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Magnetic age hardening of cold-deformed bulk equiatomic Fe–Pd intermetallics during isothermal annealing
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 270 (2004) 157–166
Magnetic age hardening of cold-deformed bulk equiatomic Fe–Pd intermetallics during isothermal annealing A.R. Deshpande*, J.M.K. Wiezorek Department of Material Sciences and Engineering, University of Pittsburgh, 848, Benedum Hall, Pittsburgh, PA 15261, USA Received 1 July 2003; received in revised form 1 July 2003
Abstract The interplay between the ordering reaction with recovery and recrystallization of the as-deformed state leads to combined reactions (CRs) during annealing of cold-deformed disordered Fe–Pd intermetallics at temperatures below the critical ordering temperature. CRs can be exploited to control the scale and morphology of the Fe–Pd alloy microstructures in order to optimize alloy properties. Here, the magnetic age hardening behavior and microstructural evolution of cold-deformed (cold rolled to 97% reduction in thickness) binary equiatomic Fe–Pd has been studied for isothermal annealing at temperatures of 400 C, 500 C, and 600 C. The evolution of the microstructure during the annealing treatments has been characterized by a combination of X-ray diffraction (XRD) and scanning electron microscopy (SEM). The magnetic age hardening behavior, the evolution of the coercivity as a function of annealing time, has been determined using a vibrating sample magnetometer (VSM). The microstructures of the transforming material have been characterized quantitatively using computer assisted image analysis methods. The CR transformed microstructures are morphologically equiaxed with average grain sizes in the sub-micron range and show coercivity up to five-fold larger than for conventionally processed equiatomic bulk Fe–Pd. During annealing the coercivity increases up to a maximum peak value and has been correlated with the increasing fraction of ordered material. The maximum coercivity obtains, as the ordering phase transformation is complete. With respect to conventionally processed material the ordering transformation in the cold-deformed material exhibits accelerated kinetics and is facilitated by a CR, which involves heterogeneous nucleation and growth processes akin to a ‘massive ordering’ reaction. Further annealing leads to decreasing coercivity, which has been attributed to the onset of grain growth in the population of CR-transformed grains. The characteristic magnetic age hardening response has been rationalized in terms of the microstructural observations. r 2003 Elsevier B.V. All rights reserved. PACS: 81.40.E; 81.40.C.E; 75.30.K; 75.50; 72.30; 61.16.B Keywords: L10-alloys; Fe–Pd; Combined reaction transformation; Annealing; Magnetic age hardening; Grain size effect; Electron microscopy
1. Introduction *Corresponding author. Tel.: +1-412-624-9750; fax: +1412-624-8069. E-mail address: [email protected] (A.R. Deshpande).
L10-ordered g1-Fe–Pd intermetallics are tetragonal binary AB compounds of near equiatomic
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.08.013
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composition. These intermetallics exhibit high uniaxial magnetocrystalline anisotropy with an ‘easy’ c-axis, large theoretical B–H products and good mechanical and corrosion properties [1]. This combination of properties renders them attractive for a range of permanent magnet and thin film applications [1]. However, while the intrinsic properties of the intermetallic g1-phase are comparable to those of rare-earth magnets, the technical properties reported for conventionally processed Fe–Pd based alloys are disappointingly low [2,3]. This implies that the technical properties of the Fe–Pd alloys are very sensitive to microstructure. Thus, it is very important to develop a fundamental understanding of relationships between processing, microstructure and properties for these ferromagnetic intermetallics. In conventionally processed equiatomic Fe–Pd based alloys, the L10-ordered phase forms upon cooling below the order-transition temperature (TC B650 C) via a thermodynamically first-order type order–disorder transformation from the disordered solid solution (g-(Fe,Pd), FCC) [3–5]. The nucleation of coherent precipitates of the tetragonal phase throughout the grains of the disordered cubic phase leads to considerable transformation strains. These strains are relaxed by the formation of self-accommodating arrays of dodecahedrally conjugated twins, i.e. the polytwin structure characteristic of conventionally processed L10-ordered Fe–Pd alloys develops [3,5]. It has been shown that the disappointing technical magnetic properties of these alloys are associated with the presence of the polytwin structure [3]. Alternative thermo-mechanical processing routes have been used to produce bulk Fe–Pd alloys with improved hard magnetic properties by successfully suppressing the formation of the polytwin structure [6–9]. These latter processing schemes involve the annealing of alloys after cold deformation in the disordered state at temperatures below the ordering temperature, TC ; and the transformation of the microstructure proceeds by a combined reaction (CR) of concomitant recovery and recrystallization of the cold-deformed state and the ordering reaction. The CR-transformed alloys exhibit microstructures that contain large fractions of equiaxed, fully L10-ordered
grains [9]. The coercivity (HC ) obtained for conventionally processed polycrystalline Fe–Pd with the polytwin structure ranges between 250 and 350 Oe [7], while the highest HC -value that has been reported for CR-transformed bulk Fe–Pd is about 800 Oe [9]. Interestingly, even larger values of HC on the order of 2000–2500 Oe have been reported for rapid-solidification processed thin films and for mechanically ball-milled and powder-processed bulk Fe–Pd with a nanoscale grain size [10]. This indicates that both the morphology and the scale of the microstructure of the bulk L10ordered Fe–Pd alloys strongly affect coercivity. Furthermore, it appears that mechanical working of the Fe–Pd prior to ordering during annealing, i.e. CR processing, may be used to alter and ultimately control the microstructural evolution in bulk Fe–Pd alloys in order to improve hard magnetic properties. The processes of cold deformation/cold working, such as cold rolling, are capable of imparting a large amount of strain energy to the material. This stored strain energy or energy of cold work provides the driving force for recovery and