What is the new discovery?
Hexagonal metals have anisotropic coefficients of thermal expansion causing grain-level internal stresses during heating. High energy x-ray diffraction microscopy, a non-destructive, in situ, micromechanical and microstructural characterization technique, has been used to determine the anisotropic coefficients of thermal expansion (CTEs) for Ti-7Al. Two samples of polycrystalline α-phase Ti-7Al were continuously heated from room temperature to 850 °C while far-field HEDM scans were collected. The results showed a change in the ratio of the CTEs in the ‘a’ and ‘c’ directions which explains discrepancies found in the literature. The CTE additionally appears to be affected by the dissolution of α2 precipitates. Analysis of the grain-resolved micromechanical data also shows reconfiguration of the grain scale stresses likely due to anisotropic expansion driving crystallographic slip.
Why is it important? What are the broader impacts of this work?
In this work, the scientists show that ff-HEDM data can be used to characterize the anisotropic thermal expansion of Ti-7Al, which is critical to understanding residual stresses that impact how and when the material ultimately fails. They also show that ff-HEDM data can measure the effects of the grain-interactions caused by the constraints of a polycrystalline aggregate.
Future work on this data will focus on modeling this experiment with both FE and FFT-based crystal plasticity methods. This will allow us to gain a better understanding of what is occurring on the grain-scale and further tune the CTEs from those calculated in this work. This data has shown that there is more happening on the mesoscale than originally expected, and understanding the anisotropy of thermal expansion is important for the accuracy of micromechanical modeling work.
Why did this research need CHESS?
Conventional, bulk methods of determining CTEs are not capable of measuring the expansion of a polycrystalline material along specific crystal directions. This leads to the need for x-ray measurements, which can differentiate between expansion along separate crystal directions. The x-ray method used here, ff-HEDM, takes this one step further than x-ray powder diffraction by allowing the measurement of the strain tensor for individual grains. The CHESS beamline where these measurements were taken has been optimized for this type of in-situ ff-HEDM experiment, from the availability of high-energy x-rays to the custom-built x-ray transparent furnace used to heat the samples.
How was the work funded?
This work is based upon research conducted at the Cornell High Energy Synchrotron Source (CHESS) which is supported by the National Science Foundation and the National Institutes of Health/National Institute of General Medical Sciences under NSF award DMR-1332208. The work is funded by the Air Force Office of Scientific Research under grant FA9550-16-1-0105.
Rachel E. Lim, Darren C. Pagan, Donald E. Boyce, Joel V. Bernier, Paul A. Shade, Anthony D. Rollett, Grain-resolved temperature-dependent anisotropy in hexagonal Ti-7Al revealed by synchrotron X-ray diffraction, Materials Characterization, Volume 174, 2021