Superplasticity and production of mechanically milled Ti-6Al-4V-0.5B

Superplastic forming of conventional titanium alloy sheet is limited commer cially by the relatively long cycle times imposed by the high temperatures and slow strain rates required. In order to minimise cycle times material w ith a fine grain size is required to allow either, an increase in the formi ng rate or a reduction in the deformation temperature. This study details t he manufacture of Ti-6Al-4V-0.5B powder with a nanocrystalline grain size, which was produced by mechanical milling. The material was consolidated by hot isostatic pressing at a range of temperatures during which similar to 2 .5 vol.-%TiB was formed by an ill situ reaction between the titanium and bo ron. The TiB particles limited the growth of the grain size in the titanium from the nanocrystalline structure in the powder to sizes in the range 600 mn-4 mum after consolidation. The consolidated material was hot tensile te sted at a range of temperatures and strain rates. A superplastic elongation of 310% was achieved when testing at 900 degreesC at a strain rate of 6x10 (-2) s(-1) compared with 220% for conventional Ti-6Al-4V sheet. However, ex tensive cavitation, induced by the presence of argon, occurred during high temperature deformation and limited the superplastic extensions achieved. M ST/4779.