CALPHAD-aided development of quaternary multi-principal element refractory alloys based on NbTiZr

Abstract

Equilibrium phase diagrams for ten MeX(NbTiZr)100-X alloy systems, where Me is Al, Cr, Fe, Hf, Mo, Re, Si, Ta, V or W and X ranges from 0 to 25 at% were calculated using the PanNb2018a database recently developed by CompuTherm, LLC. By the type of phases and sequence of formation, these alloy systems can be divided into two groups. The first group of quaternary alloys, containing Al, Hf, Mo, Ta, or V has a single-phase BCC region below the solidus line over the entire X range and can be considered as potential candidates for the development of single-phase high entropy alloys. The second group consists of the quaternary alloy systems in which the fourth element (Cr, Fe, Re, Si or W) has limited solubility in BCC NbTiZr, which leads to the formation of an additional phase below the solidus and above the solubility limit. These quaternary alloy systems can be used for the development of precipitation or dispersoid strengthened complex concentrated alloys. To verify CALPHAD calculations three representative alloys, Cr10Nb30Ti30Zr30, Ta10Nb30Ti30Zr30 and Re10Nb30Ti30Zr30, were prepared by arc melting. The alloy densities were 6.56, 7.81 and 7.85 g/cm3, respectively. The phase compositions of the produced alloys agreed satisfactorily with CALPHAD calculations. Mechanical properties were also studied and compared with those of NbTiZr. At room temperature (RT) all the alloys showed high hardness exceeding 350 Hv and high compression yield stress exceeding 1000 MPa. RT compression ductility of Re and Ta containing alloys was above 50%, but Cr-containing alloy showed low ductility of 5%. With an increase in temperature ≥800 °C, compression strength decreased more rapidly for the Cr-containing alloy, which at 1200 °C became softer than NbTiZr. The least temperature dependence of the strength was observed for the Ta-containing alloy, which became the strongest at 1200 °C.