Physical properties of some iron based alloys in liquid and amorphous states

The atomic structure and the physical properties of amorphous ribbons depend strongly on the state of the melt before quench. It is known that slightly above liquidus metallic melts can preserve a non-equilibrium metastable state for a long time. Moreover some structural transformations in liquid metallic alloys, similar to phase transitions in solids, may take place with an increase of the temperature. In this paper we report measurements of the viscosity, magnetic susceptibility and surface tension in some Fe-based melts. Amorphous ribbons of the same alloys were prepared by standard planar method from different states of the melt. The electrical resistivity, the kinetics of crystallization and the magnetic properties of the ribbons were investigated. It was found that the properties depending upon nanoscale inhomogeneities are different for ribbons produced after different heat treatments of the melt before quench.     

Growth of extrusions in localized cyclic plastic straining

Cyclic strain localization into persistent slip bands produces on the surface of cyclically deformed materials persistent slip markings in the form of extrusions and intrusions. The localized cyclic plastic straining in persistent slip bands leads to the production and annihilation of dislocations as well as point defects. Production, annihilation and migration of point defects, preferably vacancies, is quantitatively characterized. The effect of vacancy production and migration on the extrusion growth is treated under simplified conditions. At low temperatures when vacancies are immobile, a static extrusion is produced. At elevated temperature the diffusion equation is solved, a general expression for stabilized rate of extrusion growth is obtained and temperature dependence of the growth rate can be predicted. The predictions of the model are compared with experimental observations.     

Deformation properties of amorphous alloys based on titanium

Translated from Fiziko-khimicehskay Mekhanika Materialov, Vol. 27, No. 3, pp. 77–79, May–June, 1991.     

On the theory of perfectly plastic anti-plane straining

A general formulation of two-dimensional elastic-perfectly-plastic anti-plane straining is presented for materials with arbitrary anisotropic convex yield surfaces. Stress and strain distributions in plastic regions adjoining portions of the boundary are obtained directly in terms of the yield surface geometry. When specialized to the classical torsion problem, results lead directly to a generalization of the well-known plastic roof construction for limit loads. Examples of the determination of fully plastic stress distributions and corresponding limit torques are given for circular and rectangular shafts with various yield conditions. Another specialization is made to the contained plastic deformation created by longitudinal shearing of a body containing a sharp edge notch. Here the determination of the elastic-plastic boundary and strain distribution is reduced to a potential theory problem for a region in the stress plane bounded by straight line segments and a portion of the yield surface, and a membrane analogy is presented which allows effective visualization of the solution. A solution valid for small scale yielding near a crack is given in terms of a conformal transformation of the yield surface to a unit circle, and some specific examples are worked. Particular attention is given to single crystal type yield surfaces made up of straight line segments corresponding to discrete slip planes.     

Oxidation of amorphous alloys

Owing to their unique short- or medium-range ordered microstructures and excellent mechanical, physical, and chemical properties, amorphous alloys have attracted significant interest in recent years. For the application of amorphous alloys, clarifying their oxidation processes and mechanisms is necessary since many of the surface-related properties of amorphous alloys largely depend on the surface oxide layer. The aim of this paper is to review the recent research on the thermal oxidation behaviors of amorphous alloys under pure oxygen or air condition. The contents are divided into three categories according to the number of components the research considers, i.e., the oxidation of binary, ternary, and multi-component (>3) amorphous alloys. Each section discusses the thermal stability of the amorphous matrix, oxidation kinetics, and the oxide layer and amorphous substrate, which are strongly affected by internal factors (i.e., alloy elements and microstructure) and external factors (i.e., oxidation temperature, duration, and oxygen partial pressure, etc.). The general features of the oxidation of amorphous alloys – from simple binary to complex multi-component amorphous alloys – will be summarized. This overview of the current scientific understanding on the fundamentals of these materials may provide guidelines for the development of strongly corrosion-resistant amorphous alloys.     

弛豫对Zr-Al-Ni-Cu基非晶合金晶化过程的影响

非晶合金的稳定性是制备大块非晶合金的关键，而弛豫后形成的短程有序结构对非晶中的晶体相形核有重要的影响。本文通过差热分析及X射线衍射法研究了预先弛豫处理对Zr60Al8Ni12.5Cu17Si2.5和Zr60Al8Ni10Cu17Si5非晶合金晶化过程的影响。结果表明：预先弛豫处理降低了上述非晶合金的稳定性。Si含量的增加则提高上述非晶合金的稳定性。根据等温转变过程日体相形核孕育期采用Arrhenius公式所确定的晶化激活能更能反映非晶晶化过程及非晶的稳定性。     

Consolidation of nanometer sized powders using severe plastic torsional straining

Severe Plastic Deformation (SPD) via torsional straining was used to consolidate nanometer-sized metallic powders and metal-ceramic nanocomposites at room temperature. Materials processed using this technique included copper, Al and nanocomposites based on these metallic nanopowders and SiO₂, SiC and Al₂O₃ nanopowders. The as-processed materials were nearly fully dense. Transmission electron microscopy (TEM) and microhardness measurements were used to characterize the materials. The as-consolidated materials had a grain size smaller than the average particle size of starting powder, indicating grain refinement during severe plastic torsional straining. Due to the oxidation of the initial nanometer sized powders, the as-processed samples were very brittle. Thermal stability was investigated by annealing the as-consolidated samples at various temperatures and then measuring their microhardness.     

