CeNF/Al-based composites were prepared using CeNFs collected by a non-woven aluminum filter, followed by hot extrusion to obtain plates. Gel-like CeNFs were collected by an aluminum non-woven filter and compacted by a warm press to obtain a compressed form with a lighter specific density than pure aluminum. The compressed forms were hot extruded to fabricate bars and plates. Both bars and plates were observed in the macro-and microstructural morphology and XRD measurements. They were not significantly carbonized to graphite only, which was inferred to be present as CeNF under the present experimental conditions. Microstructural observations show that CeNFs are aggregated and present in the pores/cracks between the Al filters in the compressed forms. Al filters and CeNF aggregates were more finely mixed when the material was fabricated into hot extruded plates with a higher extrusion ratio. The maximum tensile strength of the CeNF/Al composite extruded plate was about 1.5 times higher than pure aluminum. In addition, the extruded plates could be cold-rolled by about 30%, and the maximum tensile strength of the extruded sheets was found to be about twice that of pure aluminum.
We investigated how dwell fatigue loading accelerates the crack propagation in bi-modal Ti–6Al–4V alloy. A fatigue test was programmed to include displacement holding for only one cycle at several ΔK. We found (1) crack tip strain evolved during the displacement holding, (2) the displacement holding increased fatigue striation spacing, and (3) the strain increment during the displacement holding was linearly correlated with spacing of the displacement-holding-extended striations. These facts indicate that the dwell loading assisted crack opening, which accelerated the crack propagation. Other analyses results and discussion are also presented, in terms of crack propagation mode, crack closure, and dislocation structure.
Non-equiatomic high-entropy alloys (HEAs) for which the mixing (Smix), configuration (Sconfig), and equivalent ideal (Sideal) entropies satisfy Smix > Sconfig = Sideal were reported for Co–Cr–V–Fe–(Al, Ru, or Ni) systems. Three Co20Cr20Fe20V10X30 (X = Al, Ru, or Ni) alloys (referred to as Al30, Ru30, and Ni30 alloys) were studied here using conventional arc melting and subsequent annealing. The X-ray diffraction profiles revealed that the Al30, Ru30, and Ni30 alloys annealed at 1600 K for 1 h exhibited B2 ordered, hcp, and fcc structures, respectively. A single structure was verified by scanning electron microscopy observations combined with elemental mapping via energy-dispersive X-ray spectroscopy. Thermodynamic calculations of Smix normalized by the gas constant (Smix/R) revealed that Al30, Ru30, and Ni30 alloys at 1600 K had Smix/R = 0.833, 1.640, and 1.618, respectively, where the latter two alloys exceeded Sconfig/R = 1.557. A compositionally optimized Al-containing HEA for Smix with a single bcc structure was computationally predicted and verified experimentally for the Al6Co27Cr34Fe19V14 alloy (Al6 alloy). The non-equiatomic Al6 alloy with Sconfig/R = 1.480 exhibited Smix/R of 1.703 at 1600 K, surpassing Sconfig/R = ln 5 = 1.609 for the exact equiatomic (EE) quinary alloy. The bcc Al6, hcp Ru30, and fcc Ni30 alloys were regarded as ultra-high mixing entropy alloys (UMHEAs) according to Smix > Sconfig. Structure-dependent Smix and the mixing enthalpy of constituent binary EE alloys are useful for future UHMEAs as a subset of HEAs.
The relationship between the mechanism of high temperature deformation and the evolution behavior of microstructure and the texture during deformation, which has been found in various solid solution alloys, is clarified. It is shown that the preferential dynamic grain growth (PDGG) mechanism proposed by the authors can explain the behavior of microstructure change as well as texture change of all studied alloys without contradiction. The essential aspect of the PDGG mechanism is the preferential growth of crystal grains with the orientation stable for deformation and with low Taylor factor in the given deformation mode. It is concluded that the Taylor factor corresponds to dislocation density and stored energy during the high temperature deformation of solid solution alloys when viscous glide of dislocations is the rate controlling process. The possibility of the occurrence of the PDGG mechanism in materials other than solid solution alloys is also discussed.
