KEY CONCEPTS
• Application of cyclic loading to a single crystal of nickel was used to show how persistent slip bands (PSBs) form, which eventually lead to the formation of small metal cracks.
• The researchers needed to run millions of cycles at the micron scale at high frequency. 
• Sequential SEM micrographs were used to track the evolution of damage on the surface of the test specimen. 

The use of metals in more high-performance applications where they are required to perform under extreme-operation conditions continues to lead to concern about how to minimize failure. Determining the origin of metal cracks that lead to fatigue, and then eventually failure, is important in predicting the operating life of a specific part.

In a previous TLT article,¹ researchers linked metal fatigue over time to an increase in disorder that is known as entropy. A new parameter known as fracture fatigue entropy was devised to predict metal failure. In running experiments on a number of different metal alloys, the researchers were able to utilize fracture fatigue entropy to predict failure no matter the material or the experimental conditions.

Metals must exhibit a proper balance of strength and ductility to meet specific application requirements. Jaafar El-Awady, associate professor of mechanical engineering at The Johns Hopkins University in Baltimore, Md., says, “The application of stresses, either in a repeated or fluctuating manner, subjects metal alloys to the imposition of cyclic loading. Such a process over time eventually can generate small microstructurally-short cracks that will gradually grow as the cyclic loading progresses. As the process continues, some of the short cracks will grow into large cracks that can propagate to the point where metal fatigue and failure occurs.”

Short cracks can be characterized based on their size, according to El-Awady. He says, “Due to the localization of damage, short cracks will propagate. Establishment of a right set of conditions is needed for this to occur. It is difficult to measure the transition from short to long cracks in macroscale experiments, because detecting short cracks is very difficult.”

Short cracks will form due to nucleation caused by the repetition of dislocations during cyclic loading. The result is the formation of long-range ordered structures that are known as persistent slip bands (PSBs). El-Awady says, “Observation of PSBs dates back to the 1950s. PSBs are well-defined arrangements of dislocation structures. The structure of PSBs is characterized by the arrangement of dislocations into a ladder-like structure. PSBs form in wavy slip metals, in large grains or crystals oriented for a single slip and along the slip plane where the atoms are most closely packed.”

One other factor that defines PSBs is their persistence. El-Awady says, “When a slip is formed on a surface due to cyclic loading and is subsequently removed by polishing, it will reappear in exactly the same spot when cyclic loading resumes.”

Previously, researchers proposed what is known as the EGM model in the 1980s to explain how PSBs will eventually cause metal fracture. El-Awady says, “The EGM model explains how stress buildup will eventually be relieved through the formation of extrusions on the metal crystal, which act as stress concentrators that lead to crack initiation. But some inconsistencies have been established with the EGM model because short cracks might form before the extrusions are created.”

A new approach has now been developed to better understand the formation of PSBs by conducting experiments at the micron scale.

In-situ experiments
El-Awady and his students evaluated how PSBs lead to small cracks at the micron scale through applying cyclic loading to a single crystal that has a cross-sectional area of 156 square microns (12 by 13 micron²). The experiments were conducted under high frequency cyclic loading in situ in a scanning electron microscope (SEM). Sequential SEM micrographs of the evolution of the damage on the surface of the test specimen were obtained as the microcrystal was cyclically loaded at a constant shear strain amplitude of 6.3 x 10^-3.

El-Awady says, “In contrast to large scale experiments that require somewhere between 1,000-100,000 cycles to see any evidence of small cracks, at the micron scale, we needed to run to millions of cycles. To achieve this, the process was speeded up by cyclic loading at high frequency.”

In running these experiments, the researchers imposed a load on the metal single crystal through the use of a nanoindenter. Stiffness was constantly measured throughout the experiments and increased to correlate with cyclic hardening.

The metal used in the experiments was nickel. El-Awady says, “We decided to use nickel in our studies for two reasons. Nickel is used as the base metal component in superalloys that are utilized in many applications, including aerospace where minimizing formation of small cracks is very important. The second reason is that we wanted to work with a pure metal to eliminate any additional complexity that can occur through the use of alloys.”

Based on data generated from these in-situ experiments, El-Awady believes that PSBs form through a series of steps starting with the activation of the weakest pre-existing dislocation sources on parallel planes with the highest resolved shear stress. He says, “This occurs within the first series of cyclic loading steps. After the application of further loading, dislocation tangles form that eventually form primary edge dipoles. The next step is for the dipoles to rearrange into a ladder structure that leads to the generation of a PSB nucleus. Disentanglement of nearby dislocations in the metal matrix lead to an expansion of the PSB nucleus into a ladder structure.”

At that point, further cyclic loading on the PSBs leads to the formation of persistent slip markings (PSMs, see Figure 2) on the surface of the nickel crystal.


Figure 2. Persistent slip markings (PSMs) that form on the metal surface due to cyclic loading of PSMs are shown. Figure courtesy of The Johns Hopkins University.

Future work will involve trying to get a more fundamental understanding of PSBs. El-Awady says, “We will be trying to understand the basics of the mechanism for nucleation of PSBs. Our objective is to develop a complete picture of the formation of PSBs and how that leads to the generation of short cracks.”

El-Awady points out that when the cyclic loads are relatively low, 90% of the operating life of a metal component occurs prior to short crack formation. He says, “It is very critical to figure out how to delay the onset of short cracks to extend the useful life of a specific metal part.”

Additional information can be found in a recent reference² or by contacting El-Awady at jelawady@jhu.edu.

REFERENCES
1. Canter, N. (2011), “Predicting metal failure,” TLT, 67 (7), pp. 10-11.
2. Lavenstein, S., Gu, Y., Madisetti, D. and El-Awady, J. (2020), “The heterogeneity of persistent slip band nucleation and evolution in metals at the micrometer scale,” Science, 370 (6513), DOI: 10.1126/science.abb2690.