What did the Scientists Discover?
At CHESS, in-situ loading experiments allowed researchers to analyze the effects of grain orientation to develop a better understanding of the crystallographic slip mechanism that contributes to the non-standard fracture mechanism known as delamination. Results established a strong correlation between grain orientation relative to the loading direction (Schmid factor) and the occurrence of localized (planar) slip. Grains with the highest Schmid factors showed the largest impact of planar slip, with differences in the shear stress required to initiate plastic deformation of up to 40% between low and high Schmid factor grains and even occurrences of strain softening were observed.
Broader Impacts of this work?
Non-standard failure mechanisms such as delamination fracture create challenges when applying traditional criteria for damage tolerant structural designs, a problem NASA is looking to address. These new X-ray data are creating a path forward for the development of new microstructurally sensitive failure criteria.
Why is this important?
Aerospace vehicles, be it airplanes or rockets, require high strength-to-weight ratio materials to decrease mass and in turn, fuel costs. The creation of Al-Li alloys through small additions of lithium to aluminum offers significant weight savings (3% density reduction for every 1 wt.% of Li). However, the use of Al-Li alloys is difficult as they exhibit significant mechanical anisotropy and complex microscale deformation mechanisms, including a tendency for grain boundary fracture known as delamination. Also, shearing of strengthening precipitates can lead to planar slip and damage localization.
Why did this research need CHESS?
Al-Li alloys have been extensively studied for over 20 years, but high energy X-ray diffraction microscopy (HEDM) techniques performed at CHESS, are now offering the unprecedented capability to directly study these deformation mechanisms in-situ at the microscale.
Collaborators:
- Wesley A. Tayon, NASA
- Kelly E. Nygren, CHESS
- Roy E. Crooks, Black Laboratories
- Darren C. Pagan, CHESS
Publication Citation:
W.A. Tayon, K.E. Nygren, R.E. Crooks and D.C. Pagan In-situ Study of Planar Slip in a Commercial Aluminum-Lithium Alloy using High Energy X-ray Diffraction Microscopy Acta Materialia 173, 231 (2019).
How was the work funded?
The research at CHESS is supported under NSF award DMR-1332208.