Faqiang Guo

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.

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.

CaAl-based bulk metallic glasses with high thermal stability

We report that binary Ca–Al alloys can be readily cast into amorphous rods of 1 mm in diameter. Upon further alloying to depress the liquidus temperature, the amorphous rod diameter is increased to 3 mm. The high glass transition temperature Tg∼210 °C and crystallization onset temperature Tonset∼240 °C or higher observed are attributed to the covalent bonding trend noted in Ca–Al alloys that exhibit complex network structures. Along with a high microhardness value of 200–235 DPH (diamond pyramid hardness) and low mass densities of 2 gm/cm3, these thermally stable light-metal alloys are recognized as potential structural amorphous metals.

Mg–Ca–Zn Bulk Metallic Glasses with High Strength and Significant Ductility

The development of Mg–Ca–Zn metallic glasses with improved bulk glass forming ability, high strength, and significant ductility is reported. A typical size of at least 3–4 mm amorphous samples can be prepared using conventional casting techniques. By varying the composition, the mass density of these light metal based bulk amorphous alloys ranges from 2.0 to 3.0 g/cm 3 . The typical measured microhardness is 2.16 GPa, corresponding to a fracture strength of about 700 MPa and specific strength of around 250–300 MPa cm 3 /g. Unlike other Mg- or Ca-based metallic glasses, the present Mg–Ca–Zn amorphous alloys show significant ductility.

Enhanced bulk metallic glass formability by combining chemical compatibility and atomic size effects

The glass formability (GFA) of Y- and Ca-based alloys is enhanced by inserting elements that yield a more uniform separation in the atomic sizes, as well as a wider atomic size distribution range, in agreement with the analytical findings of Miracle, Senkov, and others. The degree of the GFA improvement depends on the chemical compatibility of the addition elements with the existing components. By combining this observation with similar results noted in other alloys that form large-size bulk metallic glasses, the importance of the atomic size effect and chemical compatibility in glass forming alloys is revealed. The atomic size effect and chemical compatibility are discussed along with other glass forming factors.

Formation of ductile Al-based metallic glasses without rare-earth elements

Al75Cu17Mg8 is a eutectic composition according to the ternary phase diagram, which can be quenched into a fully amorphous phase by adding 2-8at.% Ni, but the addition of a similar percentage of Gd failed to form the amorphous phase. The amorphous alloys obtained exhibit two broad diffuse peaks in the X-ray diffraction curves and, correspondingly, two halo rings in the electron diffraction patterns, implying that two types of local atom configuration exist. Thermal analysis of the amorphous alloys indicates that the primary crystallization peak shifts to higher temperatures with increasing Ni content. The occurrence of a nucleation and crystal growth peak during isothermal crystallization reveals the amorphous nature of the quenched ribbon alloys. The quenched amorphous ribbons do not break after bending by 180°. Mechanical testing yielded a tensile strength of 810MPa for (Al75Cu17Mg8)95Ni5, and a vein structure, characteristic of amorphous fracture, is apparent in scanning electron micrographs. The different effects of Ni and Gd on the glass formation indicate that the large atomic size of Gd is not critical to the glass formation.

Networking amorphous phase reinforced titanium composites which show tensile plasticity

Titanium matrix composites reinforced by an in situ formed networking amorphous phase have been prepared directly in the form of as-cast ingots. The composites show significant compressive and tensile plasticity. In contrast to the highly localized plasticity for most monolithic amorphous alloys, the tensile deformation of the current composites distributes uniformly over the entire specimen as a consequence of substantial work (strain) hardening. The formation mechanism of the networking structure, as a result of well-separated two-step solidification of the current alloy melts, have possible implications for composite synthesis in other alloy systems.