Kenji Wakashima

Mean-field micromechanics model and its application to the analysis of thermomechanical behaviour of composite materials

Abstract This overview article is concerned with the mean-field micromechanics model that has been developed by several investigators as a powerful modification of Eshelbys theory for the study of the macro-mechanical behaviour of composite materials. First, a general framework of the model is reformulated with emphasis on clarification of the key approximation involved. The model is then applied to the prediction of overall thermoelastic properties of some specific composites of practical interest. For the system with randomly oriented spheroidal particles in particular, the overall bulk and shear moduli, computed as functions of the particle aspect ratio, are shown to vary systematically within the margins which coincide with those bounded by Hashin and Shtrikman. The remainder of this article is devoted to the analysis of anisotropic elastic-plastic responses of metal matrix composites to multiaxial thermomechanical loads. The energy approach that has been proposed is generalized and its applications to specific problems are presented and discussed in comparison with experimental data.

Composition dependent crystallography of α″-martensite in Ti–Nb-based β-titanium alloy

Composition dependence of crystallographic features of α″-martensite in Ti–Nb-based alloy was systematically examined in Ti–Xmol%Nb–3mol%Al alloy, where X is 10–30. One of the lattice deformation strains, η3, became 0 at the critical Nb concentration of 23 mol% and the sign of η3 changed across the composition. The phenomenological theory of martensite crystallography (PTMC) predicted that the lattice invariant shear (LIS) would vanish at the critical composition. Transmission electron microscopy revealed that non-twinned martensite was formed with a habit plane of {755}b when η3 = 0, and that martensite with an internal twin of {111}o type I was formed with a {443}b habit plane for η3 = +0.008. Subscript b and o indicate β and α″ lattice, respectively. The crystallography of these two α″-martensites were well explained by the PTMC. On the other hand, non-twinned martensite with {111}b–{775}b habit plane was formed even when η3 was not zero. It was found that there is a critical value of |η3| for the formation of twinned martensite and that class B transformation, in which LIS is a {011}o compound twin, hardly occurs in β-titanium alloys.

Material design and shape memory properties of smart composites composed of polymer and ferromagnetic shape memory alloy particles

Abstract Ferromagnetic shape memory alloys (FSMAs) such as NiMnGa are expected to be new practical actuator materials with high driving frequency by magnetic field and large strain due to the shape memory effect (SME). However, the brittleness and poor workability of FSMAs, especially at a polycrystalline state, are serious problems and should be improved for a practical use. From this viewpoint a smart composite has been designed by a combination of a polymer matrix and FSMA particles (FSMAP), and a systematic investigation has been done for a NiMnGa-FSMAP/epoxy smart composite. This paper summarizes the design concept and some experimental results of the smart composite. It is pointed out that the single-crystal NiMnGa-FSMAP are easily made by mechanical crush due to the brittleness of FSMAs, and microstructural control is also possible by applying magnetic field during curing. Experimental study revealed that the NiMnGa-FSMAP/epoxy smart composites exhibit both tensile ductility and SME, and that shape memory properties become improved by decreasing particle size of FSMAP. It is concluded that the FSMAP/polymer smart composite has a large potential to be a new practical actuator material.

Steady-state creep rate of a composite: Two-dimensional analysis

A new method is proposed for calculating the steady-state creep rate of a composite, when the inclusion geometry and the matrix creep law are known. The method is demonstrated in a simple two-dimensional problem. During steady-state creep, an increment in plastic strain in the matrix causes a jump in displacement on the interface between the matrix and an individual elastic inclusion. The jump is counterbalanced by that due to diffusion and sliding on the interface. The rate of diffusion is determined by a normal force distributed on the interface and that of sliding by a tangential force. From these forces, the average stress in the inclusions is calculated; it is proportional to the steadystate creep rate. From the condition that an external stress is a volumetric average of the stress in the inclusions and that in the matrix, the constitutive equation of stationary creep of the composite is formulated.

Reinvestigation of phase equilibria in the system Ni-Al-Mo and its implication to the elevated temperature stability of γ/γ′-Mo aligned eutectics

Abstract An erroneous interpretation of phase equilibria in the system Ni-Al-Mo, which is due to stabilities of the mixtures containing Ni-rich f.c.c. solid solution (γ-phase) and almost pure b.c.c. Mo (α-phase), has been pointed out. Coexistence of the γ- and α-phases at high temperatures becomes unstable at lower temperatures; they transform peritecto-eutectoidally at ca. 1130°C into compounds based on Ni3Al and NiMo (γ′- and δ-phases respectively). This transformation is, however, virtually inhibited when certain microstructures develop having massive γ′ separating and preventing direct contact between the γ and α phases. Systematic experimental studies, by means of transmission electron microscopy and electron diffraction along with differential thermal analysis, have been carried out on a series of Ni-Al-Mo alloys containing approximately 65 at. % Ni. Based on the experimental observations, the phase diagram of the system is reconstructed with emphasis on the isotherm at 1000°C.

Bauschinger effect in particulate SiC-6061 aluminum composites

Abstract The work-hardening behavior of a particulate metal matrix composite (MMC) was studied in terms of the Bauschinger effect. The particulate MMC system used in this study is SiC particulate in 6061 aluminum alloy with three different aging conditions: unaged (solution-treated T4) and T4 aged at 175 °C for 4 and 8 h. The experimental results of the stress-strain behavior of the MMCs, which were processed by the casting route, are considered in terms of the Bauschinger effect, the difference ( Δσ f ) and sum ( Σσ f ) and of flow stresses in the forward ( σ t f ) and backward loading ( σ c f ) directions It was found that Δσ f increases linearly with plastic strain ϵ p up to some point ( ϵ b p ) and then bends to a non-linear curve, while the relation between Σσ f and ϵ p gives rise to a flat curve. These results are confirmed by an analytical study based on Eshelbys model. These findings imply that the work hardening of this particulate MMC system is mainly due to back stress and not to forest dislocations or the source-shortening stress effect.

