In-situ characterization of the solution heat treatment of B319 aluminum alloy using x-ray diffraction and electron microscopy

Abstract The solution heat treatment of industrial B319 Al alloy is a complex process, involving many phase compositions and morphologies. However, most studies of the associated phenomena have utilized ex-situ methods, which may obscure the true mechanisms of dissolution. In this work, several innovative in-situ techniques were performed to characterize the sequence and kinetics of dissolution during a temperature ramp and isothermal holding in a typical 490–500 °C solution heat treatment for the as-cast alloy. This included the use of high-temperature x-ray diffraction as well as scanning, transmission, and scanning-transmission electron microscopy to provide a unique and comprehensive assessment of the microstructural transformations at various length scales. For the first time, quantitative dissolution transformation curves were developed from diffraction data, that demonstrated that the global Al2Cu and Al5Mg8Cu2Si6 phase concentrations reduced synchronously during the heat treatment. With electron microscopy, it was revealed that radial dissolution was predominant for Al5Mg8Cu2Si6 as well as both the blocky and eutectic Al2Cu morphologies. Furthermore, the progression of the dissolution front was attributed to several factors, including the particle size, three-dimensional morphology, surface quality, local composition gradient, distribution and shielding from adjacent phases. The direct observations produced by this study considerably enhanced the current understanding of the solution heat treatment of multiphase alloys. Additionally, these results demonstrated the effectiveness of such in-situ techniques for the advanced characterization of solid-state transformations.