The high-temperature synthesis of Al-Cr perform in microwave field
Krzysztof Naplocha, Kazimierz Granat Politechnika Wrocławska, Instytut Technologii Maszyn i Automatyzacji, ul. Łukasiewicza 5, 50-371 Wrocław, Poland
Quarterly No. 3, 2008 pages 236-241
DOI:
keywords: SHS, perform, Al-Cr, infiltration, composite, fibre
abstract Production method for composite materials reinforced with Al-Cr intermetallic compounds was elaborated. At the beginning proper amount of metallic powders were mixed to make specimens with stoichiometric ratio of Al to Cr ranges from 7:1 to 1:1. Next using cold isostatically pressure (CIPed) cylindrical specimens with a diameter of 23-mm and a height of 4.5 mm were prepared. The specimens were placed in microwave chamber to carry out sef-propagating high temperature synthesis. Quartz tube with flowing argon protected specimens from atmosphere and oxidization. Microwave energy was transferred from magnetron via standard waveguide and with the help of tuner focused on the specimen. After initiating high-temperature synthesis magnetron was turned off and reaction spread through entire specimen. Alumina blanket partly insulated reacted material, which were ignited by SiC susceptor. Using one-color optical pyrometer Raytek, model Marathon MM with 0.6 mm spot in diameter, profile of temperature during synthesis were measured and recorded. Due to the low exothermic character of the reaction synthesizing Al-Cr intermetallic compounds, cylindrical specimens were preheated to about 870 K. Analyze of profile for different compositions and stoichiometric ratio revealed that with increasing of Cr powder content maximum temperature is higher and heat from exothermic reaction is liberated in a shorter time. In specimens containing smaller amount of Cr, with stoichiometric ratio 7:1, propagation front was unstable and moved with two temperature gradient stops. Microscopic observation of fracture revealed porous morphology with rounded Al-Cr compound covered by small cuboid particles. In some cases when adiabatic conditions was not reached and heat leak rate was too high, cores of Cr particles were surrounded by layer of other Al-Cr compound. This layer was enriched by ca. 50% wt. of Cr. EDS analysis confirmed original concentration of chromium in the centre, which decreased toward outer perimeter. During synthesis enriched layer consumes Cr cores and spreads over external area with smaller Cr content. In order to synthesize Cr with Al completely, material should be insulated or higher amount of chromium powder embedded in aluminum matrix. Some of the synthesized materials with regular morphology and sufficiently open porosity were infiltrated with casting aluminum alloy. Using direct squeeze casting method and typical processing parameters, casting was reinforced with intermetallic structure. It confirmed usefulness of preforms and reduces their natural brittleness.