An advantageous imaging perspective for quantitative evaluation of 7075 aluminum alloy grain boundary precipitates using scanning electron microscope.

Abstract

7075 Aluminum alloy (AA7075) samples undergone four aging sequences were examined using a scanning electron microscope (SEM) and a transmitted electron microscope (TEM). The measurements results validate the correlation between stress corrosion cracking (SCC) resistance and the size and inter-distance of the grain boundary precipitates (GBPs). To evaluate the size and inter-distance of GBPs, we demonstrate in this study a highly efficient SEM imaging technique that can unfold grain boundary in a two-dimensional view. Compared to TEM, imaging with backscattered electrons in SEM (SEM-BSE) is more advantageous for GBPs presentation and measurements. The major reason is that about 900 times more sampling area can be imaged with SEM from the same specimen for TEM observation, thus enabling frequent appearances of GBPs at normal top view perspective, a planar view best for GBPs quantitative analysis but not well-documented. The acceleration tension of SEM for imaging was optimized at 10kV with an information depth of around 330 nm. RESEARCH HIGHLIGHTS: Scanning electron microscope (SEM) imaging using backscattered electrons is efficient for AA7075 grain boundary precipitate imaging. The precipitate size and inter-distance can be more accurately measured with the perspective of normal top view under SEM than transmitted electron microscope.