First Investigations on a TNM TiAl Alloy Processed by Spark Plasma Sintering
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First Investigations on a TNM TiAl Alloy Processed by Spark Plasma Sintering Thomas Voisin1, Jean-Philippe Monchoux1, Helmut Clemens2 and Alain Couret1 1
CEMES/CNRS, 29 Rue J. Marvig, BP 94347, 31055 Toulouse Cedex 4, France
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Department of Physical Metallurgy and Materials Testing, Montanuniversitaet Leoben, A-8700, Leoben, Austria. ABSTRACT The processing of a TNM powder (Ti-43.9Al-4Nb-0.95Mo-0.1B, in at.%) by Spark Plasma Sintering (SPS) was investigated for the first time. SPS experiments were performed at varying temperatures. The microstructures of the products were analyzed and interpreted in reference to the available phase diagram. Results of tensile tests conducted at room temperature are also presented. INTRODUCTION Among TiAl alloys, TNM alloys, which are containing niobium and molybdenum as alloying elements, attract a great deal of interest because of an excellent hot-workability and balanced mechanical properties [1,2]. The high workability is attributed to the presence of βphase which provides independent slip systems at elevated temperatures. This is of primary interest for forging, especially when forging of small parts is concerned. However, this β-phase is suspected to be detrimental to the creep resistance at service temperature and to the ductility at room temperature. The phase diagram and the solidification sequence of TNM alloys have been largely investigated for a wide range of chemical compositions [3,4]. Here, attention will be focused on alloys with the chemical composition of the powder used in the present work: Ti-43.9Al-4Nb0.95Mo-0.1B (Figure 1). Consistently with the phase diagram, this alloy solidifies via the β phase following the sequence [4]: L
L+β
β
β+α
α
α+γ
α+γ+β
α + α2 + γ + β0
α2 + γ + β0
The existence of the single α-phase field region was experimentally confirmed in a previous study by heat-treating a sample at 1265 °C for 1h [5]. The addition of Nb and Mo leads to the formation of a four-phase field region corresponding to the ordering of the α phase into α2 phase [3]. During the last few years, several TiAl alloys were successfully sintered by SPS [6,7]. SPS is a powder metallurgy technique for which the densification is due to the simultaneous application of a pulsed direct current and an uniaxial pressure. Because it involves strong high temperature deformation as in the forging process, SPS is assumed to be adapted for processing TNM alloys. This paper presents a first investigation concerning the consolidation of a TNM
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powder by SPS. In particular, it will be studied whether the β solidification route allows to adjust refined microstructures by means of sintering.
Figure 1. Phase diagram of the Ti-(35-50)Al-4Nb-1Mo alloy [3]. Points a to d situate the dwell temperatures of the SPS densification performed in the present work. EXPERIMENTAL DETAILS SPS experiments were conducted on a Ti-43.9Al-4Nb-0.95Mo-0.1B powder produced by argon gas atomization using the Electrode Induction Melting Gas Atomization (EIGA) technique [8]. The diameter of the powder
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