Observing Changes Occurring Inside Metals

Metals consist of collections of tiny crystals, or grains, that measure up to several tens to several hundreds of micrometers. A defining feature of these metallic crystals is their capacity to undergo plastic deformation when subjected to external forces. When a force is applied, linear crystal defects known as dislocations frequently form within the grains, shifting regularly arranged atoms and generating deformation. This property enables metals to be shaped into many forms; therefore, they have been used across numerous industrial fields.

“However, we still lack a full understanding of where slip is concentrated and how internal changes unfold on a microscopic scale. Plastic deformation can certainly be mathematically expressed using a crystal plasticity model and can be simulated to some extent. But I question whether these simulations truly reflect reality. My motivation is to directly observe what is happening inside metals,” explains Professor Masakazu Kobayashi.

To date, practical experience in metal processing has indicated the existence of regions in which deformation tends to concentrate, and efforts have been made to locate these regions and create high strength materials. Obtaining the ability to predict such locations in advance could guide the development of even stronger materials in the future.

“When metal is repeatedly bent and stretched, it becomes harder and eventually fractures, but heating softens it again. During this process, atoms slide internally, forming crystal defects that alter the metal’s properties and increase its hardness, but heat application results in recrystallization and grain growth, causing the crystals to enlarge. However, these changes do not progress evenly across the metal, with some areas undergoing intense deformation and others not. Why does this happen? I would like to identify the regions where deformation preferentially takes place and investigate the reasons behind it.”

Approaching Microscopic Internal Changes in Metals at SPring-8

Conventional observation techniques such as electron microscopy have facilitated only surface level inspection of metals. Professor Kobayashi believes that metals are constrained by neighboring crystals; thus, internal changes in the metal do not necessarily match what appears on the surface.

“Observing a metal’s interior in three dimensions without destroying it requires X rays of extremely high intensity, far exceeding the levels used in medical CT. Therefore, beginning around 2004, we started conducting experiments at SPring-8, the world’s highest performing synchrotron radiation facility in Hyogo Prefecture. In these experiments, we examine changes that occur while pulling square bars of aluminum alloy roughly 500 to 600 µm thick.”

Aluminum alloys are selected because they are widely used, possess a relatively simple crystal structure (a face centered cubic (FCC) lattice) with slip as the primary deformation mechanism, and have a low melting point that permits many alloy variations to be fabricated in the laboratory. Another aim is to clarify the differences in deformation mechanisms among materials by modifying the alloying elements.

However, only a few methods are capable of nondestructively observing the interiors of metals in three dimensions.

“Understanding deformation in detail requires markers that indicate the location and magnitude of movement. We therefore developed a technique that tracks the amount of movement by adding lead particles to aluminum or by using the numerous micropores (voids) inside the material as markers. The sample undergoes repeated processes of slight stretching and imaging, which allows us to obtain three dimensional data describing the crystal orientation in each region, slip deformation directions, deformation concentration zones, and strain changes over time, which we then convert into images through computer analysis.”

These experimental techniques were created through many years of joint research with the Technical University of Denmark. Professor Kobayashi spent roughly a year conducting research at the Technical University of Denmark about 10 years ago through the university’s overseas training program, and has continued the collaboration to the present day, steadily refining his observational methods.

“The group involved in this research later launched a venture company and produced a commercial measurement instrument. Our laboratory is now collaborating with this company while conducting research using SPring-8.”

Discovery that Initially Deformed Regions Continue to Deform

What have the researchers begun to learn from the three dimensional data obtained in these observations? Professor Kobayashi carefully chooses his words as follows:

“It had been commonly assumed that once a region deformed, it hardened and became less likely to deform, and that other regions would then deform instead. However, our observations revealed that the region that deformed first continued to deform afterward. Conversely, the areas that had not deformed remained unchanged indefinitely. In short, metals contain regions that continue to move and others that barely move at all. These differences are thought to arise from specific interactions between adjacent crystal grains, influenced by factors such as grain shape and orientation. These findings will help advance our understanding of the mechanism of complex plastic deformation of crystal grains inside metals. If conventional simulations cannot reproduce this behavior, the simulation methods themselves will also need revision.”

If interactions between grains that initiate deformation can be clarified and the regions that continue to deform can be identified, it may become possible to design metals with greater resistance to fracture.

Professor Kobayashi emphasizes fundamental research to uncover underlying principles, but he is also engaged in collaborative projects with automotive parts manufacturers and others.

“With the rise of electric vehicles, companies that once specialized in producing automobile engines are seeking new fields. One promising area is manufacturing body components made from cast aluminum. In this process, recycled aluminum must be used for carbon neutrality, but recycled materials are more susceptible to impurities that can reduce toughness, raising concerns about ensuring safety in collisions. We are therefore studying why impurities make materials more brittle and to what extent such impurities are acceptable. This research is possible only because we possess the rare capability to observe crystal deformation directly.”

Another advantage of the Kobayashi Laboratory is its strong connection to SPring-8. Although the facility is open to researchers from Japan and overseas, many companies hesitate to use it because of the advanced expertise required and the need to apply in advance.

“I have used this beamline for over 20 years and know the staff well, so if any company wishes to use it, we can help connect them with the appropriate personnel.

I believe future materials research will focus not only on durability but also on recyclability and controlling how materials fail. Achieving this requires a thorough understanding of deformation and fracture mechanisms. Materials research in metals is one of Japan’s strengths, and I hope to continue contributing to its progress,” says Professor Kobayashi. Further exploration of these fundamental principles is eagerly anticipated.

Reporter's Note

Professor Kobayashi has been fascinated by machines since childhood and studied mechanical engineering in university with the idea that he might eventually work in the automotive sector. “I discovered the compelling intersection of physics and chemistry found in metallic materials research while studying crystal growth simulations in graduate school,” he recalls. This period also coincided with advances in two dimensional crystal orientation measurement tools and computer-based image analysis using digital data. “My enduring interest has been in uncovering what cannot be seen,” he says. This curiosity aligns with one of humanity’s deepest fascinations. We look forward to seeing what future research reveals about the unseen worlds that lie inside metals.