recrystallization (e.g. [11]). Considering all other processing parameters to remain constant, for disordered alloys the recrystallized and refined average grain sizes generally decrease as the amount of stored energy of cold work is increased (e.g. [11]). The ordering transformation in equiatomic Fe–Pd exhibits the so-called ‘c-curve’ kinetics, which are typical of nucleation and growth processes. Hence, the rate of ordering depends on the annealing temperature ToTC and is fastest at about 525 C, while slower ordering kinetics pertain at significantly higher and lower isothermal transformation temperatures below TC [6–9]. Thus, both the strain of cold deformation and the annealing temperatures are expected to influence strongly the evolution of the microstructures of bulk Fe–Pd alloys during CR processing. While it appears reasonable to propose that the final grain size of the CR-processed Fe–Pd should decrease with increasing strain of cold deformation, the effect of the annealing temperature on the microstructural evolution is expected to be more complex and may be different for different amounts of cold deformation. Using large
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amounts of cold deformation and a suitable annealing temperature CR processing may enable the synthesis of bulk Fe–Pd alloys with microstructures of equiaxed morphology and average grain sizes in the sub-micron to nanometer scale, which could reasonably be expected to exhibit significantly enhanced hard magnetic properties [8–10]. This paper presents the results of an experimental study of the effect of the annealing temperature on the microstructural evolution and the magnetic age hardening behavior of bulk Fe– Pd during CR processing for a constant amount of cold-deformation strain. Here, cold rolling to a 97% reduction in thickness at room temperature has been used to impart large amounts of strain energy to equiatomic Fe–Pd in the disordered state. The evolution of the microstructures during isothermal annealing at three different temperatures has been studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM), while the magnetic properties and behavior have been determined using a vibrating sample magnetometer (VSM). The effect of microstructural metrics, such as the fraction CR transformed, the grain size and the long-range order parameter, on the coercivity or magnetic hardness as a function of annealing time, i.e. the magnetic age hardening behavior, is reported and discussed.
2. Experimental procedure The equiatomic Fe–Pd alloy used in this study has been prepared from high-purity elemental starting materials (Pd 99.95% and Fe 99.98% purity) using vacuum arc melting in a residual atmosphere of purified argon gas. Sections from the as-cast buttons have been cold rolled to about 50% reduction in thickness, followed by a homogenization treatment at 950 C (1223 K) for 6 h (21.6 ks) and quenching into ice-brine. In the asquenched state the material consisted of stress free grains of the disordered FCC g-(Fe,Pd) solid solution with an average diameter of approximately 130 mm [12]. A billet 6 mm thick (4 mm 24 mm in width and length) was cold rolled to 97% reduction in thickness to 180 mm
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with multiple passes. Isothermal annealing after cold rolling has been performed at temperatures of 400 C (673 K), 500 C (773 K) and 600 C (873 K) for times ranging between 0.5 h (1.8 ks) and 8 days (691.2 ks). The evolution of the magnetic properties was monitored as a function of annealing time for each of the three temperatures using a VSM producing a maximum field of 15 kOe at room temperature. The evolution of the microstructure as a function of annealing time has been studied by XRD and SEM using a Philips X’pert XRdiffractometer and a field-emission gun equipped Philips XL30 SEM, respectively. The grain size analysis has been performed using the NIH image analysis software.
3. Results 3.1. Evolution of microstructure—SEM Fig. 1a shows a SEM backscatter electron mode (BSE) micrograph of the polytwinned microstructure developed during conventional processing (100 h of annealing at 500 C without prior deformation) of the Fe–Pd alloy. Fig. 1b shows a SEM BSE micrograph of the as deformed state after 97% reduction in thickness by cold rolling. The origin of contrast in the BSE micrographs is due to differences in crystallographic orientations. In Fig. 1a different polytwinned grains and individual c-domain lamellae are discernable, while Fig. 1b indicates that a dislocation cell or substructure developed in the heavily deformed alloy. Examples of SEM BSE micrographs for annealing conditions associated with the maximum coercivity and in the slightly magnetically overaged condition for the three different isothermal annealing temperatures explored here are shown in Figs. 2–4. The microstructures of the CR-processed Fe–Pd alloys consisted of essentially equiaxed grains of fully ordered L10-phase. The larger grains in the population of CR-transformed grains frequently appeared to exhibit annealing twins. A significant fraction (X5%) of grains with the PT-structure has not been observed for any of the processing conditions explored here.
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(a)
(b)
Fig. 1. SEM BSE micrographs of (a) polytwinned microstructure of polycrystalline Fe–Pd and (b) Fe–Pd alloy after 97% thickness reduction by cold rolling.
(a)
(b)
Fig. 2. SEM BSE micrographs of microstructures of coldrolled Fe–Pd after annealing. (a) 400 C, 7 days, maximum coercivity HC ¼ 1389 Oe and (b) 400 C, 8 days, HC ¼ 1151 Oe.
3.2. Evolution of magnetic hardness—VSM Vibrating sample magnetometer experiments have been performed on isothermally annealed samples with the direction of rolling (RD) aligned parallel to the externally applied field. Complete hysteresis graphs have been obtained for each sample. The change in the magnetic hardness, represented by the coercivity, HC ; with annealing time for the three annealing temperatures is shown in Fig. 5. In order to assess the potential effect of crystallographic textures, VSM measurements have also been performed for systematically different orientations of the applied field relative to the RD of the Fe–Pd samples. These experiments produced essentially the same hysteresis behavior as those used to collate the data shown in Fig. 5 and are not shown for brevity. This either implies that textures are probably not very strong in these CR-processed samples or, at least, that textures did not have a significant effect on the magnetization behavior measured by VSM.
