1388 IEEE TRANSACTIONS ON MAGNETICS, VOL. 26, NO. 5, SEPTEMBER 1990

STRUCTURE AND YKUPERTIES OF RAPIDLY-SOLIDIFIED IRON-PLATINUM AND IRON-PALLADIUM ALLOYS

B. Zhang and W. A. Soffa Department of Materials Science and Engineering

University of Pittsburgh, Pittsburgh, PA 15261 U.S.A.

Abstract-Fe-Pt and Fe-Pd alloys can develop an attractive combination of hard magnetic properties similar to the Co-Pt permanent magnet alloys. This family of alloys derives its properties from CuAuI (L1 -type ordering and exhibits a polytwinned microstructure d i c h is expected to have a strong influence on the domain structure and mechanism of coercivity. In this study rapid solidification techniques and systematic heat treatment have been employed to vary the microstructure of equiatomic Fe-Pt and Fe-Pd alloys to gain a better understanding of the fundamental processes controlling the magnetic properties in this genre of alloy. Preliminary results on the Fe-Pd alloy indicate that the magnetically hardened material is fully ordered and exhibits a well-developed polytwinned state. Melt-spinning was found to enhance the coercivity compared to a bulk alloy and it is suggested that this change in properties is related to microstructural refinement within the polytwinned state. The initial results on the Fe-Pt alloy indicate that the details of the mechanism of hardening in this alloy may be somewhat different than in the Fe-Pd system although both develop the polytwinned state after prolonged aging.

INTRODUCTION

Fe-Pt and Fe-Pd alloys can develop coercivities in the range Hc-500 Oe to 5 kOe and energy products (BxH) -5-20 MGOe. Both alloys derive their properties from C u A r ( L 1 )- type ordering and the ordered tetragonal Phase exhibits' a uniaxial magnetocrystalline anisotropy-2-7x10 ergs/cc similar to the well-known Co-Pt alloys where K - 5 ~ 1 0 ~ ergs/cc [1,2]. The Fe-Pt, Fe-Pd and Co-Pt family of alloys is characterized by a polytwinned structure which emerges during ordering to relax the strain energy attendant to the cubic-tetragonal transformation [3]. However, the twin structure has often been ignored in analysis of the origin of coercivity. For example, Gaunt [4] has concluded that the coercivity of CO-Pt alloys stems from domain wall pinning produced by large gradients in the domain wall energy within a fine mixture of the disordered cubic and ordered tetragonal phases. The role of the twin microstructure has been the focus of discussion by a number of investigators particularly in the Russian literature [5,6]. The unique hierarchy of possible domain structures characterizing these materials has been clearly recognized and discussed but more theoretical and experimental work are required to elucidate the role of the twins in magnetization reversal.

In this paper the preliminary results of a new study aimed at re-examining the relationship between microstructure and magnetic properties in Fe-Pt and Fe-Pd alloys are reported with special emphasis on the development and influence of the polytwinned state. Rapid solidification has been employed since there is some suggestion that the scale of the microstructural elements might be extremely important in governing the properties of this family of alloys.

EXPERIMENTAL PROCEDURE

The Fe-Pd and Fe-Pt alloys used in this study were prepared in bulk form and in the form of melt-spun ribbon. The alloys were prepared by arc melting and/or melt spinning in an argon atmosphere using high-purity materials. Melt-spinning was camed out using a copper wheel. The alloy compositions were very nearly equiatomic (5050) as verified by analytical electron microscopy (AEM) and atom probe field-ion microscopy (APFIM). The arc melted Fe-Pd alloy button was coldworked and then homogenized and recrystallized at 950 "C for 6hrs and quenched into ice water. X-ray and electron diffraction showed that both the as-quenched bulk and as-melt-spun Fe-Pd alloys were disordered whereas the melt-spun Fe-Pt ribbons were highly ordered. Alloy specimens for aging heat treatments were

encapsulated in evacuated quartz capsules which had been back- filled with argon.

The magnetic measurements were performed at room temperature using a vibrating sample magnetometer with a maximum field of 15 kOe. The electron microscopy was camed out on either a JEOL 2OOCX or JEOL 2000FX instrument. Thin foils for transmission electron microscopy were prepared using a twin-jet electropolishing technique.

