The importance of cryogenic toughness in metals
One of the most critical measures of materials designed for space exploration, cryogenic storage, ice breaking, and other extreme applications is their ability to maintain impact toughness and tensile strength when exposed to frigid temperatures. However, most metals and materials become increasingly brittle as temperatures drop due to restricted movement of atoms in the crystal lattice. Put simply, cold weather makes it easier for materials to crack.
But not all materials are equally susceptible to the effects of temperature. For example, cryogenic metals like aluminum, titanium, and some steel alloys are well suited to colder applications. Some of the most common of these cryogenic metals are austenitic steel alloys, which are characterized by the presence of alloying elements like nickel, chromium, and—in some varieties—molybdenum and titanium.
Analyzing cryogenic properties of metals
When searching for the right metals for cryogenic applications, researchers rely on experimental methods that can characterize brittle behavior at the atomic level to help them understand which metals possess good cryogenic toughness. They often use two approaches: mechanical testing to understand how the material responds to stress, strain, and impact and high-resolution imaging to understand how those forces affect the material’s atomic structure.
The mechanical tests include strength, elongation, and Charpy impact toughness, which measures the energy absorbed by a sample while breaking under impact. These tests can be performed both at room temperature and under cryogenic conditions.
Imaging techniques include scanning electron microscopy, electron backscatter diffraction, and X-ray computed tomography, all of which can investigate material structures down to the micrometer scale. With the right image processing and segmentation software, X-ray tomography in particular can visualize a full 3D image, decompose it into individual layers, and numerically characterize every defect. Ultimately, this granular detail is invaluable to improving material performance.
Enhancing cryogenic metal toughness
Using a combination of these techniques, researchers recently found that a low-cost, fine-grained variety of austenitic steel known as Fe-30Mn-0.11C possesses an unusual degree of cryogenic toughness. The study subjected a sample of the fine-grained steel as well as a sample with coarser grain to impact testing and then used micro-computed tomography to see what cracks formed. The testing was conducted at both room temperature and liquid nitrogen temperature.
Ultimately, the coarse-grained sample followed the typical toughness-temperature dependence: it decreased in toughness by about 38% when the temperature was lowered. The fine-grained sample, on the other hand, showed inverse temperature dependence: the impact toughness increased by 36% as the temperature dropped. These results show that simply changing the grain size of a relatively cheap material with high manganese content delivers properties that are crucial for cold applications.
While many of the materials designed to resist cryogenic impacts use high-entropy alloys that are expensive to manufacture, the smaller particle size of Fe-30Mn-0.11C offers an alternative that is both more affordable and stronger. This principle could potentially be applied to other steel alloys as well, which could lead to even more abundant, affordable alloys for applications like cryogenic storage.
3D fracture surface morphology using micro-computed tomography of fine- and coarse-grained samples.
Avizo Software for metals and alloys characterization
The study also highlights the game-changing capabilities of Thermo Scientific Avizo Software, which was used to visualize and segment imaging data and to measure features of interest. The software sorted fracture types to help the team better understand the position, shape, and dimensions of the cracks formed during impact tests.
Avizo Software automates and simplifies many of the complex steps involved in image processing and analysis, helping researchers maximize data insights and fostering new possibilities in materials science. From a single environment, it provides optimized workflows for advanced materials characterization and quality control.
Learn more about Avizo Software >
Luigi Raspolini is a Product Marketing Manager at Thermo Fisher Scientific
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