2021 Volume 62 Issue 7 Pages 1046-1051
This paper presents a current research trend based on the best papers awarded in 2019 and 2020 by Materials Transactions. The summary includes the 8 papers carefully selected from the articles in the research areas of materials physics, microstructures of materials, mechanics of materials, materials chemistry, materials processing, and engineering materials and their applications. Four out of the 8 best papers are those specially selected for young scientists who are the age of 35 or younger. Here, brief summary is given for the introduction of high quality papers published in Materials Transactions.
Fig. 1 Schematic diagram of the procedure for the crystal plasticity modeling of lath martensitic steel based on a multi-scale anisotropic tessellation.1)
Materials Transactions selected the 8 best papers from the articles published in 2019 and 2020. The papers were carefully reviewed and scored by the members of the 6 selection committees representing the research areas such as (1) materials physics, (2) microstructures of materials, (3) mechanics of materials, (4) materials chemistry, (5) materials processing, and (6) engineering materials and their applications. The papers which earned the highest scores won the best paper award. However, if the score is low (below 3.5 out of 5), the paper is not qualified for the award. Four papers out of the 8 papers are for young scientists of age 35 and under, and the award is also intended to encourage their future research activity.
Here, as a current trend in research, it is a good opportunity to introduce all the awarded papers, giving their brief summaries provided by the corresponding authors. All papers are based on cutting-edge research work fulfilling one or more of the following requirements for the best paper award: (a) sufficient originality, (b) highly influential (good potential for significant research advancement), (c) solving long-term problem, and (d) being far ahead of others. We anticipate that the awarded papers promote the activity of the related research and provide readers of Materials Transactions with high-quality research information.
Lath martensite is a multi-scale material often described in terms of packets, blocks and laths based on the Kurdjumov-Sachs (KS) orientation relationships (ORs).2) In such a material, strain localization and failure initiation are strongly related to the inherent relationship between morphology and crystallography of blocks. Therefore, reliable prediction of its behavior can only be achieved if these relations are explicitly accounted. In this study, a numerical approach based on a multi-scale anisotropic tessellation was developed for the modeling of lath martensite3) (Fig. 1). The proposed method allowed a precise control of the prior austenite grain size, packet size and block widths as well as their mutual crystal orientation relationships. Crystal plasticity finite element simulations were then conducted to evaluate the influence of the different modeling hypotheses on the strain distribution under fatigue conditions.4) To this end, a scale-independent phenomenological crystal plasticity model, calibrated on low-cycle fatigue tests was used. A slip-band averaged critical plane fatigue criterion following the dislocation-based Tanaka-Mura model5) was finally investigated. Fatigue crack initiation was predicted to occur in blocks whose active slip systems matched that of their elongation direction as commonly observed experimentally.
Schematic diagram of the procedure for the crystal plasticity modeling of lath martensitic steel based on a multi-scale anisotropic tessellation.1)
For the first time in the world, a team of Tohoku University has succeeded in developing a single hcp phase in high-entropy alloys (HEAs) that are composed of transition metals only in multicomponent systems with elements of 5 or more through solidification from melts.6) Of the three representative simple crystallographic structures of solid solutions (bcc, fcc, or hcp) of alloys, it had long been difficult until 2014, 10 years since 2004 when HEAs were firstly presented, to fabricate HEAs with a single hcp structure7) where these hcp-HEAs are comprised of the elements of heavy lanthanides with and without Y.7) In strong contrast, the present study demonstrated the success in fabricating HEAs with a single hcp structure6) from constituent elements of 4d and 5d transition metals in Ir26Mo20Rh22.5Ru20W11.5 and Ir25.5Mo20Rh20Ru25W9.5 alloys as demonstrated in Fig. 2. The present work led to a subsequent report about the formations of HEAs with bcc, hcp, and fcc structures simultaneously in a composition line.8) Further relevant thermodynamic researches were performed in terms of mixing entropy of single-phase HEAs itself9) based on CALPHAD (CALculation of PHAse Diagram) scheme to evaluate whether or not the HEAs truly possess a large magnitude of mixing entropy. These achievements with a knowledge of thermodynamics and relevant theoretical aspects assisted by computational approach have offered profound insights into the nature of HEAs.
