Structures and mechanical properties of Ti-Nb-Cr-V-Ni-Al refractory high entropy alloys

Abstract

Four refractory high entropy alloys with different chemical compositions, which can be calculated as Ti (50-1.5625x) Nb (30-0.9375x) Cr 10 V 10 Ni 1.5x Al x (x ¼ 0, 5, 7, 10), were prepared by arc melting to determine the effect of Ni and Al on the phase composition, structure and mechanical properties. Each alloy was studied in both the as-cast and annealed at 1000 � C for 24 h conditions; compression tests at room temperature or at 800 � C was used to examine mechanical behavior of the alloys and the effect of deformation on microstructure. The replacement of Ti and Nb with Al and Ni resulted in the formation of the Ti, Ni-rich σ-phase and Ti 2 Ni phases in the bcc matrix in contrast to the ThermoCalc predictions which suggests the formation of the bcc solid solution, B2, and Laves phases. The Ti, Ni-rich σ-phase can't also be expected from the analysis of the corresponding binary and ternary phase diagrams. Annealing did not result in noticeable changes in the microstructure. The formation of the σ and Ti 2 Ni phases resulted in a considerable increase in strength (from 745 to 1600 MPa) and decrease in ductility (from a thickness reduction �50% to fracture in the elastic region) at room temperature. An increase in the testing temperature to 800 � C resulted in softening to and a substantial increase in ductility of all the alloys. Complex relationships between the fraction of the second phases and mechanical properties were discussed.

Development of Precipitation-Strengthened Al0.8NbTiVM (M = Co, Ni) Light-Weight Refractory High-Entropy Alloys

“…According to the constitutive elements and the composition of the sigma phases for present alloys, the atomic structure of possible sigma phase is believed to be Al(Nb, Co) 2 and Al(Nb, Ni) 2 phases, since sigma phases in current alloys are enriched with Nb, Al, Ni, and Co. Note that the AlNb 2 sigma phase also could contain ~11.4 at.% of Co, ~30 at.% of Ti, ~20 at.% of V, or ~10 at.% of Ni as solute elements [ 18 , 31 , 32 ]. Furthermore, the formation of AlNb 2 sigma phase was often observed in Al-Nb-Ti-V HEAs at temperatures below 1200 °C, but the fraction was very limited [ 17 ].…”

“…In the present study, the addition of Co or Ni stimulates the homogeneously distributed sigma precipitates in the BCC solid-solution matrix, and it was demonstrated that the sigma precipitate plays a crucial role in the improved yield strength at room and elevated temperatures. There have been extensive efforts to improve the yield strength of RHEAs by controlling the size, volume fraction, and the feature of precipitates' distribution [18,36,37], and it has been reported that the homogeneous distribution of fine precipitates is effective to improve the strength [37] compared to other control factors. As observed in the current case, the uniformly dispersed sigma precipitates in the Al 0.8 NbTiVM 0.2 (M = Co, Ni) alloys are believed to improve the yield strength at room and high temperatures.…”

“…To meet the increasing demand of high-temperature alloys, there have been many efforts to design the RHEAs through the new alloy design approaches [ 15 , 16 , 17 , 18 , 19 ]. For example, RHEAs reinforced by formation of secondary phases, such as precipitates, which results in considerable improvement of the yield strength at high temperatures [ 17 , 20 ].…”

“…The heat-treated Al 0.8 NbTiVCo 0.2 alloy showed less plasticity than the Al 0.8 NbTiVNi 0.2 alloy at 700 • C.In contrast to the deformation at room temperature, the stress-strain curves of the alloys at 700 and 800 • C indicated pronounced softening during plastic deformation. The sharp decrease of the strength at 800 • C is believed to be associated with the thermal softening of the sigma phase[18]. The detailed high-temperature mechanical properties of the current alloys are listed in Table5.…”

A Review of the Latest Developments in the Field of Refractory High-Entropy Alloys

“…These BCC, sigma, and Ti2Ni phases were found to improve the material's mechanical characteristics, and the nanoindention of those phases is shown in Table 3 [39]. Panina et al reported that yield strength and ultimate tensile strength improved after replacement [39]. NbTaTiV refractory high-entropy alloys were synthesized using powder metallurgy as the sintering temperature was increased from 1500 to 1700 °C, and the Vickers hardness increased (as shown in Figure 2) [31].…”