The effect of ball to powder ratio on the processing of a novel Mo-Cu-Al2O3 composite

Abstract

In the present article a 45Mo-45Cu-10Al2O3 (wt%) nano-composite was produced by mechanical alloying, shaped by cold pressing then by hot pressing at 950 °C. Our aim was to investigate the effect of different ball to powder ratio (BPR) on the particle size and shape, as well as on the material composition. Investigations were done with XRD, SEM and for hot pressed specimens Brinell hardness measurement. Lower BPR resulted in nano-sized crystallites in the powder, a homogenous phase distribution and higher BPR resulted in even smaller crystallite sizes, but with ZrO2 contamination from the milling equipment. The milling resulted in two separate fractions in the metallic phases separated by crystallite size, strain and lattice parameter. After hot pressing, the lower BPR powders developed a homogeneous, evenly distributed microstructure. Cu recrystallized during hot pressing, but still remained nanocrystalline, the crystallite size of Mo and α-Al2O3 decreased even more due to crystallite deformation and in the case of Mo, the lattice strain values indicates that recovery had also happened. The heating eliminated the crystal defects at highly strained areas and the subsequent cooling evened out the stresses in the respective metallic phases. The inner grain structure was revealed by etching, showing that α-Al2O3 particles congregated only at the Mo grain boundaries, but penetrated the Cu grains. The lattice strain values combined with the BSE images reveal that the Cu is the main matrix of the α-Al2O3 particles. The results indicate that the α-Al2O3 particles induced Particle Stimulated Nucleation (PSN) in the Cu explaining its moderate crystallite size increase. The hot pressed samples of higher BPR had higher ceramic (ZrO2 and α-Al2O3) content, resulting in higher hardness, but lower relative density (92.4%), compared to the samples of lower BPR, which reached 97.2% relative density and hardness still significantly higher than MoCu alloys.