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This work represents an attempt to understand the nature of micron and attrition milled nano-sized titanium powders on two different aspects, i.e., pressure-induced phase change and thermal expansion. Contraction in the volume of unit cell in terms of decrease in interplaner spacing (d) has been observed in both powders and tends to restore upon annealing. At a given pressure, nano titanium shows a smaller decrease in d relative to micron titanium. The stress analysis of the compacts indicates higher value of residual stresses and deformations in micron powder than in nano powder. The dilatometric study reveals, first, the release of internal stresses and entrapped gases causes huge expansion in nanopowder compacts during heating. Secondly, there is no significant difference in the expansion coefficients of sintered micro- and nanocrystalline titanium samples.

I. INTRODUCTION

Materials under high pressure often lead to phase transformation, deformation, amorphization, etc.1–8 During the powder metallurgical process of compaction, similar physical changes on powders would also be expected. Some of the physical properties of the material may not remain the same as in the starting powder (before compaction). Powder may undergo transformation partially or completely to other metastable or stable phases during compaction. Further, when the powder compacts are subjected to the thermal treatment for sintering, some additional reactions take place within the compact, such as release of internal stresses, entrapped gases, and thermal expansion, etc. These factors often lead to errors in determining the exact expansion or shrinkage of the compact material. With increasing demand for nanocrystalline materials, it is important to study these issues associated with the processing of nano powders, which have not been studied thoroughly in many materials. A brief review on the pressure-induced phase transformations and thermal expansion behavior of nanocrystalline materials is given below. A. Pressure-induced phase change

Pressure-driven transitions from the ␣ [hexagonal close packed (hcp)] to the ␻ phase in Ti, Zr, and Hf at a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0075 580

http://journals.cambridge.org

J. Mater. Res., Vol. 20, No. 3, Mar 2005 Downloaded: 09 Feb 2015

about 60 GPa were reported by Jamieson1 in 1963 and later by others.2–4 This phase transition is attributed to the transfer of an s electron from an s band to a d band and a possible modification of the Fermi surface under compaction. Similar transitions have been reported on several other metals5,6 and on nonmetallic materials7,8 at very high pressures. Pressure-induced structural transformations were observed also in the nanocrystalline materials. Nano-anatase (TiO2) phase was transformed to an amorphous phase above 16.4 GPa pressure, which is reverted upon release of pressure.9 A significant broadening in x-ray peak of nanocrystalline Fe (∼50 nm) was observed at 800 MPa pressure compared to micrometric p