This study reports the structure and mechanical properties of new refractory Ti₃₇.₅Nb₁₂.₅Hf₂₅Al₂₅, Ti₄₀Nb₃₀Hf₁₅Al₁₅, and Ti₄₀Nb₂₀Ta₁₀Hf₁₅Al₁₅ (at.%) high entropy alloys. After annealing at 1200 °C for 24 h, the program alloys had a single-phase B2 structure. Further annealing at 600 °C for 24 h resulted in the formation of Widmanstatten orthorhombic particles (O-phase) in the bcc matri

Panina, E.

Salishchev, G.

Stepanov, N.

Yurchenko, N.

Zherebtsov, S.

DSpace at Belgorod State University

IOP Conference Series: Materials Science and EngineeringPAPER • OPEN ACCESSPrecipitation-hardened refractoryTi-Nb-Hf-Al-Ta high-entropy alloysTo cite this article: E Panina et al 2021 IOP Conf. Ser.: Mater. Sci. Eng. 1014 012041 View the article online for updates and enhancements.This content was downloaded from IP address 188.170.217.49 on 15/04/2021 at 10:22Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distributionof this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.Published under licence by IOP Publishing LtdASYS 2020IOP Conf. Series: Materials Science and Engineering 1014  (2021) 012041IOP Publishingdoi:10.1088/1757-899X/1014/1/0120411      Precipitation-hardened refractoryTi-Nb-Hf-Al-Ta high-entropy alloys  E Panina*, N Yurchenko, S Zherebtsov, G Salishchev and N Stepanov  Laboratory of Bulk Nanostructured Materials, Belgorod State University, Belgorod 308015, Russia  *Corresponding author: panina_e@bsu.edu.ru  Abstract. This study reports the structure and mechanical properties of new refractory Ti37.5Nb12.5Hf25Al25, Ti40Nb30Hf15Al15, and Ti40Nb20Ta10Hf15Al15 (at.%) high entropy alloys. After annealing at 1200 °C for 24 h, the program alloys had a single-phase B2 structure. Further annealing at 600 °C for 24 h resulted in the formation of Widmanstatten orthorhombic particles (O-phase) in the bcc matrix. The Ti40Nb30Hf15Al15 and Ti40Nb20Ta10Hf15Al15 alloys annealed at T = 1200 °C showed moderate strength and good ductility (>50%) at 22 and 600 °C; while the Ti37.5Nb12.5Hf25Al25 alloy was stronger, but less ductile at both temperatures. Subsequent annealing at T = 600 °C significantly increased the strength of the Ti40Nb30Hf15Al15 alloy at 22 and 600 °C, maintaining compressive sufficient ductility at room-temperature .   1.  Introduction Recently introduced refractory high entropy alloys (RHEAs), which demonstrated an outstanding capability to maintain high strength at temperatures up to T = 1600 °C [1], seem promising candidates for next-generation turbines. Despite the attractive high-temperature strength, one of the major drawbacks of most RHEAs is the modest mechanical performance at ambient temperatures.  It is believed that balanced properties can be obtained by developing precipitation-strengthened RHEAs. One of the promising examples of such RHEAs are alloys with the bcc/B2 structure [2, 3]. The superalloy-like microstructure with a bcc solid solution, strengthened by coherent cuboidal B2 nanoparticles, provides a good combination of strength and ductility at temperatures T≤600 °C. However, the development of bcc/B2 RHEAs is a non-trivial task due to weakly established composition-structure relationships and poor fidelity of thermodynamic modeling in the case of the B2 phase [4, 5]. In this work, we presented a series of RHEAs strengthened by nanosized Widmanstatten orthorhombic (O-phase) particles. The O-phase precipitation was found to be an attractive option to achieve a notable strength enhancement both at room and elevated temperatures with only a slight loss in ductility.   