ISSN 1392–1320 MATERIALS SCIENCE (MEDŽIAGOTYRA). Vol. 9, No. 1. 2003 The Formation of Structure and Change of Properties During Heat Treatment of Boron Carbide Composites L. Kommel ∗ , R. Veinthal, E. Kimmari Department of Materials Technology, Tallinn Technical University, Ehitajate tee 5, Tallinn 19086, Estonia Received 10 September 2002; accepted 30 January 2003 In this work, we present a new method of heat treatment to change structure and properties of boron carbide composites. During self-propagating high temperature synthesis (SHS-process), pressing and heat treatment of composites on base of boron carbide and aluminum alloy refractory compounds with specific properties were formed. These new formed nano- and microstructured refractory compounds change the structure and as a result, mechanical, physical and protective properties of the composites. Keywords: boron carbide based composites, self-propagating high temperature synthesis, structure and properties. 1. INTRODUCTION ∗ Boron carbide is chemically inert, excellent light- weight and hard technical material [1]. It has high thermal neutrons attenuation factor [2], and high elasticity. At present time production of the boron carbide composites or cermets is limited, because the boron carbide grains do not wet with metal of binder phase at sintering temperatures. Only by using self-propagating high temperature synthesis (SHS) process, the manufac- turing of composites or cermets on base of boron carbide and aluminium alloy powders is possible [3]. The formed structure and as a result, the properties changes are possible only during heat treatment before [4] or after [5] SHS-process. In this work we study the composites structure formation and properties changes depending on temperature, time and environment of the heat treatment. 2. EXPERIMENTAL In this work the composites were produced from boron carbide and aluminum alloy powders by self-propagating high temperature synthesis (SHS) with hot pressing and subsequent step-by-step heat treatment under high temperatures up to 1500 °C in different environments and in vacuum. The specimens for testing were produced from the SHS-processed polished billets with dimensions of 6 × 12 × 20 mm and 5 × 6 × 19 mm. The composite structure was studied by optical (Nikon CX) and scanning electron microscopes (JOEL JSM-840 A). X-ray diffraction analyser (D5005, Bruker AXS) was used for determination of phase composition. The mecha- nical properties of composites were measured by universal hardness tester (Zwick Z 2,5/TS1S). The Rockwell and Vickers hardness testers were used for hardness testing of composites. For the microhardness testing the “Mikromet- 2001” was used. To define yield stress and flexural modulus by three-point, the bend testing on the Instron Corporation Series IX Automated Materials Testing System INSTRON-8516 was used. 3. RESULTS After SHS-process the microstructure of boron carbide cermet is typical, as for cermets on tungsten or titanium carbide base (Fig. 1. a). This microstructure consists of grains of hard phase and binder phase between these grains. The boron carbide grains have a view of white angular grit. These grains are ambient from boron aluminum carbide thin layers. Between these grains aluminum alloy is filled as the binder metal. The structure of composite is shown in Fig. 1. a. The chemical composition of this material is shown in Fig. 2. (curve A). Only boron carbide (1), aluminum (2) and graphite (3) can be seen. After the heat treatment of densified powder at temperatures over 600 °C the only Al 4 BC was formed on the boron carbide and aluminum base. By heat treatment of densified powder at temperature up to 800 °C the substantially free AlB 12 , AlB 12 C 2 and Al 4 C 3 were formed. After SHS-process and heat treatment at temperature of 1080 °C during 5 – 10 h BN, Al 4 C 3 and Al 3 BC chemical compounds (Fig. 2., curve B) were formed in boron carbide/aluminum composites. As result of temperature increase up to 1300 – 1500 °C hard phase carcass from this refractory compounds (Fig. 1, c) was formed. These compounds were aluminum oxide (Al 2 O 3 ) and aluminum boron carbide (Al 8 B 4 C 7 ), as shown in Figure 2 (curve D). This structure changes from initial boron carbide aluminum content (a) up to refractory carbides formation (b) and the hard carcass formation (c). Curve A (initial) corresponds to the heat pressed powders of boron carbide and aluminum alloy; curve B – after SHS-process and heat treatment at temperature 1080 °C during 5 hours; curve C – after heat treatment in vacuum at temperature 1150 °C during 30 minutes; curve D – after dual-temperature heat treatment at 1150 and 1300 – 1500 °C during 15 minutes. 99 ∗ Corresponding author. Tel.: + 372-620-3356; fax.: +372-620-3196. E-mail address: kommel@edu.ttu.ee (L. Kommel)