory microstructural requirements. So far, there exist no ductile RHEAs that possess reasonable oxidation resistance, and the formation of protective oxide scales such as alumina in them is never seen. Here we report, for the first time, a strategy to form protective alumina scale in ductile RHEAs upon high temperature exposure, using a tailor-designed two-step pack aluminizing process. Very importantly, the oxidation resistance of ductile RHEAs improves tremendously, as evidenced from harsh cyclic oxidation tests, thus providing a huge impetus to push forward the further development of RHEAs, towards ultrahigh-temperature applications. • a two-step pack aluminizing process forms protective alumina scale in ductile RHEAs. • a TiAl 3 /TiAl/Ti 2 AlNb and substrate multi-layered gradient structure is achieved. • Crack-free TiAl 3 coating forms on the top surface of Al 0.5 Cr 0.25 Nb 0.5 Ta 0.5 Ti 1.5 • The oxidation resistance of ductile RHEAs improves tremendously after aluminizing. [ABSTRACT FROM AUTHOR]