Study On Pressureless Reaction Sintering Al₃Ti/Mg Composite

In this paper,the thermodynamics of Mg-Al-Ti system was calculated and analyzed by Miedema's model.Al₃Ti/Mg composite was fabricated through pressureless sintering of Mg-Al-Ti system.XRD,SEM,EDS and TEM were used to analyze phases, microstructure and composition of composite.And the mechanical properties and wear resistant behavior of the composite were also determined.The study of the reactive kinetics of Mg-Al-Ti system and the effect of system's parameters and sintering rule on the microstructure and properties are the emphasis.And discussed reactive formation mechanism of Mg-Al-Ti system.The calculated results of Miedema's model and Gibbs' free-energy show that Al-Ti reaction is primary in Mg-Al-Ti system and Al₃Ti intermetallics could be formed in Al-Ti reaction by Gibbs' free-energy calculation.And Al₃Ti/Mg composite was fabricated successfully through optimization of sintering process.The experimental results show that Al₃Ti/Mg composite posses with dense microstructures and composites are characterized by a relatively homogeneous distribution of Al₃Ti particulates with a size range of 2～5μm.The distribution of Al₃Ti particulates is more homogeneous with Al₃Ti volume fraction increased.And the best sintering process is that the temperature is 720～800℃and the holding time is 90～120min.With the increasing of Al₃Ti volume fraction,the coefficient of thermal expansion of Al₃Ti/Mg composite is decreased and the hardness,strength and wear resistance is improved.When Al₃Ti volume fraction is 20～30Vol%,the strength and toughness and wear resistance of composite is higher than AZ91D alloy.It is found that the reaction of Mg-Al-Ti system is simimarly those of Al-Ti system.Mg is a catalytic agent and a diluent as well in the reaction.Mg shifts the reaction of Al and Ti to the lower temperatures and delay reactive time.The additional amount of Al is needed for eliminating residual Ti and solid solution strengthening for Mg matrix.The suitable addition amount is 6～10wt.%.The reactive formation mechanism of Mg-Al-Ti system is summarized. As reaching around melting points of Al and Mg,Al and Mg start to melt.Meanwhile Ti particles start to dissolve in the melt and start to reaction.The reactions may be 3Al(I)+[Ti]→Al₃Ti(s),Al₃Ti(s)+[Ti]→AlTi(s)and AlTi(s)+Al(I)→Al₃Ti(s)respectively. Because of the lowest Gibbs' free-energy of Al₃Ti,Al₃Ti particles stably exist and in Mg-Al melt.After cooling,Al₃Ti/Mg composite can be achieved.