Sintering mechanisms of attrition milled titanium nano powder
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V.V. Dabhade and P. Ramakrishnan Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology, Bombay, 400076 India (Received 14 August 2004; accepted 4 November 2004)
Detailed sintering studies have been carried out on attrition milled nanocrystalline titanium powder through isothermal dilatometry over a temperature range of 300– 1250 °C along with microstructural and x-ray diffraction studies. The sintering behavior of attrition milled nanocrystalline titanium appears to be characterized by: (i) very low activation energies, (ii) high shrinkage anisotropy, (iii) very rapid grain growth in the beta range, and (iv) two kinds of densification processes, namely, intra-agglomerate and inter-agglomerate. Analysis of the kinetic data through sintering diagram approach indicates the operation of particle sliding and grain boundary rotation, type of mechanism in addition to the grain-boundary diffusion, and lattice diffusion as the dominant mass transport mechanisms.
I. INTRODUCTION
Sintering behavior of titanium powder has not been well understood due to its high reactivity and complex diffusion behavior. Titanium has a high melting point (1668 °C) and undergoes a phase transformation [hexagonal close-packed (hcp) to body-centered-cubic (bcc)] at 882 °C.1 It shows diffusional anisotropy in the ␣ (hcp) range and shows an anomalous curved Arrhenius behavior in the bcc range as in the case of other group IVB elements.2,3 The activation energy for self diffusion in the beta range continuously increases as a function of temperature (131–328 kJ/mol2,3). Further, some of the transition elements (Fe, Ni, Co, Cr, etc.) tend to dissolve partially interstitially and also diffuse by an interstitial mechanism.3 Sintering studies of P/M grade titanium powder have shown a low activation energy, which have been attempted to be explained in terms of additional defect generation during ␣ →  transition.4–6 Nanocrystalline powders have a large specific surface area and a high defect density (specially in milled powders), which can act as additional driving forces during sintering.7,8 They show an onset temperature of sintering as low as 0.2–0.3 Tm (Tm is melting temperature) and a high sintering rate.7–9 The full density could be achieved at relatively much lower temperatures.8–10 However, a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0116 J. Mater. Res., Vol. 20, No. 4, Apr 2005
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there could be serious interference from the problems of agglomeration, adsorbed gases, difficulties in compaction, and grain growth during sintering.8,9,11 Nano powders tend to yield a relatively low activation energy of sintering.9,12 Enhanced sintering rates in nanocrystalline powders, however, cannot often be explained in terms of the additional driving forces alone and often new mechanisms, such as particle sliding, grain boundary rotation for rapid densification, and a rapid grain growth etc., have been prop
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