Grain Boundary Cracks

Much like the grain in wood, steel has a grain structure, and when steel cracks it tends to do it along the boundaries between grains. At high-strength levels, steel is very sensitive to impurities, and even hydrogen absorbed from the air can cause the grain boundaries to crack. Research has shown that when phosphorus, a common steel impurity, is located at a grain boundary, the steel is more susceptible to this hydrogen-induced cracking. "We know what the bad actors are," says Olson, "but with the supercomputer, we're getting at the electronic mechanism of how they do it."

In collaboration with solid-state physicist Art Freeman of Northwestern, Olson has done quantum theory calculations at Pittsburgh that delve into the roots of the problem --how the electrons associated with iron and phosphorus atoms interact at the grain boundary. Using a highly precise and computationally demanding approach (known as the full-potential linearized augmented plane wave method, FLAPW for short), they have addressed this question of why phosphorus atoms at the grain boundary reduce resistance to cracking. Their results, says Olson, detailed in an article in Science (July 15, 1994) provide "the first definitive electronic-level answer."

Phosphorus and Grain Boundary Cracking
These contour plots from Greg Olson's quantum calculations at the Pittsburgh Supercomputing Center (with Art Freeman and Ruqian Wu) show how phosphorus at a grain boundary affects electron distribution in steel. In the top panels, color corresponds to electron density with yellow representing the highest density decreasing through green, blue and purple. The left panel shows iron atoms and a phosphorus atom at the grain boundary (the boundary runs horizontally through the center of the panel). The right panel shows the same steel fractured along the boundary with the top segment removed, so that phosphorus is at the "free surface" of the crack.

The lower panels are difference plots --showing how the electron distribution changes when phosphorus is in the steel compared to only iron. Electron gain is shown in pink (greatest), red, yellow and green, and electron loss is purple and blue (more negative). These plots show that phosphorus bonds much more strongly with the iron atom below it at the free surface (large pink area) than it does at the grain boundary. "Phosphorous at the free surface has lowered energy," explains Olson, "which means that phosphorus at the grain boundary reduces the work of fracture-promoting embrittlement of the steel."

As part of this work, Olson and SRG colleagues developed a new method to remove phosphorus from the grain boundaries, and steel they produced this way has significantly higher resistance to hydrogen-induced cracking. With future calculations on the CRAY C90 at Pittsburgh, they will explore how other alloy-elements like boron and molybdenum affect grain-boundary cracking. The objective, says Olson, is to design steel the way semiconductors are designed, with elements deliberately added to get the electronic structure that will provide precisely the properties needed to satisfy the demands of a particular application.

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