IN-SITU X-RAY INVESTIGATION OF IMMISCIBLE PHASES IN Al-Bi ALLOYS

摘要

In-situ synchrotron X-ray video microscopy has been undertaken to investigate two monotectic systems. When a hypermonotectic alloy solidifies, a temperature range exists where the liquid undergoes separation into two immiscible liquid phases that, depending on their density difference, may cause settling and subsequent agglomeration of the denser phase. Monotectic alloys with a hardenable matrix and a soft immiscible component embedded as homogenously dispersed domains are of particular interest for bearing applications, yet most casting attempts have failed to hinder gravity induced macrosegregation of the liquids prior to solidification. Al-Bi alloys undergo a monotectic reaction during solidification and have been identified as a potential candidate material for slide bearings. A high temperature gradient strip casting method has been devised to counterbalance sedimentation by exploiting the combination of Marangoni induced motion anti-parallel with gravity and the reduction of the immiscibility length. By using directional solidification in combination with X-ray imaging, Al-6%Bi and Al-8%Zn-6%Bi alloys were investigated to observe phase separation, liquid motion, morphology evolution and coagulation of the liquid phases during cooling through the miscibility gap, as well as engulfment of the Bi domains at the monotectic solidification front. In the case of the Al-6%Bi alloy, it was found that droplet size after phase separation played an important role in droplet motion and larger droplets were observed to settle towards the monotectic front. The Bi droplets in the ternary alloy behaved some what differently and were observed to dissolve when the local concentration of Zn was increased.