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Mesoscale identification of beta phases in aluminum magnesium alloys
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Mesoscale identification of beta phases in aluminum magnesium alloys

Jasmine Mary George
Master of Science (M.S.), Drexel University
Jun 2017
DOI:
https://doi.org/10.17918/etd-7439
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Abstract

Aluminum-magnesium alloys Corrosion and anti-corrosives--Research Materials Science
Aluminum-magnesium alloys are known for their good weldability, high specific heat, and excellent corrosion resistance, making them a suitable material for naval welded structures. When exposed to elevated temperatures for prolonged periods of time and operating under a corrosive environment such as sea water, the material becomes sensitized to intergranular corrosion and stress corrosion cracking. Sensitization occurs when magnesium precipitates out of the matrix material at the grain boundaries, forming a precipitate that is susceptible to corrosion. Of particular interest in the alloy precipitation behavior is greater understanding of the formation and characterization of the constituent phases, [beta]' and [beta]. To create a clear delineation between the two precipitates and to provide further characterization, nanoindentation and etching studies were performed on samples that underwent different sensitization treatments. These mesoscale techniques allow for the identification of key phases in sensitized alloys, thus providing a less invasive means of stress-corrosion cracking prevention. Although neither nanoindentation nor etching studies created a clear delineation between the two [beta] precipitates, etching studies did show interesting results that with continued work may lead to further characterization and understanding of these precipitates. With the phosphoric acid etchant, the HAGB etched in the LTA sample but there were no [beta] precipitates located along the boundary, possibly due to gallium from FIB. In the HTA sample, the HAGB was unetched but [beta] precipitates were seen along the boundary. In an attempt to directly visualize and differentiate between [beta] precipitates under SEM, samples were etched with ammonium persulfate dissolved in water. However, grain boundaries and precipitates remained unetched, resulting in no direct visualization under SEM. Due to the contradiction of these results with those shown in literature, these etchants warrant further study.

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