Sandwich strategy for designing HEAs with hcp structure in transition metals.6)
Following a previous report,6) a group from Tohoku University has succeeded in finding a composition line in a multicomponent alloy system that satisfies the simultaneous appearance of the bcc, hcp, and fcc structure of high-entropy alloys (HEAs).8) The composition line, Ir0.415254(100−2x)Rh0.415254(100−2x)Ru0.169492(100−2x)WxMox (x: 0–50 at%), was decided by using CALPHAD (CALculation of PHAse Diagram) scheme and by referring to valence electron concentration (VEC). As illustrated in Fig. 3, at 2100 K, four types of phases were predicted: (i) a single bcc, fcc, and hcp phase, respectively, at x = 35 (Alloy A, VEC = 6.849), 15 (Alloy C, VEC = 7.981), and 5 (Alloy E, VEC = 8.574); (ii) a mixture of bcc+hcp and hcp+fcc at x = 24 (Alloy B, VEC = 7.472) and 8 (Alloy D, VEC = 8.378), respectively; (iii) a triple mixture of bcc+hcp+fcc; and (iv) a mixture of bcc+fcc in Alloys A–E at low temperature. Experiments at 2100 K revealed that Alloys C–E tended to exhibit better reproducibility: Alloy A exhibited a mixture of bcc+hcp and that Alloys B–D were formed into HEAs with a single hcp structure whereas Alloy E can be regarded as a new refractory HEA with fcc structure. Besides, Alloy C annealed at 1273 K for 200 h maintained a single-hcp structure. These achievements combined with a subsequent thermodynamic analysis of mixing entropy of single-phase HEAs itself based on CALPHAD scheme9) will open the door for the promising fundamental researches of HEA.
Alloy design of HEA by using CALPHAD scheme and VEC, and experimental results.8)
Silicon (Si) has attracted considerable interest as a negative electrode material for next-generation lithium (Li)–ion batteries because of its high capacity density.11–14) In this study, ex situ electron microscopy was applied to observe Si negative electrodes under different charge states within an actual battery structure to reveal the Li intrusion direction and the effects of Li concentration on the electrode structure. All of the processes from disassembly of the charged battery and preparation of specimens for use in electron microscopy observation to specimen transport to the electron microscopes were performed under non-atmospheric exposure conditions. The orientation of the single-crystal Si powder in the charged state was observed by electron backscatter diffraction, indicating that lithiation occurred preferentially along the (110) plane of Si. The initial stage of amorphization was observed by high-angle annular dark field-scanning transmission electron microscopy as shown in Fig. 4, demonstrating that the Li atoms occupied the tetrahedral sites of Si crystals, and that the crystal structure was destroyed via the severing of Si–Si bonds between the {111} planes. During the charge reaction, Li occupied the tetrahedral sites via intrusion along the ⟨110⟩ direction of Si, and amorphization proceeded as the Li concentration increased. Thus, the amorphous region grew preferentially in the ⟨110⟩ direction of Si.
Visualization of Li penetration direction by charging reaction using lithium-ion battery Si single crystal negative electrode by HAADF-STEM observation.10)
The hydrogen is one of the attractive and alternative energy sources for fossil fuels to realize the sustainable development of industry without exhausting a carbon dioxide and/or a sulfur oxide. The safe operation is the most important point in whole processes for a hydrogen society, for instance production, transportation, and consumption. The selective gas sensors for hydrogen gas are the key components for safety usage of hydrogen gas. The nanostructured noble metal fabricated by dealloying method is one of the suitable materials as the gas sensing material and/or catalytic materials, because of its large specific surface area and open-cell structure.16) In this study, we developed the high-purity palladium film with a three-dimensional nanoporous structure fabricated from a reactive sputtered Al–N–Pd alloy film by a dealloying method that used citric acid chelation. This fabricated material has been expected both sensitivity and selectivity for hydrogen gas which derived from the characteristic three-dimensional nanostructure. Its characteristic porous structure could be controlled by the concentration of nitrogen gas in the Ar sputtering gas (Fig. 5). The added nitrogen gas inhibited the formation of the intermetallic Al4Pd phase in the as-deposited film, thereby improving the purity of Pd in the dealloyed nanoporous Pd film up to 99 at%. Furthermore, the formed structure of the dealloyed film changed with the nitrogen gas concentration during initial sputtering, i.e., the structure of the film could be controlled from a three-dimensional nano-network to an aggregated nanoparticle-like structure with increasing N2 gas concentration. These developed nanostructured Pd film and its morphological controlling method are expected that realize a practical inexpensive gas sensor with equipped both high sensitivity and selectivity of hydrogen gas for the hydrogen society.17,18)
STEM-SE images of specimen of different adding N2 concentration in sputtering Ar gas.15) Upper column corresponds to before dealloying specimen (As-sputtered Al–Pd alloy film), and lower column corresponds to after dealloying specimen (Nanostructured Pd film). STEM-SE image: Scanning transmission electron microscope – secondary electron image.
