Yorinobu Takigawa

Tensile Properties of Bulk Nanocrystalline Ni and Ni-W Fabricated by Sulfamate Bath

Nanocrystalline materials with high strength have been reported in large numbers. In particular, there has been considerable research on electrodeposited nanocrystalline Ni (nc-Ni) and nc-Ni alloys. However, reported data vary widely especially in ductility. Therefore, it is necessary to obtain the true characteristic value of nc-Ni and nc-Ni alloys. In the present study, nc-Ni and nc-Ni-W was electrodeposited under different conditions in order to obtained bulk nc-Ni and nc-Ni-W with high tensile strength and good ductility. At first, bulk nc-Ni-W was fabricated using a sulfamate bath. Although the resulting bulk nc-Ni-W had inhomogeneous grain size and W-concentration, this sample exhibited plastic deformation behavior. Then, nc-Ni was fabiricated by four types of sulfamate baths. As a result, the nc-Ni obtained from a sulfamate bath containg added saccharine and 2-butyne-1,4-diol exhibited brittle behavior. In contrast, bulk nc-Ni obtained from sulfamate bath with a grain size of about 60 nm exhibited a tensile strength of about 1000 MPa and ductility of 8.8 %.

Contribution of interstitial solute strengthening in aluminum

Enthalpies of solutions and misfit strains for the Al–X (X = H, B, C, N, and O) binary alloys were determined by first-principles calculations to estimate the strengthening of solid solutions caused by interstitial atoms. The results indicate that interstitial solute atoms produced large misfit strains. Electrodeposited Al containing 0.12–1.32 at.% C was used to assess the validity of solid-solution strengthening by interstitial solute atoms. The role that interstitial carbon plays in strengthening electrodeposited Al is discussed.

High Tensile Ductility in Electrodeposited Bulk Nanocrystalline Ni–W Alloys

Bulk nanocrystalline Ni–W alloys were electrodeposited from a sulfamate bath that contained saccharin sodium as a gloss agent, and propionic acid and sodium gluconate as a complexing agent (SPG bath) to understand the tensile behavior. SPG bath with 1.0 and 5.0 g/L saccharin sodium at 45 ºC produced the bulk specimens with W content of 3.4 and 1.5 at.%, respectively. The electrodeposited alloys had a nanocrystalline structure with grain sizes of approximately 20 nm and a stronger (111) texture. The bulk nanocrystalline Ni–3.4 at.%W alloys deposited from an SPG bath with 1.0 g/L saccharin sodium exhibited a tensile strength of 1.6 GPa and tensile ductility of 1.8%. The bulk nanocrystalline Ni–1.5 at.%W alloys deposited from an SPG bath with 5.0 g/L saccharin sodium exhibited a tensile strength of 1.4 GPa and tensile ductility of 1.7%. The bulk nanocrystalline Ni–W alloys with a stronger (111) texture showed high strength and low plasticity.

Effect of Small Addition of Si on Superplastic Elongation at Room Temperature in Zn-Al Eutectoid Superplastic Alloy

Recent experimental data have revealed that a small amount of impurity can significantly influence the superplastic behavior in Zn-Al eutectoid superplastic alloy. However, the effect of Si content on the superplastic behavior in Zn-Al alloy has not been reported. In this study, the superplastic behavior at a room temperature of two grades of the Zn-Al eutectoid superplastic alloy was studied under identical conditions of grain size, temperature, and strain rate. These two grades were prepared from high-purity Zn, Al and Al-Si alloy using the same procedure but different Si impurity levels; Zn-Al-10Si and Zn-Al-1000Si contain 10 and 900 wt. ppm of Si, respectively. As a result of annealing treatments, an average grain size of 0.6 μm in both grades. To investigate the effects of Si content on superplastic properties, the tensile tests were performed at a room temperature of 298 K and a constant strain rate of 1×10-3 s-1. Microstructures before and after the tensile tests was observed using a scanning electron microscope. The experimental results show that the elongations decreased with increasing the Si content. In contrast, the flow stress of Zn-Al alloys was not affected by the Si content. On the microstructure observation of the two grades of the Zn-Al alloy before and after the tensile tests, cavities existed at grain boundaries and strain enhanced grain growth was observed.

Mechanical Behavior of Electrodeposited Bulk Nanocrystalline Fe-Ni Alloys

Bulk nanocrystalline Fe-Ni alloys with Ni content of 40-55 at.%, grain size of 12-15 nm, and hardness of 3.9-4.6 were prepared by an electrodeposition. The decrease in the hardness values as grain size decreased was discussed in terms of grain refinement effect, solid solution strengthening, and grain boundary relaxation strengthening. It was found that the change of Ni content could not significant solid solution strengthening, and there were no difference in the state of grain boundary for each sample. Micro X-ray diffraction analysis on the sample after tensile tests showed that the (200) texture was developed but full width at half maximum was not changed. This indicated the potency that the grain boundary activity would be induced by a tensile loading. The softening behavior of electrodeposited Fe–Ni alloys would be related to the transition of the dominant deformation mechanisms.

