Plasticity of Nanotwinned Aluminum and Aluminum Alloy

Nanotwinned (nt) metals have been intensively studied and has shown unique mechanical properties, including high strength and high ductility. Although twins can be introduced into face-centered-cubic (fcc) metals by annealing (annealing twins), deformation (deformation twins) and growth (growth twins), most of these twinned metals have low stacking fault energy (SFE). The twinnability of fcc metals remains largely controlled by their SFE. Consequently, the high SFE of Al typically prohibits the formation of twins in aluminum (Al). This dissertation focuses on the introduction of several innovative strategies that can introduce high density growth twins in Al and Al alloys and study the influence of twinnability on strengthening and plastic deformation of these twinned alloys. The growth twins were observed in a polycrystalline Al thin film fabricated by magnetron sputtering. And the twin formation mechanism was discussed in a thermodynamic view. Then, we show that high-density twin boundaries can be introduced in Al films by tailoring the texture of the films without any seed layers. Transmission Kikuchi diffraction and transmission electron microscopy studies on (111), (110) and (112) textured Al films. Epitaxial Al (112) film has the highest density of ITBs, because the twin variants (335) and (535) are separated by Al (102) islands, promoting the formation of ITBs. The smaller domain size can thus be achieved by introducing HAGBs into the twinned bicrystal structure to inhibit the abnormal growth of single variant. Furthermore, twin boundaries in Al appear to be stronger barriers to dislocations than conventional high angle grain boundaries. Besides tailoring the twin structure by changing the growth orientation, alloying has been used in an Al matrix. The high strength epitaxial AlMg alloy has been fabricated with a high density twinned structure. The strong ITB barriers play an important role to strengthen the film. Combined with the solid-solution strengthening, the calculated flow stress correlated well with the experimental data. The knowledge derived from this study may facilitate the design of high-strength, light-weight, and ductile Al alloys

Nanoscale stacking fault-assisted room temperature plasticity in flash-sintered TiO2.

Ceramic materials have been widely used for structural applications. However, most ceramics have rather limited plasticity at low temperatures and fracture well before the onset of plastic yielding. The brittle nature of ceramics arises from the lack of dislocation activity and the need for high stress to nucleate dislocations. Here, we have investigated the deformability of TiO2 prepared by a flash-sintering technique. Our in situ studies show that the flash-sintered TiO2 can be compressed to ~10% strain under room temperature without noticeable crack formation. The room temperature plasticity in flash-sintered TiO2 is attributed to the formation of nanoscale stacking faults and nanotwins, which may be assisted by the high-density preexisting defects and oxygen vacancies introduced by the flash-sintering process. Distinct deformation behaviors have been observed in flash-sintered TiO2 deformed at different testing temperatures, ranging from room temperature to 600°C. Potential mechanisms that may render ductile ceramic materials are discussed

9R phase enabled superior radiation stability of nanotwinned Cu alloys via in situ radiation at elevated temperature

Nanotwinned metals exhibit outstanding radiation tolerance as twin boundaries effectively engage, transport and eliminate radiation-induced defects. However, radiation-induced detwinning may reduce the radiation tolerance associated with twin boundaries, especially at elevated temperatures. Here we show, via in-situ Kr ion irradiation inside a transmission electron microscope, that 3 at. % Fe in epitaxial nanotwinned Cu (Cu97Fe3) significantly improves the thermal and radiation stability of nanotwins during radiation up to 5 displacements-per-atom at 200 °C. Such enhanced stability of nanotwins is attributed to a diffuse 9R phase resulted from the dissociation of incoherent twin boundaries in nanotwinned Cu97Fe3. The mechanisms for the enhanced stability of twin boundaries in irradiated nanotwinned alloys are discussed. The stabilization of nano-twins opens up opportunity for the application of nanotwinned alloys for aggressive radiation environments. Includes supplemental Appendix. Video files are attached below

Deformation mechanisms of flash sintered yttria-stabilized zirconia via in situ micromechanical testing

Flash sintering has been applied to sintering a variety of ceramic materials. However, the mechanical behavior of flash-sintered ceramics is less well understood. In this study, the deformation mechanisms of flash-sintered yttria stabilized zirconia (YSZ) were investigated via in-situ microcompression test at temperatures of 25 to 650oC. The flash sintered YSZ exhibits high fracture strain due to transformation induced toughening below the test temperatures of 400oC. At higher temperatures, crack nucleation and propagation are significantly retarded, and no more catastrophic failures are observed. Strain rate jump tests were also performed at elevated temperature (450 ~ 650oC) to investigate the temperature dependent deformation mechanisms. The activation energy for deformation and its implication are discussed

