LATERALS GEGE ELSEVIER Materials Science and Engineering A232( 1997)39-46 Kinetics of directed oxidation of Al-Mg alloys into Al,O3 preforms H. Venugopalan, T. DebRoy x Department of Materials Science and Engineering, Pennsylvania State University, University Park, PA 16802, USA Received 16 December 1996: received in revised form 4 March 1997 Abstract Synthesis of oxide matrix composites by the directed metal oxidation process offers significant advantages over traditional composite processing routes. Much of the previous work on directed oxidation has been focused on the understanding of the microstructural evolution during the process. In this work, growth kinetics of AlO3 Al composites through Al2, preforms has been studied. The mechanism of oxidation of Al-Mg alloys into Al2O, preforms has been investigated theoretically and experimentally. Analysis of the oxidation kinetics for various preform particle sizes and durations of oxidation demonstrates that he preform provides preferential sites for Al2O, nucleation. Furthermore, the weight gain rate increases with decreasing Al2o3 particle size. With increasing oxidation time, liquid metal transport to the oxidation front slows down and becomes a factor in controlling the weight gain rate. The oxidation rate of Al alloys into Al O, preforms can be tailored by the control of preform particle size. g 1997 Elsevier Science S.A Keywords: Kinetics; Directed oxidation; Al-Mg alloys 1. Introduction heat exchangers, and furnace components [6] In the directed melt oxidation(DIMOX)process, a elements like Mg or Zn is crucial for directed oxidation molten aluminum alloy is oxidized to produce ceramic/ of aluminum alloys lo take place [1, 7]. Dopants like Mg metal composites [1]. Under appropriate conditions of or Zn are believed to hinder the formation of a protec- temperature, oxygen pressure, and alloy composition, tive alumina film on the alloy surface and thus allow apid reaction of the molten alloy with the oxidant to continued oxidation of the alloy. Additional elements form a-alumina occurs and the reaction product grows such as Si arc usually added to improve alloy/preform outward from the original metal surface. In many cases, compatibility. These dopants can be either applied to the reactio sustained by the transport of liquid the surface of the aluminum exposed to the oxidant or metal through the reaction product [l]. The resulting if soluble, alloyed with the parent metal. Three distinct product is an Al2 O3/Al composite with an intercon- stages can be observed in the oxidation of Al-Mg nected network of unoxidized metal [2]. Reinforced alloys at a given temperature [8]. When Al-Mg alloys mposites with the desired structural properties can are heated in argon to a given temperature and then obtained by growing the composite into preforms con- exposed to an oxygen atmosphere, tial stage of sisting of reinforcing whiskers or fibers of AlO3 and rapid weight gain occurs [8]. During this SiC [3-5] Composites made by directed oxidation can forms by oxidation of Mg vapor and, subsequently,it be tailored to have good toughness, thermal shock falls back on to the melt surface [9 Formation of a resistance, wear resistance, high stiffness, and high ter thin, dense layer of MgAl,O beneath the MgO perature stability. They are being used or evaluated for the initial stage of oxidation and corresponds to the use in turbine engine components, armor applications, start of incubation [8 ] During incubation, metal chan nels are observed to form in the spinel. The arrival of Corresponding author. Tel +1814 8651974; fax: +1 814 these metal channels at the top of the spinel believed 8652917 to correspond to the end of incubation and the start of 921-5093/97/S17.00@ 1997 Elsevier Science S.A. All rights reserved. PIS0921-5093(97)00088-9MATERIALS SCIENCE & ENGINEERING A ELSEVIER Materials Science and Engineering A232 (1997) 39-46 Kinetics of directed oxidation of Al-Mg alloys into A&O3 preforms H. Venugopalan, T. DebRoy * Department of Materials Science ma Engineering, Pennsylrania State University, University Park, PA 16802, USA Received 16 December 1996; received in revised form 4 March 1997 Abstract Synthesis of oxide matrix composites by the directed metal oxidation process offers significant advantages over traditional composite processing routes. Much of the previous work on directed oxidation has been focused on the understanding of the microstructural evolution during the process. In this work, b orowth kinetics of Al,O,/Al composites through Al,O, preforms has been studied. The mechanism of oxidation of Al-Mg alloys into AlLO preforms has been investigated theoretically and experimentally. Analysis of the oxidation kinetics for various preform particle sizes and durations of oxidation demonstrates that the preform provides preferential sites for A&O, nucleation. Furthermore, the weight gain rate increases with decreasing Al,O, particle size. With increasing oxidation time, liquid metal transport to the oxidation front slows down and becomes a factor in controlling the weight gain rate. The oxidation rate of Al alloys into Al,O, preforms can be tailored by the control of preform particle size. 0 1997 Elsevier Science S.A. Keywords: Kinetics; Directed oxidation; Al-Mg alloys 1. Introduction In the directed melt oxidation (DIMOX) process, a molten aluminum alloy is oxidized to produce ceramic/ metal composites [l]. Under appropriate conditions of temperature, oxygen pressure, and alloy composition, a rapid reaction of the molten alloy with the oxidant to form a-alumina occurs and the reaction product grows outward from the original metal surface. In many cases, the reaction is sustained by the transport of liquid metal through the reaction product [l]. The resulting product is an Al,O,/Al composite with an intercon￾nected network of unoxidized metal [2]. Reinforced composites with the desired structural properties can be obtained by growing the composite into preforms con￾sisting of reinforcing whiskers or fibers of Al,O, and SIC [3-51. Composites made by directed oxidation can be tailored to have good toughness, thermal shock resistance, wear resistance, high stiffness, and high tem￾perature stability. They are being used or evaluated for use in turbine engine components, armor applications, *Corresponding author. Tel.: + 1 814 8651974; fax: + 1 814 8652917. 0921-5093/97jS17.00 8 1997 Elsevier Science S.A. All rights reserved. PII SO921-5093(97)OOOSS-9 heat exchangers, and furnace components [6]. It is now recognized that the presence of volatile elements like Mg or Zn is crucial for directed oxidation of aluminum alloys to take place [1,7]. Dopants like Mg or Zn are believed to hinder the formation of a protec￾tive alumina film on the alloy surface and thus allow continued oxidation of the alloy. Additional elements such as Si are usually added to improve alloy/preform compatibility. These dopants can be either applied to the surface of the aluminum exposed to the oxidant or, if soluble, alloyed with the parent metal. Three distinct stages can be observed in the oxidation of Al-Mg alloys at a given temperature [8]. When Al-Mg alloys are heated in argon to a given temperature and then exposed to an oxygen atmosphere, an initial stage of rapid weight gain occurs [8]. During this period, MgO forms by oxidation of Mg vapor and, subsequently, it falls back on to the melt surface [9]. Formation of a thin, dense layer of MgA1204 beneath the MgO halts the initial stage of oxidation and corresponds to the start of incubation [8]. During incubation, metal chan￾nels are observed to form in the spinel. The arrival of these metal channels at the top of the spine1 is believed to correspond to the end of incubation and the start of