Dislocations in Copper Observed to Form Preferentially at Twin Grain Boundaries
- PDF / 39,795 Bytes
- 1 Pages / 612 x 792 pts (letter) Page_size
- 80 Downloads / 156 Views
match to the NTSC-recommended blue. In the mean time, the researchers believe that they have found a material with a blue emission already suited for many other video display applications. PAMELA JOHNSON
Dislocations in Copper Observed to Form Preferentially at Twin Grain Boundaries A team of scientists from the University of Idaho, Washington State University, and the Idaho National Engineering and Environmental Laboratory (INEEL) has discovered that dislocations in metals tend to form at twin grain boundaries. The research team further reported that while grain size determines at what stress level a material begins to deform, it has little effect on the ultimate strength a material can attain through deformation processing. As published in the June issue of Acta Materialia, the researchers created copper 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 developed during recrystallization, 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. Using high-magnification microscopy, they periodically analyzed the deformed microstructure of each sample for clues to the formation of dislocations, dislocation density, and dislocation behavior. “This is a basic mechanism of dislocation behavior not previously reported,” said John Flinn, adjunct professor with the University of Idaho and retired INEEL researcher. 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, the researchers said. By observing when and where dislocations develop, the research team 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
MRS BULLETIN/SEPTEMBER 2001
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 µm to 60 µm 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 behavior 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 INEEL scientist Tom Lillo. For this research, t
Data Loading...
