Crystallization Behaviors Of Late Transition Metal-Early Transition Metal Metallic Glasses At Temperatures Below Calorimetric Glass Transition

Crystallization has been found to be one of the most intriguing problems in metallic glasses. Metallic glasses with excellent glass-forming ability and wide supercooled span are believed to be stable at the temperature below glass transition (Tg). Crystallization are readily occurred at the temperature higher than Tg, i.e., in the supercooled region. Below Tg, the motion of atoms is not completely frozen in, and the metallic glass systems can lower energies by structural relaxation. The diffusion of constituent elements in the glassy state has been extensively studied in a wide variety of metallic glasses, and it has been found that decoupling in diffusion of constitutes with small atom size is a universal phenomenon. The fast diffusion of small atoms in metallic glasses has an important implication on the crystallization at temperatures below Tg.Ni-Ta, Ni-Nb and Ni-Zr are typical late transition metal-early transition metal (LTM-ETM) metallic glasses. These metallic glasses exhibit many unique properties, such as high thermal stability, excellent corrosion resistance and good hydrogen permeability, which are potentially useful for hydrogen permeation. The operating temperature for hydrogen permeation lies in the range from573to873K, which is not far from the glass transition temperatures of these metallic glasses. Their structural stabilities at these temperatures are therefore being a major concern in the hydrogen permeation application.In the present study, the crystallization behaviors of Ni60Ta40, Ni60Nb40, Ni40Nb60and Ni40Zr60metallic glasses at the temperatures far below the glass transition have been studied by means of X-ray diffraction (XRD), differential scanning calorimetry (DSC), differential thermal analysis (DTA), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). The experimental results are summarized as follows:(1) Ni-Ta and Ni-Nb metallic glasses over a wide composition range were made using melt-spun technique. Among them, a Nb-based Ni40Nb60metallic glass, which was previously thought not to be a glass-former, has been successfully obtained.(2) Tg of Ni60Ta40and Ni60Nb40metallic glasses were measured to be998K and913K, respectively, by DTA at a constant heating rate of40K/min. The glass transition of Ni40Nb60metallic glass is detectable, followed by the onset of crystallization (Tx) at940K. The Tx of Ni40Zr60metallic glass was measured to be725K by DSC at a constant heating rate of20K/min. (3) Ni60Ta40and Ni6oNb4o metallic glasses preserve an amorphous structure after isothermal annealing168h at the temperatures about150K below Tg. The temperatures are close to their respective Vogel-Fulcher-Tammann temperatures. In the Ni6oTa4o metallic glass, a Ta-enriched face-center cubic (a=0.38nm) metastable phases precipitated from the amorphous matrix after3min annealed at903K. After annealing at903K for30min, the number density of nanocrystals increases, and some of them are closely neighbored. This indicates that spinodal decomposition cannot be an operable precursor mechanism for the nanocrystallization. The grain size is about5to10nm, which is indicative of grain growth. Comparison of the DSC curves of the annealed and as-quenched samples reveals subtle shift of the crystallization peak to a lower temperature in the annealed state. The profile of the crystallization peak is almost preserved, and reduction of the integral crystallization enthalpy is calculated to be about3%, which indicates that only a small portion of the amorphous body has devitrificated. In Ni6oNb4o, the formation of face-centered cubic (a=0.38nm) nanocrystals of about5nm was observed after1h annealing at873K. STEM analysis reveals the similar composition separation phenomenon in the nanocrystallized microstructures. In the Ni4oNb6o metallic glass, the face-center cubic (a=0.37nm) metastable phase was also formed after48h annealing at763K. In the Ni4oZr6o metallic glass, an unknown big unit cell phase was observed after isothermal annealing at673K, and a FCC structure (a=2.09nm) was tentatively assigned to this phase. In addition, in long time annealed Ni6oNb4o and Ni4oZr6o samples, two unknown crystalline phases were found:a primitive triclinic phase (a=0.58nm, b=0.77nm, c=0.40nm, a=92°, B=127°, y=102°) in873K-2h annealed Ni60Nb40and a base-centered orthorhombic phase (a=0.93nm, b=0.5nm, c=1.11nm) in673K-0.5h annealed Ni4oZr6o samples. These results are not consistent with those accounted in previous studies.An underlying physical mechanism is proposed for the composition separation-like nanocrystallization. In particular in Ni6oNb4o and Ni6oTa4o metallic glasses, the decoupled fast diffusion of Ni atoms is assumed to occur in the glassy states. By virtue of the existence of liquid-like regions in metallic glasses at temperatures below Tg, local β processes are readily percolated, and hence cooperated atomic diffusion can occur in these liquid-like regions. If the big heavy Ta and Nb atoms are rather immobile than Ni atoms in the liquid-like domains, the long-range cooperative diffusion of Ni atoms will leave behind nanometer scaled domains which are enriched in Ta or Nb. A lattice of vacancies are formed immediately with Ni depleting, and hence structural rearrangement may occur in the nanometer scaled amorphous domains. Notice the small self-diffusion coefficients of atoms at temperatures well below glass transition, typically～10-22-10-24m2s-1. The cooperative shear displacement of atoms into a FCC lattice can be kinetically favorable in these domains at these temperatures. As a consequence, the final grain sizes of nanocrystals in the annealed microstructures would be limited by the length scale of the liquid-like domains.