Albertus D. Setyawan

Nanoscale multistep shear band formation by deformation-induced nanocrystallization in Zr-Al-Ni-Pd bulk metallic glass

A single Zr65Al7.5Ni10Pd17.5 bulk metallic glass exhibits a large plastic strain of 6.6% during the compressive deformation process, which is attributed to the deformation mode with nanoscale multistep shear bands. We have observed that nanocrystals with a metastable fcc Zr2Ni structure containing several distorted icosahedral clusters are arranged in a “bandlike” formation in the glassy matrix around the multistep shear bands. This is recognized as direct evidence of the novel phenomenon of the restraint of shear band propagation owing to the dynamic precipitation of the nanocrystals.

Work-Hardening Induced Tensile Ductility of Bulk Metallic Glasses via High-Pressure Torsion

The mechanical properties of engineering materials are key for ensuring safety and reliability. However, the plastic deformation of BMGs is confined to narrow regions in shear bands, which usually result in limited ductilities and catastrophic failures at low homologous temperatures. The quasi-brittle failure and lack of tensile ductility undercut the potential applications of BMGs. In this report, we present clear tensile ductility in a Zr-based BMG via a high-pressure torsion (HPT) process. Enhanced tensile ductility and work-hardening behavior after the HPT process were investigated, focusing on the microstructure, particularly the changed free volume, which affects deformation mechanisms (i.e., initiation, propagation, and obstruction of shear bands). Our results provide insights into the basic functions of hydrostatic pressure and shear strain in the microstructure and mechanical properties of HPT-processed BMGs.

Magnetic properties of 120-mm wide ribbons of high Bs and low core-loss NANOMET® alloy

A 120-mm wide amorphous ribbon of a Fe-Co-Si-B-P-Cu NANOMET® alloy has been successfully produced by a single roll melt spinning technique. The optimally annealed samples exhibited low coercivity (Hc) of 5–7 A/m and high saturation magnetic flux density (Bs) of 1.83 T. The plots of Hc and Bs vs. annealing temperature (Ta) revealed basin-like and plateau-like characteristics, respectively, indicating the good annealing controllability for nanocrystallization and for obtaining soft-magnetic properties with high Bs. The excellent magnetic softness was attributed to the nanocrystalline structure composed of homogeneously dispersed α-Fe grains (with a size of 15–20 nm in diameter) emerged from the amorphous structure after optimum annealing. The nanocrystalline ribbons also exhibited low core-losses (W at 50 Hz) of 0.37 and 0.64 W/kg under maximum flux density of 1.5 T and 1.7 T, respectively. The magnetic properties were comparable with those of laboratory-scale small-width ribbons and confirmed to be indepen...

Glass formation dependence on casting-atmosphere pressure in Zr65Al7.5Ni10Cu17.5−xPdx(x=0–17.5) alloy system: A resultant effect of quasicrystalline phase transformation and cooling mechanism during mold-casting process

A different dependence of apparent glass-forming ability (GFA) on casting-atmosphere pressure is observed in a Zr65Al7.5Ni10Cu17.5−xPdx (x=0–17.5) alloy system. Low-Pd alloys (x<12.5) reveal low GFA dependence, while high-Pd ones (x⩾12.5) show high GFA dependence on atmosphere pressure applied during casting. The phenomenon is suggested to originate from the fact that for the high-Pd alloys, the transformation region of supercooled-liquid to quasicrystalline phase is located at the low-temperature side of the undercooled-liquid region where the atmosphere-pressure dependence of cooling characteristic coincidently takes place. The on-cooling phase transformation in the alloy system and the cooling mechanism during mold-casting process are discussed comprehensively.

Thermal conductivity of an alloy in relation to the observed cooling rate and glass-forming ability

The glass-forming ability (GFA) of an alloy in this case is the largest diameter of a rod which can be cast fully glassy. The present work shows that the thermal conductivity of a liquid alloy has a strong effect on GFA by influencing the cooling rate upon mould casting. The initial cooling rates (for the first 70–100 K of temperature decrease), obtained for Cu-, Zr- and Au-based bulk glass-forming alloys in the liquid state, are found to scale linearly with the thermal conductivities of the liquid base elements. However the low cooling rate found for Ni-based alloy suggests that the heat transfer at the melt–mould interface may also influence the cooling rate. The low thermal conductivity of Ni-based alloys and the correspondingly low cooling rate obtained compared to Cu-based counterparts explains their lower GFA. In the literature, many factors influencing the GFA of alloys have been discussed. To these factors, the present study adds the thermal conductivity of the molten alloy and the melt–mould heat-transfer coefficient. Moreover, the cooling rate depends on temperature and, thus, the critical cooling rate itself is not a suitable parameter for indicating GFA. The cooling can be better described by an appropriate fitting of the cooling curve to an exponential temperature decay function.

