Author: Jin Soo Kim, Dilip Roshan Das, Yanting Li, Hsin-Yeong YenPresenter: Jonathan Tsai, Ling Chen, Benjamin Yeung, Jeffrey Best, Rena Yang, Walter Cisneros

Graduate student Instructor : Ming-Je Sung, Advisor : Farghalli A. Mohamed Department of Chemical Engineering and Materials Science, The Henry Samueli School of Engineering

University of California, Irvine

Grain Growth and Corrosion Behavior of Ultrafine-Grained 5083 Al

Methods

Introduction

ResultsGrain Size AnalysisOM Vs. SEM

Number of Grains vs. Grain Sizes (AR)

Number of Grains vs. Grain Sizes (Annealed)

CorrosionSEM

Conclusionsa. High annealing temperature causes grain growth.b. Refining the grains size of 5083Al to the UFG range has

enhanced its corrosion resistance.

Results and DiscussionThe average grain size of annealed sample 5083Al was measured to be 303 μm and that of the consolidated sample was 244 nm. Additionally, the salt spray test results showed that the UFG 5083Al had better corrosion resistance than the CG 5083Al as shown in figure 5, 6, and 7.

 

Figure 3: Salt spray machine that was used to perform corrosion test.

Grain size, d (mm) 10-1 10-2 5x10-3

4 8 12 16d-1/2 (mm-1/2)

200

150

30

20

10

0

Yie

ld st

reng

th (M

PA)

Yie

ld st

reng

th (k

si)

100

50

0

Figure 2: Hall-Petch relation: The influence of grain size on the yield strength of a 70 Cu-30

Zn brass alloy [1]

Further StudiesThe strength of 5083Al samples will be measured as a function of grain size, and corrosion resistance will be investigated by giving different time exposures.

AcknowledgementThe Laboratory for Electron and X-ray Instrumentation. (LEXI) facility in UCIUndergraduate Research Opportunities Program (UROP)

References[1] Callister Jr., William D. Fundamentals of Materials Science and Engineering: An Integrated Approach. 2nd ed. Hoboken, NJ: John Wiley & Sons, 2005. Print.[2] "Cost of corrosion to exceed $1 trillion in the United States in 2012." Generation 2 Materials Technology. Metallurgical Services, 01 Jun 2011. Web. 6 May 2012. http://www.g2mtlabs.com/2011/06/nace-[3] ASTM B117. 2005. Ascott Analytical Equipment Limited and National Exposure Testing. http://www.ascott-analytical.com/ASTMB117/astmb117testconditions.htm 23 April 2013.

AbstractDamages caused by corrosion tremendously scar the infrastructure of today’s society. The United States alone spends about 1 trillion dollars every year due to corrosion damage up to this date. In an effort to reduce this cost, due to the Hall-Petch properties of metals in which the yield strength increases as the grain sizes decreases, ultra-fine grained aluminum was examined to find out whether the same principle applies in significantly decreasing damages caused by corrosion. For this experiment, two samples were prepared: 5083 bulk aluminum sample that was fabricated by cryomilling in order to achieve grain sizes in the ultra-fine scale and conventional commercial grade 5083 aluminum. Both of these samples were placed under the salt spray machine to emulate corrosion damage. The result was that the cryomilled ultra-fine grained 5083 aluminum had less pits than the commercial grade 5083 aluminum, and thus concluding that ultra-fine crystalline metals resist corrosion damage better. 

Figure 1: Singlecrystalline Vs. Polycrystalline

A grain is the microstructure of a material within which the atoms are arranged in the same manner. This type of crystal material is called single crystalline. However, the atomic orientation of adjacent grains are different, known as polycrystalline. This difference creates regions of atomic mismatch, called grain boundaries. Increasing the number of grains and the amount of grain boundaries by reducing the grain size enhance the mechanical properties of a material. The relationship between the grain size and the yield strength isrepresented by the Hall-Petch relation:

σy = σ0 + kyd-1/2

where σy is the yield strength, σ0 is the initial stress, ky is a constant, and d represents the diameter of average grain size

[1].

However, this relationship only applies to mechanical properties, and chemical properties such as corrosion are still needed to be identified. By the end of 2012, the U.S. government reports an estimated loss of almost 1 trillion dollars due to corrosion in materials[2]. The direct costs of corrosion reported by companies split into two categories: costs related to design, manufacturing, and construction and costs related to management. Not only does the costs come from buying new and better corrosion resistant materials and the technological uses of them but also the costs to constantly replace them with better materials and managing the materials annually. For this reason, it is important to start looking for new materials that last far longer than any of the commercial materials used today by companies to decrease the costs of corrosion damage in the long run.

Samples were cut from a 5083Al bulk material, and they were heated at 673K for 30 minutes to release internal stresses in the material. After cooling, half of the samples were additionally annealed at 873K for one hour to observe if the grain sizes changed. All samples were polished with silicon carbide paper starting from grit size 22μm to 5μm. They were further polished with 1 micron to 0.3 micron alumina and colloidal silica suspensions till 100μm thickness of mirror-like surfaces were obtained. Then the annealed sample was etched chemically, and the consolidated sample was etched physically to reveal the grain under microscope. Grain sizes of annealed sample was measured using Optical Microscopy (OM) and Scanning Electron Microscope (SEM) for the consolidated sample. Lastly, an consolidated sample and an annealed sample were placed into salt spray machine for 48 hours to test their corrosion resistance. The surface of the samples were examined by using Scanning Electron Microscope (SEM) and Energy-Dispersive X-ray Spectroscope (EDS) [3] .

grain grain

Figure 4: Images of grains: (Left) Annealed sample, (Right) Consolidated sample

Figure 5: Corroded surface of 5083Al samples: (left) Annealed sample, (right) Consolidated sample

EDS

Figure 6: Corroded surface of 5083Al sample: Annealed

Figure 7: Corroded surface of 5083Al sample: Consolidated

The above images compare the grain sizes of the two samples before being exposed to corrosive environment.

The two images above show the microstructures after undergoing corrosive environment. It is apparent that the CG sample has more surface damages than the UFG sample.