ASYS 2020IOP Conf. Series: Materials Science and Engineering 1014  (2021) 012041IOP Publishingdoi:10.1088/1757-899X/1014/1/0120412      2.  Materials and methods Ingots of the Ti37.5Nb12.5Hf25Al25, Ti40Nb30Hf15Al15, Ti40Nb20Ta10Hf15Al15 alloys were produced by vacuum arc melting of pure (≥ 99.9 wt%) elements. The alloys were annealed in quartz tubes at 1200 °C for 24 h. Some samples were further annealed at 600 °C for 24 h. The phase composition and microstructure of the alloys were studied using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The densities of the Ti37.5Nb12.5Hf25Al25, Ti40Nb30Hf15Al15, Ti40Nb20Ta10Hf15Al15 alloys, determined by the hydrostatic weighing method, were 7.23± 0.03, 7.07 ± 0.03, 7.87± 0.03 g/cm3, respectively. Isothermal compression was carried out in air at 22 °C or 600 °C using an Instron 300LX test machine.  3.  Results 3.1.  Microstructure  After annealing at 1200 °C, the Ti37.5Nb12.5Hf25Al25,Ti40Nb30Hf15Al15, and Ti40Nb20Ta10Hf15Al15 alloys had a coarse-grained single-phase microstructure with the average grain size ~300 μm, (Figure 1a-c); no secondary phases were observed. After further annealing at 600 °C, precipitation of profuse Widmanstatten second phase particles was revealed. The particles located adjust to grain boundaries were coarser than the particleswithin the grain interior. The volume fraction of the second phase was nearly equal in all program alloys and estimated to be ~ 35%.        Figure 1. SEM images of the Ti37.5Nb12.5Hf25Al25 (a, d), Ti40Nb30Hf15Al15(b, e), Ti40Nb20Ta10Hf15Al15 (c, f) alloys after annealing at 1200°C, 24 h (a, b, c) and further annealing at 600°C, 24h (d, e, f).  After annealing at 1200 °C, all the program alloys had a single-phase B2 structure as revealed by the selected area electron diffraction (SAED) patterns. TEM analysis revealed numerous anti-phase boundaries in the matrix grains (denoted in figure 2a-c as APBs); no secondary phases were found in all alloys. ASYS 2020IOP Conf. Series: Materials Science and Engineering 1014  (2021) 012041IOP Publishingdoi:10.1088/1757-899X/1014/1/0120413      The phases found after further annealing at 600 °C, were identified using TEM analysis: the matrix had the B2 (Ti37,5Nb12,5Hf25Al25) or bcc (Ti40Nb30Hf15Al15 and Ti40Nb20Ta10Hf15Al15) structure, whilst the particles were defined as the O-phase.        Figure 2. TEM bright-field images of the Ti37.5Nb12.5Hf25Al25 (a, d), Ti40Nb30Hf15Al15(b, e), Ti40Nb20Ta10Hf15Al15 (c, f) alloys after annealing at 1200 °C, 24 h (a, b, c) and further annealing at 600 °C, 24h (d, e, f). Selected area electron diffraction patterns used for phase identification are shown in the inserts. 3.2.  Mechanical properties Stress-strain curves of the program alloys annealed at 1200 °C, i.e. with the single B2 phase structure, are presented in figures 3a, b. At 22 °C, the Ti40Nb30Hf15Al15 and Ti40Nb20Ta10Hf15Al15 alloys demonstrated moderate strength (830 and 1075 MPa, respectively), high strain hardening capacity and compressive ductility. Meanwhile, the Ti37.5Nb12.5Hf25Al25 alloy was the strongest (1645 MPa), albeit fractured after 1% of plastic deformation. At 600 °C, the strength of the Ti40Nb30Hf15Al15 and Ti40Nb20Ta10Hf15Al15 alloys reduced to 635 and 700 MPa, respectively. The Ti37.5Nb12.5Hf25Al25 alloy exhibited a notable decrease in yield strength to 810 MPa with a simultaneous increase in compressive ductility to 12%.  