Experimental study and thermodynamic re-assessment of the Al–B system

• Published in: Journal of alloys and compounds Vol. 384; no. 1-2; pp. 168 - 174
• Main Authors: Mirković, Djordje, Gröbner, Joachim, Schmid-Fetzer, Rainer, Fabrichnaya, Olga, Lukas, Hans Leo
• Format: Journal Article
• Language: English
• Published: Lausanne Elsevier B.V 14.12.2004; Elsevier
• Subjects: Borides; Calorimetry; Condensed matter: structure, mechanical and thermal properties; Exact sciences and technology; Intermetallics; Phase diagram; Physics; Thermal analysis; Thermal properties of condensed matter; Intermetallics; Calorimetry; Borides; Thermal analysis; Phase diagram; Differential scanning calorimetry; Crystal twin; Powders; Melting; Cooling; Enthalpy; Sublimation; Heat flow; Entropy; XRD; Eutectoids; Heat treatments; Heat flux; Heat of formation
• ISSN: 0925-8388; 1873-4669
• DOI: 10.1016/j.jallcom.2004.04.100

Controversial thermodynamic data in previous assessments of the Al–B system were used to identify a key experiment regarding the enthalpy of formation of AlB2 and its peritectic decomposition temperature. Experiments were performed using AlB2 powder purified by vacuum sublimation. Differential scanning calorimetry (DSC) in a heat-flux twin cylindrical Calvet-type calorimetric system was utilized to measure the enthalpy of decomposition and the reaction temperatures. Samples were characterized by X-ray powder diffraction (XRD). Based on that, a comprehensive and consistent re-assessment of the Al–B system was performed. The incongruent melting of AlB2 occurs at 972°C and a eutectoid decomposition of AlB2 at 213°C is predicted from the thermodynamic description. The present enthalpy data are supported by realistic values of absolute entropies of solid Al–B phases. The formation of AlB2 was never observed on cooling, it is extremely sluggish. Upon heating AlB2, the peritectic reaction does not produce perfectly crystalline AlB12. Based on that and constraints in the thermodynamic data, it is derived that the stability range of AlB2 in equilibrium with perfect crystalline AlB12 may be even more narrow than 972–213°C.