Characterization of beta phase growth and experimental validation of long term thermal exposure sensitization of AA5XXX alloys.

摘要

The United States Navy has a need for fast, light-weight ships to provide rapid deployment in its operations. Strong and corrosion-resistant aluminum alloys, such as AA5083 (UNS A95083) as well as other AA5XXX alloys, have properties that are well-suited for such applications. However, AA5XXX alloys are susceptible to intergranular corrosion (IGC) and stress corrosion cracking (SCC) because of sensitization which is a consequence of the formation of the grain boundary β-phase, Al3Mg2, and the anodic dissolution of the β-phase. Significant research has been performed to measure and understand the effects of time, temperature, stress, and sea water on sensitization and associated intergranular corrosion and stress corrosion cracking under steady-state conditions.;In the present work, the behaviors of β-phase nucleation and growth were characterized using optical and electron microscopy, the relationship between preexisting particles and β-phase, as well as the effect of different heat treatment times and temperatures on IGC and SCC susceptibility of 5XXX alloys were investigated. Grain boundary β-phase thickness was measured with high resolution transmission electron microscopy (TEM). The corrosion sensitization susceptibility was evaluated according to the American Society for Testing and Materials (ASTM) standard G67 tests, that is, nitric acid mass-loss testing (NAMLT).;Diffusion of Mg is manifested by the thickening of β-phase along the grainboundary because the grain boundary is considered as the preferential site for β-phase nucleation. The β-phase growth rate was monitored using high resolution TEM.;The variety of precipitates and their subsequent effects on β-phase nucleation and growth kinetics was investigated. The existence of various intermetallic particles was observed in both baseline and thermally exposed (70°C and 175°C) samples. These particles are usually either rod-shaped or equiaxed, and rich in Mn, Fe, and Cr. Indexing of lattice planes observed in a few of these particles suggested the composition is Al6Mn or Al6(Mn, Fe, Cr). This research also shows that the β-phase precipitation occurs between the preexisting Mn rich particles.;The basic model for the determination of diffusivity values, the prediction of β-phase thickness growth, and corrosion sensitization prediction have been improved by new data from this research.