Characteristics of nanophase TiAl produced by inert gas condensation
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Nanophase TiAl, with grain sizes in the range of 10-20 nm, was synthesized by magnetron sputtering in an inert gas atmosphere and consolidated, in situ, under vacuum. The properties of the powders and sintered compacts were studied by transmission electron microscopy, scanning electron microscopy, calorimetry, Rutherford backscattering, and x-ray diffraction. Samples compacted at 1.0 GPa at room temperature had a large fraction of amorphous phase, while samples compacted at the same pressure and 250 °C were predominantly the equilibrium y phase. An enthalpy change of 22 kJ/g-atom was measured during a DSC scan over the temperature range 125-450 °C, which is approximately the range over which crystallization occurs. Nearly full density could be achieved by sintering at 450 °C without significant, concomitant grain growth. The Vickers microhardness of these samples at room temperature and at —30 CC revealed an inverse Hall-Petch relationship at small grain sizes, 10-30 nm, and the usual Hall-Petch behavior at larger grain sizes. A small component of indentation creep was also observed. The maximum hardness is ~ 4 times larger than that of a cast TiAl specimen of the same composition. The Vickers hardness was also observed to decrease rapidly with temperature above =200 "C.
I. INTRODUCTION In the last several years, intermetallic compounds have been the subject of considerable interest for a new generation of high temperature structural materials. These compounds of metals have crystal structures that are often different from those of the component metals, and because of the strong bonding of the unlike atoms, they have useful mechanical and chemical properties. For aerospace applications, the titanium or aluminum based intermetallic compounds are particularly attractive since they are characterized by high melting temperatures, low densities, high strengths, and good oxidation resistances.1 However, there is a limitation in the forming methods of these alloys because of their brittleness at low temperatures. Many attempts to improve ductility of intermetallic compounds have been made by micro-2 or macro-alloying3 or microstructural control through fiber strengthening.4 It has also been found that single crystals as well as polycrystals with very fine grain size exhibit a fair degree of ductility.5'6 By a modification of the process used to produce vapor deposited thin-film structures, ultra-fine-sized (nanophase) powders7 can be produced by condensing metal vapor in an inert gas. More recently, much interest has developed in synthesizing and processing of nanoscale, 3-50 nm grain size, bulk materials by consolidating the condensed powders. Because of their high potential for useful properties, several nanophase ceramics and metals have been studied. However, despite 2962 http://journals.cambridge.org
J. Mater. Res., Vol. 7, No. 11, Nov 1992 Downloaded: 16 Mar 2015
this promising research, little effort has been directed, as yet, toward producing nanophase intermetallic compounds. The possibility of superplastic fo
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