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تعداد صفحات این فایل: ۱۲ صفحه

بخشی از مقاله انگلیسیعنوان انگلیسی:On the Role of Atomic Ordering in the Formation of a HighCoercivity State in Iron–Cobalt–Vanadium Alloys~~en~~

Abstract

Processes that occur in the iron–cobalt–vanadium alloy with 52% Co and 7% V (vicalloy) upon heating to 1000°C and cooling have been studied by the dilatometric and magnetometric methods. It has been found that upon heating two phase transitions occur in the alloy, namely, B2type atomic ordering in the phase and a polymorphic transformation, whose temperature intervals differ considerably. An analysis of the dependences of the coercive force and the hardness of the alloy on the annealing temperature has shown that the maximum magnitudes of these parameters correspond to the ordering temperatures in the phase. There was made an assumption that the magnetization reversal process in the alloy tested is related to the displacement of ferromagnetic domain walls and their pinning at the boundaries of antiphase domains ordered by the B2 type.

۱ Introduction

The magnetic alloys of the iron–cobalt–vanadium system with 50–۵۲% Co and 5–۱۳% V revealed in the first half of the 20th century, which are called “vical loy” [۱], belong to the group of magnetically hard materials. Depending on the amount of vanadium, the coercive force of the alloys varies between 4.0 and 24.0 kA/m, the residual induction is between 1.25 and 0.6 T [1, 2].

A distinguishing feature of vicalloys is their high plasticity, which allows one to obtain the aboveindi cated magnetic properties in metallic ribbons up to 100 m thick. This fact, along with the high tempera ture stability of magnetic characteristics, makes the vicalloys virtually irreplaceable for producing active parts of rotors of synchronous hysteresis motors (SHM) [3]. The dependence of the coercive force of the alloys on the vanadium concentration allows using them for producing a wide assortment of the SHMs with working fields of 4–۲۴ kA/m [4].

A large number of studies has been devoted to the investigation of the nature of a highcoercivity state in the vicalloys [5–۸]. Today, the most widespread notion is the one, according to which a high coercive force of these alloys is related to the forward and reverse transformations. The hightemperature fcc phase transforms into a bcc phase either upon air cooling from 1000°C (alloys with 5–۹% V) or upon plastic deforma tion to no less than 80% (alloys with 10–۱۳% V). The subsequent tempering in the temperature range of 550–۶۷۰° leads to a partial transformation, which results in the formation of a twophase struc ture, in which disperse (probably, singledomain) regions of the ferromagnetic phase are surrounded with the paramagnetic phase.

According to the existing magnetichysteresis the ory [9], the magnetization reversal in an ensemble of singledomain particles is realized by the magnetiza tionvector rotation, and the magnitude of the coer cive force is determined by the energy of stray fields of the particle. Generally, such mechanism of magnetiza tion reversal is typical of many magnetically hard alloys, in particular, of the alloys of Fe–Ni–Al–Co–Ti (YuNDKT) [10, 11] and Fe–Cr–Co (KhK) [12, 13] systems.

In addition to the polymorphic transfor mation, atomic ordering in the phase occurs in these alloys upon heating, with the formation of a 2 super structure. This fact was mentioned in [5], and the existence of ordering was testified by neutron diffrac tion in [7, 8] for the alloy containing 10.5% V.

However, it was not possible to unambiguously evaluate the role of atomic ordering in the formation of a highcoercivity state of the vicalloys, since in the alloy chosen for experiments in [7, 8] the temperature intervals of ordering and transformations coincided. In this work, the effect of atomic ordering on the coercive force was studied on the alloy with 7% V, the temperature intervals of atomic ordering and poly morphic transformation in which differ considerably, as will be demonstrated below.

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