Microstructure evolution during solid-state foaming of titanium

Solid-state foaming of commercially pure titanium was achieved by high-temperature expansion of high-pressure argon bubbles trapped in titanium by a powder-metallurgy technique. The foaming step was performed at constant temperature, where creep of the titanium matrix controls pore expansion, or during thermal cycling around the (alpha)/(beta) allotropic temperature, which induces transformation superplasticity of the matrix. Superplastic foaming led to significantly faster pore growth and higher terminal porosity than isothermal creep foaming. During thermal cycling, the porosity remains nearly fully closed to the surface of the specimen up to the point where the maximum porosity (44%) is obtained, despite the presence of some internal pore coalescence. With continued thermal cycling, pores coalesce further by fracture of thin interpore walls and pores finally open to the surface, but without a significant increase in the amount of total porosity. The remnants of these walls result in a jagged pore morphology. Under isothermal conditions, pores remain small, equiaxed and unconnected with no pore surface roughness. However, after long annealing times, they exhibit faceting due to surface diffusion.