In situ synchrotron radiation measurements of orthorhombic phase formation in an advanced TiAl alloy with modulated micr
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In situ synchrotron radiation measurements of orthorhombic phase formation in an advanced TiAl alloy with modulated microstructure M. Rackel, A. Stark, H. Gabrisch, F.-P. Schimansky, N. Schell, A. Schreyer, F. Pyczak Institute of Materials Research, Helmholtz-Zentrum Geesthacht, Max-Planck-Straße 1, D-21502 Geesthacht, Germany
ABSTRACT New low aluminium high niobium TiAl alloys exhibit a nano scale modulated microstructure consisting of lamellae with a tweed substructure. These tweed like appearing lamellae are a modulated arrangement of at least two phases. One constituent of the crystallographic modulation in the lamellae is an orthorhombic phase, which is closely related to both the hexagonal α2-Ti3Al phase and the cubic B2 ordered βo-TiAl phase. In this study the nature and formation of this orthorhombic phase has been investigated by high-energy X-ray diffraction. Measurements have shown that the newly formed orthorhombic phase is structurally comparable to the O phase (Ti2AlNb). It forms in the temperature range of 550 °C to 670 °C from the α2 phase by small atomic displacements and chemical reordering. The in situ experiments yielded information about the thermal stability of the orthorhombic phase. After dissolving at temperatures above 700 °C the phase can be re-precipitated by annealing within the temperature range of formation. INTRODUCTION Light-weight TiAl alloys are suitable for high-temperature applications in the automotive and aero industries. However, mass production and use in high-temperature applications is still hampered by the limited room temperature ductility, which makes optimisation of alloy composition and microstructure crucial [1]. One way to reach this goal is by modification of the microstructure. Appel et al. [2-4] describe a Ti-(40-44) Al-8.5 Nb alloy in which a nano-scale modulation within a lamellar microstructure exists (all alloy compositions in the text are given in atomic percent). This advanced TiAl alloy shows an excellent balance of room temperature ductility and strength. Several authors [2-7] have identified orthorhombic phases as one of the constituents of the modulated lamellae but there is no general agreement about their formation path and temperature range of formation. The crystallographic similarity to related phases in this type of TiAl alloy [8] makes an unambiguous identification of the formation path difficult. For example Nguyen-Manh et al. [9] proposed that the orthorhombic B19 phase formed from the ordered βo phase by a shuffle displacement of neighbouring B2 planes in opposite B2 directions. Song et al. [7] observed that the B19 phase formed out of the α2 phase as a transitional phase that further transforms to γ phase in a Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y alloy. Muraleedharan et al. [10] found that the orthorhombic O phase forms from α2 in an Al lean and Nb rich TiAl alloy. Many of the above studies used transmission electron microscopy (TEM) characterisation
measurements for phase identification. However, high-energy X-ray diffraction (HEXRD) is suitable
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