Microstructure characterization and creep deformation of an Al-10 Wt Pct Ti-2 Wt Pct Cu nanocomposite
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I. INTRODUCTION
MANY recent articles have reported attractive physical and mechanical properties for nanocrystalline materials, as well as the development and optimization of techniques to process them.[1–8] A challenge has been to prepare bulk nanocrystalline samples free of defects and contaminants.[9] Many of the techniques use powder metallurgy (P/M) routes to produce nanometer or submicrometer grains in bulk samples. The P/M techniques, however, do not result in truly pure materials. The approach is therefore to carefully characterize the oxides and other impurities and incorporate them in the analysis of the experimental results. Very often the superposition of several effects makes the analysis complex. In the current article, we study an Al-10 wt pct Ti-2 wt pct Cu nanocomposite produced by mechanical milling in a liquid nitrogen bath. The steps involved in the preparation of the material are described from the details of cryogenic milling to the consolidation of the milled powder into a bulk dense nanocomposite. The chemical composition of the material is used to calculate the volume fraction of the different particulates. The creep deformation of the cryomilled and consolidated Al-10 wt pct Ti-2 wt pct Cu is investigated at temperatures from 533 to 644 K (260 °C to 371 °C) and at stresses from 55 to 117 MPa. Typically, the creep strength of an alloy at a given temperature decreases with decreasing grain size, as evidenced by the extensive work reported on superplasticity. Furthermore, creep strength is often extremely low in aluminum alloys, due in large part to their low melting points. However, the improved properties of oxide-dispersion-strengthened (ODS) aluminum alloys and of discontinuously reinforced aluminum (DRA) metal matrix composites together with the increasing importance of superplastic forming in industry have created a R.W. HAYES, President, is with Metals Technology Inc., Northridge, CA 91324. P.B. BERBON, Researcher, is with Rockwell Scientific Company, Thousand Oaks, CA 91360. Contact e-mail: [email protected] R.S. MISHRA, Professor, is with the University of Missouri, Rolla, MO 95409-0340. Manuscript submitted November 20, 2003. METALLURGICAL AND MATERIALS TRANSACTIONS A
need for the study and understanding of aluminum-based materials at elevated temperatures. II. MATERIAL PROCESSING AND EXPERIMENTAL METHODS The starting material is prealloyed Al-10 wt pct Ti-2 wt pct Cu (Al-Ti-Cu) powder purchased from Valimet, Inc. (Stockton, CA). The raw powders were inert gas atomized from a mixture of aluminum, titanium, and copper, and sieved to under 50 m (250 mesh). The typical appearance of such powders can be seen in Figure 1, where the specimen was prepared by sprinkling powder over a fine carbon conductive tab. The powder was mechanically milled in a high-energy attritor while in a liquid nitrogen (LN2) bath. A laboratory scale Szegvari Model 1-S attritor manufactured by Union Process and installed at Rockwell Scientific (Thousand Oaks, CA) was used. This system processes batches in 1-kg
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