Sub-Micrometer Spatially Resolved Measurements of Mechanical Properties and Correlation to Microstructure and Compositio
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Sub-micrometer spatially resolved measurements of mechanical properties and correlation to microstructure and composition M. Kunert 1, B. Baretzky1, S. P. Baker2, and E. J. Mittemeijer1 1 Max Planck Institute for Metals Research, Seestrasse 92, 70174 Stuttgart, Germany 2 Cornell University Department of Materials Science and Engineering, 129 Bard Hall, Ithaca, NY 14853-1501, USA ABSTRACT The variations of hardness, composition, and microstructure within a carbon implanted region – about 350 nm thick – of a Ti-6Al-4V alloy were measured using nanoindentation, Auger electron spectroscopy and transmission electron microscopy, respectively. Correlations among hardness, composition, and microstructure were made with a spatial resolution of about ±20 nm. The variation in hardness within the implanted regions was quantitatively explained as due to the formation of an almost continuous TiC layer and precipitate hardening. The problems that may arise in measuring and correlating spatial variations in such a complex material on this scale are outlined and a successful method to solve them is proposed. The need for highly spatially resolved measurement techniques is emphasized. INTRODUCTION Most engineering materials are inhomogeneous on the micro- or nanometer scale. For example, the medical prostheses alloy Ti-6Al-4V consists of two phases with different mechanical properties [1]: a harder hexagonal α-phase and a softer cubic (bcc) β-phase (see Fig.1). When such an alloy is surface engineered by ion implantation, additional large variations in composition and microstructure are generated within less than 500 nm of the sample surface. The near-surface microstructure of such an ion implanted Ti-6Al-4V alloy is thus very inhomogeneous in both lateral and vertical directions (see Fig. 2). Implantation of carbon into Ti-6Al-4V is known to improve the wear resistance of this alloy against polyethylene [2]. Although often attributed to an increase in surface hardness [3], this effect is not yet understood. An understanding of the relationship between ion implantation and resulting near-surface mechanical properties requires (i) knowledge of the influence of the ion implantation parameters on the composition and microstructure within the implanted region and (ii) knowledge of the effect of these compositional and microstructural variations on the mechanical properties within this region. The aim of the present work was the investigation of the latter. The scale of the variations within the implanted region requires that measurements be made with a depth resolution on the order of 10 nm. Additionally, because of the two-phase microstructure of the Ti-6Al-4V sample, the measurements must be performed with a high lateral resolution. In the present paper some problems will be pointed out that may arise in measuring and correlating the variations with depth of composition, microstructure and mechanical properties on nanometer scale of a material with a very complex microstructure. A successful way to overcome those problems is presented.
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