Ingots of six Nb-(TiNiCoHf) alloys, i.e. unalloyed Nb, Nb99(TiNiCoHf)1, Nb98(TiNiCoHf)2, Nb95(TiNiCoHf)5, Nb90(TiNiCoHf)10 and one binary Nb99Ni1 alloy were synthesized by arc-melting a mixture of constituent ingredients with a purity above 99.9 % in a Ti-gettered high-purity argon atmosphere. All cast ingots were re-melted at least six times, to ensure chemical homogeneity of the blocks. Afterwards the materials were subjected to high-pressure torsion (HPT) for grain refinement. For the HPT process, disks with a diameter of 10 mm and an initial thickness of 0.8 mm were cut directly from the ingots and then polished. The HPT process was conducted at room temperature at a nominal pressure of 6 GPa and a rotational rate of 1 rotation per minute for altogether 5 rotations under quasi-constrained conditions46.
2. Thermal annealing treatment.
It is reported that the microhardness in the HPT-processed pure Nb, exposed to 6 GPa for 5 rotations, begins to saturate at a radius of about 1.5 mm from the center and extends towards the outer rim of the HPT disk, due to microstructural inhomogeneity47. In the current study specimens for heat treatment were taken from the same radial location across the HPT disks of above 1.5 mm from the center.
a) Isochronal annealing: The HPT-processed NC pure Nb, 1HEA, 2HEA, 5HEA, 10HEA and binary 1Ni samples were placed into a vacuum quartz tube and then annealed at temperatures of 673, 773, 873, 973, 1023, 1073, 1123 and 1173 K for 2 h, respectively. The NC 10HEA sample was additionally further annealed at 1273 K for 2 h, to study the material’s grain growth response.
b) Isothermal annealing: The HPT-processed NC unalloyed Nb samples were placed into a vacuum quartz tube and then annealed at 973 K for 2, 10, 40 and 100 h, respectively. The binary NC 1Ni samples were isothermal annealed at 973 K for 2 and 100 h and the NC 1HEA, 2HEA, 5HEA and 10HEA samples were annealed at this temperature for 2 and 100 h and even longer time to 600 and 1000 h. Specifically, the NC 1HEA, and 10HEA alloys were annealed at 973 K for 10, 20 and 40 h. The NC 1HEA was also annealed at 973 K for longer time for 1300 and 2200 h. In addition, the NC 1HEA samples were also isothermal annealed at higher temperature of 1023 K for 100h.
3. Nano-hardness measurement
Nano-hardness tests of samples before and after annealing were performed using a MTS DCM nanoindentation system equipped with a triangular-pyramid indenter. Experiments were conducted at a constant strain rate of 0.05 s-1 to a total indentation depth of 2000 nm. Nano-hardness values were averaged with at least 5 indentations. The spacing between two neighboring indents was above 50 μm to avoid any overlapping effect.
4. Microstructural characterization and grain boundary structure
Structural characterization was conducted using synchrotron XRD probing on the 11-ID-C beam line of the Advanced Photo Source at Argonne National Laboratory, USA. The wavelength of the X-ray was 0.1173 Å. Microstructure and morphology were characterized by a Zeiss Supra 55 field emission scanning electron microscope which was equipped with a backscatter electron (BSE) detector and an AZtecHKL electron back-scatter diffraction analysis system. The scanning electron microscope specimens were polished with 5000-grit SiC paper. Subsequently they were mechanically polished using 0.04 μm silica suspensions. The electron back-scattering diffraction specimens were initially polished with 2,000-grit SiC paper and subsequently electrochemically polished using 12 g manhydrous magnesium perchlorate per 100 ml methanol at a direct voltage of 60-70 V at 228-243 K. Bright-filed images were taken in the Tecnai F30 transmission electron microscope (TEM) operated at 300 kV, as well as high-resolution TEM images. STEM-HAADF images were taken in an aberration-corrected JEOL-ARM200F TEM operated at 200 kV and a FEI Titan G260-300kV S/TEM equipped with an aberration corrector. Energy-dispersive spectroscopy (EDS) was used to construct the elemental map. The TEM specimens were prepared using a focused ion beam (FIB, FEI Helios Nanolab 600i). Both SEM and TEM were used to photograph grains. The average grain size was calculated using Image J software. The distribution of GB planes was computed by the collected EBSD mapping data.
5. Grain boundary segregation and composition analysis
Atom probe tomography (APT) was employed to investigate the distribution of elements in the HPT-processed and annealed samples, placing attention on the GB decoration. Specimens with a sharp tip of about 50-60 nm for APT probing were fabricated by focused-ion-beam (FIB) milling using a FEI Helios Nanolab 600i instrument. APT experiments were then performed on a Cameca LEAP 5000XR local electrode instrument with magnetically enhanced flight path for highest mass-to-charge resolution at a specimen temperature of 70 K, under pulsing UV laser exposure using a laser energy of 80 pJ. The pulse repetition rate was 200 kHz at an ion collection rate of 3 ions per 1000 laser pulses. APT data three-dimensional (3D) atomic reconstruction and quantitative analysis were carried out using the CAMECA IVAS version 3.8.2 software.