Thermal Stability, Glass-Formation Ability, and Mechanical Properties of (Zr41.2Ti13.8Cu12.5Ni10Be22.5)100?xNbx Amorphous Alloys

Bulk amorphous alloys of (Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5)_100−x Nb _x with x = 0, 5, 11, and 13 were prepared by water quenching. Differential scanning calorimeter (DSC) analysis revealed that the addition of Nb enhances the thermal stability but appreciably decreases the glass-forming ability (GFA) of the alloys. Scanning electron microscope (SEM) and compression tests indicated that the Nb addition effectively improves the strength and plasticity of a Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 amorphous alloy, which benefits from multiple shear bands induced by ductile crystalline phase dispersing in the amorphous matrix. The bulk amorphous alloy with x = 5 exhibits a fracture stress of 2070 MPa and total strain to fracture of 25.8 pct, respectively. This article is based on a presentation given in the symposium entitled “Bulk Metallic Glasses IV,” which occurred February 25–March 1, 2007, during the TMS Annual Meeting in Orlando, FL, under the auspices of the TMS/ASM Mechanical Behavior of Materials Committee.

---

A Study on Synthesis, Crystallization, and Magnetic Behavior of Nanocrystalline Fe-Based Metallic Glasses

In the present work, structure of the as-cast melt-spun ribbons, nonisothermal crystallization kinetics, and the effect of heat treatment on the magnetic properties have been studied. X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses have revealed the presence of amorphous and partly crystalline structures in the as-cast Fe₆₇Co₁₈Si₁B₁₄ and Fe₅₇Co₂₆Cr₃B₁₄C_0.2 metallic-glass ribbons, respectively. The crystalline phase present in the as-cast Fe₅₇Co₂₆Cr₃B₁₄C_0.2 metallic-glass was identified as α-Fe. Direct transformation from liquid to α-Fe has been analyzed from a thermodynamic and kinetics point of view. The differential scanning calorimetry (DSC) studies have shown two-stage crystallization behavior. The primary and secondary crystallization phases were identified as bcc-Fe(Co) and bct-(Fe,Co)₃(Si,B), respectively. Kissinger and Gao et al. methods were employed for nonisothermal crystallization kinetic studies. The activation-energy values obtained by the two models were in good agreement. The nucleation and growth morphologies of crystalline phases have been explained on the basis of the Avrami exponent, which were found to be consistent with the observed microstructures. The magnetic properties of as-cast amorphous ribbons showed low coercivity, and this has been attributed to averaging of magnetocrystalline anisotropy over grains coupled within an exchange length, i.e., based on a random anisotropy model. The influence of microstructure on magnetic properties was studied by crystallizing the amorphous phase at 400 °C for 3 hours. The saturation magnetization and coercivity had increased after crystallization for both alloys. This article is based on a presentation given in the symposium entitled “Materials Behavior: Far from Equilibrium” as part of the Golden Jubilee Celebration of Bhabha Atomic Research Centre, which occurred December 15–16, 2006 in Mumbai, India.

---

Preparation and thermal stability of Zr-Ti-Al-Ni-Cu amorphous powders by mechanical alloying technique

This study examined the amorphization feasibility of Zr_70−x−y Ti _x Al _y Ni₁₀Cu₂₀ alloy powders by the mechanical alloying (MA) technique. According to the results, after 5 to 7 hours of milling, the mechanically alloyed powders were amorphous basically in the ranges of 0 to 12.5 at. pct Ti and 2.5 to 17.5 at. pct Al. These ranges are larger than those of bulk amorphous alloys prepared by a squeeze mold casting technique. Most of the amorphous mechanically alloyed powders exhibited a wide supercooled liquid region of more than 60 K before crystallization. The glass-transition and crystallization temperatures of mechanically alloyed samples were different from those prepared by squeeze casting. It is suspected that different thermal properties arise from the introduction of impurities during the MA process. The amorphization behavior of Zr₅₀Ti_7.5Al_12.5Ni₁₀Cu₂₀ was examined in detail. The X-ray diffraction and extended X-ray absorption fine structure (EXAFS) results show the fully amorphous powders formed after 5 hours of milling. A kinetically modified thermodynamic phase transformation process was observed for the glass-transition behavior in the Zr₅₀Ti_7.5Al_12.5Ni₁₀Cu₂₀ amorphous powder.     

Preparation,mechanical strengths,and thermal

Ni-based amorphous wires with good bending ductility have been prepared for Ni₇₅Si₈B₁₇ and Ni₇₈P₁₂B₁₀ alloys containing 1 to 2 at. pct Al or Zr by melt spinning in rotating water. The enhancement of the wire-formation tendency by the addition of Al has been clarified to be due to the increase in the stability of the melt jet through the formation of a thin A1₂O₃ film on the outer surface. The maximum wire diameter is about 190 to 200 μm for the Ni-Si (or P)-B-Al alloys and increases to about 250 μm for the Ni-Si-B-Al-Cr alloys containing 4 to 6 at. pct Cr. The tensile fracture strength and fracture elongation are 2730 MPa and 2.9 pct for (Ni_0.75Si_0.08B_{0.17 99}Al₁) wire and 2170 MPa and 2.4 pct for (Ni_0.78P_0.12B_0.1)₉₉Al₁ wire. These wires exhibit a fatigue limit under dynamic bending strain in air with a relative humidity of 65 pct; this limit is 0.50 pct for a Ni-Si-B-Al wire, which is higher by 0.15 pct than that of a Fe₇₅Si₁₀B₁₅ amorphous wire. Furthermore, the Ni-base wires do not fracture during a 180-deg bending even for a sample annealed at temperatures just below the crystallization temperature, in sharp contrast to high embrittlement tendency for Fe-base amorphous alloys. Thus, the Ni-based amorphous wires have been shown to be an attractive material similar to Fe- and Co-based amorphous wires because of its high static and dynamic strength, high ductility, high stability to thermal embrittlement, and good corrosion resistance.