J.H. Na

Role of nanometer-scale quasicrystals in improving the mechanical behavior of Ti-based bulk metallic glasses

The effect of the precipitation of nanosized quasicrystals on the mechanical properties of Ti–Zr–Cu–Ni–Be bulk metallic glasses (BMG) has been investigated. The Ti40Zr29Cu8Ni7Be16 BMG crystallizes by forming a few nanometer-size quasicrystals in the amorphous matrix, enabling the fabrication of quasicrystal-reinforced BMG matrix composites. Simultaneous improvement of strength and ductility can be obtained when 3–5-nm-size quasicrystals are isolated and homogeneously distributed in an amorphous matrix. The fracture strength and global strain, respectively, increase from 1921 MPa and 5.1% for as-cast BMG to 2084 MPa and 6.2% for partially crystallized BMG with the volume fraction of ∼7% quasicrystals. These improvements may be attributed to the structural similarity between quasicrystalline and amorphous phases. Stable low-energy interface between two phases may act as a source for multiple-shear-band formation.

Correlation between fragility and glass-forming ability/plasticity in metallic glass-forming alloys

In the present study, the authors draw attention to the relationship among fragility index (m), glass-forming ability (GFA), and plasticity in various metallic glass-forming alloys (MGAs), and show that the m value is closely related to both characteristics. In particular, m can be formulated with ν (Poisson’s ratio): ν=−0.179+0.312logm, which means that high m as well as large ν values might be regarded as indicators of the MGAs ductility. As an example, it can be rationalized that the lowest m value (21.5 at 5K∕min) of Ca65Mg15Zn20 supports both outstanding GFA and extreme brittleness.

Abnormal behavior of supercooled liquid region in bulk-forming metallic glasses

A metallic glass is often viewed as an amorphous alloy exhibiting a single endothermic reaction in the supercooled liquid region (SCLR, ΔTx=Tx−Tg). Here we discuss the origin and consequences of abnormal behavior of SCLR in various bulk-forming metallic glasses (BMGs). The two-stage-like endothermic reaction in Ni-based, Cu-based, Zr-based, and Mg-based BMGs can originate from the local immiscibility of liquids, which is closely related to chemical heterogeneity in as-cast BMG. These inflections can be attributed to the overlap of the exothermic reaction for the formation and growth of clusters in SCLR. The abnormal behavior of SCLR can be modulated by controlling cooling rate as well as by tailoring alloy composition, with the consequence that the modulated local heterogeneity in these BMGs can lead to enhanced flexibility of the BMGs. This correlation assists in understanding toughening mechanism and in guiding alloy design to alleviate brittleness of BMGs.

Poisson’s ratio and fragility of bulk metallic glasses

The correlation among apparent global plasticity, Poisson’s ratio, and fragility in monolithic bulk metallic glass (BMG) alloys has been investigated in the present study. The shear and bulk moduli in monolithic Cu-based BMG alloys have been measured by resonant ultrasound spectroscopy (RUS) and ultrasonic technique. The Cu 43 Zr 43 Al 7 Ag 7 BMG alloy showing a large apparent global plasticity (∼8%) exhibits a high Poisson’s ratio when compared with that of Cu 43 Zr 43 Al 7 Be 7 BMG alloy. In addition, the fragility of Cu-based BMG alloys can be obtained by differential scanning calorimetry (DSC). The fragility index m of Cu 43 Zr 43 Al 7 Ag 7 BMG alloy is slightly larger than that of Cu 43 Zr 43 Al 7 Be 7 BMG alloy. The correlation between Poisson’s ratio and fragility in BMG alloys can be presented by a simple relation of m − 17 = 14 ( B ∞ / G ∞ − 1). Poisson’s ratio and fragility might be regarded as an important parameter that controls global plasticity of glass-forming alloys.

Development of Ni-Zr-Nb-Al-M(=Ta,Ti,Y) Metallic Glass

The effects of Ta, Ti and Y addition substituting Zr in the Ni-Zr-Nb-Al metallic glass alloys have been investigated by using thermal analysis, X-ray diffractometry and transmission electron microscopy. Partial replacements of Zr with M(=Ta, Ti, Y) in Ni61Zr28Nb7Al4 alloy significantly enhance the glass forming ability and enlarge the undercooled liquid region during continuous heating of glassy ribbons. Fully glassy rods with the diameter of up to 2 mm can be fabricated by a copper mold casting method.