TY - THES

T1 - Characterization of an orthorhombic phase in a water-quenched Ti-44Al-3Mo (at.%) alloy using in situ synchrotron diffraction and transmission electron microscopy

AU - Musi, Michael

N1 - embargoed until 04-09-2020

PY - 2018

Y1 - 2018

N2 - In this work the thermally activated formation of an orthorhombic phase in a β-homogenized and water-quenched Ti-44.5Al-3.2Mo-0.1B (at.%) alloy is studied. The goal was to identify the type of this orthorhombic phase and to investigate the temperature interval of its formation and dissolution as well as its phase stability. For this purpose, in situ heating and ageing high-energy X-ray diffraction experiments starting from the water-quenched state were carried out at the synchrotron radiation source PETRA III at the DESY in Hamburg, Germany. Additionally, scanning and transmission electron microscopy was used to study the evolution of the microstructure at different length scales. During the reheating of the β-homogenized and water-quenched samples the splitting of the α2 peaks could be observed in a temperature range from 590 °C to 724 °C in the synchrotron data. This could be attributed to the formation of an orthorhombic phase, which consumed a significant amount of α2 phase. Using Rietveld analysis, the synchrotron data were evaluated to characterize the formed orthorhombic phase in terms of lattice parameters and phase fraction. After exceeding 724 °C the orthorhombic phase vanished and a large amount of γ phase formed. The in situ heating and cooling experiments showed that the orthorhombic phase only formed upon reheating after the water quenching and not during a subsequent cooling. In combination with the formation pathway α2→orthorhombic phase→γ it could be concluded that the orthorhombic phase is a metastable transition phase between the α2 and the γ phase for this type of alloy. Furthermore, higher heating rates lead to the shift of the formation and dissolution of the orthorhombic phase to higher temperatures. Using Rietveld refinement on the basis of the implemented structural phase models the lattice parameters could be evaluated. All of them were in good agreement with literature. The TEM investigation revealed that after water quenching from the β single-phase field region the microstructure mainly consisted of large α2 laths with a mixture of α2’ martensite and βo phase in between. Upon the reheating process the orthorhombic phase precipitated in the form of small lamellae within the α2 laths and the α2’ martensite. After exceeding the dissolution temperature of the orthorhombic phase, the γ phase formed fine lamellae, similar to the orthorhombic phase, within the α2 laths and the α2’ martensite.

AB - In this work the thermally activated formation of an orthorhombic phase in a β-homogenized and water-quenched Ti-44.5Al-3.2Mo-0.1B (at.%) alloy is studied. The goal was to identify the type of this orthorhombic phase and to investigate the temperature interval of its formation and dissolution as well as its phase stability. For this purpose, in situ heating and ageing high-energy X-ray diffraction experiments starting from the water-quenched state were carried out at the synchrotron radiation source PETRA III at the DESY in Hamburg, Germany. Additionally, scanning and transmission electron microscopy was used to study the evolution of the microstructure at different length scales. During the reheating of the β-homogenized and water-quenched samples the splitting of the α2 peaks could be observed in a temperature range from 590 °C to 724 °C in the synchrotron data. This could be attributed to the formation of an orthorhombic phase, which consumed a significant amount of α2 phase. Using Rietveld analysis, the synchrotron data were evaluated to characterize the formed orthorhombic phase in terms of lattice parameters and phase fraction. After exceeding 724 °C the orthorhombic phase vanished and a large amount of γ phase formed. The in situ heating and cooling experiments showed that the orthorhombic phase only formed upon reheating after the water quenching and not during a subsequent cooling. In combination with the formation pathway α2→orthorhombic phase→γ it could be concluded that the orthorhombic phase is a metastable transition phase between the α2 and the γ phase for this type of alloy. Furthermore, higher heating rates lead to the shift of the formation and dissolution of the orthorhombic phase to higher temperatures. Using Rietveld refinement on the basis of the implemented structural phase models the lattice parameters could be evaluated. All of them were in good agreement with literature. The TEM investigation revealed that after water quenching from the β single-phase field region the microstructure mainly consisted of large α2 laths with a mixture of α2’ martensite and βo phase in between. Upon the reheating process the orthorhombic phase precipitated in the form of small lamellae within the α2 laths and the α2’ martensite. After exceeding the dissolution temperature of the orthorhombic phase, the γ phase formed fine lamellae, similar to the orthorhombic phase, within the α2 laths and the α2’ martensite.

KW - Orthorhombische Phase

KW - in situ

KW - Synchrotron

KW - TEM

KW - TiAl-Legierung

KW - orthorhombic phase

KW - in situ

KW - synchrotron

KW - TEM

KW - TiAl alloy

M3 - Master's Thesis

ER -