Yoshihito Kawamura, Kazuki Ougi, Shin-ichi Inoue, Takanori Kiguchi, Makoto Takafuji, Hirotaka Ihara, Donald S. Shih
High thermal conductivity, high strength and non-flammability have successfully been achieved concurrently for the first time in Mg–4.5Al–2.5Ca, Mg–5Al–3Ca, Mg–6Al–4Ca, and Mg–4Al–2Ca–0.03Be (at%) alloys, which are produced by hot extrusion of the heat-treated cast alloys. These alloys consist of α-Mg, and C36, C14 and C15 compounds, and exhibit a high thermal conductivity of 111–119 Wm−1K−1, a high ignition temperature of 1343–1408 K and a high tensile yield strength of 318–363 MPa.
Hiroki Kanai, Yoshiharu Kariya, Hiroshige Sugimoto, Yoshiki Abe, Yoshinori Yokoyama, Koki Ochi, Ryuichiro Hanada, Shinnosuke Soda
A method for predicting the lifetime of fatigue crack network formation in die-attach joints is considered through experiments on high-speed thermal cycling using a Si/solder/Si joint specimen and the mechanism is identified. Equibiaxial stresses are generated in the solder layer because thermal deformation of the solder is constrained by the Si, which causes continuous initiation and propagation of crisscross-shaped cracks. When the crack density is sufficiently high, crack growth is arrested by collisions between cracks, and the formation of the fatigue crack network is completed. Based on these results, development of the damaged area and arrest of the development by collisions between the cracks is expressed in terms of extended volume theory incorporating crack initiation and propagation functions for solder as well as considering the damage rate equation. The experimental result for the relationship between the damage ratio in the die-attach joint and the number of cycles under each thermal condition are reproduced by the damage rate equation.
The temperature dependence of fatigue crack growth in stage IIb was investigated in Ti–0.49 mass%O with an alpha single phase. It was found that the fatigue crack growth rate in stage IIb was temperature independent at temperatures above 300 K. EBSD maps beside the fatigue crack showed that prismatic slips with 〈a〉 dislocations were dominantly activated during the fatigue crack growth. The reason why the prismatic slips were dominantly activated is discussed with numerical crystal plasticity analysis. It was shown that the local strain rate at the notch tip was higher than the macroscopic strain rate, which leads to the suppression of non-prismatic slips at the fatigue crack tip.
An improved combination of strength and ductility is generally a trade-off relationship, and it remains a major research topic in the field of structural materials. A bimodal grained microstructure, consisting of a coarse-grained region “core” and a surrounding fine-grained region “shell”, exhibits a good balance between strength and ductility. Therefore, the exact reason for the strengthening mechanism needs to be investigated. In the present study, we conducted nanomechanical characterization to evaluate the individual strengthening factors, including matrix strength (σ0), and grain boundary effect (k), in the Hall–Petch model for each region of the core and shell to clarify the strengthening mechanism in the bimodal grained microstructure. The nanoindentation technique was applied locally in the “grain interior” to evaluate σ0, and “on grain boundary” and “near grain boundary” to assess the grain boundary effect associated with the k value. The grain interior nanohardness was found to be higher in the core region than that in the shell, which is explained by the higher pre-existing dislocation density in the core region. The nanomechanical characterization of the “on grain boundary” and the “near grain boundary” regions show a higher barrier effect due to the grain boundary in the shell than that in the core, which is presumably dominated by the higher internal strain at the shell grain boundary. Furthermore, a Hall–Petch plot was constructed using nanohardness, Vickers hardness, and grain size to estimate the k value. The plot showed a higher k value in the shell, which is consistent with the higher strengthening effect of the shell grain boundary that is evaluated independently in the local region. Therefore, the macroscopic strength of the shell region is significantly affected by both grain boundary effect as well as fine grain size.
