Mitsuhide Matsushita

Novel hexagonal structure and ultrahigh strength of magnesium solid solution in the Mg–Zn–Y system

A magnesium (Mg) solid solution with a long periodic hexagonal structure was found in a Mg 97 Zn 1 Y 2 (at.%) alloy in a bulk form prepared by warm extrusion of atomized powders at 573 K. The novel structure has an ABACAB-type six layered packing with lattice parameters of a = 0.322 nm and c = 3 × 0.521 nm. The Mg solid solution has fine grain sizes of 100 to 150 nm and contains 0.78 at.% Zn and 1.82 at.% Y. In addition, cubic Mg 24 Y 5 particles with a size of about 7 nm are dispersed at small volume fractions of less than 10% in the Mg matrix. The specific density (ρ) of the extruded bulk Mg–Zn–Y alloy was 1.84 Mg/m 3 . The tensile yield strength (σy) and elongation (δ) are 610 MPa and 5%, respectively, at room temperature, and the specific yield strength defined by the ratio of σy to ρ is as high as 3.3 × 10 5 Nm/kg. High σy values exceeding 400 MPa are also maintained at temperatures up to 473 K. It is noticed that the σy levels are 2.5 to 5 times higher than those for conventional high-strength type Mg-based alloys. The Mg-based alloy also exhibits a high-strain-rate superplasticity with large δ of 700 to 800% at high strain rates of 0.1 to 0.2 s −1 and 623 K. The excellent mechanical properties are due to the combination of the fine grain size, new long periodic hexagonal solid solution containing Y and Zn, and dispersion of fine Mg 24 Y 5 particles. The new Mg-based alloy is expected to be used in many fields.

Formation and properties of Zr-based bulk quasicrystalline alloys with high strength and good ductility

The crystallization mode of the Zr–Al–Ni–Cu amorphous alloys changed from a single stage to become two stages by the addition of Ag or Pd, and the first-stage exothermic reaction was found to result from the precipitation of nanoscale icosahedral particles with a size of 20 to 50 nm. The precipitation took place by high nucleation and low growth rates in a polymorphous mode for the Ag-containing alloys, and a diffusioncontrolled mode for the Pd-containing alloys. The nanoscale mixed structure alloys exhibited improved strength and ductility as compared with the corresponding amorphous single-phase alloys. The findings of the dispersion strengthening as well as the dispersion ductilization gave a future opportunity to fabricate a new bulk nonequilibrium phase alloy by use of the new phenomenon.

Direct observation of icosahedral cluster in Zr70Pd30 binary glassy alloy

The medium-range order in the Zr70Pd30 binary glassy alloy, where the nanoicosahedral phase precipitates as a primary phase, was examined using the high-resolution electron microscopic technique. The ordered region in the diameter of ∼2 nm was observed in the as-quenched glassy state. This region grows slightly to the diameter of 3–4 nm by annealing for 120 s at 690 K, where the amorphous structure remains. The nanobeam electron diffraction pattern taken from the medium-range order shows the fivefold symmetry, indicating that this region has the icosahedral structure. This result is recognized as a direct evidence for the existence of the icosahedral cluster in the alloy. For further annealing for 600 s at 690 K, the icosahedral quasicrystalline phase in the diameter of 5–8 nm precipitates by assimilating the icosahedral cluster. The formation of the nanoicosahedral phase originates from the existence of the quenched-in icosahedral clusters in the glassy state followed by their easy growth to the icosahed...

Formation of icosahedral quasicrystalline phase in Zr70Ni10M20(M=Pd, Au, Pt) ternary metallic glasses

An icosahedral quasicrystalline phase was confirmed as a primary precipitation phase in melt-spun Zr70Ni10M20 (M=Pd, Au and Pt) ternary metallic glasses with two-stage crystallization process. The onset temperature of the transformation from amorphous to icosahedral phase are 687 K for Pd, 754 K for Au and 783 K for Pt at a heating rate of 0.67 K s−1. The size of the icosahedral particles is in the range of 5–20 nm. The second crystallization reaction results in the formation of Zr2Ni+Zr2Pd and Zr2Ni+ZrPt phases in the Zr–Ni–Pd and Zr–Ni–Pt alloys, respectively, and Zr3Au phase in the Zr–Ni–Au alloy through a single exothermic reaction. The formation of nanoscale icosahedral phase indicates the possibility that icosahedral short-range order exists in the glassy state.

Ductile quasicrystalline alloys

An icosahedral (I) quasicrystalline phase with a grain size below 40 nm was formed as a metastable phase in crystallization of the bulk glassy Zr65Al7.5Cu17.5−xNi10Mx (M=Ag, Pd, Au, or Pt; x=5 and 10 at %) alloys. The volume fraction (Vf) of the I phase is about 85% for the 5% M alloy and nearly 100% for the 10% M alloy. The I phase changes to Zr2Cu+Zr2Ni+Zr2Al3 in a fully annealed state. Compressive fracture strength (σc,f) and fracture elongation (ec,f) of the 10% Pd cylinder with a diameter of 2 mm are respectively 1640 MPa and 2.2% for the glassy phase and increase to 1830 MPa and 3.1% for the I phase. The increase in σc,f is due to the suppression effect of the I particles against the shear deformation of the intergranular glassy phase, and the increase in ec,f results from the localization effect of deformation into the glassy layer. The precipitation of the I phase implies that the glassy alloys include randomly oriented I configurations. The present work shows promise for the new class of high-str...

