Selected-area diffraction ring patterns in Al-Zn-Mg powders

Attempts were made to prepare thin foils of Al--6.5 Zn--2.5 Mg--1.5 Cu--0.40% Co, manufactured by four different vendors. The average dendrite spacing indicated solidification rates 104-105 deg K/sec, five orders of magnitude slower than that producing an amorphous structure. A micrograph of a thin foil prepared by ion-milling using 98.5% pure Ar, and a continuous three-ring selected-area diffraction pattern are shown. The was identified as from gamma -Al2O3. Even with an oxygen trap rings were obtained, but too diffuse for positive identification. Chlorine in the Ti-sponge O trap may have produced AlCl3; 5N Ar was then used. diffuse ring patterns resulted. Tests with 7075 (the ally minus Co) erosion of the stainless-steel holder, which was found to have an Ar-plasma shroud. Three solutions are being studied, increasing the accelerating (risking deep Ar penetration), using a sacrificial carbon coating on the specimen holder, and fabricating a holder from a machinable ceramic.7 refs.--EPAA/AF.