The coercive force of an iron powder core decreases with an increase in the circularity of the base iron powder. Microstructural observation reveals that the crystal grain size of the iron powder core is reduced by recrystallization during stress relief annealing after powder compaction. An analysis of the work hardening behavior during the compaction process shows that a rounder particle shape leads to smaller particle deformation, resulting in a reduced grain refinement effect during recrystallization. A grain boundary pinning model convincingly describes the reduction of coercive force with the increase in the eventual grain size.
This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 79 (2015) 315–323.
In this study, the formation mechanism of the stress-induced α″ martensitic phase in the deformation twinning (DT) band of metastable β-type Ti-27Nb-0.5Ge (at.%) alloy under tension was investigated. A preferential nucleation site for the stress-induced α″ martensitic transformation is inside a DT band with high dislocation density. The α″ martensitic phase developed in the DT band with increasing applied plastic strain. The α″ martensitic phase subsequently grew to intersect other DT bands, forming secondary DT bands themselves in the parent β grains.
Fig. 3 SEM images of (a) region 3 in Fig. 2 and (b) TEM sample via FIB cutting. (b) The bright-field TEM image of the α″ martensite and DT formed on the β matrix. The SADPs of the (d) α″1, (e) DT, and (f) α″2 regions marked by the white circles in (c). (g) A schematic image representing the SADPs consisted of the β, α″, ω1, and ω2 phases having the specific orientation relationships.
Mechanical properties of magnesium alloys under dynamic loading are still unclear. To evaluate the impact fracture behavior of magnesium alloys, we constructed a novel impact three-point bending test apparatus using three elastic bars with Charpy standard-size specimen, and investigated the impact fracture properties of as-cast Mg-3Al-1Zn (hereafter denoted as AZ31) alloy. Finite element (FE) analysis were carried out to estimate the effect of inertial force of the specimen during the impact three-point bending. Based on the FE analysis, we successfully developed a small-scale apparatus for examining a quarter-size specimen, which was capable of carrying out the impact three-point bending test with minimized influence of the inertial force. Impact fracture behavior of Mg-6Al-1Zn-2Ca (hereafter denoted as AZX612) alloy was estimated and compared by using small-scale apparatus. The experimental results pointed out that the AZX612 had similar energy absorption capability to AZ31 against the dynamic loading, however, the crack propagation speed of the Ca bearing alloy was almost twice as fast as that of the AZ31 alloy.
This Paper was Originally Published in Japanese in J. Japan Inst. Light Metals 66 (2016) 258–265.
The hot-rolled Mg-14.3Li-0.8Zn (HR LZ141) alloy exhibits anisotropic tensile properties with an average value of the normal anisotropic parameter, ravg, of 0.6. From microstructural observation, it is proposed that the mechanical fibering, which is caused by the preferred alignment of the small α-phase particles in the rolling direction, results in this anisotropic property. Both strain rate $\dot \varepsilon $ and temperature T influence the tensile properties. At 6.67 × 10−5 s−1 to 6.67 × 10−2 s−1 and room temperature, the tensile properties are all linear and sensitive to the log-scale $\dot \varepsilon $. The strain-rate sensitivity exponent, m, is 0.055 at all testing $\dot \varepsilon $. The work-hardening exponent, n, is positive, and the higher the $\dot \varepsilon $ is, the larger the n-value is, but the increase in the n-value is quite low for $\dot \varepsilon $ < 3.33 × 10−4 s−1 and $\dot \varepsilon $ > 6.67 × 10−3 s−1. The yield point phenomenon appears in σ-ε curves at room temperature when $\dot \varepsilon $ = 6.67 × 10−5 s−1, and at 343 K when $\dot \varepsilon $ = 3.33 × 10−3 s−1. The work-softening phenomenon occurs at T ≥ 343 K with $\dot \varepsilon $ = 3.33 × 10−3 s−1. The yield stress and UTS increase but the elongation decreases as the testing temperature decreases. The Charpy impact test indicates that different notch orientations influence the impact energy due to the formation of mechanical fibering, and the result of impact energy vs. temperature shows no significance of transition temperature.