The magnetic age-hardening curves collated in Fig. 5 show that the coercivity of the cold-rolled samples increases up to a maximum value prior to a characteristic decrease for each of the three isothermal treatments. These observations follow a similar pattern to those reported previously for somewhat different thermo-mechanical treatments of bulk equiatomic Fe–Pd [6–9]. The maximum magnetic hardness, HC ; attained in these bulk intermetallics was 1389 Oe for an isothermal annealing temperature of 400 C for a time of 7 days (10,080 min or 604.8 ks). This is a significant increase in coercivity of more than 1.0 kOe relative to conventionally processed Fe–Pd with the polytwinned microstructure (HC B2502350 Oe). 3.3. Evolution of L10-order—XRD XRD data has been used to monitor the progress of the ordering reaction. A representative
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Fig. 3. SEM BSE micrographs of microstructures of coldrolled Fe–Pd after annealing. (a) 500 C, 6 h, maximum coercivity HC ¼ 1251 Oe and (b) 500 C, 10 h, HC ¼ 958 Oe.
Fig. 4. SEM BSE micrographs of microstructures of coldrolled Fe–Pd after annealing. (a) 600 C, 3 h, maximum coercivity HC ¼ 984 Oe and (b) 600 C, 8 h, HC ¼ 705 Oe.
set of symmetric (y 2y-XRD data for an Fe–Pd sample annealed at 400 C after 7 days is shown in Fig. 6. Plots similar to that of Fig. 6 are obtained for the maximum coercivity conditions for isothermal annealing treatments at 500 C (360 min) and 600 C (180 min). The diffraction peak, 2y ¼ 83:43 Corresponds to {3 1 1} and {1 3 1}, whereas that at 2y ¼ 84:81 Corresponds to {1 1 3}. The splitting of these latter peaks is evidence of the transformation to the tetragonal L10-ordered phase. Also, the diffraction peaks (0 0 1) at 2yB24:5 and {1 1 0} at 2yB34 ; as well as the splitting of the {2 0 0}/(0 0 2) pair of peaks around 2yB48 and the pair of peaks {2 2 0}/{0 2 2} at 2yB70 indicate the transformation from the disordered FCC to the L10-ordered Fe–Pd phase. The (c=a)-ratio has been obtained from the symmetric XRD patterns using the {3 1 1} and {1 1 3} peaks and was used to monitor the evolution of long range L10-order. Beginning at
values closer to unity the (c=a)-ratio attains the value of 0.966, which represents the equilibrium (c=a)-ratio, for annealing times corresponding approximately to the coercivity peak. This implies that the long-range order parameter and thus the volume fraction of ordered L10-phase increases with annealing time up to the time corresponding approximately to that associated with the coercivity maximum. This behavior of the (c=a)-ratio has been observed consistently in XRD experiments for all of the three annealing temperatures. Thus it appears reasonable to conclude that the ordering reaction is essentially complete at the instance when the maximum in magnetic hardness has been achieved. 3.4. Grain size evolution—image analysis Grain size analysis has been performed using the ‘NIH image’ software. Some pertinent results of
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Coercivity vs Annealing time ( 97% Cold deformed samples) 1600
1389
1400
1251
Coercivity (Oe)
1200
1151 984
1000
989
843
800 600 400 200 0 1
10
100
1000
10000
100000
Annealing time [log (min)] Coercivity 400 deg C
Coercivity 500 deg C
Coercivity 600 deg C
Fig. 5. The magnetic age hardening behavior, HC [Oe] against annealing time [min] (log-scale), for isothermal annealing at 400 C, 500 C and 600 C (lines connecting the relevant data points for each data set are only meant to guide the eye of the reader).
in the magnetically overaged state with respect to the microstructural state associated with the peak coercivity. However, the minimum grain sizes remain the same for these two microstructural states. The microstructural changes give rise to significantly different values of coercivity for each of the annealing temperatures.
Intensity vs 2 Theta (400 deg C - 7days)
Intensity (arbi. units)
600 500 400 300 200
47.05 83.43
100
48.87 84.81
0 0
10
20
30
40
50
60
70
80
90
100
4. Discussion
2 Theta Intensity vs 2 Theta
Fig. 6. Example XRD pattern, Intensity against 2y for Fe–Pd annealed at 400 C for 7 days.
