Fabrication and Characterization of Nano-Y2O3 Dispersed W-Ni-Mo and W-Ni-Ti-Nb Alloys by Mechanical Alloying and Spark Plasma Sintering

Abstract

In the present investigation, nano-Y2O3 dispersed W-Ni-Mo and W-Ni-Ti-Nb alloys with nominal composition of W79Ni10Mo10(Y2O3)1 (alloy A) and W74Ni10Ti5Nb10(Y2O3)1 (alloy B) (all in wt.%) were synthesized by mechanical alloying (MA) for 20 h followed by spark plasma sintering (SPS) at 1000°C, 1200°C and 1400°C for alloy A and at 1400°C for alloy B, respectively for 5 min at 75 MPa pressure. Microstructure evolution and thermal behavior of milled powders and consolidated samples, were examined by scanning electron microscopy with energy-dispersive X-ray detection (SEM/EDX), high resolution transmission electron microscopy (HR-TEM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD). Minimum crystallite size of 29 nm and 23 nm, and maximum lattice strain of 0.44% and 0.491% were observed for alloy A and B, respectively milled for 20 h. The dislocation density for both alloys significantly increased after 10 h of milling and marginal increase was exhibited at 20 h of milling. The density, hardness, compressive strength and wear resistance increased with increase in SPS temperature for alloy A and maximum values of 99%, 10.91 GPa, 2.24 GPa, 1.28×10-15 m3/N m (wear rate), respectively were obtained for 1400°C sintered sample. However, alloy B sintered at 1400°C achieved higher hardness (11.89 GPa), compressive strength (2.26 GPa) and wear resistance (wear rate: 1.14 ×10-15 m3/N m) owing to finer crystallite size and precipitation of higher volume fraction of hard NbNi and Ni3Ti intermetallic phases (Fig 1. (b)) as compared to single hard MoNi intermetallic phase in alloy A (Fig. 1(a)). The hardness and strength of both the alloys are 2-3 times higher than the recently investigated W based alloys. The considerable improvement in the mechanical property for both alloys was attributed to dispersion strengthening mechanism contributed by the nano-Y2O3 dispersoids precipitated at the grain boundaries. The wear test results confirm that abrasive wear is the dominant wear mechanism in both the alloys. Observation of increased texture intensity as a function of SPS temperature for (110), a harder orientation, also confirmed the increase in hardness in alloy A with increase in SPS temperature. A regular decrease in residual stress with increase in SPS temperature was observed for both alloys. However, the alloy B had a higher residual stress compared to alloy A, both sintered at 1400°C.