The kinetics of phase transformation in the Fe–C system during cooling are of great importance for controlling its microstructure and mechanical properties. However, real-time observation of the phase transformation has been a challenging task because of experimental difficulties. Here, we developed an analytical technique for time-resolved observation of the phase transformation of an Fe–C system during cooling via X-ray absorption spectroscopy with a time resolution of 200 µs. The technique was applied to model specimens: Fe, Fe–0.044C, and Fe–1.24C. The incubation time before phase transformation and the multiple steps of phase transformation from γ-Fe to α-Fe (+ Fe3C) could be clearly observed by the proposed technique, and the behaviors were significantly different among the specimens. Through the developed technique, change in atomic structures at a short-range scale was detected, which is complementary to X-ray diffraction that detects atomic structures only in long-range order. The developed technique can detect structure changes at early stage of phase transformations, where structures changes often begin at a short-range scale, and provide inevitable information on the kinetics.
Tubasa Suzuki, Masaki Tanaka, Tatsuya Morikawa, Jun Fujise, Toshiaki Ono
Silicon single crystals were deformed in tensile tests along the  direction between 1173 K and 1373 K. The yield point phenomenon was observed in the specimens deformed below 1273 K, while a continuous yield was observed in the specimens deformed above 1323 K. The values of work-hardening rate in stage II were the same as those reported in other single crystals. Orientation maps of the specimen obtained by using electron backscattered electron diffraction method indicated that stage II starts before the Schmid factor of the secondary slip system became larger than that of the primary slip system. Because of the constraint due to the gripping of the test piece, kink bands are formed during stage I before the onset of stage II, and then the stress state becomes non-uniaxial. This suggests that the formation of kink bands triggers the activation of the secondary, i.e., conjugate slip system to increase the resolved shear stress on the conjugate slip systems.
Measurements of the open circuit potential of AA1050 were performed in 10 mM CrO3, and the Al-matrix around the intermetallic particles was found to dissolve locally. Pre-immersion in 1 M NaOH promoted the dissolution of the Al-matrix around intermetallic particles, resulting in the improvement of pitting corrosion resistance in NaCl solutions. The pitting corrosion resistance provided by molybdate treatment is effectively improved by pre-immersion in NaOH. The maximum pit depth of the specimen treated in MoO3–HNO3 solution after pre-immersion in 1 M NaOH was small compared to the chromate-treated specimen. It was determined that immersion in an acidic solution containing an inhibitor after pre-immersion in NaOH is an effective method to improve the pitting corrosion resistance of aluminum alloys.
The complex atomic structure of a high-order approximant to face-centered icosahedral quasicrystal in Al–Pd–TM (TM = transition metal) systems can be deconvoluted into two kinds of atomic clusters pinned at the vertices of a tiling composed of four basic polyhedra, called the canonical cells. As a result, thousands of atoms per unit cell can be registered in fifteen orbits associated with vertices, edges, faces, and cells of the relevant canonical-cell tiling. This geometrical framework facilitates a rational guess of an atomic jungle in an unknown approximant structure, could an underlying tiling be postulated. A novel approximant phase in the Al–Pd–(Mo–Fe) system is discussed within the present framework.
Recent discovery of various magnetism in Tsai-type quasicrystal approximants, in whose alloys rare-earth ions located on icosahedral vertices are coupled with each other via the Ruderman-Kittel-Kasuya-Yosida interaction, indicates an avenue to find novel magnetism originating from the icosahedral symmetry. Here we investigate classical and quantum magnetic states on an icosahedral cluster within the Heisenberg interactions of all bonds. Simulated annealing and numerical diagonalization are performed to obtain the classical and quantum ground states. We obtain qualitative correspondence of classical and quantum phase diagrams. Our study gives a good starting point to understand the various magnetism in not only quasicrystal approximants but also quasicrystals.
Search for high-order semiconducting quasicrystalline approximants can play an essential role in finding clues to the discovery of semiconducting quasicrystals. According to the previous theoretical work, a model of Al–Pd–Co 1/1 cubic quasicrystalline approximant was predicted to be semiconductor from a calculation based on the density functional theory. We noticed that the F phase in the Al–Pd–Co system is a 2 × 2 × 2 superlattice structure of the calculated model. To verify this prediction, we synthesized the F phase sample, and measured its thermoelectric properties. The measured electrical conductivity linearly increases with increasing temperature. The magnitude of measured Seebeck coefficient is smaller than the typical semiconductor. These properties indicate that the prepared sample of the F phase has a pseudogap rather than a finite band gap. To investigate this discrepancy between the theoretical prediction and experimental results, we calculated the electronic structure for the three structural models using density functional theory. The most energetically stable model has a semimetallic electronic structure.
