Thermal Behavior Of AZ91 Alloy And SiCw/AZ91 Composite

This paper studies the thermal behaviours of the squeeze-casting ,quenching,and aging (175℃,5h,15h,30h) states AZ91 alloys and SiCw/AZ91 composites. The thermal cycling experiment was done by dilatometer. The high limit of temperature was 400℃, the rate of caloric test was 5℃/min, and we do 3cycles.The microstructure that begin and after of the materials were examined by means of optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM).To compared the thermal cycling curve of squeeze-casting alloy with die-casting alloy ,it is shows that the thermal behavior of the squeeze-casting alloy impacted by the residual strain and the precipitated phase. Do the solid solution to the alloy, the residual strain reduce the CTE sharply. Do the aging(5h,15h) to the alloy, the more the quantitative of the precipitated ,the lower the CTE. Do the aging(5h) to the alloy, it is shows that the residual strain do the chief effection.To compared the thermal cycling curve of the squeeze-casting SiCw/AZ91 composite with the squeeze-casting alloy, the composite's hysteresis ring shows a irregularity and width sharp, the CTE is obvious falling down, the positive strain occurs during the thermal cycling. All these changes result in the input of whiskers,which reduce the CTE, while do little influence to the cycle stability. Do the solid solution to the composites, the residual strain reduce the CTE sharply. Do the aging(5h,15h) to the composites, the more the quantitative of the precipitated ,the lower the CTE. Do the aging(15h) to the composites, it shows the low CTE.All of the composites show a elastic-plastic transition during the special temperature interval(320~370℃). There are both valley and peak appearing at thermal cycling curve. The valley results in the quick relaxation of great residual stress, while the peak results in the reduce of speed that caused by the elastic-plastic transition.