The preparation process of nanocrystalline materials is studied in depth to manufacture lighter, stronger and more durable parts and components, thereby improving the performance and longevity of products.
Nanomaterials exhibit extraordinary strength and certain ductility, which gives them special physical properties in some cases. To take full advantage of these material properties in structural applications, nanomaterials need to be fabricated in bulk form, and over the past few decades, scientists have employed severe plastic deformation techniques.
Nanomaterials and nanolayered materials have been successfully fabricated using methods such as wire drawing, cumulative rolling bonding, channel angle extrusion, and channel angle extrusion and cumulative rolling bonding, both processes that can produce nanomaterial sheets and bars in sizes suitable for structural applications.
Small-sized nanostructures, mainly for cubic crystal metals, and the composite combination of the two, namely FCC-FCC and FCC-BCC, many metals with practical application value, have hexagonal dense structures, such as zirconium, magnesium and titanium.
Researchers have tried to use the cumulative rolling bonding method to prepare nanolayered Zr-Nb composites, but on an engineering scale, it has been very difficult to fabricate nanoscale hexagonal dense stacked metals, and previous methods can only obtain a small amount of nano-Zr materials.
At present, nanolayered Zr-Nb composites can be prepared using magnetron sputtering or other deposition methods, and nanograin Zr can be prepared by high-pressure torsion.
However, high-pressure torsion is not suitable for large-scale production and will cause the α-Zr phase to be converted into the ω-Zr phase and the β-Zr phase, although the ball milling method can produce about 5 nanometer crystalline Zr, but some Zr will also be transformed into a face-centered cubic phase.
Previously, the method of cumulative rolling bonding could be used to manufacture Zr-Nb layered composites, but when the layer thickness reached 4 microns, it was stopped by the formation of shear bands, and then the microstructure of ultrafine grains was formed during the cumulative rolling bonding process of single-phase Zr.
This nanocrystalline Zr is formed during annealing, not refinement, and shows poor thermal stability.
With the increase of annealing temperature, the volume fraction of nanocrystals decreases sharply, and in order to meet the needs of engineering applications, researchers need to explore more efficient ways to fabricate large-scale nanoscale hexagonal dense metal materials to achieve their excellent mechanical properties and thermal stability.
To prepare nanolayered Zr/Nb materials, the researchers first used high-purity Zr and Nb sheets that have a thickness of 2 mm and can show preferential orientation, and equiaxed grains with an average grain size of 40 microns.
Second, the researchers used a process called ARB for material processing, sandwiching a 2 mm thick Nb sheet between two 1 mm thick Zr sheets, through this sandwich technology, the new Nb-Zr interface is formed only at the first roll, and the number of layers gradually increases as the roll joining step is repeated, while the thickness of the individual layers decreases.
During this process, various instability factors such as clamping, disbonding and shearing bands can hinder the refinement of the layer. Based on early mathematical mechanical models that showed that these problems could be avoided when the ratio of flow stresses between the two components was low, the researchers performed a static annealing step in the process that lasted 60 minutes.
And after each roll connection, annealing is required, corresponding to an incremental strain interval of 1.4. To study the chemical clarity of the Zr-Nb interface, the researchers used energy dispersive spectroscopy, which was analyzed in transmission electron microscopy, while EDS line scans performed on samples with a layer thickness of 92 nanometers showed no signs of mixing.
In layers within 4.5 nanometers of the interface, different pure phase chemistries are shown, and EDS lines are included to scan the results, which clearly show the rapid phase transition between Zr and Nb.
To detect fast phase transitions, the researchers measured the volumetric texture of each phase at all length scales using neutron diffraction.
Unlike the nanostructure Zr produced by HPT, the nanostructure prepared by ARB did not undergo measurable allotrope phase transitions with Zr, and in order to study the effect of annealing on the microstructure and texture development of ARB process, the researchers observed the texture changes of the 26 micron layer before and after annealing.
After annealing, both Zr and Nb showed a similar response to already rolled HCP and bcc materials, with Nb maintaining a weaker rolling texture before and after annealing, while Zr showed signs of reorientation after annealing, deviating from the initial orientation.
To further evaluate the morphological changes of grains and layers, the researchers scanned the sample multiple times using electron backscatter diffraction.
In the preparation of nanograin Zr blocks, it is an effective method to suppress instability through the biphasic interface, while the instability of grain size is a common problem during the preparation of nanocrystalline materials, because small grains have high surface freedom and large microstrain.
This preparation method can be widely used in the field of materials science and engineering for the preparation of high-performance nanocrystalline materials. It not only provides better mechanical properties and thermal stability, but is also expected to be used in fields such as additive manufacturing, electronics, and catalysts, advancing materials science.
