On the fabrication of atom probe tomography specimens of Al alloys at room temperature using focused ion beam milling with liquid Ga ion source.

Abstract

In this work, a simple rectangular milling technique was demonstrated to prepare needle shape atom probe tomography (APT) specimens from Al alloys by focused-ion-beam (FIB) milling using Ga+ ions at room temperature. Ga has high miscibility in Al owing to which electropolishing technique is preferred over Ga+ ion FIB instruments for the fabrication of APT specimens. Although, site specific sample preparation is not possible by the electropolishing technique. This led to the motivation to demonstrate a new rectangular milling technique using Ga+ FIB instrument that resulted a significant reduction of Ga+ ion impregnation into the specimens. This is attributed to the reduction of milling time (<30 s at 30 kV acceleration voltage) and the use of lower currents (<0.3nA) compared to the conventional annular milling method. The yield of specimens during field evaporation in APT was also significantly increased from around 8 million ions to more than 86 million ions due to the avoidance of Ga+ ion embrittlement. Therefore, the currently demonstrated rectangular milling technique can be used to prepare APT specimens from Al-alloys and obtained accurate compositions of matrix, phases, and hetero-phase interfaces with Ga < 0.1at%. RESEARCH HIGHLIGHTS: Feasibility of using Ga+ ions for the preparation of needle shaped specimens at room temperature from aluminum alloys. Demonstration of using a rectangular milling technique instead of annular milling technique that led to a significant reduction in impregnation of Ga+ ions into the specimen needles. Due to very low Ga+ ion damage, the yield of the APT data increased by 10-12 times.