Y. Soejima, A. Heima, H. Akamine, T. Inamura, M. Nishida
To quantitatively understand the thin foil effect in in situ observations of the B2 to B19′ transformation in Ti–Ni alloy, the microstructure of the B19′ martensite in thin foil and bulk specimens was compared. The transformation temperatures decreased with decreasing specimen thickness. There were large habit plane variants more than several tens of micrometers in size in the area of the specimen less than 10 µm thick. The critical thicknesses for reproducing the transformation behavior in the bulk material was about 20 µm based on the self-accommodation morphology and 4 µm based on the twin width ratio of the 〈011〉 type II twin.
A new reduction process for producing titanium (Ti) with an ultra-low oxygen concentration directly from TiO2, employing yttrium (Y) as the reductant, was developed in this study. Several methods for the direct reduction of TiO2 have been proposed to lower the cost of production of Ti. However, none of them have yet been applied industrially. In addition, Y has never been used as a reductant for reducing TiO2 although its reducing ability is the highest among the reductants capable of reducing TiO2 to Ti with low oxygen concentration. In this study, the reduction reactions of TiO2 using Y/Y2O3 equilibrium and Y/YOCl/YCl3 equilibrium, employing various molten salts as solvents, were investigated. TiO2 pellets and metallic Ti pieces were placed in several types of solvents along with sufficient Y and heated at 1300 K for 86 ks. When YCl3 or CaCl2 was used as the solvent, the TiO2 pellets were reduced to metallic Ti. The oxygen concentrations in the Ti pieces after heating in YCl3 and CaCl2 were 90 ± 40 mass ppm O and 350 ± 60 mass ppm O, respectively. However, when NaCl or KCl was used as the solvent, a small amount of metallic Ti and a large amount of complex oxides were obtained. It is considered that the reduction reaction did not proceed sufficiently owing to the low solubility of the oxide ions in molten salts. It was experimentally demonstrated that Ti with an ultra-low oxygen concentration (100 mass ppm O or less) can be directly produced from TiO2 by using Y as the reductant in an appropriate solvent. This method is expected to lead to the development of a new industrial process for the production of Ti with an ultra-low oxygen concentration directly from its ore.
Co–Cr– and Co–Cr–Mo-based alloys are commercially used in the industry especially for high wear resistance and superior chemical and corrosion performance in hostile environments. These alloys were widely recognized as the important metallic biomaterials. Here, the first development of Co–Cr–Mo–Fe–Mn–W and Co–Cr–Mo–Fe–Mn–W–Ag high-entropy alloys (HEAs) based on Co–Cr–Mo metallic biomaterials is reported. Ingots of six-component Co2.6Cr1.2Mo0.2FeMnW0.27 (Co41.5Cr19.1Mo3.2Fe16Mn16W4.3, at%) HEAs with a minor σ phase and of seven-component Co4.225Cr1.95Mo0.2FeMnW0.2Ag0.5 (Co46.6Cr21.5Mo2.2Fe11Mn11W2.2Ag5.5, at%) and Co2.6Cr1.2Mo0.1FeMnW0.1Ag0.18 (Co42.1Cr19.4Mo1.6Fe16.2Mn16.2W1.6Ag2.9, at%) HEAs without an σ phase were fabricated. The alloy was designed by a taxonomy of HEAs based on the periodic table, a treelike diagram, predicted phase diagrams constructed by Materials Project, and empirical alloy parameters for HEAs. The σ phase formation prevented the formation of solid solutions in Co–Cr–Mo-based HEAs without a Ni element. The σ phase formation in as-cast ingots was discussed based on the composition dependence and valence electron concentration theory.
