Development of Oxide Dispersion Strengthened Mo Alloys by Mechanical Alloying and Hydrogen Sintering

Abstract

Molybdenum-based alloys are highly valued for their elevated melting points, excellent mechanical properties like yield strength, higher thermal conductivity, which make them crucial for high-temperature application such in aerospace, defence and nuclear sectors where maintaining strength and structural integrity is very essential. Nevertheless, Mo also faces some drawbacks, such as brittleness at room temperature and rapid oxidation at elevated temperatures, which can lead to failure, necessitating the use of protective coatings or alloying to enhance oxidation resistance. The present study investigates the various ceramic oxide dispersoids that can be used as reinforcements to alter the microstructural, mechanical, and oxidation resistance properties of Mo-based alloys. 1 wt.% of four different oxides, namely Al2O3 (alloy A), Y2O3 (alloy B), La2O3 (alloy C), and TiO2 (alloy D) were added to the base composition of Mo89Ni5Cr5. Nickel enhances ductility and densification, enabling the formation of durable oxide protective coatings at elevated temperatures through enhanced atomic diffusion during sintering. Chromium simultaneously contributes oxidation resistance by forming adhering Cr2O3 scales. This work is primarily motivated by the concept that oxide type has a significant impact on mechanical performance, high temperature oxidation resistance, and densification behaviour. Three consecutive steps were used in powder metallurgy to create oxide-dispersed alloys: uniaxial compaction at 350 MPa to form green pellets, high-energy ball milling for 20 h to promote particle refinement and uniform phase dispersion, and hydrogen sintering at 1500 °C for 1.5 h with controlled heating rates (5 °C/min to 1000 °C, then 3 °C/min beyond). To promote densification in a reducing environment and to prevent molybdenum from oxidising, hydrogen sintering was chosen. Archimedes' density measurements, HRTEM for nanostructural analysis, SEM/FESEM with EDS for microstructural evaluation, and XRD for phase identification were all used in the thorough characterisation. Vickers microhardness testing was used to examine mechanical properties under a 100 gf load, and isothermal oxidation resistance was studied at 1000 °C for 10 h, with weight measurements taken every 30 min. The minimum crystallite size of around 25 nm and maximum lattice strain of 0.16 % has been reported for Mo-Ni-Cr-TiO2 at 20 h of milling. The minimum average particle size of 624 nm as determined by particle size analysis after 20 h of milling, is achieved for Alloy D. Fine-sized particles will eventually improve sintering kinetics. TiO2 reinforced alloy D shows the highest relative sintered density (90.8%), while the lowest was recorded for Alloy C (78.2%). The microhardness values reached their highest for the TiO2 samples (768 ± 36 HV), while the Al2O3 (Alloy A) sample showed the minimum hardness value (640 ± 28 HV). During the oxidation test at 1000 °C, Alloy C displayed the highest weight loss (834.96 mg/cm2) due to severe material breakdown into powders, while Alloy D displayed the lowest weight change (484.48 mg/cm2). Alloy A shows almost nearly similar trend as alloy D. Spallation was observed in all the alloys. The changes in the oxide morphology after oxidation were analysed by SEM, where Alloy C showed uneven and coarser grains (~3.14 µm) due to poor oxide scale adhesion and oxygen ingress, while fine and comparatively compact oxide grain arrangement (~1.37 µm) was observed in the case of Alloy D. Though all the alloys shows degradation against oxidation alloy A and alloy D exhibited comparatively improved resistance against oxidation. Molybdenum-based complex oxides like NiMoO4, CrMoO4 were evident in all the alloys and additional other oxides such as NiCr2O4 (in alloy A, alloy B, alloy C), MoO3 (in alloy A, alloy B, alloy D), retained Al2O3 (in alloy A), Y6MoO12 (Alloy B), La2Mo2O9 (Alloy C), retained TiO2 (Alloy D) were verified using XRD investigation of the samples after oxidation. The stability and protective nature of the oxides formed strongly impact the overall oxidation kinetics. TiO2 dispersed samples exhibit the best synergistic advantage, characterised by high densification and hardness, resulting from the combination of dispersoids and fine grain strengthening. In the case of Alloy C, the formation of La2Mo2O9, which is thermodynamically unstable, results in a coarse microstructure during oxidation, leading to underperformance. These findings highlight the need for oxide-alloy compatibility evaluation in ODS material design and offer practical design guidelines for creating advanced Mo-based alloys appropriate for high-temperature industrial tooling, nuclear reactor structures, and aerospace turbine components where both mechanical loading and extreme oxidising conditions are present.