Deformation mechanisms in Ti-6Al-4V/TiC composites
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8/8/03
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Deformation Mechanisms in Ti-6Al-4V/TiC Composites A.J. WAGONER JOHNSON, K.S. KUMAR, and C.L. BRIANT The compressive behavior at room temperature of Ti-6Al-4V/TiC composites was examined at strain rates from 0.1 to 1000 s1. As little as 1 vol pct TiC particulates provided greater than a 20 pct increase in strength over that of the monolithic Ti-6Al-4V, while further additions of TiC did not provide proportional benefits. Microstructural examination before and after compression testing was instrumental in understanding the relative importance of the primary strengthening mechanism in the composites as compared to the monolithic material. A comparison of the various possible mechanisms clearly showed that the dominant mechanism was due to carbon in solid solution. At low strain rates, the failure process consisted of a progression of damage in the matrix and at particle-matrix boundaries, while at high strain rates, failure occurred along adiabatic shear bands. The composites had a greater susceptibility to adiabatic shear-band formation than did the monolithic material. I. INTRODUCTION
TI-6AL-4V (hereafter called Ti64) is one of the most widely used Ti alloys, because of its high specific strength and corrosion resistance.[1,2] The alloy has also been considered as a matrix for particulate- and fiber-reinforced composites which would have higher abrasion resistance than the base material[3] and, also, for potential applications in the area of armor.[4] Due to the high reactivity of titanium and its alloys, both in the liquid and solid state, the reinforcement phase and processing conditions must be carefully chosen to minimize the formation of reaction products at the particle- or fibermatrix interface. Reinforcements that have been considered for the Ti64 matrix include SiC, TiB2, TiB, and TiC.[5–11] Of these, SiC is the only one available in fiber form. Fibers are typically coated in order to create a diffusion barrier between the fiber and the matrix and to aid in the relief of residual stress generated by thermal mismatch.[5] However, even these coated fibers are found to react with the matrix to form a brittle silicide layer or other reaction products.[5,6] TiB2 has been considered for particulate reinforcement of Ti-based materials, but it decomposes to form TiB, which results in a reaction product at the particle-matrix interface. There is also evidence that TiB is readily oxidized and can, thus, be the site of failure initiation in the composite.[5] TiC particulate additions have been shown to increase the strength of monolithic Ti64 in tension.[9] Furthermore, TiC is now readily available in particle sizes as small as 1 to 5 m, which reduces the likelihood that defects in the particle can lead to failure. Such defects have been a source of damage and failure in Ti64/TiC composites when larger particles were used.[9] Although TiC is thought to be thermodynamically stable in Ti, there have been studies that indicate that carbon diffuses from the particles into the matrix of Ti64
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