Pseudoelastic Properties of Cold-Rolled TiNbAl Alloy

Microstructures and pseudo-elasticity of as-rolled Ti-24mol%Nb-3mol%Al, a newly developed functional biomedical β-Ti alloy (bcc), at room temperature were characterized. The material was homogenized at 1273K after arc-melting and quenched into water to obtain single phase of β. Cold-rolling of 99% reduction in thickness was carried out and then, microstructures and pseudo-elastic properties were examined. θ-2θ XRD measurement revealed that the as-rolled material was consisted with the parent β phase and martensite phase (α’’, C-center orthorhombic). X-ray pole figure measurements revealed that the rolling texture was a mixture of <110>β{001}β-type and <112>β{111}β-type textures. Shape recovery strain of 4% appeared along TD without any intermediate annealing after the cold-rolling.

Steady-state creep of a composite analysed by an energy balance method

The purpose of this paper is to analyse steady-state creep of a composite with spheroidal inclusions aligned along a tensile direction. It is an extension of a previous study of two-dimensional creep by Mori et al . (1997, Phil. Mag. Lett ., 75, 359). As in the previous study, interfacial diffusion and sliding play essential roles in inducing steady-state creep. A new method of analysis is introduced. Basically, it calculates energy dissipation rates from the rates of diffusional flow and sliding. Both rates are geometrically connected to a creep rate, in a manner depending on the shape of inclusions. By specifying the matrix creep law, a simple relation between the steady-state creep rate and an external tensile stress is obtained. Coble creep is also analysed as a simple and extreme case to which the present method can apply: grains in a polycrystal are treated as inclusions and deformation of plastic character is achieved by boundary diffusion and sliding.

Effect of Co addition on oxidation behavior of IrAl

Abstract Although iridium is promising to be as an effective oxygen diffusion barrier (ODB) for ultrahigh-temperature structural materials, iridium itself is not applied to be a practical oxidation resistant coating due to its poor oxidation resistance caused by gaseous IrO 3 formation. Recently the authors proposed B2 IrAl-base smart coatings which can form an iridium layer as ODB and an aluminum layer as a protective coating on the iridium layer by oxidation. However, the oxidation resistance of binary IrAl alloys is still insufficient. In this study, the effect of Co addition to IrAl on oxidation behavior was studied where Co addition was expected to enhance B2 phase stability. Binary IrAl and IrAl containing 10 mol% Co were fabricated by reactive hot pressing method and oxidation behavior was evaluated using simultaneous differential scanning calorimetry (DSC) and thermogravimetry (TG) in O 2 up to 1873 K. X-ray difrractometry (XRD) and Scanning electron microscopy (SEM) were also carried out for alloy characterization. It was revealed for heating oxidation up to 1863 K in O 2 that the oxidized products confirmed by XRD were Ir, IrO 2 and Al 2 O 3 for both the alloys and that Co 2 AlO 4 was recognized only for Co-added IrAl. It was also found for isothermal oxidation up to 1673 K that thin and continuous Al 2 O 3 layer was formed at surface only for Co-added IrAl. It is concluded that oxidation resistance of IrAl alloys was dramatically improved by Co addition.

Phase Stability and Mechanical Properties of Ti-Ni Shape Memory Alloys Containing Platinum Group Metals

In order to develop Ti-Ni base shape memory alloys (SMAs), the effects of ternary additions on phase constitution and mechanical properties were investiga ted for TiNi alloys containing some platinum group metals: Ir, Rh and Pt. All the al loys fabricated were made by Ar arc melting method using high purity elemental materials fol owed by hot-forging at 1173-1673K in Ar and furnace cooling. Then X-ray diffraction analysis a nd tensile tests were carried out at room temperature (RT). It was found that, whe n the amount of ternary is lower than 10mol%, all the additions reduce martensitic transf ormation temperature ( Ms) of TiNi and B2 phase becomes stable. Besides monoclinic, L1 0 and B19 phases appear for TiNi containing 40-50mol%Ir, 30-50mol%Rh and 20-50mol%Pt, respectively. The t ensile ductility at RT decreases with increasing the amount of terna ry additions, and the ductility becomes very limited when monoclinic (TiIr), L1 0 (TiRh) and B19 (TiPt) phases appear in the Ti-Ni-Ir, Ti-Ni-Rh and Ti-Ni-Pt systems, respectively . Strength strongly depends on Ms and crystal structure of the apparent phase. Small work hardening is recognized in all the alloy systems. Introduction Ti-Ni SMAs exhibit several smart functions represented by shap e memory effect and superelasticity. The applications of the Ti-Ni alloys are, how ever, still limited mainly because the martensitic transformation temperature ( Ms) is below 400K in the binary systems [1]. Then, in order to expand the applications related with SMAs, shape memory alloys actuated at higher temperature than binary Ti-Ni are required. Several investigations have been done for the improvement of actuation temperature of Ti-Ni by a lloying additives [2-4]. Most of additional elements such as Co, Fe, Mn, Cr and V reduce Ms. On the other hand, some refractory metals such as Hf and Zr and platinum group meta ls (PGMs) such as Pd and Materials Science Forum Online: 2003-08-15 ISSN: 1662-9752, Vols. 426-432, pp 2333-2338 doi:10.4028/www.scientific.net/MSF.426-432.2333