Internal stresses measured in sub-micrometer volumes

Sheet metal destined to become part of a car body is given the desired shape by bending the metal over a die. Unfortunately, the shape changes after it is removed from the die, a phenomenon known as spring-back.  This bugaboo of the metal forming industry is hard to predict and control due to a lack of understanding of stress distribution in the material. Working with DOE and automotive industry partners, the Metallurgy Division in MSEL is developing new techniques to measure local stresses to allow an understanding that could help improve springback control and other phenomenon.

At the atomic level, during deformation crystalline imperfections, or “dislocations,” arrange themselves to produce micrometer-sized cells of relatively perfect material bounded by walls of highly defective material. The result is an inhomogeneous distribution of internal stresses.  For twenty years, the macroscopic behavior of deformed metal has been understood in terms of the so-called “composite model” of this non-uniformity of stress in dislocation cell structures.  However, there was never any direct, experimental confirmation, as stresses could only be measured on a macroscopic scale, requiring averaging over thousands of dislocation cells.

The Metallurgy Division has been working with a team of researchers at ORNL and the DOE Advanced Photon Source (APS) to develop an x-ray measurement technique that probes the stresses inside individual dislocation cells. The technique is capable of acquiring data from regions as small as 0.07 μm³, which allows one to “look inside” an individual cell. The team presented their first results on the distribution of stresses within cells in a 2006 Nature Materials paper. They demonstrated that the averaged behavior measured using bulk techniques was indeed made up of the contributions from individual dislocation cells, which confirmed one of the central assumptions of the composite model. In addition, they found that the stresses exhibited large unpredicted fluctuations over micrometer length scales.  In recognition, the Advanced Photon Source named their work one of its Outstanding Science Accomplishments of 2006.

Since that first publication the team has extended the technique to measure the distribution of stresses within cell walls. In 2008, they refined the technique even further, and are now able to measure the distribution of stresses inside an individual cell. This capability will enable them to test other critical assumptions of the composite model.