Environment related surface phenomena and their influence on properties of Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo : oxidation at elevated temperature and corrosion during chemical treatment

• Main Author: Sefer, Birhan
• Other Authors: Universitat Politècnica de Catalunya. Departament de Ciència dels Materials i Enginyeria Metal·lúrgica, Luleå tekniska universitet, Antti, Marta-Lena, Mateo García, Antonio Manuel, Pederson, Robert M.
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: Universitat Politècnica de Catalunya 2016
• Subjects: Àrees temàtiques de la UPC::Enginyeria dels materials; Titanium alloys; Oxidation; Oxide; Alpha-case; Chemical milling; Corrosion; Aleaciones de titanio; Oxidación; Óxido; Capa alfa; Fresado químico; Corrosión; Luleå
• Online Access: http://hdl.handle.net/2117/105568; http://hdl.handle.net/10803/403884; https://doi.org/10.5821/dissertation-2117-105568

Cotutela Universitat Politècnica de Catalunya i Luleå University of Technology This thesis covers investigation of the surface phenomena of Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo alloys related with oxidation at elevated temperature and corrosion during chemical treatment in hydrofluoric-nitric acid (HF/HNO3) solutions. The explored phenomena are related to manufacturing and service conditions of aero-engine components. During manufacturing and operation, the alloys are sunjected to elevated temperatures in oxygen environment. In such conditions an oxide on the surface and an oxygen enriched layer below the oxide, commonly referred as alpha-case is formed. Alpha-case is a hard and brittle layer that is detrimental to the mechanical properties and must therefore be completely eliminated. A conventional method for elimination of alpha-case is chemical processing in HF/HNO3 solutions, i.e. chemical milling. Isothermal oxidation treatments in air at 500, 593 and 700 ºC for up to 500 hours were carried out in this thesis. Both alloys developed rutile type of oxide structure and Ti-6Al-2Sn-4Zr-2Mo exhibited stronger oxidation resistance than Ti-6Al-4V. Transition from parabolic to linear oxidation rate was observed at 700 ºC and = 200 hours for both alloys. The difference in the oxidation kinetics of the two alloys is suggested to be related with the chemical composition of the alloys. The oxygen enriched layer, i.e. alpha-case layer, was characterised and its thickness was measured using conventional metallographic and microscopic techniques. The parabolic rate constants, the oxygen diffusion parameters and activation energies were estimated in the temperature range of 500-700 ºC for both alloys. Moreover, the oxidation for 500 hours at 700 ºC resulted in microstructural changes and element re-distribution. The bulk and alpha-case layer hardness at micro- and nano-scale were measured using microhardness and nanoindentation techniques. The alpha-case layer had higher hardness due to the solid solution strengthening effect ...