TEM characterisation of fatigue tested lamellar Ti-44Al-8Nb-1B
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TEM characterisation of fatigue tested lamellar Ti-44Al-8Nb-1B H Jiang, D Hu* and I P Jones Department of Metallurgy and Materials and *IRC in Materials, The University of Birmingham, Edgbaston, B15 2TT UK ABSTRACT Detailed microstructural examination by TEM of fine-grained polycrystalline lamellar Ti-44Al-8Nb-1B after fatigue testing (R=0.1) at room temperature has been carried out. The operative slip systems were identified as 1/2 ordinary dislocations and 1/6 twinning. The results showed no strong relation between operative slip systems and macroscopic Schmid factor and it is believed the local stress conditions control the operation of the slip systems. Translamellar cracking was observed to be associated with fine transverse twins in the gamma lamellae.
INTRODUCTION Gamma (TiAl)-based titanium aluminides have received wide attention over the past two decades as possible high temperature aerospace materials due to their low density, excellent high temperature strength and oxidation resistance [1]. The fatigue behavior of TiAl-based alloys has been investigated extensively, mainly focused on the property-microstructure relationship. Many investigations of the fatigue crack propagation of γ-TiAl alloys [2-4] have shown that the fully lamellar microstructure presents superior crack growth resistance as compared to the other possible microstructures. It has been shown that deformation under fatigue conditions at room temperature is through the glide of mainly ordinary dislocations and via twinning in the TiAl phase [7-10]. Some efforts have been directed to relating the defect characteristics to the macro-stress condition in PST TiAl crystals because they offer simplicity in controlling the stress orientation [5]. However, such efforts have not been extended sufficiently into polycrystalline lamellar microstructures, although some work on local deformation behavior near cracks has been reported [6]. The purpose of this paper is to report a detailed TEM study on fatigue tested polycrystalline fine-grained lamellar Ti-44Al-8Nb-1B in an attempt to provide a general picture of the defects’ nature, density distribution and relation to the crystal orientations.
EXPERIMENTAL PROCEDURE The alloy, with a nominal composition of Ti-44Al-8Nb-1B (at.%), was prepared by double plasma cold hearth arc melting and cast into a 100mm diameter ingot weighing about 25kg. The oxygen content was about 700 wtppm. A section of the ingot was isothermally forged at 1150°C at a strain rate of 5x10-3 s-1 to a reduction in height of about 70%. The forged alloy was heat treated at 1310°C for 2 hours followed by furnace cooling to room temperature. The heat treatment gave rise to a near fully lamellar microstructure with about 12% grain boundary gamma grains. As-machined cylindrical samples with a diameter of 4mm and a gauge length of 20mm were subjected to fatigue testing at 500MPa (80% of the 0.2% proof stress) in air with a
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stress ratio R=0.1 at 82Hz. The volume fraction of gamma grains was examined by optical microscope (OM
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