Stress corrosion cracking (SCC) evolution is controlled by a changing predominant factor from electrochemical to mechanical depending on its progression. The issue of how to visualize and monitor the early stage of crack evolution still lacks a clear solution. In this work, acoustic emission (AE), which is sensitive to the steel corrosion,20–22) was correlated with microscopic observation to investigate the evolution of a single crack in SUS420J2 stainless steel through a modified in-situ droplet corrosion testing. The main AE results are presented in Fig. 6 highlighting the AE feature of the crack evolution that developed with a transition from a slow initiation of active path corrosion-dominant cracking to a rapid propagation of hydrogen-assisted cracking, which was confirmed with in-situ observation of the corrosion evolution. In addition, a cluster analysis of the traditional AE waveform parameters and fast Fourier transform (FFT)-derived frequency components with k-means algorithms extracted two distinct AE clusters. Compared with the EBSD-derived KAM map of crack path, two AE clusters were associated with the plastic deformation and cracking process, respectively. The correlations between AE feature and SCC progression is expected to provide a nondestructive testing (NDT) signals-based insight into the SCC monitoring.
The AE activity of amplitude and cumulative events and the profile of pit size plus dominant crack length over the time evolution.19)
The turbine disks used in the jet engines or power generators are mainly made of cast and wrought, precipitation strengthened superalloys because of their high heat resistance and high strength properties. During the forging process of the turbine disk, the material is preliminarily heated to decrease its deformation resistance by the small amount of precipitates. However, the material temperature decreases due to the material being in contact with the forging dies. The temperature drop during the forging process causes an additional precipitation, which leads to an increase of deformation resistance. Prior to the trial and error approach of prototype production, numerical modeling and simulation was used as a very strong tool to find the optimum forging conditions because the prototype production of large forged parts such as turbine disks cost too much. Therefore, the high temperature deformation and microstructure evolution of Ni–Co base superalloy TMW-4M3 during the isothermal forging process were studied.24,25) A uniform compression test of TMW-4M3 where both the strain rate and compression temperature were controlled showed dynamic recrystallization flow stress.26) The peak stress and steady stress of the deformation resistance curve were characterized with the Zener-Hollomon parameter.27) The average grain size after dynamic recrystallization was also correlated with the Zener-Hollomon parameter, but this relationship changed with compression temperature. In previous studies, the effect of precipitates on recrystallization are only considered qualitatively.28,29) We found that this temperature dependency was related to the pinning effect of the γ′ precipitates in the γ matrix and proposed a new prediction model for dynamic recrystallization grain size considering not only the Zener-Hollomon parameter but also the volume fraction of the γ′ precipitates. This enables us to predict the average grain size after isothermal forging within an error of 12%. (Fig. 7)
Prediction accuracy of developed numerical model for dynamic recrystallization grain size considering pinning effect.23)
The demand for electric vehicles (EV) and hybrid electric vehicles (HEV) has been increasing due to the growing interest in environmental issues in recent years, and the permanent magnets with high coercivity (HcJ) and high remanence (Br) are required for use under high-loads and high-temperatures driving. However, the supply of heavy rare earth (HRE) elements such as Dy and Tb, which are the elements that increase the coercivity, depends on a specific country, and this has become a serious issue in terms of cost escalation and supply risk. As a solution to these problems, many researchers have been studied the grain boundary diffusion (GBD) process, in which HRE is selectively diffused only at the grain boundary that are responsible for the coercivity of the Nd–Fe–B sintered magnets. This technique can effectively increase the coercivity while reducing the amount of HRE used.
In our previous papers, we have reported the GBD process with sputtered Tb metal and mixed powders of HRE compounds and CaH2.31,32) Later, similar GBD processes using various HRE-compounds, vapor deposition, and alloy powders have been reported.33,34) However, these techniques had problems in terms of raw materials cost, process costs and equipment investment.
In this study, the grain boundary of the Nd–Fe–B sintered magnets was modified via reduction-GBD (r-GBD) process with HRE-compounds such as oxide or fluoride with Ca metal vapor for the purpose of effective use of inexpensive and stable raw materials and to establish a simple process. Ca metal has a low boiling point (b.p. = 1757 K) and can easily be vapor deposition under vacuum condition, so it is expected to serve as a reducing agent that reduces HRE-compounds to HRE metals during heat treatment. After modification, the magnetic properties were measured accurately by superconducting magnet-based vibrating sample magnetometer (SCM-VSM). (Fig. 8)
Demagnetization curves of untreated magnet and r-GBD magnet with TbF3 + Ca metal vapor, measured by SCM-VSM.30)