Sensitivity calibration of post-yield strain gauges under cyclic plastic straining

A plastic sensitivity calibration procedure is outlined for strain gauges under cyclic, four point bending. Tests show that the gauge resistance changes in an approximately linear manner with longitudinal strain for the first quarter cycle of loading. Calibration curves for subsequent reversals to the direction of deformation display zero-shift and non-linearity. Under balanced strain cycling, there is evidence of a cyclically-stable, sensitivity calibration loop. Theoretical considerations are given in which it is proposed that separate sensitivity factors apply to the elastic and plastic components of strain. It is shown that the plastic sensitivity factor is a function of (i) plastic strain induced hardening and softening in the gauge foil and (ii) any apparent change to the gauge resistivity due to imperfect bonding. The elastic component sensitivity factor equals the manufacturer's value only in the absence of hardening. The two sensitivities may be combined to give a total sensitivity factor when a post-yield strain gauge suffers elastic-plastic straining.     

The Fe--B--Ge amorphous alloys

The crystallization behavior, the saturation magnetization, and the Curie temperatures of Fe-B-Ge amorphous alloy ribbons are reported and compared to previously reported results for Fe-B, Fe-B-C, Fe-B-Si, Fe-B-Si-C, and Fe-B-Ga.     

Switching in amorphous alloys

In this paper we present a survey of recent developments in the field of memory switching and of recent insights into the mechanism of threshold switching, in “ovonic” multicomponent chalcogenide alloys.     

Nanostructured amorphous Si-Au alloys

Nanostructured amorphous Si_1-XAu_X-alloys (0.1 ≤x_Au ≤ 0.3) were produced by an inert-gas condensation and in-situ consolidation technique and investigated by Small Angle Neutron Scattering (SANS) and density measurements. The results are interpreted in terms of a microstructure which consists of nanometer-sized amorphous grains embedded in a matrix of lower density, formed by grain boundaries and free volumes. The relative densities were found to be dependent on the average particle sizes, but independent of the alloy composition.In all samples the SANS intensity decreases at low Q according to a power law ∼Q^−3.37, which is attributed to a surface fractal structure of dimension D_S=2.63. The surface fractal structure is traced back to the interface between the open porosity and the amorphous material. This is supported by a contrast variation experiment.     

Magnetic annealing of amorphous alloys

Amorphous alloys with nominal composition of Ni40Fe40P14B6are shown to respond to annealing in a magnetic field. Coercive forces are reduced by a factor of 10 to 50 during annealing of straight ribbons to values of 0.003 Oe, as low as ever reported for potentially useful materials. Concurrently the ratio of the magnetization in 1 Oe applied field, to saturation, increases from about 0.5 to 0.95. These changes during annealing correlate with measured stress relief changes. It thus appears that most of the strain-magnetostriction contribution to the anisotropy is removed during annealing. Magnetic annealing at temperatures as low as 100°C results in noticeable changes in properties. From measurements transverse to the magneticaliy induced anisotropy axis, the induced anisotropy is calculated to be about 800 ergs/cm³, considerably smaller than obtained in crystalline Ni50Fe50. This field-induced anisotropy is reversible in direction and magnitude by reheating the sample to its Curie temperature and then cooling in a field. Annealing of 1.5 cm diameter toroids, made from 50 μm thick tapes, increases the initial permeability by more than a factor of 10 and decreases losses by more than a factor of 10. Losses and permeabilities after heat treatment compare favorably to the Permalloys with similar saturation magnetizations.     

Nanocrystallization of amorphous CoSiBFeNb alloys

The effect of Fe and Nb contents on the crystallization behavior of CoSiBFeNb alloys was studied using Mössbauer spectroscopy, transmission electron microscopy and X-ray diffraction. It was found that the structure formed by primary crystallization in the (Co₇₇Si_13.5B_9.5)₈₈Fe₇Nb₅ alloy is similar to that of the famous Fe_73.5Si_1.35B₉Cu₁Nb₃ nanocrystalline alloy. The data obtained are discussed regarding a random network of associates as a base structural element of amorphous metallic alloys.     

Intensification of diffusion saturation of iron subjected to cyclic straining

It was shown that if nitriding of armco iron is carried out on specimens subjected to cyclic elasto-plastic torsional strains, the rate of diffusion is increased 3–4 fold. Hardness and corrosion resistance of coatings obtained under these conditions are not lower than those of coatings produced in the normal way; in certain cases they may be higher.