The main target of this study is to optimize the microstructure and to achieve an optimization for the mechanical properties in a biomedical Co-Cr-Mo (CCM) alloy with the nominal composition of Co–28Cr–6Mo (mass%) subjected to high-pressure torsion (HPT) and subsequent short annealing. The γ → ε phase transformation and grain refinement occur in the CCM alloy subjected to HPT processing at an equivalent strain (εeq) of 2.25 (CCMHPT). The HPT processing causes a decrease in the elongation due to the formation of an excessive amount of ε phase. For removal of the excessive amount of ε phases, the CCMHPT was subjected to a short annealing (CCMHPTA). The effect of the short annealing temperature (1073 K, 1273 K, and 1473 K; annealing time was fixed at 0.3 ks) on CCMHPT was investigated. In addition, the effect of the length of duration for the short annealing (0.06 ks, 0.3 ks, and 0.6 ks;) for a fixed annealing temperature of 1273 K on CCMHPT was studied. CCMHPTA(1273 K) annealed for 0.3 ks shows a good optimization of mechanical properties that include high strength and large elongation owing to its ultrafine-grained microstructure, and removal of excessive ε phases.
Micro-tensile tests of pure copper and a precipitation strengthening-type Cu alloy, Cu-Ni-Si alloy, were performed using micro-sized tensile specimens with 10 × 10 µm2 in cross-section and 40 µm in length and a micro-gripper, which were fabricated by a focused ion beam system. The obtained experimental results were compared with the results of Cu-Ni-Si alloy bulk sample. The micro-tensile tests of both pure Cu and Cu-Ni-Si alloy showed the typical serrations caused by moving of dislocations and a decrease of flow stress by the necking. In the Cu-Ni-Si alloy, characteristic deformation of work-hardening was observed. Electron back scatter diffraction analysis showed a gradual change in crystal orientation at the necking area.
To develop high-temperature Ti alloys, microstructure and oxidation behavior were investigated in Ti-(10,15)Al-2Nb-(1,2)Zr (at%) alloys. For reference, ternary Ti-15Al-2Zr alloy was also investigated. A single α phase exists in Ti-10Al-2Nb-(1, 2)Zr alloys; on the other hand, an α and α2 two-phase was observed in Ti-15Al-2Nb-(1, 2)Zr alloys. Fine α2 precipitates of about 10 nm were observed in the Ti-15Al alloys. The oxidation behavior of Ti-Al-Nb-Zr and Ti-Al-Zr ternary alloys was investigated to understand the effect of Zr on oxidation behavior at 1023 K. It was found that Zr improves the oxidation resistance of the α phase more effectively than Nb. TiO2 was formed in all tested alloys. The addition of Zr drastically decreased the growth rate of TiO2. It was also found that the addition of Nb improved adhesion of the oxide layer.
The electrodeposition behavior of Zn-Ni alloys produced from acidic sulfate solutions was investigated from partial polarization curves obtained during alloy electrodeposition. At the current density at which the co-deposition of Zn-Ni alloys produced anomalous results, we found that Zn deposition is polarized and is affected by the bath Zn concentration and flow rate. This indicates that Zn deposition is controlled by diffusion at high current densities. Under the conditions for increased Zn deposition, Ni deposition was not suppressed even in the region of anomalous co-deposition. With a low pH bath, the Ni concentration in the deposit did not increase under a high current density because of strong suppression of Ni deposition under the low pH condition.
This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 79 (2015) 398–403.
