Phase transformations in Ti 3 Al and Ti 3 Al + Mo aluminides
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We have analyzed the phase relationships in two titanium aluminides containing 3.4 at. % Mo with different aluminum compositions. The alloys were first homogenized in the /3 field, then cooled continuously at different cooling rates from 80 °C/s to 0.1 °C/s. The continuous cooling transformation diagrams (CCT) show that phase transformations and resulting microstructures are highly dependent on cooling rate. The microstructure consists of ordered a2 (DOi9), ordered y30 (B2), and athermal a (hexagonal) phases. The "tweed microstructure" is observed. The evolution of microhardness was determined as well as the relative partitioning of Al and Mo in (a 2 ', a2) and /30 phases as a function of cooling rate.
I. INTRODUCTION
II. EXPERIMENTAL
The intermetallic compound Ti3Al has three major properties, low density, high specific strength, and good chemical resistance to oxidation and corrosion.12 Thus, one application for this material may be the replacement of a superalloy like INCO718 in the fabrication of compressor disks for turbomachines in the temperature range 550-650 °C. However, the titanium aluminides are brittle mainly because of their ordered structure. In order to overcome this brittleness and improve its ductility and deformation modes, /6 phase stabilizing elements such as Nb are added to Ti3Al. This procedure results in a two phase a2/p0 microstructure at room temperature.3 In the a2 phase, two types of morphology structure are present, the a2 martensite with very fine needles, or the a2 phase in plates. To each of these morphologies is attributed the ordered hexagonal crystal structure a2 (DOi9).4 The fio phase is ordered with the B2 crystal structure. Anthony et al.5 present this structure by considering four sublattices (cubic face-centered). However, in titanium aluminides containing niobium two new phases are reported in the literature: the ordered orthorhombic phase O which is derived from the a2 phase 6 " and the co phase9 that forms in the /3o phase retained in the metastable state at room temperature. Today, most studies in /3 phase stabilizing elements are concerned with the influence of Nb.10"12 In this paper, we investigate the effects of molybdenum to Ti3Al; two compositions were considered: Ti-21.6Al-3.4Mo and Ti-24.6Al-3.4Mo at. %. Since the mechanical properties of titanium aluminides are strongly affected by their chemical composition and their microstructure, we first studied their structural transformations under different cooling rates.
The alloys we are considering were fabricated in small plates 80 x 50 x 10 mm3 by consumable electrode arc melting. Chemical compositions of the intermetallic compound Ti3Al and ternary aluminides are shown in Table I. The /3 solution treatment and cooling operations were conducted in an Adamel LK02 dilatometer (Adamel Instruments S.A., 15 Avenue Jean Jaures, Ivry-sur-Seine, 94203 Cedex, France). The specimens (section 2 x 2 mm2, length 12 mm) were placed in a protective pure helium atmosphere ( a2 + j80.
T°C
AL
100 200 300 400 500 600 700 800 900
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