Microstructure evolution of a novel low-density Ti–Cr–Nb–V refractory high entropy alloy during cold rolling and subsequent annealing

Abstract

Refractory high entropy alloys represent a new class of metallic alloys attractive for high-temperature applications. However, most of the developed alloys have either low ductility at room temperature or high density. In this work, we report structure and mechanical properties of a novel non-equiatomic Ti 1.89 CrNbV 0.56 alloy produced by vacuum arc melting. The density of the alloy was 6.17 g/cm 3 . In the as-cast condition, the alloy had a single-phase bcc structure enabling room temperature deformation in compression to ε > 50% or cold-rolling to a thickness strain of 80%. Rolling resulted in the formation of a dislocation substructure and development of kink and shear bands. Meanwhile, microhardness measurements have revealed only a moderate increase from 396 HV in the as-cast condition to 454-469 HV after 40-80% rolling. After 80% cold rolling the alloy had yield strength of 1020 MPa, ultimate tensile strength of 1535 MPa, and elongation to fracture of 3.5%. The cold rolled alloy was annealed at 800, 1000 or 1200°C for 1-100 h. Microstructural response to the annealing strongly depended on temperature. Annealing at 800°C mostly resulted in Cr-rich fcc (C15) Laves phase particles precipitation. Annealing at 1000°C led to the bcc matrix recrystallization along with the precipitation of the Laves phase particles, thereby producing a fine duplex microstructure. Finally, annealing at 1200°C resulted in a coarse-grained recrystallized single-phase bcc microstructure. Microhardness of the alloy lowered with an increase in the annealing temperature while the annealing time had a small effect on hardness.

Laser Beam Welding of a Low Density Refractory High Entropy Alloy

“…It is generally believed that the second phase particles precipitation should result in strengthening. However, in the present case nano-hardness of the particles-containing areas (480 HV) was found to be lower than that of the particle-free zone (560 HV) after welding at 800 • C. This result can most likely be related to (i) lower solid solution hardening of the bcc matrix due to depletion in Cr and Nb [11,33] and (ii) low (er) hardness of the Laves phase itself [33]. That is why the Laves phase particles precipitation in the present alloy resulted in both some softening and an increase in ductility after welding at 800 • C. It should be also noted, that the size of the facets in Figure 10b is correlated with the size of precipitation-free areas (Figure 5d) thereby suggesting that the more ductile behavior can be associated with a temporary crack arrest in these Cr and Nb depleted regions.…”

“…In addition, a small amount (~1%) of lens-shaped second phase particles were found predominantly in the inter-dendritic areas (see the high magnification insert in Figure 1a). According to XRD (Figure 1c) and taking into account previous results for the same alloy [33], these particles can be identified as the C15 (cubic) Laves phase. Note that the presence of the C15 Laves particles with a similar morphology was recently revealed in the Cr 10 Nb 30 Ti 30 Zr 30 alloy [41].…”

“…However, in the present case nano-hardness of the particles-containing areas (480 HV) was found to be lower than that of the particle-free zone (560 HV) after welding at 800 °C. This result can most likely be related to (i) lower solid solution hardening of the bcc matrix due to depletion in Cr and Nb [11,33] and (ii) low (er) hardness of the Laves phase itself [33]. That is why the Laves phase particles precipitation in the present alloy resulted in both some softening and an increase in ductility after welding at 800 °C.…”

“…Note that the particles precipitated in the inter-dendritic areas were observed mainly nearby the coarser particles, inherited from the as-cast condition. The Laves phase in this alloy is supposed to be composed of Cr and Nb predominantly [33] and, therefore, the "additional" particles formed in the inter-dendritic areas which were rich in these elements (Table 1), similarly to the Laves phase particles formation in the NbCrMo0.5Ta0.5TiZr alloy [43].…”

“…Therefore, in the present work we have explored the structure and mechanical properties of the Ti 1.89 CrNbV 0.56 RHEA butt joints obtained by LBW. This recently introduced RHEA [33] had a low density of 6.17 g/cm 3 and can be cold rolled to a high thickness reduction. The LBW technique was previously used successfully to join Ti alloys [34][35][36][37][38][39], and therefore, can be considered a proper welding method for the program Ti-rich RHEA.…”