Solid-state foaming of titanium by superplastic expansion of argon-filled pores

Solid-state foaming of commercial purity titanium was achieved by hot-isostatic pressing of titanium powders in the presence of argon, followed by expansion of the resulting high-pressure argon bubbles at ambient pressure and elevated temperature. The foaming step was performed under isothermal conditions or during thermal cycling around the alpha/beta allotropic temperature of titanium. Such thermal cycling is known to induce transformation superplasticity (TSP) in bulk titanium due to the complex superposition of internal transformation stresses and an external biasing stress; TSP was found to be active during foaming, where the deviatoric biasing stress was provided by the internal pore pressure. As compared to isothermal control experiments where foam expansion occurred by creep only, TSP foaming under thermal cycling conditions led to significantly higher terminal porosity (41% as compared to 27%). The foaming rates were also higher for the TSP case before pore growth ceased. Additionally, foaming experiments were conducted under an externally applied uniaxial tensile stress of 1 MPa. This procedure resulted in foaming kinetics and porosities similar to those achieved without an external stress and, for the TSP case, led to high aspect ratio pores elongated in the direction of the applied external stress.