The present study utilized W nanopowders (prepared by a two-stage reduction process consisting of steps of carbothermic pre-reduction and hydrogen deep reduction) to produce fine-grained 90W-7Ni-3Fe and 93W-4.9Ni-2.1Fe alloys. The effects of sintering temperature [in a range of 1573 K to 1723 K (1300 °C to 1450 °C)] and time on the microstructure and mechanical properties of W-Ni-Fe alloys were studied in detail. The microstructures of the prepared alloys at 1573 K and 1673 K (1300 °C and 1400 °C) indicated that the grain sizes of W grains are < 5 μ m. However, the grain size dramatically increases to > 5 μ m after sintering for > 60 minutes at 1723 K (1450 °C). The observation shows the as-sintered alloys with different holding times exhibit different fracture modes. With increasing holding time, the hardness of two alloys first increases and then decreases at the same sintering temperature. It is also concluded that the bending strength of alloys increases with the increase of sintering temperature. The maximum hardness of 381 and 393.6 HV was obtained after sintering at 1573 K (1300 °C) for 60 minutes for 90W-7Ni-3Fe and 93W-4.9Ni-2.1Fe alloys, respectively, while the highest bending strength of 1381.4 and 1278.8 MPa could be achieved at 1673 K (1400 °C) for 120 minutes. The excellent mechanical properties of W-Ni-Fe alloys could be attributed to the grain refinement strengthening.
The conventional sintered W-Ni-Fe alloy encounters challenges in achieving both high strength and plasticity simultaneously, primarily due to its inadequate organizational compatibility. To address this issue, we developed a novel approach combining in situ freeze-drying and straightforward liquid-phase sintering techniques to effectively control the microstructure of 93W-4.9Ni-2.1Fe alloys. The corresponding W-Ni-Fe alloy in this work simultaneously takes into account ultrafine W grains (28.3 µm), low W-W connectivity (CW-W = 0.31), and suitable γ phase volume fraction (20.2%), which is currently difficult for traditional W-Ni-Fe alloys to achieve. The minute dimensions of nanopowder play a crucial role in inhibiting Ostwald ripening and facilitating W grain refinement. Additionally, the extensive surface area of nanopowder enhances capillary penetration, leading to effective homogenization of high γ phase volume. As a result, the freeze-dried W-Ni-Fe alloy simultaneously achieves high strength (1010 MPa) and high plasticity (31%), which provides a new insight for the further development of W-Ni-Fe alloy industries.
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