Production and Characterization of Nanostructured Ti-Based Intermetallics
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ABSTRACT Mechanical alloying and plasma assisted sintering are used to produce nanocrystalline Ti-Al alloys. Microstructural characterization and measurement of mechanical properties are reported. The alloys investigated include AITi+X and Al 3Ti+X where X represents either Cr, Mn or Fe. Mechanical alloyed powders have a microstructure, which consist of an amorphous matrix containing small crystalline domains. Sintered specimens have a crystalline microstructure with nano-sized grains. Alloys based on AlTi-X are composed mainly of the phases a 2 (DO1 9 ) andy (Li 0 ) distributed homogeneously in the form of nanograins (100-300 nm) resembling a globular structure. Al3Ti-X alloys show a cubic ordered phase (L1 2) with grain sizes ranging from 16 to 40 nm. Vickers hardness measurements of sintered alloys show a considerable increase with respect to as-cast alloys. Compression tests show a high mechanical strength of these alloys, although in the case of A13Ti-X, no ductility is found. INTRODUCTION Intermetallic compounds are materials with interesting properties for high temperature applications [1]. The most promising intermetallic compounds for commercialization are Ti-based alloys because they offer significant improvement in properties over disordered alloys currently in use. However these intermetallic compounds are generally brittle at ambient temperature which limits their application. Modifying the crystal structure through the addition of a third alloying element can increase the ductility of some of these materials. In addition, synthesis of intermetallics with a fine grain size is likely to increase the ductility further [2]. Novel processing techniques such as mechanical alloying (MA), offer the potential of producing useful intermetallic materials engineered for specific applications [3]. Mechanical alloying does not involve high temperature melting but requires consolidation and/or further processing to create a final shape. This can introduce unwanted variations in the microstructure and properties of the material. Nevertheless the plasma assisted sintering technique (SPS) allows short holding times at high temperature and thus little microstructural variation is induced. This technique uses the phenomenon of microscopic electric discharges between particles under pressure [4]. EXPERIMENTAL PROCEDURE The starting materials are elemental powders of high purity (99.9%) and particle sizes smaller than 200 gtm. Mixtures of these powders have been prepared to produce the following materials (numbers refer to atomic percentages): Al67Ti25Fe8, A167Ti25Mns, AI67Ti25Crs, Ti5oAl50, TisoA148Cr2, Ti50A146Cr4, Ti50A148Mn 2 and Ti50 A146Mn 4.Mechanical alloying was carried out in a horizontal ball mill for various periods of time with a ball to powder weight ratio of 100:1. Methyl alcohol is added as a process control agent. The charge and discharge operations have been carried out in a glove box with a protective argon atmosphere. Sintering is performed by SPS at 1073 and 1373 K and by applying 50 MPa with a holding
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