Microstructure And Fatigue Behavior Of Alumina Reinforced Al-Si Matrix Composites

This paper studied the microstructures, tensile strain, fatigue and fractography ofAl-Si matrix alloy strengthened by high-content alloy elements, such as Cu, Ni, Mg, etc.,and its composites which are produced by squeeze casting with the measurement oftensile test, fatigue test, optical microscope, scanning and transmission electronmicroscope. The mechanical properties, fatigue and fracture mechanisms of thesematerials are evaluated and analyzed as well in this paper.It can be concluded from the tensile test results that the tensile strength at roomtemperature of the Al2O3f/Al-Si composites decreases as the increasing fiber volumefraction （10vol.%,17vol.%and25vol.%）, which changes evidently from17vol.%to25vol.%. While it is totally different if the temperature is more than275°C. The tensilestrength at the temperature over275°C increases evidently from10vol.%to17vol.%.The Ai-Si composite hybrid with17vol.%Al2O3fiber and5vol.%particulates, in whichthe particulates adhere to the surface of fibers, increases the resistance force when thefibers are pulled out from the matrix and enhances the tensile strength. At roomtemperature, the crack in the Al-Si alloy propagates along the boundary between the Siplate and the Al matrix in the eutectic and breaks up the blocking intermetallics, whichexhibits brittle fracture characteristics. The crack in the composite propagates along theAl/Si or Al/Intermetallic boundary due to the constraint effect from the fibers, whichresults in a large number of small facets in the fracture surface. At the temperature from200°C to350°C, the crack in the Al-Si matrix alloy propagates along the gatheringmicropores, which exhibits ductile fracture with dimples. The composite shows similarductile fracture characteristics with dimples which have smaller size than the Al-Si alloy.The low cycle fatigue tests show that the Al-Si matrix alloy peculiarly exhibitstwo-stage cyclic hardening at room temperature. The first cyclic hardening stage happensat1～20cycles, while the second one after the200th cycle. At100°C and200°C, thematerial shows cyclic hardening behavior at the beginning of low cycle fatigue test andafterwards presents slight cyclic softening behavior. When the temperature is over275°C,the alloy shows only cyclic softening. The Al2O3f/Al-Si composites reinforced with10vol.%,17vol.%and25vol.%fibers all have better fatigue properties than the Al-Si alloy.The composite with17vol.%fibers has best fatigue property among the three ones.Based on this composite （17vol.%fiber）, Al2O3particulates （5vol.%） are added byadhering to the fibers, which results in an excellent fatigue property for the compositesimultaneously reinforced with Al2O3fibers and particulates. During the low cycle fatigue testing, all the composites show slight cyclic hardening at the first10cycles andthen presents cyclic softening behavior at room temperature. At the temperature range of200°C～350°C, the composites show obvious cyclic softening behavior.The microstructural analysis reveals that the eutectic Al-Si alloy strengthened withhigh-content Cu, Ni and Mg has a lot of intermetallics, such as Al₇Cu₄Ni、Al₅Cu₂Mg₈Si₆、Al₃CuNi、Mg₂Si、Al₂Cu with various morphologies. The plentiful intermetallics togetherwith plate-like eutectic Si separate the Al alloy matrix, which leads to disappearance ofslip bands due to inhomogeneous deformation during fatigue cycling. The first hardeningstage initiates from the interaction between the dislocations and the precipitates. And thesecond one is related to the interaction among the dislocations.The fracture analysis shows that the fatigue crack initiates close the surface of largeprimary silicon particles or intermetallic agglomerates in the Al-Si matrix alloy duringlow cycle fatigue test（the temperature is less than200°C）. When the temperature ishigher than275°C, the bulky primary Si particles are the mainly crack source. For thecomposites, cracks frequently begin at a huge fiber or a fiber agglomerate. While thetemperature under200°C, the crack in the Al-Si matrix alloy propagates mainly along theAl/Si boundary with many facets formed at the fracture surface, and the crack in thecomposites propagates mainly along the Al/Intermetallic boundaries with smaller facets.When the temperature is higher than275°C, debonding of Al/Si or Al/Intermetallicboundaries as well as breaking of fibers result in appearance of micropores and dimplesat the fracture surface. And the dimple size of Al-Si alloy is bigger than that of thecomposites.