Metallurgy: Role of grain boundaries highlighted

IN METAL tiny imperfections within the crystalline structure control whether it bends, stretches, or ultimately breaks in response to stresses produced by heavy physical loads. Researchers are offering up a new explanation for where imperfections form initially and how they affect material properties such as tensile strength in metals. These new ideas from an INEEL-led research team about where imperfections form may enable researchers to design high-strength alloys.

The team suggests that imperfections preferentially form on twin grain boundaries-areas where the atoms of one crystal grain lie in mirror image position to the atoms of a neighbouring crystal grain. What's new about this research is that researchers believe such boundaries play a far greater role in material strength than previously thought. Further, researchers argue that other than determining at what stress level a material begins to deform, grain size has little effect on the ultimate strength a material can attain through deformation processing. The team includes scientists from the University of Idaho, Washington State University, and the Department of Energy's Idaho National Engineering and Environmental Laboratory. They report their results in Acta Materialia.

"It's crucial for us to understand the formation and behaviour of imperfections in metal so that we can develop new strategies to control material properties," said INEEL scientist Tom Lillo.

Scientists know that imperfections in metal allow the material to move or flow at the atomic scale to accommodate stresses that develop under heavy physical loads.

This research team is interested in what happens after the elastic limit for the material is exceeded-the point at which the stress level is high enough to generate new imperfections and the material will no longer "spring back" to its original shape when the load is removed.

Straining metal beyond this elastic limit is called plastic deformation and increases the strength of metal once the density of imperfections becomes sufficient that they begin to interact with one another.

Researchers set out to identify where imperfections, called dislocations, form using commercially available, pure copper. They chose copper because the properties of the metal are already very well documented. The team created metal samples with a range of crystal sizes by first straining the samples to create a high density of imperfections, and then heat-treating the metal at different furnace temperatures. Different grain sizes develop during recrystallisation depending on both the temperature and the number of imperfections in the original material.

The team then tensile tested the material-stretching it at room temperature and monitoring the load generated by the applied strain. Researchers periodically analysed the deformed microstructure of each sample for clues to the formation of dislocations, dislocation density, and dislocation behaviour using high-magnification microscopy.

"The idea behind the research is to determine where the defects are coming from so we can design materials with better stress tolerance," said John Flinn, adjunct professor with the University of Idaho and retired INEEL researcher.

By analysing microscopic changes in the metal at various stages during tensile testing, researchers found clear evidence that dislocations preferentially form at twin grain boundaries.

"It's always been difficult to tell whether a grain boundary acts as a source or sink for dislocations, but in this work, we think we have a strong case for twin boundaries in particular, acting as sources of the dislocations," said Lillo. Very few dislocations formed at non-twin grain boundaries. "This is a basic mechanism of dislocation behaviour not previously reported," said Flinn.

This observation is a departure from conventionally accepted materials science theory stating that dislocations can form within the crystal grain itself or at any grain boundary-not just primarily at twin grain boundaries.

This is the crux of understanding the role grain size plays in material strength. Researchers know that materials with smaller grain size are stronger than materials with larger grain size-but haven't been able to explain why. The INEEL-led team, however, has demonstrated deformation behaviour that is clearly not explained by grain size alone.

By observing when and where dislocations develop, researchers documented that grain size plays a role only when plastic deformation begins. Materials with very small grain size can remain elastic longer than materials with larger grain size, and it takes more strain and higher stress to cause dislocations to develop.

However, after dislocations have developed, grain size makes little difference. The increasing resistance to further deformation (strengthening) as a function of strain once plastic deformation was initiated was the same for materials of all grain sizes. The team analyzed samples with grain sizes ranging from 3 to 60 micrometers and found that strain hardening from plastic deformation was completely independent of grain size.

"Once you exceed the elastic limit of a material, the deformation behaviour of the metal and improvements in mechanical strength from hardening is controlled by the interaction of one dislocation with another and not through interactions with grain boundaries," said Lillo.

For this research, the team used a relatively new technique to initially strain the copper samples to induce a very high density and uniform distribution of imperfections before heat treatment.

The combination of a high number of imperfections and low heat treatment temperatures enables researchers to create a range of grain sizes for experimentation.

Using the technique called Equal Channel Angular Extrusion (ECAE), metal is extruded through a die with an internal 90o corner, rotated, and re-extruded-a process roughly akin to kneading.

ECAE was much more effective in creating a high density of twin grain boundaries during heat treatment and subsequent recrystallization than the traditional cold-rolling. With ECAE, researchers achieved a density of almost 50 per cent twin grain boundaries.

The team plans a variety of follow-up research to further test and document their ideas. Their refined interpretation of both grain size and the role of twin grain boundaries could have important implications for materials science. "The reason we are pursuing this research is to better understand the fundamental mechanisms of defect formation and growth in metals, to be able to predict mechanical properties," said Flinn.

"The challenge now will be learning how to use this knowledge to design new high-strength, light-weight alloys." Such knowledge and will also help researchers to develop techniques for fabricating metals into various geometric forms for industrial utilization.

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