Effect of grain size upon flow and fracture in a precipitation-strengthened Ti-8 wt % Al-0.25 wt % Si alloy

Abstract

The interaction of grain size and precipitation strengthening has been studied in a Ti-8 wt % Al-0.25 wt %Si alloy. Grain sizes varying from 9 to 90μm were produced by warm-working and annealing the alloy in the single-α-phase field. A uniform distribution of the coherentα ₂ particles in theα matrix was produced by ageing the alloy in the two-phase (α +α ₂) field. The yield strength Hall-Petch slopes of the alloys with and without the α₂ precipitates were found to be nearly equal, indicating that the precipitation and grain-boundary strengthening are linearly additive. While specimens containing no precipitates exhibited a high ductility for all grain sizes, the ductility of the specimens with theα ₂ particles decreased drastically with increasing grain size. TEM examination of the specimens containing the precipitates revealed a highly planar, localized slip and SEM examination of the fracture surfaces of these specimens revealed a transition in fracture behaviour from highly dimpled to mixed cleavage and intergranular with increasing grain size.