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Editor's Note: A hypertext-enhanced version of this article is available on-line at www.tms.org/pubs/journals/JOM/0405/Chandran-0405.html.Discontinuously reinforced titaniumtitanium boride whisker (TiB w ) composites are becoming a commercial reality for a number of demanding automotive, aerospace, and other advanced applications. This article presents the general microstructural characteristics and mechanical properties of Ti-TiB w composites of various matrix compositions and TiB w volume fractions based on the authors' research as well as work done elsewhere. The directions for future development in terms of the choice of titanium matrix compositions, microstructure control, and mechanical property optimization, as well as the realistic possibilities of near-term commercial applications in industry are discussed.

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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 TiB 2 . 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/mm 2 in the ␤-Ti layer to 1600 kgf/mm 2 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.