The present work reports the effect of elemental combination on microstructure and mechanical properties of quaternary refractory medium entropy alloys (RMEAs) having equi-atomic compositions. As-cast RMEAs ((1) HfNbTaTi, (2) HfNbTaZr, (3) HfNbTiZr, (4) HfTaTiZr, and (5) NbTaTiZr) were fabricated by vacuum arc-melting of pure elements under Ar atmosphere, homogenization was then performed at 1150°C for 24 hours with Ar atmosphere. Firstly, microstructures of both as-cast and homogenized RMEAs were observed by SEM-BSE. Three kinds of microstructures consisting of annealed grains (AG), granular morphology (GM) and dendritic morphology (DM) were found to be distributing along solidification direction in the as-cast RMEAs. Inter-dendritic segregation in the as-cast RMEAs was characterized by SEM-EDX. At the same time, grain boundary precipitates were found in the as-cast (2) HfNbTaZr and (4) HfTaTiZr alloys. After homogenization at 1150°C, a fraction of AG greatly increased while that of DM largely decreased. It was also found that the degree of segregation was largely reduced after homogenization. In addition, grain boundary precipitates having equiaxed morphology and HCP structure were observed in the homogenized (2) HfNbTaZr alloy. Subsequently, tensile tests of both as-cast and homogenized RMEAs were performed at room temperature (RT) to characterize mechanical properties of the RMEAs. After homogenization, ductility of (1) HfNbTaTi, (3) HfNbTiZr, (4) HfTaTiZr, and (5) NbTaTiZr alloys was highly improved while (2) HfNbTaZr alloy still showed early brittle fracture. Better ductility of the homogenized (1) HfNbTaTi, (3) HfNbTiZr, (4) HfTaTiZr, and (5) NbTaTiZr alloys could be attributed to the elimination of inter-dendritic segregation as well as grain boundary precipitates through homogenization.
Non-equiatomic high entropy alloys (HEAs) and medium entropy alloys (MEAs) are expected to have the potential to exhibit good mechanical properties due to abundant composition designs compared to equiatomic alloys. It has been reported that an equiatomic CoCrNi MEA shows better strength-ductility balance than CoCrFeMnNi HEA, and there is a possibility that the mechanical properties can be further improved by changing chemical composition. Among the constituent elements, cobalt (Co) has the effect of decreasing stacking fault energy (SFE). In this study, we clarified the effect of Co-content on mechanical properties of non-equiatomic Co–Cr–Ni MEAs with different amounts of Co through investigating deformation behaviors and deformation microstructures. Cox(CrNi)(100−x) (x = 20 (Co20), 40 (Co40), 60 (Co60) at%) MEAs were processed to very high plastic strains by high-pressure torsion (HPT) and subsequently annealed under proper conditions to obtain FCC single-phase and uniform fine grain sizes. Mechanical properties of the specimens with fully recrystallized microstructures were characterized by tensile tests at room temperature. Their deformed microstructures at different tensile strain levels were observed by electron microscopy. The result of the tensile tests showed that the work-hardening rate was enhanced with increasing the Co-content although early fracture before reaching plastic instability condition occurred in Co60. Planar slip of dislocations and deformation twinning were observed in Co20 (SFE = 30 mJ/m2), while, in addition to them, deformation-induced martensitic transformation to HCP ε-martensite was observed in Co40 having lower SFE (SFE = 10 mJ/m2), leading to higher work-hardening rate. By increasing Co-content (decreasing SFE) further, phase fraction of ε-martensite greatly increased in Co60 (SFE = 0 mJ/m2) compared with Co40, and early fracture occurred due to stress concentration at intersects between martensite and grain boundaries. The present results suggested that the mechanical properties of the present materials could be effectively designed by controlling the SFE.
We have investigated the face-centered cubic (FCC) single-phase formability of non-equiatomic Cr–Mn–Fe–Co–Ni HEAs as well as equiatomic derivative medium/high-entropy alloys (M/HEAs) considering their valence electron concentration (VEC) and mean-square atomic displacement (MSAD). While VEC remains the most decisive parameter to predict phase formation, MSAD can be a complementary parameter that modifies the VEC boundary. Multiplicity of constituent elements was beneficial to accommodate a larger MSAD, which resulted in a downward shift of the VEC boundary for the FCC single phase. This offers information about the correlations between the phase formation preference, VEC, and MSAD of M/HEAs with various compositions.