Influence of thermal conductivity on the glass-forming ability of Ni-based and Cu-based alloys

In the present letter we compare the thermal conductivities of Cu- and Ni-based alloys in relation with their cooling rates and glass-forming abilities. The cooling rates obtained for Cu- and Ni-based bulk glass-forming alloys are found to scale with the thermal conductivities of the base elements. Relatively low thermal conductivity of Ni-based alloy compared to the Cu-based one explains its lower glass-forming ability. The results also indicate that the thermal conductivity of the molten alloy should be also used as an indicator of the glass-forming ability among the other factors.

Phase transformation behaviour in continuously cooled Zr65Al7.5Ni10Cu17.5-xPdx (x = 0 -17.5) glass-forming alloys and consequences for structure and property control

Continuous cooling transformation (CCT) diagrams were successfully constructed for Zr65Al7.5Ni10Cu17.5− x Pd x (x = 0 − 17.5) glass-forming alloys, comparing phase-transformation features in the alloy system to composition. While a low-Pd alloy (x = 5) showed a single transformation curve, corresponding to the formation of a crystalline phase on the high-temperature side of the undercooled-liquid region, for a given time-scale, a high-Pd alloy (x = 17.5) revealed an additional curve, corresponding to quasicrystalline phase formation on the lower temperature side. The result provides a clue to the structural and property control on the alloy system. Glassy specimens of the same size but with different intrinsic structure, evaluated by structural relaxation during continuous heating, could be fabricated for the low-Pd alloy (x = 5). Plasticity was found to increase proportionally with the relaxation enthalpy. On the other hand, the critical size for glass formation could be improved considerably from 5 to 7 mm in diameter for the high-Pd alloy (x = 17.5).

Performance of a prototype power transformer constructed by nanocrystalline Fe-Co-Si-B-P-Cu soft magnetic alloys

To clarify the feasibility and performance of Fe81.2Co4Si0.5B9.5P4Cu0.8 alloy (with trade name NANOMET®) for electrical power applications, a prototype transformer was constructed. After surface treatment, as-quenched ribbons with a constant width of 50 mm and a thickness of ∼30 μm were wound into a toroidal shape. Nanocrystallization of toroidal core was performed by immersing it in a salt at 673 K for 180 s. The transformer constructed in the present work exhibit low core loss similar to the transformer constructed by a commercial Fe-based amorphous alloy. In spite of issues related to the annealing/nano-crystallization of the core, the feasibility for commercialization of NANOMET in power transformer applications can be confirmed. We believe the potential of NANOMET as core material for next generation of power transformer seems to be huge.

Effect of relaxation state on nucleation and grain growth of nanoscale quasicrystal in Zr-based bulk metallic glasses prepared under various cooling rates

Zr65Al7.5Ni10Cu12.5Pd5 bulk metallic glasses (BMGs) in various relaxation states were prepared under different cooling rates. The grain growth rate of the primary quasicrystal was examined near the crystallization temperature. It was approximately 1 × 10−9 m/s in less relaxed BMGs and approximately twice as large in relaxed BMGs. In contrast, the calculated homogeneous nucleation rate of the less relaxed samples was five to ten times higher (5 × 1019–1 × 1020/m3s) than those in the relaxed BMGs. The results indicate that the relaxation state of glassy alloys has a marked effect on nucleation and grain growth behaviors.

Deformation-induced structural transformation leading to compressive plasticity in Zr 65 Al 7.5 Ni 10 Cu 12.5 M 5 (M = Nb, Pd) glassy alloys

Zr 65 Al 7.5 Ni 10 Cu 12.5 Nb 5 glass was found to exhibit a large plastic compressive strain of over 10% and the property was suggested to be due to deformation-induced nanocrystallization. A transmission electron microscopic observation, however, only revealed obscure ordered clusters with a size of ˜2 nm in the fracture surface of a deformed sample, instead of well-identified crystals as previously reported for the Zr–Al–Ni–Cu–Pd system. This phenomenon is suggested to correlate with the higher viscosity of supercooled liquid and the slower grain growth of icosahedral phase during primary crystallization in the Zr 65 Al 7.5 Ni 10 Cu 12.5 Nb 5 compared to those in the Zr 65 Al 7.5 Ni 10 Cu 12.5 Pd 5 alloy. The role of the deformation-induced nanoclusters on the enhanced compressive plasticity was discussed.