Irradiation- Induced Extremes Create Hierarchical Face- /Body- Centered- Cubic Phases in Nanostructured High Entropy Alloys

A nanoscale hierarchical dual- phase structure is reported to form in a nanocrystalline NiFeCoCrCu high- entropy- alloy (HEA) film via ion irradiation. Under the extreme energy deposition and consequent thermal energy dissipation induced by energetic particles, a fundamentally new phenomenon is revealed, in which the original single- phase face- centered- cubic (FCC) structure partially transforms into alternating nanometer layers of a body- centered- cubic (BCC) structure. The orientation relationship follows the Nishiyama- Wasser- man relationship, that is, (011)BCC || (- 1¯1¯1)FCC and [100]BCC || [- 11¯0]FCC. Simulation results indicate that Cr, as a BCC stabilizing element, exhibits a tendency to segregate to the stacking faults (SFs). Furthermore, the high densities of SFs and twin boundaries in each nanocrystalline grain serve to accelerate the nucleation and growth of the BCC phase during irradiation. By adjusting the irradiation parameters, desired thicknesses of the FCC and BCC phases in the laminates can be achieved. This work demonstrates the controlled formation of an attractive dual- phase nanolaminate structure under ion irradiation and provides a strategy for designing new derivate structures of HEAs.A nanoscale hierarchical dual- phase structure is reported to form in a nanocrystalline NiFeCoCrCu high- entropy- alloy film via ion- irradiation- induced face- centered- cubic to body- centered- cubic phase transformation. Both kinetic and thermodynamic conditions for the phase transformation are explored. The results provide a new strategy for tailoring material structures on the nanometer or sub- nanometer scales.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/162803/3/adma202002652_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/162803/2/adma202002652.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/162803/1/adma202002652-sup-0001-SuppMat.pd

Evolution of microstructure and nanohardness of SiC fiber-reinforced SiC matrix composites under Au ion irradiation

Abstract(#br)Evolution of microstructure and nanohardness of a new type of SiC f /SiC composite under a 6 MeV Au ion irradiation up to 90 displacements per atom at 400 °C was studied. Scanning transmission electron microscopy reveals that the irradiation has induced enrichment of carbon at the grain boundaries in the fibers. This is attributed to the accumulation of C interstitials generated by the irradiation. The disappearance of {200} diffraction ring of 3C–SiC indicates that a phase transition from 3C–SiC to Si has occurred during irradiation. In addition, the hardness of SiC fiber increased after irradiation, which is due to the pinning effect caused by irradiation-induced defects. The pyrolytic-carbon interphase that contains Si-rich nano-grains in the composite has the highest irradiation tolerance as it maintained its basic morphology and graphitic nature after a radiation damage dose up to 90 dpa. Twins are the main internal defects in the SiC matrix of the SiC f /SiC composite, which grew up and resulted in the decrease of the number of twinning boundaries under irradiation. No significant microstructure change has been observed in the SiC matrix except a limited number of dislocation loops at the peak irradiation damage region. The entire matrix still maintained its hardness after irradiation

Effects of Sink Strength and Irradiation Parameters on Defect Evolution in Additively Manufactured HT9

