Hajime Iwasaki

The effect of solid solution W additions on the mechanical properties of nanocrystalline Ni

Abstract Although pure metals with grain sizes below about 10 nm are very difficult to prepare, alloying enables the realization of finer grain sizes, often down to the amorphous limit. In this work, the role of solid solution additions of ~13 at% W are considered with respect to the structure and mechanical properties of electrodeposited Ni alloys with grain sizes below 10 nm. Structure of the nanocrystalline alloys is analyzed by high-resolution transmission electron microscopy, and related to the mechanical properties assessed by instrumented nanoindentation and nano-scratch experiments. The Ni-W alloys exhibit higher hardness and scratch resistance as compared to the finest pure nanocrystalline Ni alloys, although the contribution of solid solution strengthening from W is expected to be essentially negligible. The improved properties are therefore most likely due to the finer length scale available in multicomponent nanocrystalline alloys, and suggest that alloying may suppress the breakdown of Hall-Petch strengthening to finer grain sizes. Finally, the present data are shown to smoothly bridge the hardness-grain size trend between nanocrystalline Ni (grain size>10 nm) and amorphous Ni-based alloys.

Quantitative assessment of superplastic deformation behavior in a commercial 5083 alloy

Abstract This paper describes the constitutive equation for the superplastic commercial 5083 (Al–Mg–Mn–Cr) alloy effective in a temperature range of 773–843 K and at a strain-rate range from 10−5 to 4×10−3 s−1. The superplastic properties have been investigated using velocity jump tests. The analysis of the experimental results reveals that the true stress exponent is two, and the true activation energy for superplastic flow is similar to the lattice self-diffusion of aluminum (142 kJ mol−1). All mechanical data can be represented by a single constitutive equation for superplasticity according to the Dorn equation.

Effect of solute convection on the primary arm spacings of Pb–Sn binary alloys during upward directional solidification

Abstract In this article, in order to investigate the effect of solute convection on the primary arm spacings of Pb–Sn binary alloys, upward directional solidification experiments have been carried out under the condition of 1× g . The primary arm spacings have been measured with the increase of fraction solidified. Results show that the primary arm spacings of hypoeutectic Pb–Sn binary alloy changed with increase of fraction solidified and the peak of distribution of primary arm spacings was shifted to the field of the narrower spacings with the increase of fraction solidified. It has been noted that the average of spacings decreased with the increase of fraction solidified. Mechanism of tip splitting of primary dendrite arm was found to be responsible for the change of the primary arm spacings with the increase of fraction solidified.

The development of cavitation in superplastic aluminum composites reinforced with Si3N4

Abstract Two metal matrix composites, an Al-Cu-Mg (2124) alloy and an Al-Mg-Si (6061) alloy reinforced with Si3N4 particulates, exhibit superplastic elongations at a true stress of 8 MPa at high strain rates at 783 K and 833 K respectively. Internal cavities are formed in these composites at the interfaces between the particulates and the matrix even at strains of only 0.2. However, the rate of cavity growth after nucleation is very low because diffusion growth is of negligible importance at these high strain rates. Most of the cavities have diameters of less than 2 μm in the early stages of deformation. Elongated larger cavities are observed at relatively high strains as a result of cavity growth and interlinkage. The nucleation rate of cavities is low because stress concentrations are relieved by the presence of a liquid phase.

Cavitation control by static annealing after deformation at a high strain rate of a Si3N4p/Al-Mg-Si composite

For a high-strain-rate superplastic Si3N4p,/Al-Mg-Si composite, post-deformation tensile properties were degraded by superplastic deformation because of development of cavitation. However, the postdeformation tensile properties were sufficiently recovered by short-time static annealing without superimposed pressure. This is because most of cavities were very small (<1 μ), and therefore the cavities could be reduced by static annealing.

The characteristics of microcavitation in high strain rate superplasticity

Abstract Experiments were conducted on an Al-2124 composite reinforced with Si 3 N 4 particulates under conditions appropriate for optimum high strain rate superplasticity (HSR SP) and at a temperature where there is experimental evidence for partial melting. It is shown that small but stable microcavities, having diameters of 3 N 4 particulates. Quantitative measurements show that the orientations of the cavities are random with respect to the tensile axis.

