Session: 01-16-02: Congress-Wide Symposium on NDE & SHM: Ultrasonic Waves for Material Characterization and Damage Assessment

Paper Number: 114423

114423 - Ultrasonic Monitoring of Sensitization in Aluminum Alloys 

Aluminum alloys of the 5xxx series (low-density, high-strength, and corrosive-resistant marine-grade alloys AA5xxx) become sensitized when exposed to natural or on-board heat sources. For a qualitative reference, the material is fully sensitized after being at 80 ^oC for 500 days. The cause for this sensitization is the metastable nature of the phase of these alloys in their as-received state. At 3-6 wt.%, magnesium is the major alloying element in the 5xxx series aluminum alloys. At this composition, at room temperature, magnesium is randomly distributed through the aluminum FCC structure, but above the solid solution limit, according to the equilibrium Al-Mg phase diagram. Ambient heat can easily provide energy to the magnesium atoms in the matrix to diffuse to the nearest grain boundary and precipitate in a stable magnesium-rich crystalline phase (Al₃Mg₂). In seawater, this phase is electrochemically more active than the magnesium-depleted aluminum matrix and it readily dissolves, creating a widely-spread network of channels which act as seeds for cracking, exfoliation, which ultimately leads to structural failure. Despite this, the 5xxx-series aluminum alloys continue to be used due to their desirable properties. The currently approved ASTM method for determining the susceptibility to intergranular corrosion of sensitized alloys is the Nitric Acid Mass Loss Test (NAMLT) (a.k.a. the G67 test). The procedure is lengthy and costly, not sufficiently sensitive to the alloy’s microstructure, and destructive.

Given the economical and safety-related importance of how sensitization affects structural health, multiple efforts have been directed toward finding an alternate method to the NAMLT test. Probes developed based on electrical resistivity and electrochemical reactions do not couple strongly enough with sensitization and only test the surface, being sensitive to the particular grain morphology of the exposed area. In contrast, ultrasound, traditionally a powerful nondestructive material characterization tool, proves to be effective in detecting and measuring the level of sensitization in affected alloys. In essence, since the atomic arrangement in the alloy changes as sensitization settles in, the propagation of acoustic waves is affected by this change. Through their nature, ultrasonic measurements integrate the properties volumetrically, amplifying the effect.

Three ultrasonic parameters (shear wave velocity, longitudinal wave velocity, and longitudinal wave attenuation coefficient) were measured for AA5083 (4–4.9 wt.% Mg) and AA5456 (4.7–5.5 wt.% Mg) aluminum alloys, as a function of heat treatment time at three separate temperatures. Two ultrasonic techniques were used for these measurements: resonant ultrasound spectroscopy (RUS) and pulse echo (PE). PE is the standard method for ultrasonic nondestructive evaluation, currently used in general structural health integrity inspections. Here, RUS was not used with the intention of extending the method into a field probe for sensitization, but simply to correlate the findings between the two methods used. However, due to the very small sample size (1-3 mm) needed, RUS works as a biopsy-like test, which could be utilized in the field if needed. The measured quantities vary monotonically with the heat treatment time, which correlates with the degree of sensitization in the alloy. The rate of change decreases as saturation is approached, in agreement with nucleation and growth kinetics. The 20% change between as-received and sensitized states measured in this initial study in the attenuation of longitudinal waves at 5 MHz is already orders of magnitude higher than what other techniques have been capable of observing. Stronger effects at higher frequencies are possible; the closer the wavelength to the size of the precipitate, the larger the attenuation.

The patented methodology identifies multiple ultrasonic parameters that can be combined into a new, on-site, nondestructive tool for quantitatively monitoring the sensitization level in magnesium-rich aluminum alloys, with possible extensions to other materials. Given the sensitivity of multiple parameters to the level of sensitization in the material, a redundancy-based model can be used for increased confidence.

Presenting Author: Gabriela Petculescu University of Louisiana at Lafayette

Presenting Author Biography: Gabriela Petculescu holds a BS in Technological Physics and an MS in Physics from the University of Bucharest, where she worked in Plasma Physics. Her Ph.D. is also in Physics, from Ohio University, with a dissertation in Fundamental Processes in Thermoacoustic Engines. She continued her career building expertise in physical acoustics. As a postdoctoral researcher, she developed low-profile Lamb-wave sensors and actuators for aircraft skin made of carbon-fiber composites, while at Northwestern University, in Evanston, Illinois. In a previous postdoctoral appointment at the National Center for Physical Acoustics, she learned the technique of Resonant Ultrasound Spectroscopy and used it to investigate magnetostrictive and thermoelectric materials. Since 2006, when she joined the Physics Department at the University of Louisiana at Lafayette, she has been working at the interface of physical acoustics and material science by using ultrasonic techniques to understand the behavior of materials and their applications.

Authors:

Gabriela Petculescu University of Louisiana at Lafayette