Influence of annealing on depth distributions and microstructure of ion-implanted Ti6Al4V
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I. INTRODUCTION
THE titanium alloy Ti6Al4V is used for load-bearing components of joint prostheses.[1] When the sliding surfaces are made from this alloy, wear can lead to abrasion of oxide particles and black staining of the surrounding tissue.[2] Various studies have shown that the wear resistance of Ti6Al4V can be enhanced by ion implantation of nitrogen or carbon (e.g., References 3 through 10). Earlier work by the present authors[4,5] revealed that implantation of noble metals (Ir, Pt, or Au) also improves the wear behavior. The improvement observed after nitrogen or carbon implantation is attributed to the formation of titanium nitride (TiN) and titanium carbide (TiC).[5–10] The compounds in ion-implanted titanium have been identified by various authors.[6–22] A systematic screening of investigations using transmission electron microscopy (TEM) (Table I) indicates the influence of different implantation parameters on the phases formed. When the fluence (dose) F exceeds 2 3 1021 m22, finely dispersed TiN or TiC precipitates are formed in the titanium matrix in the region of high enrichment, i.e., at a depth d . 50 nm. The size of the precipitates ranges from 3 to 50 nm depending on the fluence.[7–9,11,13,17] Implantation of high fluences (F . 6 3 1021 m22) leads to the formation of continuous TiN or TiC layers.[9,15] Although the configuration of the subsurface region affects the wear properties, investigations on the phase formation in the transition region between the oxide film on the surface and the region of high enrichment of implanted nitrogen or carbon have not been reported in the literature. The concentration of implanted atoms can exceed the solid solubility in a phase of the substrate.[23] The alteration of the microstructure of Ti6Al4V during heat treatment after implantation was investigated by Vardiman.[8,16,17] During annealing of nitrogen-implanted samples (F 5 2 3 1021 m22, temperature T 5 500 8C, and duration t 5 4 hours), the size of the TiN precipitates increases only slightly from H. SCHMIDT and G. MIEHE, Senior Scientists, and A. SCHMINKE, Research Associate, are with the Faculty of Materials Science, Darmstadt University of Technology, D-64287 Darmstadt, Germany. M. SOLTANIFARSHI, Research Associate, is with the Institute for Nuclear Physics, University of Frankfurt, D-60486 Frankfurt, Germany. Manuscript submitted October 23, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
20 to 30 nm.[17] Carbon-implanted Ti6Al4V was annealed at different temperatures for 1 hour.[8] The TiC precipitates grow when the temperature exceeds 300 8C, reaching sizes up to 60 nm at T 5 400 8C and 0.5 to 2 mm at T 5 500 8C.[8] Wenzel et al.[18] show that the size of the TiC crystals in carbon-implanted pure titanium (F 5 6.5 3 1021 m22) increases from 90 to 110 nm during annealing (T 5 450 8C and t 5 2 hours). The compound formation during implantation of metallic ions into titanium was studied by Sugizaki et al.,[19,20] Tomashov et al.,[21] and Pham et al.[22] Implantation of b-isomorphous elements (V, Nb,
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