GoKawiilMaterials preparation
To ensure chemical homogeneity of the final ingot over a few kilograms, we first prepared pre-alloyed small ingots of about 125 g by arc-melting using high-purity raw materials of Ta, W, Re and HfB 2 . Before melting, the vacuum chamber was evacuated to a pressure of 3 × 10−3 Pa and subsequently backfilled with 99.995% pure argon. A titanium getter was melted for 2 min to remove residual impurities in the chamber. The raw materials were melted in a water-cooled copper crucible. Each ingot was flipped and re-melted more than eight times. The as-cast small ingots were ultrasonically cleaned in an isopropyl alcohol bath for 10 min. Subsequently, the pre-alloyed ingots were melted to form a large 4.5 kg ingot in an induction furnace under an argon atmosphere, and then flipped and re-melted more than five times to ensure homogeneity. The as-cast ingots were finally homogenized at 1,600 °C for 4 h.
Microstructural characterization
X-ray diffraction measurements were performed using a Bruker D8 ADVANCE diffractometer with Cu Kα radiation with a step size of 0.01° and a scanning rate of 1° min−1. Electron backscatter diffraction (EBSD) was conducted on a Thermo Scientific Helios 5 UX DualBeam focused ion beam microscope at an acceleration voltage of 30 kV and a beam current of 5.5 nA. The EBSD data were analysed using OIM Analysis 8 software to generate inverse pole figure maps. TEM, high-resolution TEM, HAADF, EDS and selected area electron diffraction analyses were carried out using Thermo Scientific Talos F200X and Spectra 300 microscopes. Thin TEM foils were mechanically polished to about 50 μm thickness, followed by twin-jet electron polishing using a Struers Tenupol-5 at a voltage of approximately 20 V and a temperature of −20 °C. The electrolyte used consisted of 80% methanol, 14% sulphuric acid and 6% hydrofluoric acid. Three-dimensional elemental mapping at the atomic scale was performed using a local electrode atom probe tomography (APT) system (CAMECA, LEAP 4000 XR). Tip-shaped specimens for 3D-APT tests were fabricated using a lift-out method and annularly milled using a focused ion beam/scanning electron microscope (FIB/SEM, FEI Scios). APT measurements were conducted at 50 K in the voltage mode under ultrahigh vacuum conditions (6 × 10−9 Pa), with a pulse repetition rate of 125 KHz and a pulse fraction of 10%. The 3D reconstruction and compositional analyses were carried out using Imago Visualization and Analysis Software (v.3.6.8).
Room-temperature tension tests
To prepare the flat specimens used for room-temperature tension, a thin plate was cut from the as-cast ingots and rolled at about 1,100 °C to achieve a 40% reduction in thickness. Oxide scales were removed through pickling and mechanical methods, followed by annealing at 1,800 °C for 1 h. The sample was then rolled at 450 °C to achieve an additional 50% reduction in thickness and finally annealed at 1,800 °C for 1 h (Extended Data Fig. 3). The dog-bone-shaped specimens were fabricated by electrical discharge machining. The gauge section has a length of 12 mm, a width of 3.5 mm and a thickness of 1 mm. The sample surfaces were polished using 2000-grit SiC paper to eliminate scratches. The tension tests were conducted on an Instron 5969 universal testing machine at a strain rate of 1 × 10−3 s−1. The strain was measured by a non-contact video extensometer.
High-temperature tension tests
To produce the large cylindrical specimens used for high-temperature tension tests, the hot extrusion was carried out at 1,700 °C in air to a 4.5 kg as-cast ingot with molybdenum cladding (Extended Data Fig. 4). The extrusion ratio was 3. The as-extruded sample was then annealed at 1,800 °C for 1 h. The dog-bone-shaped specimens were cut from the annealed ingot for uniaxial tension at elevated temperatures. The gauge section has a length of 40 mm and a diameter of 5 mm (Extended Data Fig. 4). Two extra lugs machined at both sides of the gauge length are used to mount the contact extensometer, which is made of carbon–carbon composite to sustain high heat flow. The high-temperature tension tests were carried out in an argon environment using an electro-thermal mechanical testing platform. Specifically, the chamber was first evacuated to less than 3 × 10−3 Pa at room temperature. Then the chamber was backfilled with a steady flow of argon gas until reaching a small positive pressure, which prevented external air (especially oxygen and water vapour) from flowing back into the chamber. Three electrodes were used to heat the sample by applying an electrical current, that is, a main electrode along the axis of the specimen and two auxiliary electrodes at the two grips of the specimen. The three electrodes worked simultaneously to ensure homogeneous heating of the whole sample. The temperature was measured using the infrared sensor throughout the tests (Extended Data Fig. 5). Three positions, at the middle and two ends of the specimen, were set to monitor the temperature. The three independent signals were fed back to the electrode to make an in-time adjustment. Samples were heated up to the target temperature at a rate of 10 °C s−1. Before loading, the sample was held at the test temperature for 20 min to ensure a uniform distribution of temperatures. The tensile strain rate is 1 × 10−3 s−1. Tensile tests were conducted at temperatures ranging from 1,600 °C to 2,400 °C. To study the influence of holding time on the strength, tensile tests with different holding times up to half an hour were performed for comparison. After tension tests, the samples were cooled down to room temperature in the chamber. For each temperature, the high-temperature tensile tests were carried out more than three times to ensure good reproducibility.
Density functional theory calculations