Fatigue Behavior of an Ag₃Mg-AgMg Eutectic Composite

The eutectic alloy Ag-32 atomic percent Mg has been directionally solidified at rates ranging from 0.8 to 27.7 cm/h to produce an aligned lamellar structure. The alloy, consisting of 40 volume percent AgMg and 60 volume percent of a solid solution of approximate composition Ag-27 atomic percent Mg, has been cycled in tension-tension loading at room temperature. Fatigue life increased sharply with both higher growth speeds and with the introduction of long range order into the Ag-27 atomic percent Mg phase by post-solidification heat treatment. Fractographic examination revealed a marked change in crack path and striation morphology as order was produced. It is concluded that improvements in fatigue life with growth speed were a consequence of decreased interlamellar spacing and the resulting rise in tensile strength of the alloy.