Development of New High-Strength and Heat-Resistant Mg-Zn-Y-X (X=Zr and Ag) Casting Alloys

In order to develop a high strength and heat-resistant magnesium alloy, we focused on controlling microstructure of Mg96Zn2Y2 (at %) casting alloy by the addition of a 4th element. Initially, we investigated the effects of zirconium addition and cooling rate for grain refinement on microstructure and mechanical properties. Consequently, Mg95.8Zn2Y2Zr0.2 casting alloy contains fine equiaxed grains (approx. 0.01 mm), and it exhibits tensile and fatigue properties equivalent to or higher than those of commercial aluminum alloys at high temperature above 473 K. At 523 K, this alloy exhibited a tensile strength of 223 MPa nearly twice that of A4032-T6 alloy used in typical automotive pistons. The Mg95.8Zn2Y2Zr0.2 casting alloy also reveals sufficient ductility and good castability, characteristics not common in current heat-resistant magnesium alloys. Next, we focused on controlling microstructure of Mg96Zn2Y2 casting alloy by the addition of Ag. Mg96Zn2Y2 cast alloy is composed of alpha-Mg phase, long-period stacking ordered phase and Mg3Zn3Y2 phase; on the other hand, Mg-Zn-Y-Ag cast alloy had 4th phase by an addition of Ag. A substantial increase in yield strength at room temperature, without grain refining, was the result.

Influence of Filler Rod Composition on the Strength of Tungsten Inert Gas Welded Magnesium Alloy Joint

The influence of filler rod composition on the strength of Tungsten Inert Gas (TIG) welded magnesium alloy joint was investigated. Samples were rolled AZ31 (Mg-3Al-1Zn) magnesium alloy as base metal and drawn AZ31, AZ61 and AZ91 magnesium alloys as filler rod. The results show that all fracture points were fusion zone (FZ), and each joint efficiency (=joint strength/ base metal strength) was 70.7%, 80.0% and 73.1% when using AZ31, AZ61 and AZ91 filler rod. When using AZ91 filler rod, 0.2% proof stress was the highest but the elongation was the lowest among the three conditions, and joint efficiency was lower than that when using AZ61 filler rod. It is thought that welded joint was strengthened by solute strengthening, but excess addition of aluminum facilitated crystallization of Mg17Al12 phase. This is the reason why elongation and joint efficiency when using AZ91 filler rod decreased. In conclusion, it is effective to use filler rod which does not excess solid solubility limit.

Design and Fabrication of New Ti-Based Ternary Metallic Glasses Based on Effective Atomic Radius in the Ti Solid Solution Calculated by Ab Initio Calculation

New Ti-based ternary metallic glasses were designed and fabricated. A new parameter called effective atomic radius in the Ti solid solution from ab-initio calculation was used to design of the Ti-based metallic glasses. From the effective atomic radius, Ti-Zr-Mn, Ti-Zr-Fe and Ti-Zr-Co systems can be considered as a new Ti-based ternary metallic glass. And the reported scheme based on the concept of binary eutectic clusters is applied to predict alloy composition which shows glass transition.We prepared the Ti42Zr22Mn36, Ti43Zr29Fe28 and Ti44Zr30Co26 alloy sheets by a single-roller method. It showed that sharp diffraction peak corresponding to crystalline phases could be observed in the XRD spectra of Ti42Zr22Mn36 and Ti43Zr29Fe28. While for alloy with Ti44Zr30Co26 no sharp diffraction peak could be found except broad diffraction halos. This result suggests that a critical eutectic temperature in the phase equilibrium for forming glass phase is around 1000°C.

Preparatory Electrodeposition Process for High Purity Bulk Aluminum

Electrodeposition for Al from a dimethylsulfone (DMSO2) bath was consecutively performed, applying two types of current waveforms such as direct current and pulsed current, to investigate the effect of a current type on the preparatory electrodeposition (pre-electrodeposition) process. Electrodeposited Al from a DMSO2 bath has a nanograined structure and high strength. However, the electrodeposits showed no plastic deformability due to the large amount of sulfur and chlorine which were incorporated into the electrodeposits as sulfide and chloride. Therefore, we obtained high purity Al from a DMSO2 bath using pre-electrodeposition process, which could decrease sulfur and chlorine contents without using additives. The sulfur and chlorine contents of electrodeposits, obtained from a DMSO2 bath applying both types current, both decreased to approximately 0.1 at.%. This result indicated that the waveforms made no difference in pre-electrodeposition process.

Influence of Impurities on Mechanical Properties of Electrodeposited Bulk Nanocrystalline Al

Effect of typical impurities such as Fe, S, and Cl on mechanical properties of Al electrodeposited from a dimethylsulfone bath (DMSO2 bath) were studied. Electrodeposition from a DMSO2 bath was conducted to produce the bulk specimens with 0.08–0.24 at.% Fe, 0.47–0.84 at.% S, and 0.59–1.06 at.% Cl, varying the purity of aluminum chloride and current density. Decreasing the current density increased S contents and Cl contents, while the purity of aluminum chloride had no effect on chemical composition of the electrodeposits. The grain sizes were approximately 40–70 nm for Al electrodeposited from a DMSO2 bath. The grain sizes decreased with increase in S contents and Cl contents. The electrodeposited bulk nanocrystalline Al exhibited hardness values of 1.56–1.92 GPa. These values were higher than predicted values based on Hall–Petch equation of pure Al. Lattice parameter of samples was less than pure Al. According to Vegard’s law, Fe solute decreases the lattice parameter of Al. These results indicated that the hardness of the electrodeposited bulk nanocrystalline Al was affected by the reduction in the grain size and solid solution strengthening from the Fe contaminant.