Fabrication of Nano-Y2O3 Dispersed Tungsten Alloys by Mechanical Alloying Followed by Conventional and Spark Plasma Sintering

Abstract

Tungsten (W) based alloys are widely used in defense, nuclear reactor, electrical and electronic application owing to high density, melting point and hardness. Elevated sintering temperature in excess of 2700 °C is required to fabricate pure W and W based alloys. Such high temperature processing is not feasible with respect to cost and deterioration of mechanical properties. Therefore sintering at reduced temperature is possible by fabrication of nanostructured W alloy before sintering. The challenges of using pure W in high temperature structural applications such as poor ductility and oxidation resistance can be counteracted by suitable alloy addition.

Present study involved the fabrication of non Y2O3 and Y2O3 dispersed W based alloys. With nominal composition of W90Mo10 (alloy A), W70Mo30 (alloy B), W50Mo50 (alloy C), W80Ni10Mo10 (alloy D), W75Ni10Mo10Ti5 (alloy E), W79Ni10Mo10(Y2O3)1 (alloy F), W74Ni10Mo10Ti5(Y2O3)1 (alloy G), W73Ni10Mo10Ti5(Y2O3)2 (alloy H) and W74Ni10Nb10Ti5(Y2O3)1 (alloy I) (all in wt.%). The alloys were subjected to mechanically alloying in a high energy planetary ball mill for 20 h. Alloy A through H were consolidated by conventional sintering at 1500 °C for 2 h and alloy F and alloy I were spark plasma sintered at 1000 °C,1200 °C,1400 °C for 5 min. The milled powders and sintered alloys were studied by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), Transmission Electron Microscopy (TEM), Differential Scanning Calorimetry (DSC). The physical properties (density/porosity), mechanical properties (hardness, compressive strength, % strain to failure, wear) and high temperature properties of the consolidated alloys were studied. Atomistic simulation of non-oxide dispersion strengthened alloy A, alloy D was also carried out and correlated with experimental study to understand the deformation mechanism by Ni addition.

XRD analysis of milled powders illustrated that crystallize size substantially reduces and lattice strain and dislocation density increases with increase in Y2O3 dispersion. Minimum crystallite size of 18.6 nm was recorded for alloy H which was also supported by TEM observation. DSC study revealed that the activation energy for recrystallization decreases with increase in Y2O3 dispersion. XRD and SEM of the sintered alloys evidenced the formation of MoNi, NbNi, Ni3Ti intermetallic phases along with the presence of Y2WO6, Y6WO12, Y10W2O21 oxides in W matrix. However no oxide phases was evident in spark plasma sintered (SPS) alloys which may be attributed to less chemical reaction between W, Y2O3 phase owing to rapid sintering cycle. SPS treated alloy F and alloy I at 1400 °C exhibited 99.51% and 99.48% densification respectively which was considerably higher as compared to conventional sintering.

The hardness, strength, % strain to failure and wear resistance enhanced with Ni and Y2O3 addition. Both experimental analysis and atomistic simulation study of alloy A and alloy D showed that Ni addition improves the plastic flow property. Interestingly, higher increase in Y2O3 dispersion (2 wt.%) decreased the strength with improvement in ductility in alloy H. The maximum hardness, strength of 11.89 GPa, 2.26 GPa was achieved in alloy I, spark plasma sintered at 1400 °C, whereas maximum % strain to failure of 23.95% was recorded for alloy F at identical spark plasma sintering temperature. The higher strength in Y2O3 dispersed alloys as compared to non Y2O3 dispersed alloys was due to oxide dispersion strengthening mechanism. The hardness and strength of spark plasma sintered alloy F and alloy I at 1400 °C was 2-3 times higher as compared to recently investigated oxide dispersion strengthened (ODS) W alloys.

High temperature oxidation study at 1000 °C showed that incorporation of Ni and Y2O3 significantly improved the oxidation resistance, although combined Nb and Ti addition was not effective in increasing the oxidation resistance. Conventionally sintered alloy displayed improved oxidation resistance as compared to spark plasma sintering.

It was concluded that spark plasma sintered Y2O3 dispersed W based alloys can promote the hardness, strength and ductility whereas conventional sintering method was comparatively efficient to improve the oxidation resistance. The fabricated Y2O3 dispersed W based alloys may be potential candidate for high temperature structural applications.