FIGURE 1: Micrograph showing pore and inter-pore structure of samples foamed by thermal cycling between 770 – 1035 °C.
FIGURE 2: Micrographs after undergoing sothermal creep at 925 °C. Note the larger pores, lower pore coalescence and thinner inter-pore regions in Figure 1.
Freeze Casting
Titanium foams with aligned, elongated pores are created using a freeze-casting method. An aqueous slurry is directionally frozen to induce ice dendrite formation. The powders are pushed ahead of the solidification front and into the interdendritic spaces. The ice is then freeze-dried, or sublimated, away leaving behind the loosely connected titanium powder foam whose pore structure is determined by the growing ice crystals. Directional freeze-casting had been used with polymers, organic materials, and most recently, in alumina, however never in a metal system, to our knowledge. Recently, in this lab, this technique was successfully used to create a titanium foam with aligned, elongated pores [1].
The final application of such a foam would be as bone implant material, making the foam’s mechanical properties and pore sizes especially important for a successful product. Initial results have been encouraging. Pore sizes in the 100 micron range, the currently accepted size needed to encourage bone growth, have been produced and initial mechanical testing show ductile behavior with low stiffness and moderate strength. Further experimentation is needed to improve the mechanical properties of this foam, and to determine the effects of powder size, powder shape, solidification rate, and sintering conditions on the final product.
Related Publications
- Chino, Y., D.C. Dunand, "Directionally freeze-cast titanium foam with aligned, elongated pores," Acta Materialia (2007).
Superplastic Expansion
Pure titanium and the titanium alloy Ti-6Al-4V have been foamed by expansion of small, high-pressure, argon-filled pores. To delay cell wall bursting, the foaming was carried out under conditions where the matrix exhibited transformation superplasticity. This mechanism does not depend on the grain size as in fine grained superplasticity but, instead, relies on internal stresses due to the density mismatch between the α and the β phases that occurs during the allotropic phase transformation.
Elastic constants (Young's modulus, shear modulus and Poisson's ratio) were measured as a function of porosity and correlated to initial and foamed pore morphology. Additional control experiments conducted at constant temperature where the material deforms by creep showed that superplastic foaming is more rapid.
Related Publications
- C. Schuh, D.C. Dunand, "Contributions to Transformation Superplasticity of Titanium from Rigid Particles and Pressurized Pores "
Scripta Materialia, 40, 11, 1305-1312 (1999).
PDF - N.G. Davis, J. Teisen, C. Schuh, D.C. Dunand, “Solid-State Foaming of Titanium by Superplastic Expansion of Argon-Filled Pores” Journal of Materials Research, 16, 51508-1519 (2001).
PDF - M. Frary, C. Schuh, D.C. Dunand
“Kinetics of Biaxial Dome Formation by Transformation Superplasticity of Titanium Alloys and Composites”
Metallurgical and Materials Transactions A,33A, 1669-1680 (2002).
PDF - N.G.D. Murray, D.C. Dunand, “Microstructure Evolution during Solid-State Foaming of Titanium” Composite Science and Technology, 63, 16, 2311-2316 (2003).
PDF - N.G.D. Murray, D.C. Dunand, “Effect of Thermal History on the Superplastic Expansion of Argon-Filled Pores in Titanium: Modeling of Kinetics”, Acta Materialia, accepted for publication.
Financial Support
This research is funded by the National Science Foundation.
