TY - THES

T1 - Thermomechanical processing and high-temperature behavior of spark-plasma-sintered near-α Ti-8242NbS alloy

AU - Obersteiner, David

N1 - no embargo

PY - 2021

Y1 - 2021

N2 - Near-α titanium alloys show high potential for high-temperature applications in aerospace, automotive and nautical industries. This alloying class combines high strength, low density as well as good creep and oxidation resistance. Current scientific research focuses on developing reinforced near-α titanium alloys which can be used above 600 °C. In this thesis the thermomechanical processing and the high-temperature behavior were investigated of a new near-α titanium alloy, called Ti8242NbS, manufactured by spark-plasma-sintering. Thermomechanical processing routes were defined in the first steps to achieve fully-lamellar, bi-lamellar and bi-modal microstructures and compare them in terms of suitability for high-temperature service. Heat treatments at different temperatures and dwell times were performed to investigate the precipitation kinetics at elevated temperatures. The results have shown that precipitation hardening effects reaches its maximum at 397 HV5 with a fully-lamellar microstructure after aging at 670 °C for 24 h. The subsequent TEM investigations have shown that the hardness increase of about 20 HV5 is based on finely distributed Ti3Al-particles inside the α-lamellae. The mechanical characterization of this work consisted of tensile and creep experiments on different specimen states. It was shown that precipitation hardening of Ti8242NbS alloy at 650 °C had increased the yield strength by 40 MPa, while simultaneously decreasing the total elongation to a minimum value of 0.24 %. The creep experiments have shown that aged Ti8242NbS alloy with fully-lamellar microstructure exhibits the minimal creep rate of 1.0∙10^(-8) s^(-1) and thus performs better than the Ti6242S alloy. Final oxidation tests also showed the advantage of the Ti8242NbS alloy over the Ti6242S alloy for high-temperature application.

AB - Near-α titanium alloys show high potential for high-temperature applications in aerospace, automotive and nautical industries. This alloying class combines high strength, low density as well as good creep and oxidation resistance. Current scientific research focuses on developing reinforced near-α titanium alloys which can be used above 600 °C. In this thesis the thermomechanical processing and the high-temperature behavior were investigated of a new near-α titanium alloy, called Ti8242NbS, manufactured by spark-plasma-sintering. Thermomechanical processing routes were defined in the first steps to achieve fully-lamellar, bi-lamellar and bi-modal microstructures and compare them in terms of suitability for high-temperature service. Heat treatments at different temperatures and dwell times were performed to investigate the precipitation kinetics at elevated temperatures. The results have shown that precipitation hardening effects reaches its maximum at 397 HV5 with a fully-lamellar microstructure after aging at 670 °C for 24 h. The subsequent TEM investigations have shown that the hardness increase of about 20 HV5 is based on finely distributed Ti3Al-particles inside the α-lamellae. The mechanical characterization of this work consisted of tensile and creep experiments on different specimen states. It was shown that precipitation hardening of Ti8242NbS alloy at 650 °C had increased the yield strength by 40 MPa, while simultaneously decreasing the total elongation to a minimum value of 0.24 %. The creep experiments have shown that aged Ti8242NbS alloy with fully-lamellar microstructure exhibits the minimal creep rate of 1.0∙10^(-8) s^(-1) and thus performs better than the Ti6242S alloy. Final oxidation tests also showed the advantage of the Ti8242NbS alloy over the Ti6242S alloy for high-temperature application.

KW - High-temperature-alloy

KW - Precipitation hardening

KW - Near-α titanium alloy

KW - Creep resistance

KW - Oxidation resistance

KW - Thermomechanical processing

KW - Hochtemperatur-Legierung

KW - Auslagerungshärtung

KW - Near-α Titanlegierung

KW - Kriechbeständigkeit

KW - Oxidationsbeständigkeit

KW - Thermomechanische Verarbeitung

M3 - Master's Thesis

ER -