NDLI: Surface hardening of biocompatible ultrafine-grained niobium zirconium alloy by two-stage oxidation treatment

Content Provider	Springer Nature Link
Author	Rubitschek, F. Niendorf, T. Karaman, I. Maier, H. J.
Copyright Year	2013
Abstract	The present study reports on an optimized surface hardening process for biocompatible ultrafine-grained (UFG) niobium 2.3 wt% zirconium (NbZr) alloy, a promising candidate implant material. The as-received material of conventional grain size (CG) was processed using multipass equal channel angular processing at room temperature to obtain an UFG microstructure featuring high strength and ductility. Subsequent surface hardening was performed by a heat treatment leading to internal oxidation. Using a thermogravimetric system, the influence of temperatures, time, and partial pressure of oxygen ( $$ p_{{\text{O}}_2} $$ ) on the oxidation kinetics were investigated. Metallographic and microscopic analyses and hardness measurements were employed to evaluate maximum hardness, penetration-depth and scale formation under various conditions. Heat treatment at 620 °C for 6 h at a $$ p_{{\text{O}}_2} $$ of 0.2 hPa led to sufficiently rapid oxidation kinetics yielding a relatively high depth of penetration without formation of loose Nb$_{2}$O$_{5}$ on the surface, which was observed at higher $$ p_{{\text{O}}_2} $$ . As compared to CG material, improved hardness profiles were reached using the same heat treatment parameters, since the UFG structure significantly changes diffusion conditions and therefore oxidation kinetics. After a second heat treatment in high vacuum the high maximum hardness of 820 HV0.01 in the UFG material was reduced effectively and a less steep hardness gradient was achieved, both contributing to a less brittle behavior under mechanical loading. High-cycle fatigue tests performed on surface-hardened UFG NbZr samples showed a substantial improvement of fatigue life in tests conducted near the endurance limit. Especially when high fatigue and wear resistance are key issues for a given application, the internal oxidation process offers an effective way to further improve the properties of UFG NbZr.
Starting Page	4549
Ending Page	4556
Page Count	8
File Format	PDF
ISSN	00222461
Journal	Journal of Materials Science
Volume Number	48
Issue Number	13
e-ISSN	15734803
Language	English
Publisher	Springer US
Publisher Date	2013-01-09
Publisher Place	Boston
Access Restriction	One Nation One Subscription (ONOS)
Subject Keyword	Materials Science Characterization and Evaluation of Materials Polymer Sciences Continuum Mechanics and Mechanics of Materials Crystallography Mechanics
Content Type	Text
Resource Type	Article
Subject	Ceramics and Composites Mechanics of Materials Mechanical Engineering Polymers and Plastics

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