High-Strength Nanotwinned Al Alloys with 9R Phase

Light-weight aluminum (Al) alloys have widespread applications. However, most Al alloys have inherently low mechanical strength. Nanotwins can induce high strength and ductility in metallic materials. Yet, introducing high-density growth twins into Al remains difficult due to its ultrahigh stacking-fault energy. In this study, it is shown that incorporating merely several atomic percent of Fe solutes into Al enables the formation of nanotwinned (nt) columnar grains with high-density 9R phase in Al(Fe) solid solutions. The nt Al–Fe alloy coatings reach a maximum hardness of ≈5.5 GPa, one of the strongest binary Al alloys ever created. In situ uniaxial compressions show that the nt Al–Fe alloys populated with 9R phase have flow stress exceeding 1.5 GPa, comparable to high-strength steels. Molecular dynamics simulations reveal that high strength and hardening ability of Al–Fe alloys arise mainly from the high-density 9R phase and nanoscale grain sizes

High-Strength Nanotwinned Al Alloys with 9R Phase

Light-weight aluminum (Al) alloys have widespread applications. However, most Al alloys have inherently low mechanical strength. Nanotwins can induce high strength and ductility in metallic materials. Yet, introducing high-density growth twins into Al remains difficult due to its ultrahigh stacking-fault energy. In this study, it is shown that incorporating merely several atomic percent of Fe solutes into Al enables the formation of nanotwinned (nt) columnar grains with high-density 9R phase in Al(Fe) solid solutions. The nt Al–Fe alloy coatings reach a maximum hardness of ≈5.5 GPa, one of the strongest binary Al alloys ever created. In situ uniaxial compressions show that the nt Al–Fe alloys populated with 9R phase have flow stress exceeding 1.5 GPa, comparable to high-strength steels. Molecular dynamics simulations reveal that high strength and hardening ability of Al–Fe alloys arise mainly from the high-density 9R phase and nanoscale grain sizes

NO reduction with CO on low-loaded platinum-group metals (Rh, Ru, Pd, Pt, and Ir) atomically dispersed on Ceria

Low-loaded platinum-group single-atom catalysts on CeO2 (M1/CeO2) were synthesized via high-temperature atom trapping (AT) and tested for the NO + CO reaction under dry and wet condition. The activity of these catalysts for NO+CO reaction follows the order Rh > Pd ≈ Ru > Pt > Ir. For Rh, Ru, and Pd single-atom catalysts, the N2O byproduct is formed but not clearly observed on Ir and Pt cases, which may result from the higher reaction temperature (> 200oC) required for Pt and Ir catalysts. The presence of water can promote the activity of these M1/CeO2 catalysts for NO + CO reaction. Under wet conditions, significant NH3 formation occurred during the reaction, which is due to the co-existence of water-gas-shift reaction on these catalysts. Compared with Pt, Pd and Ir, the Rh and Ru single-atom catalysts show higher selectivity to NH3 species, resulting from the more hydride species on the surface. Among all tested catalysts, Ru1/CeO2 shows the highest production of ammonia and highest CO conversion due to excellent water-gas-shift activity, whereas Pd1/CeO2 shows lowest ammonia production

9R phase enabled superior radiation stability of nanotwinned Cu alloys via in situ radiation at elevated temperature

Nanotwinned metals exhibit outstanding radiation tolerance as twin boundaries effectively engage, transport and eliminate radiation-induced defects. However, radiation-induced detwinning may reduce the radiation tolerance associated with twin boundaries, especially at elevated temperatures. Here we show, via in-situ Kr ion irradiation inside a transmission electron microscope, that 3 at. % Fe in epitaxial nanotwinned Cu (Cu97Fe3) significantly improves the thermal and radiation stability of nanotwins during radiation up to 5 displacements-per-atom at 200 °C. Such enhanced stability of nanotwins is attributed to a diffuse 9R phase resulted from the dissociation of incoherent twin boundaries in nanotwinned Cu97Fe3. The mechanisms for the enhanced stability of twin boundaries in irradiated nanotwinned alloys are discussed. The stabilization of nano-twins opens up opportunity for the application of nanotwinned alloys for aggressive radiation environments. Includes supplemental Appendix. Video files are attached below

High-velocity projectile impact induced 9R phase in ultrafine-grained aluminium

Aluminium typically deforms via full dislocations due to its high stacking fault energy. Twinning in aluminium, although difficult, may occur at low temperature and high strain rate. However, the 9R phase rarely occurs in aluminium simply because of its giant stacking fault energy. Here, by using a laser-induced projectile impact testing technique, we discover a deformation-induced 9R phase with tens of nm in width in ultrafine-grained aluminium with an average grain size of 140 nm, as confirmed by extensive post-impact microscopy analyses. The stability of the 9R phase is related to the existence of sessile Frank loops. Molecular dynamics simulations reveal the formation mechanisms of the 9R phase in aluminium. This study sheds lights on a deformation mechanism in metals with high stacking fault energies