the analysis of a statistically sufficient number of grains, about 800 grains per isothermal annealing treatment, are summarized in Table 1. For annealing times significantly shorter than those required to attain peak coercivity at a given annealing temperature the microstructures were complex (e.g. [12]). A meaningful correlation of microstructural metrics, such as grain size, with properties is difficult for these complex, only partially transformed microstructures. Hence, here, only results of grain size analyses for the essentially fully CR-transformed microstructures are shown. It is clear from Table 1 that the average grain size and especially the maximum grain size in the population of CR transformed grains increases
Comparison of the BSE SEM micrographs obtained for the as-deformed and the annealed microstructural states of the Fe–Pd alloys studied here (e.g. Figs. 1b and 2) reveals much more diffuse contrast for the as-deformed state (Fig. 1b). The high defect density and the large amount of stored strain energy present in the as-deformed material may be responsible for this contrast effect. Conversely, a reduction in the level of internal stress associated with the overall defect content of the material during subsequent annealing would be consistent with the improved BSE image contrast of Figs. 2–4. The evolution of the coercivity for the various microstructural states developed during isothermal annealing of the cold-deformed Fe–Pd shown in Fig. 5 confirms previous work [6–9, 12]. Due to their similarity to the development of the mechanical hardness in precipitation strengthened alloys
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Table 1 Some pertinent results of the computer assisted grain size analyses for peak magnetic hardness and magnetically overaged conditions for the three different isothermal annealing temperatures Treatment temp. ( C)
Treatment time (min)
Coercivity (Oe)
Min grain size (mm) Dmin
Max grain size (mm) Dmax
Avg. grain size (mm) Davg
1
400 400
10080 11520
1389 1151
0.035 0.036
0.901 1.83
0.234 0.255
2
500 500
360 600
1251 958
0.05 0.05
2.18 2.38
0.407 0.601
3
600 600
180 480
984 705
0.05 0.08
1.99 6.36
0.493 1.13
for structural applications during annealing or aging treatments, i.e. mechanical age hardening curves, the three data sets depicted in Fig. 5 may be considered to represent ‘magnetic age hardening’ curves. During isothermal annealing or aging the coercivity initially increases (magnetic hardening) up to a maximum value (peak magnetic hardness) followed by a regime of decreasing coercivity with increasing annealing time (magnetic softening). In analogy to the mechanical age hardening curves commonly used to describe precipitation hardened alloys, the magnetic softening, i.e. the decrease in HC after the peak magnetic hardness (maximum HC ) has been reached, may be considered as magnetic overaging. With the aid of Fig. 5 the main effects of the isothermal annealing temperature in the range of 400–600 C on the magnetic age hardening behavior of CR-processed Fe–Pd can be identified. The effects of annealing temperature appear to be twofold. Firstly, the increase in annealing temperature results in an acceleration of the kinetics of the microstructural transformations responsible for the characteristic magnetic age hardening behavior (Fig. 5). Secondly, as the annealing temperature is lowered from 600 C to 500 C to 400 C, the maximum values of coercivity measured increase from 984 to 1251e to 1398 Oe, respectively, and occur after longer annealing times. The maximum value of coercivity obtained here for bulk equiatomic Fe–Pd by CR processing, HC ¼ 1389 Oe, is about four to five times larger than the coercivity typical of Fe–Pd with the PT-structure after conventional processing, HC ðPTÞ ¼ 2502350 Oe,
and also exceeds the highest coercivity reported previously for bulk CR-processed Fe–Pd, HC ðCRÞE800 Oe [6,7]. Thus, after further optimization of the processing parameters it appears that CR processing may be a suitable method to fabricate bulk Fe–Pd alloys that exhibit even larger coercivity in the range of 2–3 kOe, which previously has been achieved only in nanostructured thin film or powder processed Fe–Pd (e.g. [10]). The characteristic magnetic age hardening behavior of CR-processed equiatomic Fe–Pd documented in the graphs of Fig. 5 can be correlated to the increasing degree of L10-order, the increase in the fraction CR-transformed, the evolution of the grain size and the microstructural morphology, including the defect content. Based on the microstructural analyses by XRD and SEM it can be concluded that the magnetic hardening up to the peak magnetic hardness documented in the graphs of Fig. 5 correlates directly with an increase in the degree of L10-order developing in the Fe–Pd alloys. The XRD data further indicated that the ordering transformation was essentially complete at about the time when the peak magnetic hardness was obtained for each of the three different annealing temperatures. The microstructures associated with the peak magnetic hardness consisted essentially entirely of equiaxed L10-ordered grains that contained at least annealing twins (Figs. 2–4) and exhibited average diameters in the sub-micron range, namely between 234 and 493 nm (Table 1). Hence, it appears reasonable to propose that the initial increase in
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the coercivity during isothermal annealing of the cold-rolled Fe–Pd, the magnetic hardening, can be attributed to the monotonic increase in the fraction of ordered material and correspondingly the increase in the degree of L10-order. This conclusion is consistent with a previous study [12]. Furthermore, it is interesting to note that the PT-structure produced by the conventional ordering mechanism of coherent nucleation of L10ordered precipitates and their strain-affected growth has not been observed for any of the CRprocessed Fe–Pd studied here. The transformation from the deformed disordered parent phase to the stable ordered intermetallic phase results only in monolithic L10-grains of equiaxed morphology (Figs. 2–4). A transformation mechanism suitable to produce such a microstructure in the CRprocessed Fe–Pd may involve the stored strain energy assisted heterogeneous nucleation of ordered grains and their growth by the migration of highly mobile, essentially incoherent boundaries, akin to a ‘massive’ ordering transformation [6,7,12]. This is the CR transformation, a process of concomitant ordering and annealing of the deformation induced defect structure by recovery and recrystallization [12]. The sites for the heterogeneous nucleation of grains that are stable with respect to growth during the CR transformation have been shown to be associated with regions of large strain-energy gradients, i.e. prior FCC-grain boundaries, shear or deformation bands and dislocation cell-wall junctions [12]. In this latter work by Deshpande et al. [12] a smaller amount of stored strain energy was imparted to equiatomic Fe–Pd than in this study. The maximum coercivity reported after annealing at 500 C, HC ¼ 523 Oe, was associated with a fully L10-ordered microstructure comprised of a minority fraction of CRtransformed equiaxed grains and a majority fraction of larger grains with the PT-structure [12]. Hence, it appears that the mechanistically different ordering processes, i.e. the conventional ordering process and the CR transformation, compete with each other kinetically. For the larger amount of strain energy imparted to the Fe–Pd alloys here by the large reduction in thickness by cold rolling the kinetics of the CR transformation appear to dominate. Thus, for the material studied