Subsequent annealing at 600 °C (which caused precipitation of the O-phase particles in the bcc/B2 matrix) changed the mechanical properties of the program alloys markedly (Figures 3c, d). The Ti40Nb30Hf15Al15 alloy demonstrated a ~50% strength increment both at 22 and 600 °C while maintaining reasonable compressive ductility at 22 °C. A similar annealing effect on room-temperature mechanical properties was also found for the Ti40Nb20Ta10Hf15Al15 alloy; however, at 600 °C, the alloy performance degraded inevitably; i.e. both strength and ductility became worse in comparison with annealing at 1200 °C. In the case of the Ti37.5Nb12.5Hf25Al25 alloy, annealing at 600 °C increased its high-temperature strength, but drastically diminished compressive ductility. The obtained findings suggest complex relationships between the chemical composition, structure, and mechanical properties of the O-phase strengthened RHEAs since similar microstructural changes (precipitation of the O-phase particles) had a drastically different effect on the mechanical performance of the program alloys. Further ASYS 2020IOP Conf. Series: Materials Science and Engineering 1014  (2021) 012041IOP Publishingdoi:10.1088/1757-899X/1014/1/0120414      studies are required to reliably establish the relationship between the composition and the structural properties of these alloys.     Figure 3. Engineering stress-strain curves of the program alloys after annealing 1200 °C (a, b) and further annealing at 600 °C (c, d) obtained during compression at 22 (a,c) and 600 °C (b,d). 4.  Conclusions Microstructure and mechanical properties of the refractory Ti37.5Nb12.5Hf25Al25, Ti40Nb30Hf15Al15, and Ti40Nb20Ta10Hf15Al15 high entropy alloys were studied. The following conclusions were drawn:   • After annealing at 1200 °C, all the program alloys have a single-phase B2 structure. Further annealing at 600 °C led to the formation of a mixture of the B2 (Ti37,5Nb12,5Hf25Al25) or bcc (Ti40Nb30Hf15Al15, Ti40Nb20Ta10Hf15Al15) matrix and nanosized Widmanstatten O-phase particles. • In the single-phase state, the Ti40Nb30Hf15Al15 and Ti40Nb20Ta10Hf15Al15 alloys demonstrated moderate strength and high compressive ductility at 22 and 600 °C. The Ti37.5Nb12.5Hf25Al25 alloy showed the highest strength, but very limited ductility. The precipitation of the O-phase enhanced the strength of the Ti40Nb30Hf15Al15 alloy at 22 and 600 °C without notable sacrificing in compressive ductility at 22 °C. However, the mechanical properties of the Ti37.5Nb12.5Hf25Al25 and Ti40Nb20Ta10Hf15Al15 alloys deteriorated after annealing at 600 °C.  Acknowledgments This study was supported by the Russian Science Foundation, grant № 19-79-30066. References [1] Senkov O N, Wilks G B, Scott J M and Miracle D B 2011 Intermetallics 19 698 [2] Soni V, Senkov O N, Gwalani B, Miracle D B and Banerjee R 2018 Sci. Rep. 8 1 [3] Senkov O N, Couzinie J-P,.Rao S I, Soni V and Banerjee R 2020 Materialia 9 100627 [4] Soni V, Senkov O N, Couzinie J-P, Zheng Y, Gwalani B and Banerjee R 2020 Materialia 9 100569 [5] Whitfield T E, Pickering E J, Owen L R, Jones C N, Stone H J and Jones N G 2020 Materialia 13 100858 

Precipitation-hardened refractoryTi-Nb-Hf-Al-Ta high entropy alloys

http://dspace.bsu.edu.ru/bitstream/123456789/41434/1/Panina_Precipitation.pdf

Precipitation-hardened refractoryTi-Nb-Hf-Al-Ta high entropy alloys

Abstract

Similar works

Full text

Available Versions

DSpace at Belgorod State University