Research Of Fe-Al Intermetallic Ompounds By Reactive Sintering

In the present dissertation was studied the reactive sintering of Fe-Al, which includes thermodynamics and kinetics of reactive sintering, intermetallics formation and pore formation. On the base of the theory studies were further researched new methods by that high density Fe-Al intermetallics are achieved.The thermodynamics and kinetics were studied by differential thermal analysis. The heat of synthesis reaction of Fe3Al and other Fe-Al intermetallics was measured. The results showed that the heat of synthesis reaction is affected by Al content, heat rate, powder size, compacts density and so on. According to the results of kinetics study, factors influencing on the reactive sintering rate are powder size, green density, adiabatic temperature and sintering temperature and heating rate.The mechanism of Fe-Al intermetallic compounds formation and pores formation in the reactive sintering of Fe and Al element powders was studied with iron - aluminum diffusion couples. The couples were thermally treated respectively below and above the reactive sintering temperatures of iron and aluminum powder compacts. It was found that in the case below reactive sintering temperature, Al-rich intermetallics formed in the iron adjacent to the interface of iron and aluminum by alviminum diffusion and grew toward the inner of Fe grains. While in the case above reactive sintering temperature where a liquid was present it occurred Fe dissolution and diffusion of Fe and Al in the liquid and solid Fe. As a result the intermetallics formed in iron and aluminum separated by the interface. The intermetallics formation is a process involving diffusion and reaction steps. A large number of pores in the intermetallics were formed owing to the difference of Fe and Al in diffusivity, mole volume difference between reactants and products, heat motion and action of gases produced during the reaction. The diffusion coefficients of iron and aluminum and the activation energies of their combination reactions were obtained. The Al diffusion coefficient in solid Fe is larger than Fe in Al by two orders, which results in formation of majority pores in Al. The Fe diffusion coefficient in liquid Al is larger than Al in solid Fe. Therefore the majority of pores will produce in intermetallics during the reaction if Al has been melted. The mechanism of theintermetallic and pore formation in the diffusion couples is discussed.A new process i.e. reactive sintering of rolled Fe and Al powders compact was researched. The experimented results showed that in this process the Kirkadell effect was weakened, oxide film was broken and the action of transient liquid sintering was strengthened, so that the sinter density was markedly increased. The pores forming in reactive sintering can be effectively decreased by addition of the third element Ni. As a result of rolling combined with addition of Ni, a sintering product with theoretical density up to 99% was obtained. The Fe3Al prepared by rolling and reactive sintering has good mechanical properties and improved ductility. 'In this research the hot quase-isostatic compaction was done in common press. The experiments showed that it achieved the same results as the hot isostatic compaction. Exerting a pressure increased largely density of Fe3Al made by reactive sintering. In this research, has been developed a reasonable quase-isostatic compaction technology, by which 98% theoretical density of Fe3Al has been reached. The sinter products with significantly higher density, tensile strength and ductility have been obtained by combination quase-isostatic compaction with Ni addition.