A ferritic-martensitic (FM) steel, HT9, has been studied for use in advanced nuclear reactors for its excellent swelling resistance and high-temperature strength. However, the effects of sink strength (SS) on tailoring the radiation responses in HT9 have not yet been fully studied. The advancement of the mechanistic understanding of how SS affects microstructural evolution is of great importance to the development of new radiation-tolerant materials in the future. Additive-manufacturing (AM) has been drawing attention due to the advantage of its ability to control the complex geometry and composition of the structural components. The SS effects on radiation responses are studied within by using the as-built (ASB) and the post-built heat-treated (called ACO3 and FCRD) AM-HT9, with their starting SSs significantly differ from each other. Heat A of AM-HT9 in this study demonstrated a SS of 12.2×1015/m2 in the ASB and a nearly 5-time reduction in SS to 2.4×1015/m2 and 2.7×1015/m2 for the ACO3 and FCRD specimens, respectively. Ion-irradiations focusing on irradiation dose and temperature are conducted to systematically study the radiation responses and defect evolutions in AM-HT9 alloys. Experimental results showed that the high SS in the ASB drastically suppresses all microstructural evolution with damage levels up to 250 dpa. A higher normalization temperature used in the ACO3 results in a reduction of SS compared to the FCRD, leading to a ten-time-higher swelling rate in ACO3 after irradiation to 250 dpa. The complicated microstructural evolution including all features contribute to evolving defect sinks that collectively tailor the swelling behavior in the AM-HT9, which is verified using a simplified rate-theory model that considers the ratio of biased to neutral SS, Q. It was found that the analytical model does highlight both the overall SS and the balance between biased and neutral sinks are strong indicating factors for increased swelling resistance in AM-HT9. In addition, the Ni/Si/Mn-rich precipitate and dislocation loop coarsening processes are captured with increasing damage levels, whereas these processes are either observed to complete at much lower damage levels, or even not observed indicating an early saturation occurring in the ACO3/FCRD that contain about 5-time lower SS in the starting microstructures. Irradiation temperatures also greatly affected the radiation responses of AM-HT9. The density of a⟨100⟩ type dislocation loops dropped from 3.2×1021/m3 to 3.0×1020/m3 in the ASB specimen and dropped from (5.9-6.4)×1021/m3 to (0.3–0.4)×1021/m3 in the ACO3/FCRD heat-treated specimens. In addition, the higher irradiation temperatures stabilize the a⟨100⟩ loops and enables higher coarsening rates than lower irradiation temperatures. The precipitate evolution is greatly accelerated by the available kinetic energy at higher temperatures to overcome the pinning effects imposed by high SS, while cavity swelling exhibits the typical bell-shaped curve in the heat-treated ACO3/FCRD specimens with varying peak swelling temperatures by 30°C tailored by SSs. The overall result of this work is a wide range of microstructural responses under irradiation that can be obtained by AM fabrication with post-build heat-treatments, through the tuning of SS in the starting and the irradiated microstructures. The radiation response then needs to balance with other factors that are tied to the sink strength of AM-HT9 alloys including the mechanical properties such as tensile strength and fracture toughness. These insights obtained will stimulate further the optimization of using AM to fabricate materials that are highly radiation tolerant for advanced nuclear reactor applications.PHDNuclear Engineering & Radiological SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/172642/1/xiupy_1.pd

Enhanced void swelling in NiCoFeCrPd high-entropy alloy by indentation-induced dislocations

The role of dislocations on ion irradiation-induced void formation is studied in a high-entropy alloy (HEA) NiCoFeCrPd. Despite previous observations that show high-entropy alloys are swelling resistant due to a high defect recombination rate, the swelling is enhanced with increasing density of pre-existing dislocations at low strain levels that shortened transient duration before the onset of void swelling. Under certain irradiation conditions, a high density of dislocations may carry the material closer to the sink-dominated regime. Compared to another HEA NiCoFeCrMn, NiCoFeCrPd has a smaller loop size and higher loop density due to the stronger lattice distortion

Irradiation‐Induced Extremes Create Hierarchical Face‐/Body‐Centered‐Cubic Phases in Nanostructured High Entropy Alloys

A nanoscale hierarchical dual- phase structure is reported to form in a nanocrystalline NiFeCoCrCu high- entropy- alloy (HEA) film via ion irradiation. Under the extreme energy deposition and consequent thermal energy dissipation induced by energetic particles, a fundamentally new phenomenon is revealed, in which the original single- phase face- centered- cubic (FCC) structure partially transforms into alternating nanometer layers of a body- centered- cubic (BCC) structure. The orientation relationship follows the Nishiyama- Wasser- man relationship, that is, (011)BCC || (- 1¯1¯1)FCC and [100]BCC || [- 11¯0]FCC. Simulation results indicate that Cr, as a BCC stabilizing element, exhibits a tendency to segregate to the stacking faults (SFs). Furthermore, the high densities of SFs and twin boundaries in each nanocrystalline grain serve to accelerate the nucleation and growth of the BCC phase during irradiation. By adjusting the irradiation parameters, desired thicknesses of the FCC and BCC phases in the laminates can be achieved. This work demonstrates the controlled formation of an attractive dual- phase nanolaminate structure under ion irradiation and provides a strategy for designing new derivate structures of HEAs.A nanoscale hierarchical dual- phase structure is reported to form in a nanocrystalline NiFeCoCrCu high- entropy- alloy film via ion- irradiation- induced face- centered- cubic to body- centered- cubic phase transformation. Both kinetic and thermodynamic conditions for the phase transformation are explored. The results provide a new strategy for tailoring material structures on the nanometer or sub- nanometer scales.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/162803/3/adma202002652_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/162803/2/adma202002652.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/162803/1/adma202002652-sup-0001-SuppMat.pd

High- Entropy Alloys: Irradiation- Induced Extremes Create Hierarchical Face- /Body- Centered- Cubic Phases in Nanostructured High Entropy Alloys (Adv. Mater. 39/2020)

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/162697/1/adma202070294.pd