Submicron-grained multiphase TiAlSi alloys: Processing, characterization, and microstructural design
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G. Fanta and T. Klassen Institute for Materials Research, GKSS Research Center Geesthacht, D-21502 Geesthacht, Germany
R. Bormann AB Material Science, Technical University Hamburg–Harburg, D-21073 Hamburg, Germany, and Institute for Materials Research, GKSS Research Center Geesthacht, D-21502 Geesthacht, Germany (Received 7 December 2000; accepted 3 April 2001)
Prealloyed powders of the intermetallic ␥–TiAl phase and the ceramic –Ti5Si3 phase were high-energy milled and hot-isostatically pressed (HIP) to produce silicide dispersed composite materials with grain sizes in the submicron and nanometer range. The amorphous state of the as-milled powders crystallizes via a multistep decomposition reaction during degassing at 440 °C and HIP. At a pressure of 200 MPa HIP-temperatures as low as 750 °C are sufficient for a complete densification of the milled powder. The microstructure of the compacts is very homogeneous and consists of equiaxed ␥–TiAl crystals and –Ti5(Si,Al)3 particles. Depending on the silicon content, these particles are interspersed within the grain boundary network of the ␥–TiAl phase or dispersed inside the ␥ grains. With respect to technical applications, submicron-grained composites are regarded as promising precursor materials that should allow for easy hot working in the as-prepared state as well as for high-temperature structural applications after a suitable transformation of the microstructure.
I. INTRODUCTION
During the last two decades, nanocrystalline materials have attracted much attention due to their unique physical and mechanical properties attributed to their special atomic structure involving a significant fraction of interfacial atoms with an arrangement deviating from the rigorous periodicity inside a single crystal.1–3 However, the nontrivial preparation methods of these solids run the risk of incorporating processing-related artefacts, which may be difficult to separate from the inherent properties of the material. Accordingly, it was soon discovered that the prefix “nano” is insufficient for classifying these materials and their microstructures.4 Samples produced by a multistep procedure consisting of powder generation and subsequent consolidation were shown to contain pores as an additional structural component besides crystalline and interfacial constituents.5 In a thermodynamic sense, the grain boundaries of cold compacted specimens may be regarded as metastable or even unstable, capable of relaxing at slightly elevated temperatures6 or even at room temperature.7,8 Furthermore, most of the powder metallurgical processing routes involve the risk of contamination due to the large surface of loose powders. 1850
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J. Mater. Res., Vol. 16, No. 6, Jun 2001 Downloaded: 17 Mar 2015
All these phenomena—pores, impurities, instabilities— may particularly affect the mechanical behavior of ultrafine-grained materials.7,9,10 Therefore, this study is mainly focused on the preparation of clean powders that were consolidated to fully dense compacts at elevated
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