Microstructure and Kinetics of the Ordering Transformation in Iron–Cobalt Alloys, FeCo, FeCo–0.4%Cr, FeCo–2.5%V

Abstract

Ordering in FeCo, FeCo-0·4Cr, and FeCo-2·5V is studied using X-ray diffraction, saturation magnetization, and optical and electron microscopy. The kinetics of the disorder to order transformation on isothermal heat-treatment of quenched alloys are determined over the temperature range 250-600°C. Between 500 and 600°C alloys order by a homogeneous process followed by coalescence of antiphase domains. Below I'V 430°C FeCo and FeCo-0·4Cr order by nucleation and growth of ordered regions at grain boundaries, this mechanism does not occur in FeCo-2·5V. The grain-boundary orderedregionsexhibita facetedmorphology.Upon ordering the equiatomic alloy of iron and cobalt changes from the A2 (bcc) to the B2 type of structure. Only a rearrangement of atoms on the same lattice sites occurs, with a very small change in volume (0'2 % expansion) and no compositional changes. A previous experimental study of ordering in this system 1 indicated that the transformation was of the homogeneous type, i.e. the degree of order within the alloy remained uniform during the transformation; however, a network of antiphase domains was formed which coalesced with prolonged annealing. Within the temperature range 400-600°C there was no detectable difference in the kinetics of ordering between the binary alloy and alloys containing up to 2·5 % vanadium with iron and cobalt in equal proportions. However, the kinetics of the subsequent antiphase domain coalescence appeared to be affected by the presence of the ternary additions, particularly at temperatures < I"V 500°C. For the binary alloy the apparent antiphase domain size, as measured by X-ray superlattice line broadening, increased very rapidly with time during isothermal annealing of quenched alloys, the domain size reaching a very large value before ordering was completed. For the ternary alloys with vanadium the antiphase domain size increased only slowly after the alloy was fully ordered. The present work involves the study of the ordering transformation in both binary and ternary alloys at temperatures of between 250 and 600°C with emphasis on the lower temperature regions and the binary alloy. ExperimentalThree alloys were examined, viz. 50 %Fe-50 %Co; 48·8 %Fe-48·8 %Co-2'5 %V; 49·8 %Fe-49'8 %Co-0'4 %Cr. It was found that the third alloy behaved in much the same way as the binary alloy except that it was more easily handled (the binary Paper No. MS 432. Manuscript received 17 May 1974; in final form 22 August 1974. R. A. Buckley, DPhil, is in the Department of Metallurgy, University of Sheffield. METAL SCIENCEalloy was exceedingly brittle). The methods for the preparation of the alloys from elemental materials have been described previously.! For X-ray and metallographic work, discs I"V O' 5 mm thick were quenched from a temperature of 810°C, and isothermal ordering was carried out in a fused-salt bath. Samples for magnetic analysis were cut from the quenched discs.X-ray measurements were made using a Philips diffracto-. meter fitted with diffracted-beam monochromator set for cobalt K a r...

Near equiatomic FeCo alloys: Constitution, mechanical and magnetic properties

“…He proposed that, while the ordering reaction occurs before and independently of recrystallisation at higher temperatures, the ordering proceeds, at lower (T < 475°C) temperature, through the formation of new ordered grains. However, according to Buckley [63,64], this does not seem to occur in FeCo-V alloys, where no recrystallisation is observed at lower temperatures.…”

“…Buckley [63,64] investigated the interactions between the ordering reaction and recrystallisation. Using an FeCo-0.4Cr alloy deformed to about 40%, he distinguished four temperature regions as illustrated in Table 2.…”

Ordering and phase separation in alloys of the Fe–Co system

“…So, the detection of a small volume fraction of the B2 phase in the bulk signifies that the existing situation that the B2 phase can have a determining effect on the magnetic properties of the Fe-Co alloys is contrary to fact. It should be also pointed out that TEM gives no way of detecting any antiphase domains, which have been previously detected by Buckley [9] and Zhu et al [5].…”

“…Some authors, for example, authors of Ref. [9] reasoned that it is possible to determine the degree of order on evidence of saturation-magnetization measurements, that some antiphase domains are formed at ordering, that heat treatment at temperatures below 730 • C brings about the simultaneous rise in the lattice parameter and order parameter.…”

Optimization of the Magnetic Properties of FePd Alloys by Severe Plastic Deformation

“…3(a)) clearly exhibits some superlattice reflections of the L1 0 ordered phase, indicating that some ordering occurred as expected. The main question, already raised by R.A. Buckley 30 years ago for the FeCo system, [23,24] is the following: how do the recrystallization and the ordering process in a concomitant way to reduce the energy of the system? Although the ordering process needs in principle only a diffusion of atoms over very short distances (typically only one atomic distance), the ordering process in the equi-atomic FePd alloy, like in the FeCo alloy, is a first order transition controlled by the nucleation and growth of ordered domains.…”