Mechanochemical processing of nanocrystalline Ti-6Al-4V alloy
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I.
INTRODUCTION
M E C H A N O C H E M I C A L processing is a novel and cost-effective method of producing a wide range of nanopowders. It involves the use of conventional high-energy ball mill to initiate chemical reactions and structural changes. Ill Production of nanoparticles using mechanochemical processing requires use of suitable precursors. I21 Very often, reagents such as oxides, carbonates, sulfates, chlorides, fluorides, hydroxides, etc. are used as the precursor material. The milling action influences the reaction kinetics in a reacting powder mixture as a result of the intimate mixing and refinement of the grain structure to the nanometer scale, allowing the reaction to occur during the actual milling. Chemical reactions, which normally require high temperatures, can thus be activated during room-temperature milling. The resulting product is formed as individual nanometersized grains in a by-product matrix in relatively short time. The nanopowder is then recovered by leaching the by-product using suitable solvents. Is'4'slMilling is occasionally followed by a low-temperature heat treatment to ensure the reaction is complete before the by-product is removed, leaving the pure, nonagglomerated nanopowder. Figure 1 illustrates how the mechanochemical process can circumvent the multistage process normally used to produce titanium. I61 Mechanochemical Processing of Titanium-Based Materials
McCormick et al. were the first to successfully demonstrate the use of mechanochemical processing (MCP) to synthesize titanium from TIC14 with magnesium as a reducing agent. Ivl Using magnesium as a reducing agent, the milling had to be carried out for an extended period of time for the process to reach completion. According to a recent study, premilling of magnesium is known to expedite the MCP of titanium; a more drastic reduction in milling time is achieved by replacing magnesium with CaH~.[sl The study also suggests that use of Call2
M.D.S. PIRZADA, Graduate Student, Department of Materials and Met allmgical Engineering, S.N. PATANKAR, Assistant Director, Institute for Materials and Advanced Processes, and F.H. (SAM) FROES, Director, Institute for Materials and Advanced Processes, and Chair, Department of Materials and Metallmgical Engineering, are with the University of Idaho, Moscow, ID 83844 3026. Contact e mail: [email protected] Manuscript submitted Febinary 18, 2002. METALLURGICALAND MATERIALSTRANSACTIONSA
in combination with magnesium makes the MCP of titanium less explosive and more controllable. CaHJMg has been successfully used as a reducing agent to synthesize titanium-based advanced lightweight materials such as Ti-A1 by co-reduction of chlorides of constituent components. [9,1°1This article is an overview of our recent work on mechanochemical processing of nanocrystalline Ti-6A1-4V alloy.
II.
EXPERIMENTAL PROCEDURE
Call2, Mg powder, anhydrous A1C13, anhydrous VC13, and TIC14 were used as the starting materials. NaC1 was used as a reaction diluent. The charge comprised of 10 g of reactant mixtures
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