Hardness-depth profile of a carbon-implanted Ti-6Al-4V alloy and its relation to composition and microstructure

The variation of mechanical properties (hardness, indentation modulus) with in a carbon-implanted region of a Ti-6Al-4V alloy-about 350-nm thick-was, f or the first time, related with the microstructure and the chemical composi tion with a depth accuracy as small as +/- 20 nm. Microstructure, chemical composition, and mechanical properties of the implanted alloy were determin ed using transmission electron microscopy, Auger electron spectroscopy, and nanoindentation, respectively. The microstructure within the implanted reg ion contains TiC precipitates, the density of which changes with depth in a ccordance with the carbon content. The hardness depends on the precipitate density: the maximum hardness occurs at the depth where an almost continuou s TiC layer had formed. The depth profiles of hardness and indentation modu lus were measured using three different methods: the cross-section method ( CSM); the constant-load method (CLM); and the load-variation method (LVM). Only the hardness-depth profile obtained using the CSM, in which the indent ations are performed perpendicular to the hardness gradient on a cross sect ion of the specimen, reflects the microstructural variations present in the implanted region.