RESULTS AND DISCUSSION

Melt-spun Fe-Pd alloy ribbon in the disordered state typically showed coercivities of 5-20 Oe. The melt-spun specimens aged at 500 "C exhibited a rapid initial age hardening res onse with the coercivity reaching about 500 Oe after 5 hrs. lubsequent aging to 48 hrs led to a very small decrease in the coercivity and for prolonged aging to 148 hrs the coercivity only dropped to 435 Oe. The as-quenched bulk Fe-Pd alloy showed a similar coercivity to the as-melt-spun ribbon. Aging of the bulk Fe-Pd alloy at 500 O C also resulted in a rapid magnetic hardening. The peak hardness of about 260 Oe was reached after 10 hrs again followed by a very slow decrease after long aging times. The age hardening curves of the Fe-Pd melt-spun and bulk alloys are shown in Figure 1.

The average grain size of the melt-spun Fe-Pd specimens was approximately lpm. Figure 2 is a dark-field micrograpk obtained using the superlattice reflection z = O O l revealing the early stages of aging in a specimen aged 3 hrs at 500 "C. The micrograph shows an array of ordered particles aligned along the <110> directions of the cubic parent phase. Electron diffraction from the same region reveals that the various rows have different c-axes in twin relationship to each other. This distribution of twin related rows preferentially occupied by differently oriented tetragonal axes is depicted in Figure 3. The modulated structure occurs as a result of strain energy effects as discussed by Khachaturyan and others [7]. Similar morphologies have been observed in Co-Pt alloys by Hadjipanayis and Gaunt [SI. Figure 4 shows the microstructure of the aged Fe-Pd ribbon after 5 hrs at 500 OC (near peak hardness). The alloy is fully ordered and clearly exhibits a polytwinned structure comprised of interpenetrating microtwins which apparently arise from coalescence of the discrete ordered regions producing a high density of APB's (antiphase boundaries) within the twin plates. The polytwinned state also shows twin clusters comprised of twin plates of different twin variants on a scale of the order of 0.25 pm. The microtwin lamellae have an average thickness of about 50 A within the clusters. During prolonged aging the microtwin structure undergoes a coarsening process with certain orientations growing at the expense of others. One of the twin lamellae or variants within a cluster preferentially grows while the other gradually disappears so that each cluster consists eventually of only one type of microtwin. Also, adjacent microtwin clusters tend to develop a twin relationship. The average cluster size does not change markedly during aging, but the thickness of the microtwins increases to about 200 A after aging for 148 hrs. The coarsened twin structure is shown in Figure 5a and b. Note that the coarsened microtwins still contain a very high density of APR's.

The microstructure of the aged bulk Fe-Pd specimens showed a very similar evolution of mor hology. The grain size of the bulk alloy was about 30 pm. Figure 6 is a dark-field image, usinga=220, of the underaged bulk alloy exhibiting the so-called "tweed" contrast [9]. The contrast striations are aligned along the (110) traces. A dark-field image using a superlattice reflection instead of a fundamental reveals an aligned array of discrete ordered regions identical to that described above. A bulk Fe-Pd specimen aged to peak hardness is fully ordered and shows the interpenetrating rizrotwins and high density of APB's

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0 - 5 RGING T I E &r>

Fig. 1 Agin curves of melt-spun and bulk Fe-Pd alloys aged

Fig. 2 Dark-field image from melt-spun Fe-Pd aged 3 hrs at

as shown in Figure 7. The microtwin lamellae are about 80-100 A thick and the twin clusters have an average size of 1-2 pm.

