Effect of thermal history on the superplastic expansion of argon-filled pores in titanium: Part I kinetics and microstructure

Titanium was foamed at high temperature by expanding pressurized argon bubbles which had been trapped in the metal during prior hot-isostatic pressing. During foaming, transformation superplasticity was induced in the matrix by thermal cycling around the α/β allotropic temperature of titanium, thus accelerating pore growth and delaying cell wall fracture and pore opening to the surface, as compared to foaming performed exclusively under isothermal creep conditions in the β-Ti field. The relative importance of creep and superplastic deformation upon foaming rate, rate of porosity opening and maximum porosity was studied by changing the thermal cycling frequency and maximum temperature. Under superplastic conditions, porosity ceased to increase when reaching levels between 29% and 44%, principally because of the decrease of internal gas pressure due to pore expansion and, to a lesser extent, because of premature gas escape from the pores. Continued thermal cycling after the maximal porosity had been reached lead to pore coalescing and opening to the specimen surface by localized pore wall rupture, which resulted in large open pores with a jagged morphology.