Mechanical Spectroscopy in Advanced TiAl-Nb-Mo Alloys at High Temperature
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Mechanical Spectroscopy in Advanced TiAl-Nb-Mo Alloys at High Temperature Pablo Simas1, Thomas Schmoelzer2, Maria L. Nó3, Helmut Clemens2 and Jose San Juan1 1 Department of Física Materia Condensada, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apdo. 644, 48080 Bilbao, Spain. 2 Department of Physical Metallurgy and Materials Testing, Montanuniversität Leoben, FranzJosef-Str. 18, A-8700 Leoben, Austria. 3 Department of Física Aplicada II, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apdo. 644, 48080 Bilbao, Spain. ABSTRACT New advanced multi-phase Ȗ-TiAl based alloys (TiAl-Nb-Mo), so called TNM alloys, have been developed to promote hot workability and to allow easier processing by conventional forging. However, to control and stabilize the final microstructure, specific processing and further thermal treatments are required. In the present work we used mechanical spectroscopy techniques to obtain a better understanding of the microstructural mechanisms taking place at high temperature applying two different heat treatments. Internal friction spectra and dynamic modulus evolution have been measured in an inverted torsion pendulum up to 1220 K. A stable relaxation peak was observed in both cases at about 1050 K for 1 Hz. Spectra acquired at several frequencies between 0.01 Hz and 3 Hz allow us to measure the activation parameters of this peak. In addition, a high temperature background (HTB) has been observed. This HTB, which has been found to be dependent on thermal treatments, has been analyzed to obtain the apparent activation enthalpy, which seems to be correlated to the creep behavior. Finally, we discuss the relaxation peak and the HTB in terms of the microstructural evolution during thermal treatments. INTRODUCTION During the last decade, the demand for more energy and cost efficient materials in the automotive and aerospace industry has led to considerable research effort concerning the development of light weight high-temperature resistant materials targeted to replacing conventional Ni-based superalloys. The most important requirement for these materials is sufficient creep resistance and toughness as well as high thermal stability of the microstructure. Intermetallic γ-TiAl based alloys are considered one of the most promising candidates to meet the required thermal and mechanical specifications [1]. However, their extensive use has been limited mainly due to poor ambient temperature ductility and difficult hot-workability [2,3]. Recent advances in alloy design and processing have established a novel production technology for Nb and Mo containing γ-TiAl based alloys (TNMTM alloy) which possess a large amount of disordered β-phase that improves hot workability and allows forging under “near conventional” conditions. By subsequent heat-treatments the β-phase fraction can be substantially reduced which leads to excellent mechanical properties [2]. Mechanical properties at high temperatures are related to creep behavior, which is determined by the microscopic mechanisms of defe
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