High-temperature stability of nanocrystalline structure in a TiAl alloy prepared by mechanical alloying and hot isostati
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High-temperature stability of nanocrystalline structure in a TiAl alloy prepared by mechanical alloying and hot isostatic pressing O. N. Senkov and N. Srisukhumbowornchai Institute for Materials and Advanced Processes (IMAP), University of Idaho, Moscow, Idaho 83844-3026
¨ ¸oglu M. L. Ovec Istanbul Technical University, Maslak 80626, Istanbul, Turkey
F. H. Froes Institute for Materials and Advanced Processes (IMAP), University of Idaho, Moscow, Idaho 83844-3026 (Received 31 March 1998; accepted 14 July 1998)
A fully dense nanocrystalline compact of the Ti–47Al–3Cr (at. %) alloy was produced by mechanical alloying and hot isostatic pressing at 725 ±C. Microstructure characteristics and grain growth behavior of this compact were studied after annealing for up to 800 h in the temperature range of 725 to 1200 ±C, using analytical transmission electron microscopy techniques. The temperature and time dependencies of the grain sizes and the grain size distributions were determined. The grain growth occurred, with a timeand temperature-invariant single-peak grain size distribution (when normalized by the mean grain size), which was consistent with normal grain growth. The experimentally measured grain growth exponent decreased from 10 to 4.6 when the temperature was increased. The grain growth kinetics was described by a single thermally activated rate process limited by a permanent pinning force on the grain boundaries. The microhardness decreased on annealing and followed the Hall–Petch relationship with the parameters Hyo 5.8 GPa and KH 1.6 MPa ? m0.5 .
I. INTRODUCTION
Alloys based on the g –TiAl intermetallic are attractive for high temperature applications because of their high specific strength and modulus retention combined with good oxidation and creep resistance at elevated temperatures.1 However, they exhibit poor ambient temperature ductility and fracture toughness which has slowed widespread applications of these materials. Microstructural refinement via ternary alloying additions and far-from-equilibrium processing methods can improve these properties.1–3 It has been recently established that amorphous phases can be produced in TiAl-based intermetallics during mechanical alloying.4–7 Once the amorphous phases are synthesized, reliable methods need to be established to consolidate these amorphous materials. The interest lies in producing a nanocrystalline structure in the compacted materials by crystallization of the amorphous phase in controlling conditions. A decrease in grain size from the micrometer to the nanocrystalline range leads to a significant increase in the fraction of atoms residing at grain boundaries.8 Hence, the contribution from grain boundaries to the overall properties of the material increases. The atomic structure of the nanocrystalline material is modified through the introduction of defects, strain fields, or shortJ. Mater. Res., Vol. 13, No. 12, Dec 1998
range correlated displacements of atoms from their ideal crystal lattice positi
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