here, the ordering and the microstructural transformation are facilitated by the CR transformation and the magnetic hardening can be attributed to an increase in the fraction of CR-transformed material, which as a consequence increases the degree of long range L10-order. The data collated in Table 1 indicate that the value of the peak magnetic hardness or coercivity achieved for the three different isothermal annealing treatments is proportional to the average grain size of the equiaxed CR-transformed grains. For a smaller average grain size a larger peak coercivity value results (Table 1). Furthermore, the average grain size clearly increases during overaging when magnetic softening is observed (Table 1 and Fig. 5). The grain size increase is most apparent for the highest annealing temperature of 600 C (Table 1 and Fig. 4). The formation of the many octahedrally conjugated annealing twins during overaging of the CR-processed Fe–Pd alloys (e.g. Fig. 4) is consistent with grain growth or ‘secondary recrystallization’ in low stacking fault energy metals with FCC-derivative crystal structure [13]. Mechanisms suitable for the formation of annealing twins by moving grain boundaries in L10-ordered structures are related to those proposed for FCC metals and alloys [14,15]. Hence, the characteristic softening observed in the magnetic hardening curves during the stage of overaging appears to be related to the thermally activated and presumably largely curvature-driven grain growth in the fraction of CRtransformed grains. Interestingly, previous studies [7] have proposed that a grain size magnetic hardening contribution to the coercivity enhancement (DHC ) may exist in Fe–Pd, which would be associated with resistance to the nucleation of reverse magnetization domains that are stable with respect to growth during the demagnetization process. To a lesser degree, the pinning resistance from the frequent grain boundaries as planar obstacles to magnetic domain wall motion may also contribute to increased coercivity. Hence, magnetic grain size hardening represents a nucleation type coercivity mechanism [7,16] and the corresponding coercivity enhancement can be given approximately by DHC ¼ ðbgÞ=ðDMs Þ:
ð1Þ
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Here b is a geometrical term with a value between 1 and 5, g is the magnetic domain wall energy (17 erg/cm2), Ms is the saturation magnetization (1100 emu/cm3) and D is the grain size for L10-ordered Fe–Pd. Inspection of the data in Table 1 shows that for reasonable values of the geometrical factor, b; between 2.0 and 3.5 coercivity enhancements on the order of the values experimentally observed here are obtained for the average grain sizes measured for the three different isothermal annealing temperatures. Thus, it appears that a large fraction of the coercivity enhancement observed in the CR-processed samples studied here may be attributed to such a grain size effect on magnetic hardness. Upper and lower bounds for a grain size regime dominated by a grain size magnetic hardening related coercivity mechanism may be evaluated based on the minimum size of a domain that is stable to grow in terms of magnetostatic energy. For grains smaller than or those considerably (perhaps 3–10 times) larger than this critical dimension this nucleation-type coercivity mechanism should not provide significant contributions to the coercivity. For Fe–Pd this single domain size dimension has been predicted to lie in the range of approximately 0.25–0.45 mm [7,17]. Hence, a grain size effect on coercivity akin to a Hall–Petch effect in mechanical strengthening by grain size may exist for CRprocessed Fe–Pd in the range of average grain sizes of about 0.3 mm to perhaps about 4.0 mm or so. Of course, in addition to the apparent grain size contribution to the coercivity in the CR-processed Fe–Pd reported here, contributions from the defect content of the equiaxed CR-transformed grains are expected. A reduction in the planar defect content of the CR grains during overaging would also be suitable to rationalize the observed magnetic softening due to a diminished contribution from a pinning-type coercivity mechanism [6,7,16]. However, in order to assess the role of planar defects, such as antiphase boundaries, stacking faults and twins inside of the CRtransformed grains for the observed magnetic age hardening additional detailed TEM studies of the evolution of the defect structures, particularly in the overaged state, are required. Such experiments are currently underway and will be the topic of a
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future communication. Hence, uncertainty regarding the role of magnetic domain wall pinning contributions to the magnetic age hardening remains. However, it may be noted that both of these microstructural origins of the magnetic softening in CRP Fe–Pd, grain growth and defect density reduction, are essentially consequences of processes associated with the classic annealing phenomenon of ‘secondary recrystallization’ or simply grain growth. The motion of mobile grain boundary segments accomplishes grain growth together with a driving force derived from curvature. The grain boundary motion facilitates the absorption of line and planar defects present in the interiors of both growing and shrinking grains and the deposition of annealing twins into the growing grains. Thus, it appears reasonable to attribute the magnetic softening during overaging of CR-processed Fe–Pd essentially to grain growth in the fraction of CR-transformed grains.
5. Summary and conclusions Combinations of XRD, SEM and VSM have been used to study the microstructural and property evolution in cold-deformed bulk equiatomic Fe–Pd during isothermal annealing. The characteristic magnetic age hardening behavior of CR-processed Fe–Pd has been rationalized in terms of thermally induced microstructural changes in the solid state. The main conclusions of this experimental work can be summarized as follows: 1. The method of CR processing enables the synthesis of bulk Fe–Pd with nanoscale microstructures that are devoid of the PT-structure and exhibit considerably improved hard magnetic properties compared with conventionally processed bulk Fe–Pd alloys. 2. A maximum coercivity of about 1.4 kOe has been obtained by CR processing for a condition with the ordering reaction complete and an average grain size of 234 nm. 3. The coercivity increase (magnetic hardening) up to the peak (peak hardness) has been attributed to the increase in the degree of long
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range L10-order, which is facilitated mechanistically by the CR transformation. 4. The CR transformation has been described as an ordering transformation that involves heterogeneous nucleation and is assisted by the strain energy stored in the defect structures of the cold-deformed state. 5. The decrease in coercivity (magnetic softening) during overaging has been attributed to grain growth or ‘secondary recrystallization’ in the fraction of CR-transformed grains and its consequences for the defect content of these fully ordered equiaxed grains. 6. An apparent grain size effect on the coercivity of the CR-transformed Fe–Pd microstructures may exist for average grain sizes in the range of about 0.3–4.0 mm. Acknowledgements The materials presented are based upon work supported by the National Science Foundation, NSF, Grant Number DMR-Metals-0094213, with Dr. K.L. Murty as program manager. Any opinions, findings, and conclusions or recommendations expressed in this article are those of the authors and do not necessarily reflect the views of the National Science Foundation. References [1] D. Weller, A. Moser, IEEE Trans. Magn. 35 (1999) 4423. [2] L.M. Magat, A.S. Yermolenko, G.V. Ivanova, G.M. Makarova, Y.A.S. Shur, Fiz. Metal. Metalloved. 26 (1968) 511.