Precipitation of icosahedral phase from a supercooled liquid region in Zr65Cu7.5Al7.5Ni10Ag10 metallic glass

The formation of an icosahedral phase from the rapidly quenched Zr65Cu7.5Al7.5Ni10Ag10 metallic glass was confirmed and the kinetics of the precipitation upon isothermal annealing in the supercooled liquid region were examined by differential scanning calorimetry. Based on the kinetic analysis, it is clarified that the precipitation in the supercooled liquid region takes place by an interfacial controlled growth with a nearly steady-state nucleation rate. The Arrhenius plot between effective time lag, τ, of nucleation and isothermal annealing temperature yields a single linear relation, in which the activation energy for nucleation is evaluated to be 366 kJ/mol. It is concluded that the transformation of amorphous to quasicrystal proceeds by a homogeneous nucleation mode, which is different from those of Pd–U–Si and Al–Cu–V quasicrystals arising from an inhomogeneous distribution of quenched-in nuclei.

Nanoscale icosahedral quasicrystalline phase formation in a rapidly solidified Zr80Pt20 binary alloy

It is found that an icosahedral quasicrystalline phase is directly formed in a Zr80Pt20 binary alloy during rapid solidification from the melt. The size of the icosahedral particles lies in the diameter range below 10 nm, and the particles are distributed homogeneously. The formation of the nanoscale icosahedral phase indicates that the icosahedral short-range order exists in the melted state of Zr–Pt binary alloy. The strong chemical affinity between Zr and Pt contributes to the restraint of the long-range rearrangement of constitutional elements to form a stable crystalline phase, which is the important factor of the stabilization of an icosahedral phase.

Nano icosahedral quasicrystals in Zr-based glassy alloys

Abstract A nanoscale icosahedral quasicrystalline phase formation was found by the slight deviation from the three-component rule for high glass-forming ability in a Zr65Al7.5Ni10Cu17.5 glassy alloy. Since the primary phase consists of fcc Zr2Ni and I-phases in the Zr65Al7.5Ni10Cu16.5(Ag, Pd, Au or Pt)1 glassy alloys, where an icosahedron is contained in both the phases, it is suggested that the local icosahedral atomic configuration exists in the glassy state and it stabilizes the supercooled liquid state. We also clarified that an I-phase precipitates in the Zr70Cu29Pd1 and Zr70Ni20Al9Pd1 glassy alloys and no I-phase formation is observed in the Zr70Ni29Pd1 amorphous alloy. Since the supercooled liquid region is confirmed in the Zr70Cu30 and Zr70Ni20Al10 glassy alloys and is not observed in the Zr70Ni30 amorphous alloy, it is concluded that the stability of the supercooled liquid state is strongly correlated with the existence of the local icosahedral order.

Nanoicosahedral quasicrystalline phase in Zr–Pd and Zr–Pt binary alloys

It is found that a nanoicosahedral phase in the diameter less than 20 nm is formed as a primary crystalline phase in the melt-spun Zr70Pd30 and Zr80Pt20 binary amorphous alloys. The nanoicosahedral phase is also formed in the as-quenched state in the Zr80Pt20 binary alloy by controlling the quenching rate. The slight redistribution of approximately 3 at % is observed during the quasicrystallization in the Zr70Pd30 alloy. In contrast, no significant compositional change between the nanoicosahedral and residual amorphous phases is observed in the Zr80Pt20 alloy. It is suggested that the precipitation of nanoicosahedral phase in the Zr70Pd30 alloy takes place by a diffusion-controlled growth mode accompanying an increase in nucleation rate. The activation energy for grain growth is calculated to be 270 kJ mol−1, which implies the growth of icosahedral phase without a long-range atomic redistribution. The icosahedral medium-range order in the diameter range less than ∼2 nm is observed in the high-resolution e...

Precipitation of icosahedral quasicrystalline phase in Hf65Al7.5Ni10Cu12.5Pd5 metallic glass

An icosahedral quasicrystalline phase was found in a Hf65Al7.5Ni10Cu12.5Pd5 metallic glass annealed in the supercooled liquid region. Upon annealing at high temperature, the quasicrystalline phase was found to decompose to regular crystalline phases, indicating that it is a metastable phase. The present alloy was compared with the previously reported Zr- and Ti-based alloys with the formation of icosahedral quasicrystalline phases. Hf, Zr, and Ti belong to the same 4A column in the element periodical table. Based on the above comparison, conditions in terms of atomic radius and alloy composition which favor the formation of icosahedral quasicrystalline phase in 4A element based alloys, were suggested.