A "Multi" Methodology for Turning Negatives into Positives

Kazuyuki Shizawa (Professor, Department of Mechanical Engineering)

The term "defective product" may conjure a negative image of man-made objects, but most of the metallic materials around us could be called inherently defective, as they contain a large number of microscopic crystal defects. However, by skillfully utilizing these defects, metallic materials can exhibit unexpectedly superior properties. This can be described as a form of reverse thinking, where instead of eliminating the negative cause, it is used in a positive way.

Crystal defects are always present in real metals and are classified into three types: point, line, and planar defects. Here, we will focus specifically on line defects (dislocations) and planar defects (grain boundaries). A dislocation is a linear disturbance in the crystal lattice and is considered one of the negative causes of phenomena such as crack propagation. Even when their number is sufficiently low, there are approximately 10 ⁹ dislocations per m ² . When subjected to external force, these dislocations move on the close-packed atomic planes, and this movement manifests as plastic deformation (irreversible deformation). In other words, if dislocations can move easily, the material can be deformed with a small force and is thus considered soft. Since ordinary structural materials are polycrystals (aggregates of fine crystal grains), they contain numerous walls called grain boundaries, which act to block dislocations. Therefore, a polycrystal is a harder material than a single crystal. Moreover, if the number of these walls (the number of grains) increases and obstructs dislocation motion everywhere, the same material will become harder and its strength will increase.

As is well known, there are high expectations in recent years for the creation of environmentally friendly materials. If light metals (such as Al and Mg) with strength exceeding that of steel materials could be produced and used as structural components for transportation equipment, it would lead to enormous improvements in fuel efficiency. To achieve this, attempts are being made to create ultrafine grains from dislocation cell structures by applying severe plastic deformation to light metals, thereby introducing a large number of dislocations into the material. However, with experimental methods alone, it is difficult to instantly visualize the effects of changes in various parameters on material properties or the distribution of various quantities in submicron-sized microscopic regions. For material design, computational materials science proves powerful as an alternative to experiments. In this article, we will introduce Multiscale and Multiphysics analysis, which have been attracting attention in recent years. Multiscale analysis is a numerical analysis method that reproduces the interaction between different scales, such as microscopic phenomena at the dislocation level and macroscopic deformation at the structural component level. Multiphysics analysis, on the other hand, refers to a method that solves coupled governing equations representing different physical phenomena, such as texture formation (like dislocation patterning and recrystallization) and crystal deformation. Below are some examples of these calculations performed in our laboratory. First, Figure 1 shows a dislocation cell structure that developed in Al due to severe plastic deformation, illustrating how the misorientation within the cells develops and the red high-density dislocation walls transform into grain boundaries. Next, Figure 2 is a predictive diagram of the strength increase in ultrafine-grained Al as the grain size decreases. Figure 3 shows the results of a simulation reproducing the process where recrystallization nuclei grow, driven by the stored energy of dislocations in the parent phase, to form a fine-grained Mg structure. All of these calculations utilize the aforementioned "multi" methodologies.

Here, we have introduced the idea of dramatically improving the mechanical properties of materials by positively applying dislocations, which were conventionally perceived as a negative presence, as well as the methodology for simulating and predicting these material properties. Instead of focusing solely on eliminating negative things, you too may find that by looking at them from a different perspective, they can surprisingly be used in a positive way. I hope this article will help you in shifting your perspective.