[3] B. Zhang, M. Lelovic, W.A. Soffa, Scripta Metall. Mater. 25 (1991) 1577. [4] A.G. Khachaturyan, Theory of Structural Transformation in Solids, Wiley, New York, 1983, p. 368. [5] C. Yanar, J.M.K. Wiezorek, W.A. Soffa, in: P. Turchi, et al., (Eds.), Phase Transformations and Evolution of Microstructure in Materials, TMS, Warrendale, 2000, pp. 39–54. [6] T. Klemmer, W.A. Soffa, in: W.C. Johnson, J.M. Howe, D.E. Laughlin, W.A. Soffa (Eds.), Solid–Solid Phase Transformations, TMS, Warrendale, 1994, pp. 969–974. [7] T. Klemmer, D. Hoydick, H. Okumura, B. Zhang, W.A. Soffa, Scripta Metall. Mater. 33 (1995) 1793. [8] Ye.I. Teytel, G.M. Gushchin, Phys. Metals Metallogr. 65 (2) (1988) 114. [9] T. Klemmer, Ph.D. Thesis, University of Pittsburgh, 1995. [10] H. Okumura, T. Klemmer, W.A. Soffa, J.A. Barnard, IEEE Trans. Magn. 34 (1998) 1015. [11] R.E. Reed-Hill, R. Abbaschian, Physical Metallurgy Principles, PWS Publishing Co., Boston, 1991, pp. 227–260. [12] A.R. Deshpande, A. Al-Ghaferi, H. Xu, H. Heinrich, J.M.K. Wiezorek, in: M.A. Meyers, R.O. Ritchie, M. Sarikaya (Eds.), Nano and Microstructural Design of Advanced Materials: A Commemorative Volume on Prof. G. Thomas’s Seventieth Birthday, Elsevier Science, London, 2002 (Special issue of Journal), in press. [13] M.A. Meyers, C. McCowan, in: C.S. Pande, D.A. Smith, A.H. King, J. Walter, (Eds.), Interface Migration and Control of Microstructure, Proceedings of a Symposium Held in Conjunction with ASM/TMS/AIME Fall Meeting, ASM International, Metals Park, OH, 1984, pp. 99–123. [14] G. Baro, H. Gleiter, F. Zeitsch, Metall K. 63 (1972) 661. [15] C. Yanar, V. Radmilovic, W.A. Soffa, J.M.K. Wiezorek, Intermetallics 9 (2001) 949–954. [16] H. Kronmuller, K.D. Durst, M. Sagawa, J. Magn. Magn. Mater. 74 (1988) 291. [17] N.I. Vlasova, G.S. Kandaurova, N.N. Shchegoleva, J. Magn. Magn. Mater. 222 (2000) 138.
Journal of Magnetism and Magnetic Materials 270 (2004) 157–166
Magnetic age hardening of cold-deformed bulk equiatomic Fe–Pd intermetallics during isothermal annealing A.R. Deshpande*, J.M.K. Wiezorek Department of Material Sciences and Engineering, University of Pittsburgh, 848, Benedum Hall, Pittsburgh, PA 15261, USA Received 1 July 2003; received in revised form 1 July 2003
Abstract The interplay between the ordering reaction with recovery and recrystallization of the as-deformed state leads to combined reactions (CRs) during annealing of cold-deformed disordered Fe–Pd intermetallics at temperatures below the critical ordering temperature. CRs can be exploited to control the scale and morphology of the Fe–Pd alloy microstructures in order to optimize alloy properties. Here, the magnetic age hardening behavior and microstructural evolution of cold-deformed (cold rolled to 97% reduction in thickness) binary equiatomic Fe–Pd has been studied for isothermal annealing at temperatures of 400 C, 500 C, and 600 C. The evolution of the microstructure during the annealing treatments has been characterized by a combination of X-ray diffraction (XRD) and scanning electron microscopy (SEM). The magnetic age hardening behavior, the evolution of the coercivity as a function of annealing time, has been determined using a vibrating sample magnetometer (VSM). The microstructures of the transforming material have been characterized quantitatively using computer assisted image analysis methods. The CR transformed microstructures are morphologically equiaxed with average grain sizes in the sub-micron range and show coercivity up to five-fold larger than for conventionally processed equiatomic bulk Fe–Pd. During annealing the coercivity increases up to a maximum peak value and has been correlated with the increasing fraction of ordered material. The maximum coercivity obtains, as the ordering phase transformation is complete. With respect to conventionally processed material the ordering transformation in the cold-deformed material exhibits accelerated kinetics and is facilitated by a CR, which involves heterogeneous nucleation and growth processes akin to a ‘massive ordering’ reaction. Further annealing leads to decreasing coercivity, which has been attributed to the onset of grain growth in the population of CR-transformed grains. The characteristic magnetic age hardening response has been rationalized in terms of the microstructural observations. r 2003 Elsevier B.V. All rights reserved. PACS: 81.40.E; 81.40.C.E; 75.30.K; 75.50; 72.30; 61.16.B Keywords: L10-alloys; Fe–Pd; Combined reaction transformation; Annealing; Magnetic age hardening; Grain size effect; Electron microscopy
1. Introduction *Corresponding author. Tel.: +1-412-624-9750; fax: +1412-624-8069. E-mail address: [email protected] (A.R. Deshpande).
