Crystallization Process of Iron-, Nickel-, and Cobalt-Based Amorphous Alloys Containing Silicon and Boron (Metallurgy)

Structural changes during heating or aging in a wide temperature range were exmained with three amorphous alloys of Fe_Si_B_, Ni_S_8B_ and Co_Si_B_ by measurements of electrical resistance, differential scanning calorie and Vickers hardness and also by transmission electron microscopy and X-ray diffraction. The results obtained are summarized as follows : (1) The crystallization process of three amorphous alloys is divided into four stages : (a) the incipient stage of crystallization where a certain short range ordering of atoms occurs, (b) the formation of primary metastable phases (MS-I), (c) the formation of secondary metastable phase (MS-II) with complex single structures, and (d) the formation of a stable phase consisting of a mixture of each equilibrium phase. The MS-I phase appears in the amorphous matrix in a manner of homogeneous nucleation and gradual growth, while the MS-II phase grows rapidly from a few nuclei and completely spreads over the amorphous matrix containing the MS-I phases. At higher temperatures, these MS-II phases transform finally to stable phases. (2) The temperature-time-transformation diagrams of three alloys were constructed. In these diagrams, distinct differences in the transformation sequence and the mode are observable in the upper and lower ranges of the critical temperature (Tx\u27). Above this boundary, crystallization by way of nucleation and growth proceeds through two metastable phases and finally to a stable phase. Below that temperature, on the other hand, progressive aging gradually changes the amorphous structure to the assembly of fine grains (about 100～200A) with a simple structure such as bcc, fcc, or hcp

Viscoelastic Behavior of Amorphous Metals

The viscoelastic behavior and its rate controlling process of amorphous metals have been examined by extensive experiments of tensile creep and stress relaxation using amorphous Pd-Si, Fe-P-C and Cu-Zr alloys. The differences in creep properties of amorphous and crystalline metals have also been discussed. Creep curves of all amorphous metals used may be classified into three stages of transient creep, steady-state creep and tertiary creep. The creep strain is composed of recoverable and irrecoverable components and it can be described in terms of viscoelastic elements in rheology. The steady-state creep is controlled by thermally activated process, and seems to be closely related with atomic diffusion in amorphous structure

Spontaneous Oxidization of an Amorphous Zr_<70>Au_<30> Alloy in Air

An amorphous Zr_Au_ alloy prepared by the melt-quenching technique was found to possess an extremely high oxidization tendency in air even at room temperature, even though the other Zr-based amorphous alloys remain unchanged in the same environment. Upon oxidization, the amorphous structure changes into a duplex crystalline structure consisting of a monoclinic ZrO_2 matrix including fcc Au particles with a size as fine as about 2 nm at an inter-particle spacing of about 4 nm. The oxidization tendency depends strongly on the humidity in air and becomes remarkable with increasing humidity. Furthermore, the oxidization causes a spontaneous pulverization of the ribbon into fine powders consisting of ZrO_2 and Au. The reason for the exceptionally high oxidization only for the Zr_Au amorphous alloy was inferred to be due to a combination effect of a weak attractive bonding nature between Zr and Au and a great catalytic ability of Au

Electrical Resistivity and Its Temperature Dependence of Al-base Quasicrystalline and Crystalline Alloys

Al-Mn and Al-Cr quasicrystalline and Al-Mn crystalline alloys were prepared by melt-quenching and their electrical resistivities were studied. The quenched samples exhibited an extremely high resistivity because of nonperiodic potential scattering and resonance scattering of the Fermi electrons. The temperature dependence of electrical resistivity of Al-22.5%Mn quasicrystalline was tried to fit by several models, and it was found that the plots of lnT (from 4.2 to 20 K) and T^2 (from 20 to 60 K) showed a straight line. The electrical resistivity of Al_6Mn crystalline alloy showed a linear temperature dependence over wide high temperatures, but that of Al_4Mn crystalline alloy showed a deviation from the linearlity. The values of P of the quasicrystalline alloys were larger than those of the crystalline counterparts

Soft Magnetic Co-Ti-B Amorphous Alloys with High Corrosion Resistance

Co-Ti-B ternary amorphous ribbons were prepared by the melt-quenching method, and their soft magnetic properties, hardness and corrosion resistance have been investigated. With increasing titanium content, the Curie temperature decreases monotonically, while the crystallization temperature gradually increases. These alloys are magnetically very soft, that is, the coercive force takes a minimum value of 0.01 Oe and the maximum permeability shows a large value of 6×10^4 around x=0.05 for (Co_Ti_x)_B_ amorphous alloys. Their linear magnetostrictions are also quite small, being of the order of about -2.5×10^. In addition, these amorphous alloys have a high hardness and an excellent corrosion resistance. Therefore. Co-Ti-B amorphous alloys are promised as the soft magnetic materials for electromagnetic devices

Formation of a Nanoscale hcp Structure by Crystallization of an Amorphous Co_<91>Zr_7B_2 Alloy

A mostly single hcp phase with grain sizes ranging from 3 to 10 nm was found to form as a metastable phase in the crystallization process of an amorphous Co_Zr_7B_2 alloy. The temperature range, in which the nanoscale hcp structure forms, extends from 800 to 900 K and the further heating above 900 K causes the phase transition to ε-Co+Co_(Zr, B)_6. No distinct grain growth in the hcp structure is seen in the temperature range of 800 to 900 K. The hcp phase has a lattice parameter of a=0.2507 nm and c=0.4066 nm which is slightly different from that of pure ε-Co presumably because of the dissolution of Zr and B. The Co_Zr_9 amorphous alloy crystallizes directly to a mixed phase of ε-Co+cubic Co_Zr_6 with a large grain size of about 0.4μm through a polymorphic-type crystallization mode. It is therefore concluded that the addition of a small amount of boron is essential for the formation of the nanoscale hcp structure

Thermal Instability and Crystallization Characteristics of Amorphous Metal-Metalloid System

Annealing effects on structure of several amorphous metal-metalloid systems (Pd-Si, Fe-P-C, Fe-Si-B, Co-Si-B and Ni-Si-B) were examined by electron microscopy, X-ray analysis and further by measurements of internal friction, electrical resistivity, specific gravity, microhardness and fracture strain. In T-T-T diagrams, distinct differences in transformation sequence were observed above and below the critical temperature. Above this temperature, crystallization proceeds through two metastable phases and finally to the stable phase by nucleation and growth mechanisms. Below this temperature, however, progressive aging gradually changes the structure through two stages ; the initial stage is due to some degree of ordering in atomic arrangements in the as-quenched state and the subsequent stage due to transformation from amorphous to single phase with the same structure as the metallic element. A remarkable decrease in fracture strain of Fe-P-C alloys occurs after aging treatments at the incipient stage. This phenomenon is not a general character of all amorphous alloys