We propose a new method of applying homogeneous low potential electron beam irradiation (HLEBI) externally to both sides of glass fiber (GF) chopped strand mat (GF-CSM) reinforced thermoplastic polypropylene (PP) interlayered samples (CSM-GFRTP) after lamination assembly and hot press with layup of [PP]6[GF-CSM]5. The external process apparently improved the Charpy impact values (auc) over that of untreated. HLEBI dose from 0.22 to 0.43 MGy apparently enhanced the auc at low cumulative probability of fracture (Pf = 0.06) 40% over the untreated from 48 to 67 kJ/m2. Moreover, based on the 3-parameter Weibull equation, applying 0.30 MGy HLEBI increased the statistically lowest impact value, as at Pf = 0 significantly 120% over that of the untreated from 29 to 64 kJm−2. The improvements of weakest samples in the data set indicate increase in reliability and safety. The improvement in auc can be attributed to HLEBI generating dangling bonds in the PP polymer evidenced by an ESR peak with inflection point at B = ～322.5 mT. The HLEBI apparently acts to generate nano-compressive stresses from repulsive forces between the severed bond electrons in the PP and GFs hence strengthening the GF and PP as well as GF/PP interface causing rise in impact energy. Therefore, the external HLEBI application aims to be a viable method to increase Charpy impact value in articles of GF-CSM reinforced PP (CSM-GFRTP) for industry such as automotive, aerospace and construction.
The optimal applied potential for the acceleration of the corrosion of hot-dip aluminized ferritic stainless steel in 5 mass% NaCl solution was investigated using electrochemical techniques (open-circuit potential measurement, electrochemical impedance spectroscopy, potentiodynamic- and potentiostatic-polarization tests). The feasibility of the use of the potentiostatic polarization test as a method for the acceleration of actual corrosion behavior was proved by the corrosion potential and the pitting potential of each layer in the potentiodynamic polarization test. The optimal applied potential for the potentiostatic polarization test was selected through comparison of the surface and electrochemical impedance spectroscopy analysis.
For many years the production of solar-grade silicon remained a costly process resulting in a large amount of carbon gas being emitted, and so the process still requires improvement to suppress carbon emission. The starting point of the processes is to produce raw silicon materials from a natural resource via mostly carbothermic reduction. However, this process is very complicated and SiO and SiC form as by-products. Further improvement of the carbothermic reduction process requires an understanding in real time of the reactions occurring and the weight change during heating. In particular, the behavior of the SiO by-product plays a major role in the production of silicon because the loss of Si is caused by the escape of SiO gas. In this study, we developed an in-situ weight measuring system for our induction heating furnace, and successfully suppressed much of the error in weight by improving the crucible configuration. The real-time monitoring of the crucible weight loss during the reaction may assist in understanding of the carbothermic reduction process in a more detailed fashion.
The direct carbothermic reduction process from high-purity silica is promising for next-generation low-cost silicon solar cells. In this process, the granulation process is essential to avoid blowout of the silica powder. In this study, we investigated the effect of binders on this reduction process using four kinds of binders. The real-time monitoring of the chamber pressure and quadrupole mass spectroscopic analysis indicated the sign of the blowout phenomena of the generated CO gas and decomposition gas of the binders. In the case of starch and sucrose, the strengths of granules were not enough to the process with the pressure of the generated CO gas, while the granules with enough strength, namely, the ones with polyvinyl alcohol (PVA) and carboxymethyl cellulose (CMC), resulted in silicon yield of 33.8% and 27.8%, respectively.
The effect of nitrogen addition on the microstructure formation and hardness during solidification and heat treatment was investigated and the possibility of nitrogen as an alloying element was discussed in terms of alloy chemistry for high-carbon high-speed tool steel type cast alloys. Nitrogen with a concentration from 48 ppm to 1542 ppm was successfully introduced by mixing Cr2N into a molten alloy. Analysis of the diffraction pattern revealed that the primary V2CN carbonitride crystallized upon the addition of nitrogen, whereas eutectic carbides mainly formed in N-free specimens. The chemical composition of the carbonitride is also affected by the addition of nitrogen. With increasing quenching temperature, the Vickers hardness gradually increased to a peak and then decreased. Nitrogen addition helps to increase the hardness similarly to carbon. A N-containing specimen also exhibited superior secondary hardening after tempering. It is known that a large amount of residual austenite finally transforms to a hard martensite phase after tempering. According to the results of XRD analysis, nitrogen addition increases the volume fraction of retained austenite in the matrix at a higher holding temperature. Furthermore, the precipitation of nanosize carbonitride was observed around the primary V2CN carbonitride in addition to the standard precipitation. Therefore, this carbonitride precipitation may induce the superior secondary hardening and ultimately increase the macrohardness of N-containing specimens.