L10-ordered g1-Fe–Pd intermetallics are tetragonal binary AB compounds of near equiatomic
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.08.013
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composition. These intermetallics exhibit high uniaxial magnetocrystalline anisotropy with an ‘easy’ c-axis, large theoretical B–H products and good mechanical and corrosion properties [1]. This combination of properties renders them attractive for a range of permanent magnet and thin film applications [1]. However, while the intrinsic properties of the intermetallic g1-phase are comparable to those of rare-earth magnets, the technical properties reported for conventionally processed Fe–Pd based alloys are disappointingly low [2,3]. This implies that the technical properties of the Fe–Pd alloys are very sensitive to microstructure. Thus, it is very important to develop a fundamental understanding of relationships between processing, microstructure and properties for these ferromagnetic intermetallics. In conventionally processed equiatomic Fe–Pd based alloys, the L10-ordered phase forms upon cooling below the order-transition temperature (TC B650 C) via a thermodynamically first-order type order–disorder transformation from the disordered solid solution (g-(Fe,Pd), FCC) [3–5]. The nucleation of coherent precipitates of the tetragonal phase throughout the grains of the disordered cubic phase leads to considerable transformation strains. These strains are relaxed by the formation of self-accommodating arrays of dodecahedrally conjugated twins, i.e. the polytwin structure characteristic of conventionally processed L10-ordered Fe–Pd alloys develops [3,5]. It has been shown that the disappointing technical magnetic properties of these alloys are associated with the presence of the polytwin structure [3]. Alternative thermo-mechanical processing routes have been used to produce bulk Fe–Pd alloys with improved hard magnetic properties by successfully suppressing the formation of the polytwin structure [6–9]. These latter processing schemes involve the annealing of alloys after cold deformation in the disordered state at temperatures below the ordering temperature, TC ; and the transformation of the microstructure proceeds by a combined reaction (CR) of concomitant recovery and recrystallization of the cold-deformed state and the ordering reaction. The CR-transformed alloys exhibit microstructures that contain large fractions of equiaxed, fully L10-ordered
grains [9]. The coercivity (HC ) obtained for conventionally processed polycrystalline Fe–Pd with the polytwin structure ranges between 250 and 350 Oe [7], while the highest HC -value that has been reported for CR-transformed bulk Fe–Pd is about 800 Oe [9]. Interestingly, even larger values of HC on the order of 2000–2500 Oe have been reported for rapid-solidification processed thin films and for mechanically ball-milled and powder-processed bulk Fe–Pd with a nanoscale grain size [10]. This indicates that both the morphology and the scale of the microstructure of the bulk L10ordered Fe–Pd alloys strongly affect coercivity. Furthermore, it appears that mechanical working of the Fe–Pd prior to ordering during annealing, i.e. CR processing, may be used to alter and ultimately control the microstructural evolution in bulk Fe–Pd alloys in order to improve hard magnetic properties. The processes of cold deformation/cold working, such as cold rolling, are capable of imparting a large amount of strain energy to the material. This stored strain energy or energy of cold work provides the driving force for recovery and recrystallization (e.g. [11]). Considering all other processing parameters to remain constant, for disordered alloys the recrystallized and refined average grain sizes generally decrease as the amount of stored energy of cold work is increased (e.g. [11]). The ordering transformation in equiatomic Fe–Pd exhibits the so-called ‘c-curve’ kinetics, which are typical of nucleation and growth processes. Hence, the rate of ordering depends on the annealing temperature ToTC and is fastest at about 525 C, while slower ordering kinetics pertain at significantly higher and lower isothermal transformation temperatures below TC [6–9]. Thus, both the strain of cold deformation and the annealing temperatures are expected to influence strongly the evolution of the microstructures of bulk Fe–Pd alloys during CR processing. While it appears reasonable to propose that the final grain size of the CR-processed Fe–Pd should decrease with increasing strain of cold deformation, the effect of the annealing temperature on the microstructural evolution is expected to be more complex and may be different for different amounts of cold deformation. Using large
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amounts of cold deformation and a suitable annealing temperature CR processing may enable the synthesis of bulk Fe–Pd alloys with microstructures of equiaxed morphology and average grain sizes in the sub-micron to nanometer scale, which could reasonably be expected to exhibit significantly enhanced hard magnetic properties [8–10]. This paper presents the results of an experimental study of the effect of the annealing temperature on the microstructural evolution and the magnetic age hardening behavior of bulk Fe– Pd during CR processing for a constant amount of cold-deformation strain. Here, cold rolling to a 97% reduction in thickness at room temperature has been used to impart large amounts of strain energy to equiatomic Fe–Pd in the disordered state. The evolution of the microstructures during isothermal annealing at three different temperatures has been studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM), while the magnetic properties and behavior have been determined using a vibrating sample magnetometer (VSM). The effect of microstructural metrics, such as the fraction CR transformed, the grain size and the long-range order parameter, on the coercivity or magnetic hardness as a function of annealing time, i.e. the magnetic age hardening behavior, is reported and discussed.
2. Experimental procedure The equiatomic Fe–Pd alloy used in this study has been prepared from high-purity elemental starting materials (Pd 99.95% and Fe 99.98% purity) using vacuum arc melting in a residual atmosphere of purified argon gas. Sections from the as-cast buttons have been cold rolled to about 50% reduction in thickness, followed by a homogenization treatment at 950 C (1223 K) for 6 h (21.6 ks) and quenching into ice-brine. In the asquenched state the material consisted of stress free grains of the disordered FCC g-(Fe,Pd) solid solution with an average diameter of approximately 130 mm [12]. A billet 6 mm thick (4 mm 24 mm in width and length) was cold rolled to 97% reduction in thickness to 180 mm
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with multiple passes. Isothermal annealing after cold rolling has been performed at temperatures of 400 C (673 K), 500 C (773 K) and 600 C (873 K) for times ranging between 0.5 h (1.8 ks) and 8 days (691.2 ks). The evolution of the magnetic properties was monitored as a function of annealing time for each of the three temperatures using a VSM producing a maximum field of 15 kOe at room temperature. The evolution of the microstructure as a function of annealing time has been studied by XRD and SEM using a Philips X’pert XRdiffractometer and a field-emission gun equipped Philips XL30 SEM, respectively. The grain size analysis has been performed using the NIH image analysis software.
