Nano-Scale Design of TiAl Alloys Based on beta-Phase Decomposition
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0980-II07-06
Nano-Scale Design of TiAl Alloys Based on Beta-Phase Decomposition Fritz Appel, Michael Oehring, and Jonathan Paul GKSS Research Centre, Institute for Materials Research, Max-Planck-Str. 1, Geesthacht, D21502, Germany
ABSTRACT Phase decomposition and ordering reactions in β/Β2-phase containing TiAl alloys were utilized to establish a novel, previously unreported, type of laminate microstructure. The characteristic constituent of this microstructure are laths with a nanometer-scale substructure that are comprised of several stable and metastable phases. The constitution and microstructure of this novel type of alloy has been assessed by high-resolution transmission electron microscopy. Finally brief mention is made of the mechanical properties of the material.
INTRODUCTION One strategy for overcoming the brittleness of intermetallic titanium aluminide alloys is to use a combination of thermomechanical processing and ternary alloying elements to induce more ductile phases. The performance is generally improved, if microstructures with nanometer scale dimensions can be produced. This can be achieved by intensive hot–working or the implementation of solid-state reactions, such as a discontinuous phase transformation. In TiAlbased systems three-phase equilibria of the α or α2, the γ and the β phases can be established by choosing suitable Al contents and alloying with other metallic elements. The disordered bcc lattice of the β phase provides sufficient independent slip systems and may thus directly act as a ductilizing constituent in the final microstructure. The β phase or its ordered counterpart B2 may also serve as an intermediate phase, through which the pathway of the solid-state transformations and thus the microstructure can be manipulated [1-5]. However, several possible product phases with lower crystal symmetry, such as ω, ω’ or ω’’, are extremely brittle and thus harmful constituents of the microstructure. This inherent complexity of the phase evolution requires a tight harmonization of alloying elements that stabilize the β/B2 phase and influence the subsequent phase decomposition. This problem is addressed in the present paper in that the constitution and microstructure achieved upon decomposition of the β/B2 phase in a Nb-bearing alloy is examined.
EXPERIMENTAl DETAILS The present study is focused on alloys with the base line composition Ti-(40-44)Al-8.5 Nb (at.%), which, according to literature data [5], should contain a significant amount of β phase. The investigations were performed on extruded ingot material, which was subjected to stressrelief annealing for 2h at 1030 °C in air, followed by slow furnace cooling. Phase analysis was performed by X-ray diffraction (XRD), utilizing a Siemens D5000 diffractometer equipped with a Cu tube and a graphite secondary monochromator. The constitution and microstructure were characterized by high-resolution transmission electron microscopy (TEM). RESULTS AND DISCUSSION As indicated by X-ray analysis the constitution of the alloys involves the β/B2
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