Development of Innovative Materials and Measurement Systems used for Radiation Environment

Research and development of innovative materials is being promoted in various fields to counter global warming and energy. In the fields of energy, nuclear power, space environment, radiation medicine, nuclear fusion, and accelerator-related equipment, radiation degradation of materials and equipment occurs, and various methods such as processing heat treatment, impurity addition, alloying, micro-crystallization, nano-clusters, Oxide Dispersive Strength (ODS) steels, composites, and nano-fiber materials [1-23] have been used to improve mechanical properties, corrosion resistance and irradiation resistance, and these techniques have produced successful results. Historical rate of improvement in the maximum operating temperature for four generations of structural steels, based on results, is summarized by Viswanathan [23]. A key strategy for designing high-performance radiation-resistant materials in many cases is based on the introduction of a high, uniform density of nanoscale particles that simultaneously provide good high temperature strength and radiation damage resistance.

While, as an innovative material, high-entropy alloys (HEAs) are known for their high strength and ductility and are expected to be used in a variety of applications. HE alloys, which are currently being developed all over the world, are mixtures of five or more elements, and since there is no state diagram, alloy design is difficult. However, due to the difference in atomic size, the crystal lattice is expected to be distorted at the atomic level compared to the normal case. In addition, due to the complexity of the structure, the conventional mechanism of formation and damage of irradiation defects cannot be fully explained. As a challenging recent study, a group at Los Alamos National Laboratory in the U.S. found that a new type of material called high-entropy (HE) W alloy (composed of W, Ta, V, and Cr, which are low-activation elements) has high irradiation resistance by using a magnetic sputtering deposition method that can control the grain size to submicron and controlling the substrate temperature and it was found to have high irradiation resistance [24]. Not satisfied with the conventional materials developed so far, including iron-based alloys, challenging researches with even higher strength and ductility are being investigated by advanced institutions around the world, using the properties of high-entropy alloys and further improving them by introducing such as intermetallic compounds [25-29].

On the other hand, the defect clusters formed by irradiation have not been found to exceed the order of 1023m-3 above room temperature [30-34]. This suggests that high-density nanoclusters, or materials with properties similar to those of nanoclusters before irradiation, may exhibit high irradiation resistance. Materials such as titanium and zirconium alloys, in which high-density nanoclusters can be controlled by a simple heat treatment method, are expected to have high strength, high ductility, and high corrosion resistance, and are expected to be applied in various ways in the future. Recently, it has been shown that metastable β-Ti alloys with a body-centered cubic lattice structure have high-density nanoclusters before and after irradiation, which act as sink sites for interstitial atoms and vacancies bounced away from their lattice positions induced by collisions with high-energy particles and have high irradiation resistance [5]. Based on these two findings, we can expect to create new special materials that contain dispersive nano particles with very high number density or ultra-fine microstructure or nanocomposite structures with revolutionary irradiation resistance.

However, previous studies have shown that existing irradiation damage models cannot be applied to these materials with complex internal structures, and in order to identify very small (sub-nano to nm order) irradiation defects formed in the materials, new measurement systems beyond the three-dimensional atom probe (APT) method and high-resolution scanning electron microscopy (including High Z-contrast, atomic weight) have been found to be necessary to identify very small (sub-nano to nm order) irradiation defects in materials. Non-destructive measurement methods utilized with developed Surface Acoustic Wave (SAW) technique [35,36] and a magnetic flux density measurement system [37] were examined in the irradiated specimens and the relation between these measurement results and irradiation hardening. Nanoindentation techniques for examination of irradiation hardening behavior were also evaluated in ion irradiation experiment [38].

Figure 1: Features of the multi-type simultaneous measurement system to be developed in this research.

The positron annihilation method and the electrical resistance measurement method are uniquely useful measurement methods because they can nondestructively detect the type and amount of atomic-level lattice defects that cannot be seen even with a highresolution electron microscope. In the conventional positron apparatus (γ-γ coincidence measurement method) [39,40], simultaneous measurement with the electrical resistance method [41,42] was difficult. However, in recent years, the development of the simultaneous β+-γ measurement method using Avalanche Photodiode Detector (APD) has progressed [43,44], enabling a variety of applications, including in the nuclear and medical fields. In addition to that Thus, we are now developing a multi-type simultaneous measurement system that combines positron lifetime measurement, electrical resistance measurement, and stress load measurement as a new method to measure irradiation defects in advanced new materials with such complex structures (Figure 1 & 2). The PHITS code [45,46], which simulates the radiation behavior of all kinds of materials, can be used to advance the basic theory of irradiation damage by incorporating the defect formation rates of complex materials obtained by MD simulation [47-49]. The combination of the multi-type simultaneous measurement system and the PHITS analysis method is expected to rapidly advance the understanding and application of materials science and engineering in a wide range of fields related to the radiation environment.

Figure 2: Principle of the multi-type simultaneous measurement system being developed in this research.

Research and development of innovative materials is being promoted in various fields to counter global warming and energy. In the fields of energy, nuclear power, space environment, radiation medicine, nuclear fusion, and accelerator-related equipment, radiation degradation of materials and equipment occurs, and various methods such as processing heat treatment, impurity addition, alloying, micro-crystallization, nano-clusters, Oxide Dispersive Strength (ODS) steels, composites, and nano-fiber materials have been used to improve mechanical properties, corrosion resistance and irradiation resistance, and these techniques have produced successful results. Recently, high-entropy alloys and their composites have been attracting attention as materials that have both excellent strength and ductility, and their research and development is progressing rapidly. In the development and prompt evaluation of such advanced materials, the role of new and better state-of-the-art analysis and analytical equipment is of primary importance. Thus, we are now developing a multi-type simultaneous measurement system that combines positron lifetime measurement, electrical resistance measurement, and stress load measurement as a new method to measure irradiation defects in advanced new materials with such complex structures.

The combination of the multi-type simultaneous measurement system and the PHITS analysis method is expected to rapidly advance the understanding and application of materials science and engineering in a wide range of fields related to the radiation environment.

This work was supported by JSPS Grants-in-Aid for Scientific Research 21H04668, 19H02652, and 21H04480, and a collaborative research program of National Institute for Fusion Science. A part of this work was also conducted at the Hokkaido University as a program of “Nanotechnology Platform” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, Grant Number JPMXP09A21HK0074.

The authors declare no conflict of interest.

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