Synthesis of ductile titanium-titanium boride (Ti-TiB) composites with a beta-titanium matrix: The nature of TiB formati

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Synthesis of Ductile Titanium–Titanium Boride (Ti-TiB) Composites with a Beta-Titanium Matrix: The Nature of TiB Formation and Composite Properties K.B. PANDA and K.S. RAVI CHANDRAN The study focused on the in-situ synthesis of titanium (Ti)–titanium boride (TiB) composites with β phase in the matrix by reaction sintering of TiB2 with Ti and alloying element powders. The goal was to examine the nature of TiB whisker formation in three different kinds of powder mixtures: (1) β-Ti alloy powders and TiB2; (2) α-Ti powder, a master alloy (Fe-Mo) powder containing the β-stabilizing elements, and TiB2; and (3) α-Ti powder, a β-stabilizing elemental powder (Mo or Nb), and TiB2. The effects of powder packing and the relative locations of powder particles on the morphological changes in TiB whisker formation and their growth were studied at processing temperatures ranging from 1100 °C to 1300 °C. The morphology, size, and distribution of whiskers were found to be influenced by the powder-packing conditions. A large particle-size ratio in bimodally packed mixtures led to the formation of a TiB monolithic layer around β grains. With a relatively finer starting powder, smaller size ratio, and trimodal packing arrangement, the TiB whiskers were found to be distributed more homogeneously in the matrix. The study also used the X-ray direct comparison method and the structure factor for the β phase to determine the volume fraction of TiB phase from X-ray data. Tensile tests and fractographic investigations were carried out on selected composites. The evolution of the composite microstructure, the influence of powder-packing variables, and the morphology and growth of TiB whiskers and their effect on mechanical properties are discussed.

I. INTRODUCTION

TITANIUM (Ti) alloys are known to have a good combination of mechanical properties for lightweight structural applications. To enhance their mechanical properties further, recent research has been focused on Ti metal-matrix composites (TiMMCs). The earlier works[1,2] on TiMMCs focused mainly on the continuous-fiber reinforcements. These TiMMCs exhibited high specific properties but were very expensive to produce in large volume, making them economically unattractive. Moreover, complex fabrication routes, anisotropy, and limited formability have restricted their widespread application. Further, due to the high reactivity of Ti, there are embrittlement problems arising from the reaction of reinforcements such as B and SiC,[3,4] and this has been an additional factor in slowing the application of TiMMCs. In recent years, novel processing routes have evolved, where the reinforcements can be grown in situ in the metal matrix, utilizing either the exothermic nature of reactions or the crystallization during solidification processing.[5–12] These techniques are cost effective and flexible enough to produce composites with the desired level of thermodynamically stable reinforcements. The problem of the reactivity of Ti with reinforcements can be circumvented to some extent by using reinfor

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