Unique precipitations in a novel refractory Nb-Mo-Ti-Co high-entropy superalloy

Figures & data

Table 1. Actual chemical composition of the Nb₃₀Mo₃₀Ti₂₀Co₂₀ alloy.

Elements, at.%
Nb	Mo	Ti	Co	Oxygen, ppm	Nitrogen, ppm	Hydrogen, ppm
28.8 ± 0.2	32.8 ± 1.6	19.4 ± 0.8	19.0 ± 1.0	As-cast
				392 ± 17	49 ± 12	22 ± 7
				Annealed
				403 ± 15	54 ± 10	29 ± 5

Figure 1. A typical microstructure of the Nb₃₀Mo₃₀Ti₂₀Co₂₀ alloy in the as-cast (a) and annealed (b) states: SEM-BSE images, SEM-EDS maps, and SAED patterns recorded from [001]_bcc, [011]_B2 (as-cast state) or [001]_B2 (annealed state), and [011]_fcc zone axes of the corresponding phases showing the dendritic microstructure consisting of (Nb, Mo)-rich bcc islands, (Co, Ti)-rich B2 network, and (Ti, O)-rich fcc particles.

Figure 2. Characterisation of the bcc islands of the annealed Nb₃₀Mo₃₀Ti₂₀Co₂₀ alloy: (a) – magnified SEM-BSE image of typical bcc islands demonstrating the embedded dispersed particles of different morphology and Z-contrast denoted as ‘hierarchical precipitates'; (b) STEM-HAADF and STEM-EDS analyses showing the presence of hierarchical nanoprecipitates (some characteristic particles highlighted with white arrows) composed of Ti-rich and (Co, Ti)-rich segments inside the (Nb, Mo)-rich matrix.

Figure 3. Detailed TEM characterisation of hierarchical nanoparticles in the annealed Nb₃₀Mo₃₀Ti₂₀Co₂₀ alloy: (a) – HRTEM image demonstrating a typical hierarchical precipitate consisting of a one (Ti, O)-rich fcc plate and two (Co, Ti)-rich B2 segments inside the bcc matrix; (b) – SAED taken simultaneously from the bcc matrix and hierarchical B2 + fcc precipitates and sketch illustrating the ORs between the phases.

Figure 4. Detailed TEM characterisation of individual nanoparticles in the annealed Nb₃₀Mo₃₀Ti₂₀Co₂₀ alloy: (a) – HAADF-STEM image and STEM-EDS maps showing the individual (Co, Ti)-rich B2 (denoted with light-green arrows) and (Ti, O)-rich fcc (denoted with pink arrows) precipitates and dislocations (denoted with dark-red arrows) in the bcc matrix; (b) – HRTEM image illustrating the typical interface between the bcc matrix and plate-like individual fcc particle supporting by Fast Fourier Transforms (FFTs) taken from regions denoted and the deciphered B-N OR between the phases.

Figure 5. Mechanical properties and post-mortem microstructure investigations of the Nb₃₀Mo₃₀Ti₂₀Co₂₀ alloy in the as-cast and annealed states: (a) – true stress – true strain curves obtained during compression test at RT; (b) and (c) – SEM-BSE images of the as-cast (b) and annealed (c) specimens after RT compression test. Orange arrows in Figure 5(b) show the presence of multiple secondary cracks in the microstructure of the as-cast specimen. The compression axis is vertical.

Data availability statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.