MICROSTRUCTURAL INVESTIGATION OF A RAPIDLY SOLIDIFIED TI-6AL-4V-1B-0.5Y ALLOY

A Ti-6Al-4V-1B-0.5Y (wt %) alloy has been prepared by consolidation of the melt-spun alloy fibres. The microstructures of the melt-spun fibr e and the consolidated alloy were examined by different techniques. It was found that in the consolidated alloy, titanium boride and yttrium oxide particles have a refined particle size and a uniform distributi on in the alpha(+ beta) matrix compared with the microstructure of the same alloy obtained by conventional ingot metallurgy. The boride phas e in the consolidated alloy mainly has a needle-shaped morphology and has been identified by electron diffraction to be orthorhombic TiB wit h a B27 structure, while the yttrium oxide has a cuboidal morphology a nd has been identified as bcc Y2O3. Detailed TEM examination also reve aled that yttriumaddition has a strong influence on the TiB morphology by comparing the microstructures of Ti-GAl-4V-1B alloys with and with out yttrium addition. Under similar processing conditions, the TiB pha se in the consolidated alloys without yttrium addition mainly has a ne arly equiaxed morphology with a finer particle size, while the TiB pha se in the consolidated al toy with yttrium addition will mainly have a needle-shaped morphology. This effect of yttrium addition on the TiB morphology has been discussed in terms of heterogeneous nucleation and the reduced undercooling.