This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 79 (2015) 169–175.
Selective laser melting (SLM) is an attractive manufacturing technique for the production of metal parts with complex geometries and high performance. This manufacturing process is characterized by highly localized laser energy inputs during short interaction times which significantly affect the densification process. In this present work, experimental investigation of fabricating 316L stainless steel parts by SLM process was conducted to determine the effect of different laser energy densities on the densification behavior and resultant microstructural development. It was found that using a low laser energy density below 50 J/mm3 produced an instable melt pool that resulted in the formation of unmelted particles, pores, cracks, and balling in the as-built parts with low densification. In contrast, the as-built parts at a high energy density above 200 J/mm3 showed irregular scan tracks with a number of pores and metal balls that decreased part density. The optimal laser energy density range was accordingly determined to be 58–200 J/mm3 by eliminating obvious SLM defects, which led to near full densification. The SLM samples fabricated using optimal parameters allowed observation of a microhardness of 280 Hv, ultimate strength of 570 MPa, and yield strength of 530 MPa that were higher than those of the as-cast and wrought 316L stainless steel.
As a part of the challenge of reducing the use of scarce rare-earth elements in magnets, a Dy-free Nd-Fe-B magnet with the remanence and coercivity of 1370 mT and 1830 kA/m, respectively, was investigated. The grain boundary was composed of mainly two phases, R6T13M and R-rich phases. The R6T13M phase formed at around 750 to 1000 K, and in that temperature range, coercivity improved and remanence decreased. By increasing the amount of grain boundary phases to 17.3 at% R addition, coercivity higher than 1990 kA/m (25 kOe) was realized.
The shear strength and failure behaviors of the Sn-2.3 mass%Ag flip-chip solder joints before and after corrosion test were investigated. The relationships between the shear strength, corrosion amount, and fracture mode are elucidated in this study. The shear strength of the Sn-Ag solder bump joints decreased with increasing amount of corrosion, mainly due to the formation of brittle corrosion products. In addition, the shear strength was changed with corrosion site. After the shear test, the failure mode switched from a bulk-related ductile fracture to a corrosion-related brittle fracture, depending on the site and amount of corrosion. The top-side corroded bump did not affect the shear strength, whereas the shear strength was decreased for the partially corroded area at the side of the bump. After prolonged corrosion reactions, the joints had extremely low shear strength values and very brittle fracture surfaces. This result was discussed in terms of the relationship between the corrosion site, the shear height, and the resulting force-displacement (F-x) curves during the shear test.
To establish a recovery method for noble metals from membrane electrode assemblies (MEAs) of spent polymer electrolyte fuel cells (PEFCs) without the use of strong acids, electrochemical dissolution tests for Pt and Ru from MEAs were conducted. By using square potential waves, 93.2% of the Pt and 98.4% of the Ru dissolved from a MEA in 1 mol L−1 HCl at room temperature when oxidation and reduction potentials were at 1.5 and 0.1 V vs. SHE and the holding times for both were 15 s per cycle. The dissolution of Pt and Ru became remarkable when the oxidation potential was 1.4 V vs. SHE and gradually decreased at more positive potentials. These results indicate that competitive reactions exist in the dissolution process. In addition, the effects of H+ and Cl− concentrations on the dissolution ratios were investigated. The dissolution ratios of Pt and Ru were small in solutions with low Cl− and high H+ concentrations ([Cl−] = 0.01 mol L−1, [H+] = 1 mol L−1); however, 50.6% of the Pt and 27.9% of the Ru dissolved in solutions with high Cl− and low H+ concentrations ([Cl−] = 1 mol L−1, [H+] = 0.01 mol L−1). Thus, we verified that the electrochemical dissolution method was adaptable to the recovery of noble metals from MEAs and that strong acids were not needed.