Mechanical and corrosion properties of a medium carbon steel (S45C) recycled by solid recycling process

Recycling of metals is one of important technologies for materials circulation because of their large consumption [1]. At the present time, typical recycling process for metals is re-melting process by using redox reaction. For re-melting process, large energy consumption is needed for re-melting. Furthermore, metals recycled by re-melting process show poorer properties than virgin ones by contaminations of oxide inclusions and tramp elements [2–4]. Thus, there are still some problems for re-melting recycling. Recently, we have proposed solid recycling process as a new recycling method for metals [5–7]. In the solid recycling process, metal scraps are recycled by plastic deformation process such as hot extrusion and forging. It should be noted that re-melting is not needed for the solid recycling. In the previous work [6], solid recycled Mg alloy showed high strength due to dispersion

Microstructure and strength of rapidly solidified magnesium alloys

Abstract Two kinds of magnesium alloys, Mg5Al0.5Si and Mg-5Al1Y0.5Si, were rapidly solidified by single-roll quenching to produce ribbons 3–5 mm wide and 50–100 μm thick. Microstructures of the ribbons consisted of long slender cells with a spacing of 0.3-0.5 μm and growing vertically from the chilled surface. They have a great deal of dislocations but no precipitates except for Mg 2 Si. Annealing of ribbons at 573 K for 10 ks led to a remarkable recovery and to the formation of very fine subgrains in the Mg5Al0.5Si alloy whereas the recovery was retarded in the Mg5Al1Y0.5Si alloy. The foil metallurgy technique to form a very thin sheet was useful for consolidation of rapidly solidified ribbons. This technique gave rise to a good bonding between ribbons at 573 K and, at 70% reduction in thickness in air, a fine microstructure associated with dynamic recrystallization, and good mechanical properties. The effect of yttrium addition seemed to retard recovery and to promote dynamic recrystallization.

High strength and high strain rate superplasticity in magnesium alloys

Mechanical properties of fine-grained Mg alloys processed by powder metallurgy (P/M) and ingot metallurgy (I/M) routes have been investigated by tensile tests at room temperature and 573 K. The superplastic strain rate range for the P/M Mg alloys was higher than that for the I/M Mg alloys. Furthermore, the P/M Mg alloys exhibited higher room temperature strength than the I/M Mg alloys. These excellent mechanical properties of the P/M Mg alloys are attributed to a very small grain size of 0.5 ∼ 1 μm.

Multiaxial Experiments and Constitutive Modeling of Superplasticity

In the first part of the paper, multiaxial experiments of 5083Al alloy are carried out at 833K at constant strain-rates to elucidate the characteristic features of superplastic deformation under general forming conditions. Monotonic tension, compression and torsion tests were performed by using solid circular cylindrical specimens together with thin-walled tubular specimens. The values of the equivalent strain-rate of von Mises type are specified as 2 × 10 -3 , 1 × 10 -3 , 5 × 10 -4 and 2 × 10 -4 s -1 . The significant strain-rate dependence of flow stress and the material softening due to cavity growth are observed under the tension tests. The results of the tension and compression tests show that the material hardens under tension while it softens under compression. The comparison between tension and torsion tests shows that the equivalent flow stress of von Mises type under torsion is much smaller than that of the tension at the same equivalent strain-rate of von Mises type. Based on these observations, a constitutive model of superplasticity is then formulated based on the finite deformation theory. The rate of deformation tensor is divided into the sum of the elastic and inelastic parts, and the former part is represented by the isotropic linear elastic law. The inelastic part is modeled based on the invariant theory by employing the steady-state creep law of hyperbolic sine type and by taking account of the effects of yielding, grain and cavity growth. The rate of drag stress is assumed to be the sum of the terms due to the grain and cavity growth. The evolution equation of the grain growth is formulated by taking account of static growth and strain-induced growth. The evolution equation of the cavity volume fraction is represented by the sum of the two terms due to cavity diffusion and viscoplastic deformation. Comparison of the results of the proposed model with those of the experiments shows that the proposed model gives satisfactory predictions.