Effect of lamellar structure on microhardness and yield stress of directionally solidified intermetallic Ti-46Al-2W-0.5Si alloy

Abstract

In recent years ordered TiAl based intermetallic alloys have been extensively studied as potential hightemperature materials in the gas turbine industry [1]. Of the numerous microstructures that can be developed in TiAl-based alloys, the fully lamellar or nearly lamellar microstructures consisting of TiAl (γ -phase and Ti3Al (α2-phase) are very promising for structural applications. Mechanical properties of fully lamellar polycrystalline TiAl alloys depend on the grain size, lamellar thickness, interlamellar spacing and volume fraction of coexisting α2and γ -lamellae [2]. In the case of directionally solidified (DS) TiAl alloys, mechanical properties such as yield stress, tensile elongation, fracture toughness and creep behavior were found to depend strongly on lamellar orientation to loading axis and interlamellar spacing [3–5]. A good combination of room temperature strength, ductility, toughness and creep strength can be achieved when the lamellar orientation is aligned parallel to the tensile direction. The aim of the present work is to study the effect of lamellar structure on room-temperature yield stress and microhardness of directionally solidified Ti46Al-2W-0.5Si (at.%) alloy. The alloy was developed by Nazmy and Staubli [6] for investment cast turbine blades with improved creep properties. Unlike previous studies on mechanical properties of DS TiAl based alloys, this study is focused on fully lamellar alloy reinforced with a constant volume fraction of hard Al2O3 particles. The alloy with the chemical composition Ti-46Al2W-0.5Si (at.%) was provided by Alstom Ltd in the form of cast cylindrical ingots with a diameter of 21 mm and length of 210 mm. Directional solidification was performed in Al2O3 molds (mean grain size of 9.3 μm and purity of 99.5%) of 11/15 mm diameter (inside/outside diameter) under argon atmosphere in a modified Bridgman-type apparatus [7]. All ingots were prepared at a constant growth rate of 1.18 × 10−4 ms−1 using experimental procedure described elsewhere [7, 8]. After directional solidification the ingots were heated to solution annealing temperature of 1633 K at constant heating rate of 0.28 K s−1, stabilized at this temperature for 1 h and then cooled to 973 K at constant cooling rates varying in the range from 0.078 to 0.61 K s−1. Further cooling to room temperature was accomplished by gas fan cooling. All annealing experiments were performed in dynamic argon atmosphere. After heat treatment Vickers microhardness measurements with applied load of 0.64 N were performed on longitudinal sections of specimens. Rectangular compression specimens with dimensions of 4 × 4 × 8 mm were cut from the ingots by electric discharge machining. Statistical evaluation of all compression specimens (8–10 specimens tested for each regime) revealed that the mean angle between lamellar orientation and the compression axis varied in the range 8 ◦ to 15 ◦ and 56 ◦ to 67 ◦ for the specimens with the compression axis perpendicular and parallel to the growth direction, respectively. Roomtemperature compression tests were performed at an initial strain rate of 1.39 × 10−3 s−1 in air using a screw driven universal testing machine Instron. The compression offset yield stress was measured at 0.2% plastic strain. Microstructural analysis was performed by optical microscopy (OM) and scanning electron microscopy (SEM). Quantitative metallographic analysis was performed on digitalized micrographs using a computerized image analyser. The microstructure of DS ingots consisted of columnar grains (3–5) aligned parallel to the growth direction. The microstructure within the columnar grains was dendritic, as shown in Fig. 1. The as-grown ingots contained regular well-aligned α2 (Ti3Al) and γ (TiAl) lamellae. A small volume fraction (about 1 vol%) of elongated B2-particles (ordered β-phase) was formed at the columnar grain boundaries and in the interdendritic region. Fine Ti5Si3-precipitates were observed at the lamellar interfaces and along the grain boundaries [9]. In addition, the as-grown ingots contained constant volume fraction of Al2O3 particles (1.5 vol%), which were formed in-situ due to a reaction between the alumina mold and the melt [8]. The ingots contained tree types of Al2O3 particles (dark color phase in Fig. 1): nearly spherical, needle-like and clusters of particles. The size of particles characterized by a mean value of minor axis calculated from statistical log-normal distribution function was 6 μm. Heat treatment of DS ingots consisting of solution annealing and subsequent cooling at constant rates ranging from 0.078 to 0.61 K s−1 resulted in formation of regular lamellar structure. Fig. 2 shows an example of typical regular lamellar microstructure formed in all heat-treated ingots. In addition, as in the case of the as-grown ingots, the heat-treated ingots contained elongated B2-particles, fine Ti5Si3-precipitates and constant volume fraction of Al2O3 particles (1.5 vol%).