Effects of interstitial oxygen on microstructure and mechanical properties of Ti-48Al-2Cr-2Nb with fully lamellar and du
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TiAl alloys are considered promising for a wide range of intermediate- to high-temperature applications because of their combination of interesting properties (low density, high specific strength, and good oxidation resistance). A considerable research effort has been carried out to establish the composition-microstructure-property relationship, notably in order to find an acceptable hightemperature strength–room-temperature ductility compromise. In particular, the Ti52Al48 (usually called Ti48Al) composition is considered to be particularly attractive in terms of ductility at room temperature.[2] Furthermore, for potential applications, substitutional alloying elements, such as chromium[3] and niobium,[4] are added to further improve ductility and oxidation resistance, respectively. For the Ti-48Al composition, essentially two phases occur: at high-temperature a (disordered hcp) and at lower-temperature a2 (DO19, ordered hcp) and g (L10, ordered fcc). On cooling, the phase transformation from a essentially leads to two types of microstructure, i.e., fully lamellar with grains consisting of an arrangement of a2 and g lamellas (with {0001}hcp//{111}fcc) and duplex, where lamellar grains coexist with equiaxed phase g grains. Both the fully lamellar and duplex microstructures lead to suitable compromises between high-temperature strength, creep, and room-temperature ductility. The duplex microstructure is significantly more ductile at room temperature than the fully lamellar microstructure. The fully lamellar microstructure, while less ductile, is not only tougher than the duplex microstructure but also more resistant to creep.[5,6] [1]
M. LAMIRAND, J.-L. BONNENTIEN, and S. GUE´RIN, are with the Centre d’Etudes de Chimie Me´tallurgique, Institut des Sciences Chimiques Seine Amont, CNRS, 94407 Vitry-sur-Seine, France. G. FERRIE`RE, is with the Chaire des Mate´riaux Industriels Me´talliques et Ce´ramiques, Conservatoire National des Arts et Me´tiers, 75003 Paris, France. J.-P. CHEVALIER, Professor, Centre d’Etudes de Chimie Me´tallurgique, Institut des Sciences Chimiques Seine Amont, is with Chaire des Mate´riaux Industriels Me´talliques et Ce´ramiques, Conservatoire National des Arts et Me´tiers. Contact e-mail: [email protected] Manuscript submitted February 11, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
Titanium and its alloys have a strong affinity to oxygen (e.g., Reference 7). Oxygen is present in TiAl alloys in significant concentrations, usually around 1000 wt ppm (;2400 at. ppm for the alloys studied) or more (e.g., References 8 through 11). Oxygen atoms occupy the octahedral interstitial sites in the close-packed structures. The volume of these sites is the same in both the L10 and DO19 structures. Nevertheless, energy-compensated tomographic atom probe (TAP) studies clearly show that the oxygen atoms in these alloys are preferentially located in the a2 phase. Indeed the TAP results show that strong partitioning occurs between g (L10 structure) and a2 (DO19 structure) phases for binary,[8] ternary, and q
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