Undergraduate Material Science Research

Undergraduate Research
ASU || 16'-17'
Concepts - 3D visualization, Corrosion Tracking, Research Methodologies

The work presented is in this case study is credited fully to the efforts of Dr. Nikhilesh Chawla and my lead graduate mentor, Tyler Stannard while I participated as an undergraduate researcher at Arizona State University. During my time at the 4D Materials Science (4DMS) Center, I facilitated the capturing of 3-D renders which were used to describe microstructural information. Specifically, non-destructive tomography-based studies to understand the failure mechanisms of the high strength aluminum alloys.

Introduction to Problem Space

Aluminum alloys are created by varying the concentration of certain alloying agents. These agents help modify the material’s desired properties - including density, ductility, tensile strength, and even corrosion resistance. The described range of material qualities is what drives the various grades of aluminum

for different tasks. For the purposes of this study we shall be examining Aluminum 7475, a high strength-to-weight ratio aluminum alloy frequently used in aircraft manufacturing.

Aircraft grade aluminum is often exposed to harsh chemical and mechanical environments while in service. Naval aircrafts for example, are exposed to corrosive saltwater spray when parked on the aircraft carrier deck and then fatigue loads during flight. These complex service environments are exacerbated with microstructural material variations, complicating the creation of models for the prediction of aircraft and leading to unpredicted component failures. Numerous factors including material impurities, defects, precipitation hardening parameters, and crystallographic orientation of the grains can play a significant role in corrosion-related fracture of high strength aluminum alloys [1].

Method of Study

In order to create the most accurate models possible, four-dimensional (three dimensions + time) studies of the alloys are required. Ideally every microstructural, chemical, and mechanical detail would be known about a sample for the most accurate understanding possible. This work presents the latest efforts in achieving 3D microstructural information using 7475 Aluminum.

The study that I was primarily involved with catered to visualizing corrosion behaviour. The study began by first soaking polished peak aged 7475 samples in uncovered 3.5 wt.% NaCl solution for fifteen days, then fatigue testing them. The 4DMS team collected the data using an X-ray tomography study conducted at the Advanced Photon Source synchrotron. The instrument helped capture the behavior of the samples as a result of the fatigue test, and created a stack of images for sample analysis.

Using Avizo, a visualization software, I was able to highlight key features of the 3D images, specifically showing corrosion-fatigue crack propagation, and the effects of air bubbles trapped inside the crack.

Pitting corrosion is a form of localised corrosion in metals. The phenomena usually occurs when these metals are exposed to a corrosive environment; as a result of usually chloride where it acts as the most common corrosive ion. Pitting can usually be seen on the surface and may even cause perforation or stress corrosion. The pits are nucleated on a microscopic scale and are covered by the

corrosion product. Therefore, pitting is one of the more destructive and undetectable forms of corrosion in metals. [2] The in situ test data showed that the corrosion-fatigue cracks (teal shaded) initiated at pre-existing “mud cracks” (brown shaded) in the corrosion products of the corrosion pits. With smaller inclusions size, the crack seems to follow the corrosion pits more closely.

Results of Study

Peak-Aged 7475 Corrosion Fatigue Within the Cycle – 5150 Cycles

Buble Expansion - Notice how the volume of bubbles slowly increases till the peak load of 94 N of tension, before slowly dissipating at the end of the cycle. Corrosion bubble evolution during the fatigue test was monitored by performing 3D tomography scans at multiple loads within the fatigue cycles. The changes in the bubbles were analysed in 3D to better understand synergistic corrosion-fatigue mechanisms.
Crack Growth - When comparing the end of the cycle to the beginning, one can notice the growth of the crack ever after one loading cycle.
Bubble Volume Growth - The bubbles expand with the crack, and the overall bubble volume appears to grow after one fatigue cycle, likely due to exposure of bare metal surfaces at the crack tip.

The above work would not be possible without the leadership of my professor Dr. Nikhilesh Chawla and my lead graduate mentor, Tyler Stannard. I appreciate the time and effort both have put in helping me develop critical research skills, and valuable life lessons.

[References]

[1] Singh, Sudhanshu S., et al. "Three Dimensional Microstructural Characterization of Nanoscale Precipitates in AA7075-T651 by Focused Ion Beam (FIB) Tomography." Materials Characterization, vol. 118, 01 Aug. 2016, pp. 102-111. EBSCOhost, doi:10.1016/j.matchar.2016.05.009.

[2] Jin-yang, Jiang, et al. "Pitting Corrosion Behaviour of New Corrosion-Resistant Reinforcement Bars in Chloride-Containing Concrete Pore Solution." Materials (1996-1944), vol. 10, no. 8, Aug. 2017, pp. 1-21. EBSCOhost, doi:10.3390/ma10080903.

[3] Mark D., Sutton. "Tomographic Techniques for the Study of Exceptionally Preserved Fossils." Proceedings: Biological Sciences, no. 1643, 2008, p. 1587. EBSCOhost, doi:10.1098/rspb.2008.0263.

[4] Cui, Tengfei, et al. "Effect of Pre-Corrosion and Corrosion/Fatigue Alternation Frequency on the Fatigue Life of 7B04-T6 Aluminum Alloy." Journal of Materials Research, vol. 31, no. 24, n.d., pp. 3869-3879. EBSCOhost, search.ebscohost.com/login.aspx?direct=true&db=edswsc&AN=000392859000007&site=eds-live.