Crack formation in sintered two-phase alloys

Al/Al₂O₃ alloys are characterised by a pronounced high-temperature brittleness which is not yet fully understood. The authors postulated that microcrack formation plays a fundamental role in determining the ductility in tensile and creep tests of these alloys. To investigate the appearance and generation of cracks, density measurements and electron microscope examinations were carried out on samples broken in tensile tests (20 to 620° C temperature range) and in accelerated creep tests (450° C) of two Al/Al₂O₃ alloys (SAP-ISML and Fibroxal). Some investigations were also made on samples deformed by swaging.The experimental results (morphology, dimensions, distribution and volume of microcracks in the broken samples as a function of temperature and deformation time) confirmed the crack-formation hypothesis and showed that during the deformation of sintered aluminium two-phase alloys, voids may be generated by two mechanisms. The first involves the opening of pre-existing pores which are due to imperfect bonding between matrix and particles, the second involves the creation of new cracks by dislocations blocked at second phase particles.