Microstructure and Tensile Ductility of a Ti-43Al-4Nb-1Mo-0.1B Alloy
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1128-U03-08
Microstructure and Tensile Ductility of a Ti-43Al-4Nb-1Mo-0.1B Alloy Laura M. Droessler1, Thomas Schmoelzer1, Wilfried Wallgram2, Limei Cha1, Gopal Das3, Helmut Clemens1 1 Department of Physical Metallurgy and Materials Testing, Montanuniversität Leoben, FranzJosef-Str. 18, A-8700 Leoben, Austria 2 Bohler Schmiedetechnik GmbH&CoKG, Mariazeller Str. 25, A-8605 Kapfenberg, Austria 3 Pratt & Whitney, 400 Main Street M/S 114-43, East Hartford, CT 06108, USA ABSTRACT The microstructural development of a forged Ti-43Al-4Nb-1Mo-0.1B (in at%) alloy during two-step heat-treatments was investigated and its impact on the tensile ductility at room temperature was analyzed. The investigated material, a so-called TNM™ gamma alloy, solidifies via the β-route, exhibits an adjustable β/B2-phase volume fraction and can be forged under near conventional conditions. Post-forging heat-treatments can be applied to achieve moderate to near zero volume fractions of β/B2-phase allowing for a controlled adjustment of the mechanical properties. The first step of the heat-treatment minimizes the β/B2-phase and adjusts the size of the α-grains, which are a precursor to the lamellar γ/α2-colonies. However, due to air cooling after the first annealing step, the resulting microstructure is far from thermodynamic equilibrium. Therefore, a second heat-treatment step is conducted below the eutectoid temperature which brings the microstructural constituents closer to thermodynamic equilibrium. It was found that temperature and duration of the second heat-treatment step critically affect the solid-state phase transformations and, thus, control the plastic fracture strain at room temperature. Scanning and transmission electron microscopy studies as well as hardness tests have been conducted to characterize the multi-phase microstructure and to study its correlation to the observed room temperature ductility. INTRODUCTION In the last decades, intermetallic γ-TiAl based alloys have been developed which can replace Ni-base alloys up to temperatures of 800°C, while maintaining acceptable mechanical properties. Their low density, high specific stiffness, high yield strength, and good creep resistance have allowed this class of materials to be used as turbocharger wheels and valves in combustion engines [1]. Drawbacks which have delayed and are still delaying their widespread use are difficult hot-workability and limited ductility at room temperature. Hot workability can be improved by stabilizing the ductile disordered bcc β-phase through additions of β-stabilizers such as Nb and Mo. These Nb and Mo bearing alloys, which have Al contents in the range of 42 to 44 at%, are so-called TNM™ alloys. Specific information on TNM™ alloys can be found in [2,3]. At elevated temperatures, a high volume fraction of β-phase can be obtained which allows forging under nearly conventional conditions [4]. Following forging the material was subjected to different heat treatments to control the microstructure. Tensile tests at room temperature on a TNMTM gamma allo
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