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8/8/03

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Page 1993

Titanium-Titanium Boride (Ti-TiB) Functionally Graded Materials through Reaction Sintering: Synthesis, Microstructure, and Properties K.B. PANDA and K.S. RAVI CHANDRAN The study demonstrates an effective method to synthesize titanium-titanium boride (Ti-TiB) functionally graded material (FGM) tiles by exploiting the simultaneous TiB whisker formation in situ and the densification occurring during the reaction sintering process. The macrostructure of the graded material was designed to have a beta-titanium (-Ti) layer on one side with the composite layers of Ti-TiB mixture having increasing volume fraction of the TiB through the thickness. The approach used an optimized tri-modal powder mixture consisting of -Ti powder, a master alloy of the -stabilizing-element powders (Fe-Mo), and TiB2. The structure and properties of both of these FGMs were systematically characterized by X-ray diffraction, electron microscopy, and microhardness measurements. Interestingly, it has been found that two different kinds of TiB whisker morphologies were observed in the FGMs. The Ti-rich layers were found to have large and pristine TiB whiskers uniformly distributed in the Ti matrix. On the other hand, the TiB-rich layer was found to have a network of interconnected and relatively smaller TiB whiskers appearing as clusters. The layers of intermediate TiB volume fractions were found to consist of both the morphologies of TiB. The effectiveness of the X-ray direct comparison method for the determination of volume fractions of phases in the FGM layers was also demonstrated. The Vickers microhardness level was found to increase dramatically from 420 kgf/mm2 in the -Ti layer to 1600 kgf/mm2 in the TiB-rich layer. The elastic residual stresses retained in the graded layers after fabrication were determined based on an elastic multilayer model. The nature of microstructure, the hardness variation, and the distribution of residual stresses in these novel FGMs are discussed.

I. INTRODUCTION

A functionally graded material (FGM) is a macroscopically inhomogeneous composite material that has a gradient in composition from one surface to another.[1] Typically, FGMs are made of a metal and a ceramic as opposite faces with the intermediate zones consisting of varying volume fractions of constituents. Originally, the concept of FGMs arose as a result of efforts to prevent the cracking in ceramic-metal joints induced by the coefficient of thermal expansion (CTE) mismatch during high-temperature processing. Direct bonding of the metal and the ceramic generates thermal stresses due to the CTE mismatch and often leads to cracking of the ceramic, thus making the bi-materials unsuitable for structural use.[2] The FGM, on the other hand, has the following advantages over the metal-ceramic joints: (1) effective reduction in the overall magnitude of the thermal stresses,[3] (2) delaying of the plastic yielding and the failure of the metallic layer by the load sharing of the ceramic,[4] and (3) a better overall use of the avail