S. Joseph Poon

Fe-based bulk metallic glasses with diameter thickness larger than one centimeter

Fe–Cr–Mo–(Y,Ln)–C–B bulk metallic glasses (Ln are lanthanides) with maximum diameter thicknesses reaching 12 mm have been obtained by casting. The high glass formability is attained despite a low reduced glass transition temperature of 0.58. The inclusion of Y/Ln is motivated by the idea that elements with large atomic sizes can destabilize the competing crystalline phase, enabling the amorphous phase to be formed. It is found that the role of Y/Ln as a fluxing agent is relatively small in terms of glass formability enhancement. The obtained bulk metallic glasses are non-ferromagnetic and exhibit high elastic moduli of approximately 180–200 GPa and microhardness of approximately 13 GPa.

Critical Poisson’s ratio for plasticity in Fe–Mo–C–B–Ln bulk amorphous steel

Fracture in compression has been investigated in Fe65Mo14C15B6 bulk amorphous steel doped with lanthanides to provide systematic variations of the elastic moduli. An onset of plasticity is observed as Poisson’s ratio approaches 0.32 from below. Combining with previous analysis reported by Lewandowski et al. [Philos. Mag. Lett. 85, 77 (2005)] using single-composition results from different sources, the findings are in support of universal critical Poisson’s ratio for plasticity in metallic glasses. The compositional dependences of elastic moduli are found to be anomalous considering the elastic moduli of the alloying elements. The role of interatomic interactions in designing ductile metallic glasses is suggested.

Effect of substitutions on the thermoelectric figure of merit of half-Heusler phases at 800 °C

The merit of thermally stable MNiSn (M=Ti, Zr, Hf) half-Heusler phases, as n-type thermoelectric materials, for high-temperature power generation has been examined. Sb doping at the Sn site is shown to increase both the figure of merit, ZT, and the temperature at which ZT is maximized. The benefits of increased alloying at the M and Ni sites, on the thermal conductivity and thermoelectric transport properties, have also been investigated. The thermoelectric figure of merit, ZT∼0.8 at T∼800°C, for select Sb-doped MNiSn alloys was found to meet or exceed the industry benchmark set by SiGe alloys.

Metallic glass ingots based on yttrium

We report a family of yttrium metallic alloys that are able to form glassy ingots directly from the liquid, as well as forming bulk-sized amorphous rods with diameters over 2 cm by water cooling of the alloy melt sealed in quartz tubes. It is apparent that, in addition to the strong chemical interaction among the components, the simultaneous occurrence of well-distributed atom sizes and a strongly depressed liquidus temperature in multicomponent metallic alloys is responsible for the formation of glassy ingots.

Ductile titanium-based glassy alloy ingots

We report that ductile and strong amorphous titanium metallic alloys (∼ twice the strength of high-strength titanium alloys) have been discovered that are in the form of glassy ingots. It is found that the suppression of a competing stable quasicrystalline phase upon solidification is particularly important in forming the current glassy ingots. While there is significant technological potential for these titanium alloys, the present findings have important implications on the design of highly processable bulk metallic glasses.

Synthesis of iron-based bulk metallic glasses as nonferromagnetic amorphous steel alloys

Iron-based amorphous metals are investigated as nonferromagnetic amorphous steel alloys with magnetic transition temperatures well below ambient temperatures. Rod-shaped amorphous samples with diameters reaching 4 mm are obtained using injection casting. Amorphous steel alloys are designed by considering atomistic factors that enhance the stability of the amorphous phase, coupled with the realization of low-lying liquidus temperatures. The present alloys are found to exhibit superior mechanical strengths. In particular, the elastic moduli are comparable to those reported for super austenitic steels.

(Zr,Hf)Co(Sb,Sn) half-Heusler phases as high-temperature (>700 °C) p-type thermoelectric materials

By substituting Sn for Sb, the potential of stable (Zr,Hf)Co(Sb,Sn) half-Heusler phases, as p-type thermoelectric materials, for high-temperature power generation has been examined. Sn concentration as much as ∼20%–30% is required to realize high power factor values. Substitution of heavier Hf, which reduces the thermal conductivity (κ) via mass fluctuation scattering, nonetheless maintains high mobility. As a result, the thermoelectric figure of merit ZT, for these not-yet-optimized materials, which we found to be ZT=0.5 at 1000K (measured) and ZT=0.6 at 1100K (extrapolated), surpasses the industry benchmark for a p-type material set by SiGe alloys.

Glass formability of ferrous- and aluminum-based structural metallic alloys

Abstract Synthesis of ferrous- and aluminum-based amorphous metals as prospective structural materials is presented and discussed in light of atomic size–composition interaction effects. The search of prospective bulk metallic glasses (BMGs) may benefit from noting that current BMG alloys can be broadly categorized into two atom size-composition classes, distinctly different from ordinary metallic glasses which can exist over a much wider atom size-composition range. The high formability of one class of BMGs is suggested to be due to the presence of a structure-reinforced network or backbone formed by tightly bound components in the undercooled liquid. For the ferrous-based BMG alloys investigated, it is proposed that zirconium-boron and molybdenum–carbon atom pairs constitute the strong backbone structures. Although aluminum-based BMG has not been reported, the good formability of some current aluminum-glasses is suggested to be due to the presence of backbone structures formed by transition metal–lanthanide and magnesium–copper pairs. The ferrous-based bulk metallic glasses obtained have a high reduced glass transition temperature reaching 0.63 and large supercooled liquid region up to 100 K. These bulk metallic glasses are found to be non-ferromagnetic above 160 K as well as having Vickers hardness and specific tensile strengths that far exceed those reported for steel alloys. Magnetization and susceptibility results are presented and compared with ab initio magnetic-structure calculations [D.M. Nicholson, M. Widom, Y. Wang, unpublished results]. Relevant factors on forming bulk metallic glasses are discussed.

Fe–Mn–Cr–Mo–(Y,Ln)–C–B (Ln = Lanthanides) bulk metallic glasses as formable amorphous steel alloys

The glass formability of high-manganese amorphous steel alloys reported earlier by us has been found to improve upon additions of yttrium and lanthanide elements, enabling the formation of bulk glassy samples with diameter thicknesses reaching 7 mm by casting. Based on extensive studies using different Ln additions and systematic measurements of alloy oxygen contents, the intrinsic roles of Y/Ln in attaining good glass formability in both the high-Mn alloys and previously reported high-Cr alloys are revealed. The yield strengths of the non-ferromagnetic glassy alloys obtained are estimated to be three times those of high-strength stainless steel alloys, and high elastic moduli in the range 150–200 GPa are measured. Furthermore, in the supercooled liquid regions, the glassy rods can be bent into various configurations by hand without fracturing. The observed plastic behavior together with the measured high mechanical strengths suggests that the present Fe–based bulk metallic glasses can potentially be developed as formable non-ferromagnetic amorphous steel alloys.

Mechanical properties, glass transition temperature, and bond enthalpy trends of high metalloid Fe-based bulk metallic glasses

Mechanical properties and glass transition temperatures (Tg) of Fe–Cr–Mo–P–C–B bulk metallic glasses containing up to 27at.% metalloids have been studied. The shear modulus (G) is found to decrease with increasing metalloid content and a maximum plastic strain of ∼3% is obtained, despite the increase in the number of strong metal-metalloid bonds. Also, Tg increases with the decrease in G, in contrast to usual behavior. By employing first-principles calculations, the results are discussed in light of atomic bonding and connectivity in the amorphous network. The findings are relevant to understanding ductility and glass transition of metallic glasses.