Effect of nanovoids on metal failure

Dr. Neil Canter, Contributing Editor | TLT Tech Beat June 2012

Researchers determine the effect nanoscale voids have on aluminum alloys.

KEY CONCEPTS
• The growth and coalescence of voids initially formed during the manufacturing of metal alloys are responsible for causing failure.
• Transition state theory and atomistic simulation were used to determine if nanoscale voids can lead to the generation of metal fractures in aluminum alloys.
• The shear needed to permanently deform nanoscale voids is too high to cause nanovoids to grow.

PROVIDING BETTER INSIGHT ON METAL FAILURE IS CRITICAL so that maintenance engineers can gain a better understanding of how lubrication extends and optimizes machinery life. This process is important in continuing to provide guidance to lubricant manufacturers for developing better products to meet the more demanding operating conditions present in today’s competitive manufacturing environment.

In a previous TLT article, a new parameter known as fracture fatigue entropy was found to be an accurate predictor of metal failure (1). Regardless of the metal alloy or the experimental conditions used in the evaluation, failure occurred when the fracture fatigue entropy of a specific material was reached.

One other result from studying metal failure is this assists with the development of new metal alloys that are more durable. An example is provided in a previous TLT article dealing with the preparation of super-strong, ductile aluminum (2). Severe plastic deformation of a common aluminum alloy (7075) using a technique known as high-pressure torsion increased the strength of the aluminum alloy to a level comparable to carbon steel without sacrificing ductility. The aluminum appears to be strengthened by the formation of smaller grains below 100 nanometers in diameter.

Derek Warner, assistant professor of civil and environmental engineering at Cornell University in Ithaca, N.Y., indicates that an important mechanism responsible for many instances of metal failure is the growth and coalescence of voids. He says, “Voids are microscopic regions of metal where no material is present. The growth and coalescence of voids is a microscopic mechanism behind the ductile fracture of metals. The macroscopic loading is amplified in the vicinity of voids, causing their growth and eventual coalescence, which forms the cracks that lead to material failure.”

Figure 2 shows how voids can grow and coalesce to form a rough fracture surface. 


Figure 2. Voids can grow and coalesce to fracture metal in the manner shown. A recent study has shown that nanovoids present in aluminum alloys are not likely to contribute to metal fracture. (Courtesy of Cornell University)

It is known that microscale voids cause metal failure, but the issue is whether smaller voids present at the nanoscale also play a role in this process. Warner says, “Traditional microscale voids are defined as those with diameters of at least a few microns. The question is whether voids of smaller diameters also contribute to metal failure.”

New research has now been done to determine whether nanoscale voids are as important a factor in influencing metal failure as larger voids. 

TRANSITION STATE THEORY
Warner, in collaboration with Linh Nguyen, conducted a study to determine the effect that nanoscale voids have on aluminum. He says, “We evaluated aluminum because this metal is relevant to many technology applications.”

When asked about whether it was important to evaluate specific aluminum alloys, Warner indicated that the findings of his study apply to most aluminum alloy types. He says, “Regardless of whether a 2,000, 5,000, 6,000 or 7,000 alloy is considered, they all contain small spheroidal defects that formed during the manufacturing process. These defects lead to voids.”

With the availability of new analytical and computational techniques, the ability now exists to determine if void features on the nanoscale play a role in facilitating failure. The researchers used transition state theory and atomistic simulation to predict whether nanoscale voids can lead to the generation of fractures.

The process used is NVT (number, volume and temperature) MD simulations. A constant temperature of 300 K was used for all of the theoretical experiments.

Face-centered cubic, aluminum simulation cells were prepared theoretically with cells composed of between 191,000 and 325,000 atoms. Nanovoids with diameters of 4, 6 and 8 nanometers were examined along with a free surface. Warner adds, “We used the simulation data to provide predictions for voids at any radius.”

The next step in the process was to theoretically deform the simulation cells to force the voids to grow. Initially, a few surface atoms were removed at one or two peak shear stress locations. Shear was then applied at increasing loads to deform the voids. The voids were found to permanently deform at shear loads of 2.16, 1.89 and 1.60 gigapascals (GPAs) for the 4, 6 and 8 nanometer voids, respectively.

Warner says, “Most aluminum alloys have tensile strengths below 1 GPa, which means that it is very unlikely for sufficient shear to be generated to cause the nanovoids to grow within the metal structure.” The only possible way for this to occur is through extreme shock loading or diffusion at high temperatures. 

For large voids with diameters above 100 nanometers, the shear generated is independent of size. The net result of this work is that nanovoids present in aluminum alloys are not likely to grow and eventually create fractures that will lead to metal failure. 

Warner is looking to use this atomistic- based transitional state theory approach to evaluate the behavior of preexisting dislocation defects, an important feature with regard to mechanical behavior. He says, “Ultimately, we are interested in determining the source of strength in metal alloys.” 

Additional information on this research can be found in a recent article (3) or by contacting Warner at dhw52@cornell.edu.

REFERENCES
1. Canter, N. (2011), “Predicting Metal Failure,” TLT, 67 (7), pp. 10-11.
2. Canter, N. (2011), “Super-Strong, Ductile Aluminum,” TLT, 67 (1), pp. 10-11.
3. Nguyen, L. and Warner, D. (2012), “Improbability of Void Growth in Aluminum via Dislocation Nucleation under Typical Laboratory Conditions,” Physical Review Letters, 108 (3) 035501.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.