Hydrogen as a temporary alloying element for the formation of specific microstructural gradients in the (alpha+beta) titanium alloy Ti-6Al-4V

• Applicant: Professor Dr.-Ing. Hans Jürgen Christ
• Subject Area: Metallurgical, Thermal and Thermomechanical Treatment of Materials ; Mechanical Properties of Metallic Materials and their Microstructural Origins ; Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials
• Term: from 2022 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 470236376

• Final Report Year: 2024

Final Report Abstract

Technical components are subjected to increasing demands with regard to durability and reliability. To meet these expectations, the development of thermochemical processes for an unerring and reproducible microstructure adjustment is necessary. Titanium alloys allow temporary alloying with hydrogen as part of the heat treatment, thus called thermohydrogen treatment (THT). The present work intended to produce a local microstructural adjustment of the commercial Ti alloy Ti–6Al–4V by means of THT by changing the local distribution of strengthening precipitates and the grain size as a function of the distance to the surface (microstructural gradient). This should improve the fatigue properties of the material. For the THT parameter values selection, the hydrogen diffusion coefficient and the hydrogen solubility were determined as material parameters. In addition, the THT parameters were evaluated by determining fatigue crack propagation resistance and fracture toughness as a function of solution heat treatment, and evaluating different microstructural gradients after hydrogen loading and degassing by means of numerically simulated hydrogen concentration profiles, hardness curves, metallographic investigations and phase analysis using X-ray diffraction and TEM. The results prove that a desired microstructural gradient can be achieved by hydrogen charging at 500°C and degassing at 750°C followed by an aging at 550°C. Modelling of kinetics and thermodynamics of the Ti–H interaction reproduces the experimentally generated microstructural gradients in a good approximation and provides the basis for further optimization of the process. The evaluation of the resulting mechanical properties was carried out by tests on a miniature testing machine, which allows in-situ observation of fatigue crack initiation and propagation by means of a confocal laser microscope, stress-controlled cyclic deformation tests to determine the fatigue life as a function of the stress amplitude as well as tensile tests. The investigation shows that a THT-induced microstructural gradient can extend the fatigue life of conventionally and additively manufactured specimens by a factor of 3 – 6 compared to a homogeneous reference microstructure under HCF conditions. Moreover, THT has also offered the prospect of enhanced fatigue strength for a near-component demonstrator by means of a further increase of a prior existing hardness gradient by 4 – 5 %.

Publications

• Kennwertermittlung zur Wasserstoffaufnahme- und - abgabe von Titanlegierungen als Basis für die Gestaltung eines THT-Prozesses, In: Christ, H.-J. (Hrsg.), Werkstoffe und Bauteile auf dem Prüfstand, Tagungsband zur Tagung Werkstoffprüfung 2019, INVENTUM GmbH, Sankt Augustin, 2019, S. 257–262.
  Schmidt, C. D. & Christ, H.-J.
  
• Generation of Microstructural Gradients for Improved Mechanical Properties via Thermo-Hydrogen Treatment of the Metastable Beta Titanium Alloys Beta CTM and Ti 10V-2Fe-3Al. MATEC Web of Conferences, 321, 12017.
  Schmidt, Christopher David; Macin, Vitali; Schmidt, Peter & Christ, Hans-Jürgen
  
• Kennwertermittlung zur Wasserstoffaufnahme und - abgabe von Titanlegierungen als Basis für die Gestaltung eines THT-Prozesses (Teil 2), In: J. B. Langer und M. Wächter (Hrsg.), Werkstoffe und Bauteile auf dem Prüfstand, Tagungsband zur Werkstoffprüfung 2020, online, VDM, Berlin, 2020, S. 185–190.
  Schmidt, C. D. & Christ, H.-J.
  
• Wasserstoff als temporäres Legierungselement zur Einstellung spezifischer Gefügegradienten in der (Alpha+Beta)- Titanlegierung Ti–6Al–4V, In: Brockmann, S., Krupp, U. (Hrsg.), Werkstoffe und Bauteile auf dem Prüfstand, Tagungsband zur Werkstoffprüfung 2021 online, Stahlinsitut VDEh, Düsseldorf, 2021, S. 295–300
  Schmidt, C. D., Christ, H.-J. & von Hehl, A.
  
• Ermüdungseigenschaften von thermohydrogen erzeugten Gefügegradienten in Ti–6Al–4V, In: Zimmermann, M. (Hrsg.), Werkstoffe und Bauteile auf dem Prüfstand, Tagungsband zur Werkstoffprüfung 2022, DGM, Sankt Augustin, Germany, 2022, S. 294–299.
  Schmidt, C. D., Christ, H.-J. & von Hehl, A.
  
• Hydrogen as a Temporary Alloying Element for Establishing Specific Microstructural Gradients in Ti-6Al-4V. Metals, 12(8), 1267.
  Schmidt, Christopher David; Christ, Hans-Jürgen & Von, Hehl Axel
  
• Fatigue properties of microstructural gradients in AM Ti–6Al–4V generated with thermohydrogen treatment, In: Jackson, M. (Hrsg.), Proceedings of the 15th World Conference on Titanium, Chapter 6: C-M-P-Fatigue
  Schmidt, C. D., El-Chaikh, A., Danzig, A., von Hehl, A. & Christ, H.-J.
  
• Feasibility of THT-Induced Microstructure Gradients and Their Effects on Fatigue Properties in Additively Manufactured Cylindrical and in Conventionally Manufactured Tube-Like Ti-6Al-4V Specimens. MDPI AG.
  Schmidt, Christopher David; El-Chaikh, Ali; Danzig, André; Christ, Hans-Jürgen & Von, Hehl Axel
  
• Wasserstoff als temporäres Legierungselement zur Erzeugung spezifischer Gefügegradienten in der (α+β)–Ti-Legierung Ti–6Al–4V, Dissertation, Universität Siegen, In: Christ, H.-J. (Hrsg.), Siegener werkstoffkundliche Berichte, Bd. 34, 2024.
  Schmidt, C. D.