Thermodynamic Condition and Behavior of Precipitates in Al sub(2)O sub(3p)/7055 Composites with Cryogenic Treatment

The thermodynamic condition and behavior of precipitates were discussed about the Al sub(2)O sub(3p)/7055 composites subjected to cryogenic treatments. A kind of software named Thermo-Calc was employed to calculate and analyze the mass amount of several main precipitates based on the thermodynamic balance theory. The results demonstrate that when the temperature decreases from 300K to 77K the precipitated amount of T(AlZnMgCu) and [theta] (Al sub(2)Cu) phases will increase. For S(Al sub(2)CuMg) phase, at 175K there is a transverse point from increasing tendency to the opposite. It should be noted that [eta] (MgZn sub(2)) is the main precipitated phases in compistes, which accounts for more than eleven weight percent in one mole material. But the balance mass amount lower a little from 300K to 77K. It is analyzed that the volume shrinkage and lattice contract are the main driving forces for precipiting. Due to the difference of heat expansion coefficient among various phases the anisotropic shirinkge lead to micro voids that will act as the nucleation cores, which faciliate the precipitation.