It is interesting to compare the microstructures of the melt- spun and bulk Fe-Pd alloys at peak magnetic hardness. The coercivity of the fine-grained melt-spun material is almost twice as high as that of the bulk alloy. Importantly, although the microtwin thickness within the clusters is similar, the twin cluster size in the optimum state is finer in the melt-spun material. Since both Fe-Pd alloys are fully ordered at peak hardness, these results indicate that indeed the scale of the polytwinned state plays an important role in the mechanism of magnetization reversal. Furthermore, during prolonged aging the macrotwins which emerge are fenerally on the scale of the initial cluster size and this doesn t change very much even after long aging times. This behavior may be responsible for the extended plateau of the age hardening curve. The coarsening of the twin structure on different scales must be related to the relaxation of strain energy at short and long wavelengths [3]. The initial twin cluster size may be related to the grain size or the long wavelength components of the strain energy and there is further evidence in the literature that the coercivity of the polytwinned Fe-Pd alloys is markedly grain size dependent [lo]. It has been suggested that the coercivity in Fe-Pd and CO-Pt alloys derives from wall pinning due to the high density of APBs in the twin plates [2]. If this mechanism is operative, the elementary retarding force acting on a mi ratin domain wall within the platelets will be - U A-'/' K3/5 M -' where U is the effective thickness of the fault, A is the exchange constant, and K and M have their usual meanings. However, the force acting on thg domain walls exerted by the applied field within clusters will be -H M D2 where D is the diameter of the twin cluster. Theref;re,'the coercivity will scale with the anisotropy as K3I2 and microstructure as 1/D2. Thus, the coercivity in this simple

500 OC.

Fig. 3 Schematic drawing of a distnhtion of tetragonal phase precipitates within alternating reylarly spaced { 110) layers (from A.G.Khachaturyan [ I).

speiimen aged 5 hrs at 500 OC. -A, B, C denotk different byin clusters.

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model is expected to be strongly influenced by the dimensions of the twin clusters since these clusters are essentially acting as the basic magnetic unit. This notion seems to be in qualitative agreement with the observed increase in the coercivity of the melt-spun Fe-Pd alloy compared to the bulk material. A more quantitative treatment will require a better metallographic characterization of the geometry of the twin clusters.

The melt-spun Fe-Pt alloys were found to be in a highly ordered state but did not exhibit a well-developed polytwinned

Fig. 6 'Tweed" contrast from a bulk Fe-Pd alloy aged 30 min. and imaged with a strong two beam condition, 2=220.

Fig. 7 Dark-field image from a-bulk Fe-Pd specimen aged 10 hrs at 500 OC, near [110] zone axis.

0

0

0

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AFCING RT 800 'C

0 INITIAL STflTE

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Fig. 8 Magnetic hardening curves of the melt-spun Fe-Pt alloy.

structure. However, dark-field microscopy showed an array of aligned, twin-related articles similar to the early stages of aging in the Fe-Pd alloy. &e magnetic properties of the as-solidified Fe-Pt alloys were quite variable as shown in Figure 8, obviously the result of different cooling rates experienced by various sections of the ribbon. One Fe-Pt ribbon specimen with the initial Hc as low as 671 Oe was aged for one hour and Hc increased to 2.5 kOe. However, aging of the Fe-Pt ribbons generally led to a gradual decrease in the level of coercivity during the initial stages followed by a broad plateau. The ordering of the Fe-Pt alloy during the melt-spinning process can be understood considering the relatively high critical temperature for ordering in the Fe-Pt system (Tc- 13OOOC). The peak hardened state in the Fe-Pt alloy may be different than in the Fe-Pd system but prolonged aging on the plateau produces a similar polytwinned state. A detailed comparison of the two systems will be the subject of a forthcoming paper.

CONCLUSIONS

1. The peak magnetic hardness in melt-spun and bulk equi- atomic Fe-Pd alloys is characterized by a fully ordered poly- twinned structure. 2. Melt-spinning resulted in an increase in coercivity in the Fe- Pd alloy over the bulk material by a factor of about two. It is suggested that this enhanced magnetic hardness stems from microstructural refmement within the polytwinned state. 3. Prolonged a 'ng of both melt-spun and bulk Fe-Pd alloys leads to very littf change of coercmty. This is suggested to be related to the behavior of the polytwinned state dunng prolong- ed aging in which the twin cluster size essentially remains constant. 4. The mechanism of hardening at peak coercivity in Fe-Pt alloys may be different than in Fe-Pd alloys although the overaged state in Fe-Pt also exhibits a similar polytwinned state.

ACKNOWLEDGMENTS

This work is sponsored by the Department of Energy, Basic

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[9] LE. Tanner, "Diffraction Contrast from Elastic Shear Strains due to Coherent Phases", Phil. Mag., vo1.14, pp.111-130, 1966. [lo] YaS. Shur, LM. Magat, A.A. Glazer, Ye.V. Shcherbakova and N.N. Shchegoleva, 'The Films of an Iron-Palladium All with High Coercive Force", Fiz. metal. metalloved., 26, No?

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