Defect removal strengthens small-volume metal crystals

Dr. Neil Canter, Contributing Editor | TLT Tech Beat January 2016

An alternative approach can eliminate dislocations without causing significant material shape changes.

 

KEY CONCEPTS
• The current techniques used to remove metal defects such as dislocations at the nanoscale can dramatically change the shape and dimension of the material.
• A new process known as cyclic healing facilitates removal of defects without impacting the shape and dimensions of a material.
• Removal of dislocations in an aluminum single crystal can increase its strength by more than three times.

ONE MECHANISM FOR MATERIAL FAILURE IS CAUSED BY THE PRESENCE OF DEFECTS within the metal. A better understanding of how these defects are formed and how to eliminate them has been a research objective for some time.

In a previous TLT article, large-scale computer simulations were used to determine the mechanism of crack formation in three dimensions (1). The researchers found that cracks propagated through glass, Plexiglas® and steel in a helical fashion and as the process unfolds, daughter cracks are formed that get bigger over time.

In crystalline materials, the traditional procedure for reducing metal defects such as dislocations is the use of thermal treatment or annealing. Dr. Ming Dao, principal investigator and director, Nanomechanics Laboratory at the Massachusetts Institute of Technology in Cambridge, Mass., says, “Monotonic loading involves application of stress on a material in one direction such as stretching the material or compressing the material with progressively increasing magnitude. Cyclic loading is conducted as the magnitude of the applied loading on a material is fluctuating (i.e., repeatedly increasing and decreasing).”

At the nanoscale, materials can behave very differently than at the macroscale due to the much larger ratio of surface area to volume. Dislocations are situated much closer to the metal surface making it easier to remove them. Dao says, “The dislocations in a nanosized material can indeed be removed using large enough monotonic loading. But the problem is the use of large forces can dramatically change the shape and dimension of the material. This process also is very hard to control.”

An alternative approach that can eliminate dislocations without causing significant material shape changes might also lead to improving the strength of the material. Such an approach has now been developed.

CYCLIC HEALING
A research team led by STLE-member Dr. Subra Suresh, president of Carnegie Mellon University in Pittsburgh, Pa., Dr. Zhiwei Shan, a professor from Xi’an Jiaotong University in Xi’an, China, and Dr. Ming Dao has now developed a new process known as cyclic healing that facilitates the removal of defects from the metal without impacting its shape or dimension. Dao says, “We started by proposing a hypothesis that the introduction of a sequence of low amplitude stresses will be able to remove the dislocations from a small-volume crystal without causing any of the unwanted aspects seen with monotonic loading.”

An analogy to what the researchers are doing is the best method for removing a stick partially buried in the ground. Dao says, “If there is a partially buried stick in the ground, then it can be removed by one large pull using a great deal of force, which can break it. But, if the process is repeated using a much smaller force, then the stick can be removed, though it will take much more time and patience.”

The hypothesis was evaluated by determining if dislocations present in a small-volume single crystal of pure aluminum can be removed. Dao says, “We worked with aluminum because it is a widely used material, and the crystal is a face-centered cubic. This structural feature is important because dislocations normally glide easier on a close-packed plane within a face-centered cubic lattice.”

Dislocation removal is evaluated by placing an aluminum single crystal that is 300 nanometers thick and 500 nanometers wide in a diamond grip. The experimental setup is placed within a transmission electron microscope (TEM) so that the movement of the dislocations can be observed in real time (see Figure 1a). A successful experiment requires precise handling of nanoscale objects and accurate control of loading forces within the TEM. 

The dislocations present in the aluminum crystal occurred either during fabrication of the crystal or when the crystal and the diamond grip were fabricated during the use of focused ion beam irradiation. In the latter case, these dislocations are known as Frank dislocation loops. As shown in Figure 1b, the dislocations are close to the 110 zone axis of the crystal. 

A sequence of six groups of cyclic loading is the key aspect of the cycling healing process. The tensile strain applied is very small with a maximum of strain found to be 0.006 in the sixth cycle. Approximately 700 loading cycles are needed to remove the dislocations from the aluminum crystal as shown in Figure 1c. Changes in the density of the dislocation lines and loops during the six groups of cyclic loading are shown in Figure 1d.


Figure 1. (a) The experimental setup for cyclic healing. (b) Dislocations are found to be close to the 110 zone axis of the aluminum single crystal. (c) The successful removal of dislocations after approximately 700 loading cycles. (d) Changes in the density of the dislocation lines and loops during the six groups of cyclic loading. (Figure courtesy of Carnegie Mellon University.)

Dao says, “We found that by using this cyclic healing process, we are able to utilize low force magnitudes to achieve the gradual removal of dislocations from the aluminum crystal without causing obvious shape changes during whole process.”

Three parameters that needed to be carefully managed in cyclic healing are the stress amplitude, number of cycles and the loading sequence. Dao says, “In general, progressively larger stress amplitude is recommended. However, new systematic studies need to be carried out to find out the optimized combination of these parameters for defect removal and strength enhancement.”

The successful removal of dislocations led to a dramatic increase in the strength of the aluminum single crystal. Dao says, “We found that by removing the dislocations, the strength of the aluminum single crystal can be increased by more than three times.”

In the future, the researchers will determine if cyclic healing can remove dislocations from other materials. Dao says, “One of the materials we will be evaluating is copper.”

Additional information can be found in a recent article (2) or by contacting Dao at mingdao@mit.edu. 

REFERENCES
1. Canter, N. (2010), “Crack formation in 3D,” TLT, 66 (7), pp. 8-9.
2. Wang, Z., Li, Q., Cui, Y., Liu, Z., Ma, E., Li, J., Sun, J., Zhuang, Z., Dao, M., Shan, Z. and Suresh, S. (2015), “Cyclic deformation leads to defect healing and strengthening of small-volume metal crystals,” Proceedings of the National Academy of Sciences,” 112 (44), pp. 13502-13507.


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.