3. Results 3.1. Evolution of microstructure—SEM Fig. 1a shows a SEM backscatter electron mode (BSE) micrograph of the polytwinned microstructure developed during conventional processing (100 h of annealing at 500 C without prior deformation) of the Fe–Pd alloy. Fig. 1b shows a SEM BSE micrograph of the as deformed state after 97% reduction in thickness by cold rolling. The origin of contrast in the BSE micrographs is due to differences in crystallographic orientations. In Fig. 1a different polytwinned grains and individual c-domain lamellae are discernable, while Fig. 1b indicates that a dislocation cell or substructure developed in the heavily deformed alloy. Examples of SEM BSE micrographs for annealing conditions associated with the maximum coercivity and in the slightly magnetically overaged condition for the three different isothermal annealing temperatures explored here are shown in Figs. 2–4. The microstructures of the CR-processed Fe–Pd alloys consisted of essentially equiaxed grains of fully ordered L10-phase. The larger grains in the population of CR-transformed grains frequently appeared to exhibit annealing twins. A significant fraction (X5%) of grains with the PT-structure has not been observed for any of the processing conditions explored here.
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(a)
(b)
Fig. 1. SEM BSE micrographs of (a) polytwinned microstructure of polycrystalline Fe–Pd and (b) Fe–Pd alloy after 97% thickness reduction by cold rolling.
(a)
(b)
Fig. 2. SEM BSE micrographs of microstructures of coldrolled Fe–Pd after annealing. (a) 400 C, 7 days, maximum coercivity HC ¼ 1389 Oe and (b) 400 C, 8 days, HC ¼ 1151 Oe.
3.2. Evolution of magnetic hardness—VSM Vibrating sample magnetometer experiments have been performed on isothermally annealed samples with the direction of rolling (RD) aligned parallel to the externally applied field. Complete hysteresis graphs have been obtained for each sample. The change in the magnetic hardness, represented by the coercivity, HC ; with annealing time for the three annealing temperatures is shown in Fig. 5. In order to assess the potential effect of crystallographic textures, VSM measurements have also been performed for systematically different orientations of the applied field relative to the RD of the Fe–Pd samples. These experiments produced essentially the same hysteresis behavior as those used to collate the data shown in Fig. 5 and are not shown for brevity. This either implies that textures are probably not very strong in these CR-processed samples or, at least, that textures did not have a significant effect on the magnetization behavior measured by VSM.
The magnetic age-hardening curves collated in Fig. 5 show that the coercivity of the cold-rolled samples increases up to a maximum value prior to a characteristic decrease for each of the three isothermal treatments. These observations follow a similar pattern to those reported previously for somewhat different thermo-mechanical treatments of bulk equiatomic Fe–Pd [6–9]. The maximum magnetic hardness, HC ; attained in these bulk intermetallics was 1389 Oe for an isothermal annealing temperature of 400 C for a time of 7 days (10,080 min or 604.8 ks). This is a significant increase in coercivity of more than 1.0 kOe relative to conventionally processed Fe–Pd with the polytwinned microstructure (HC B2502350 Oe). 3.3. Evolution of L10-order—XRD XRD data has been used to monitor the progress of the ordering reaction. A representative
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(a)
(b)
(b)
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Fig. 3. SEM BSE micrographs of microstructures of coldrolled Fe–Pd after annealing. (a) 500 C, 6 h, maximum coercivity HC ¼ 1251 Oe and (b) 500 C, 10 h, HC ¼ 958 Oe.
Fig. 4. SEM BSE micrographs of microstructures of coldrolled Fe–Pd after annealing. (a) 600 C, 3 h, maximum coercivity HC ¼ 984 Oe and (b) 600 C, 8 h, HC ¼ 705 Oe.
set of symmetric (y 2y-XRD data for an Fe–Pd sample annealed at 400 C after 7 days is shown in Fig. 6. Plots similar to that of Fig. 6 are obtained for the maximum coercivity conditions for isothermal annealing treatments at 500 C (360 min) and 600 C (180 min). The diffraction peak, 2y ¼ 83:43 Corresponds to {3 1 1} and {1 3 1}, whereas that at 2y ¼ 84:81 Corresponds to {1 1 3}. The splitting of these latter peaks is evidence of the transformation to the tetragonal L10-ordered phase. Also, the diffraction peaks (0 0 1) at 2yB24:5 and {1 1 0} at 2yB34 ; as well as the splitting of the {2 0 0}/(0 0 2) pair of peaks around 2yB48 and the pair of peaks {2 2 0}/{0 2 2} at 2yB70 indicate the transformation from the disordered FCC to the L10-ordered Fe–Pd phase. The (c=a)-ratio has been obtained from the symmetric XRD patterns using the {3 1 1} and {1 1 3} peaks and was used to monitor the evolution of long range L10-order. Beginning at
values closer to unity the (c=a)-ratio attains the value of 0.966, which represents the equilibrium (c=a)-ratio, for annealing times corresponding approximately to the coercivity peak. This implies that the long-range order parameter and thus the volume fraction of ordered L10-phase increases with annealing time up to the time corresponding approximately to that associated with the coercivity maximum. This behavior of the (c=a)-ratio has been observed consistently in XRD experiments for all of the three annealing temperatures. Thus it appears reasonable to conclude that the ordering reaction is essentially complete at the instance when the maximum in magnetic hardness has been achieved. 3.4. Grain size evolution—image analysis Grain size analysis has been performed using the ‘NIH image’ software. Some pertinent results of
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Coercivity vs Annealing time ( 97% Cold deformed samples) 1600
1389
1400
1251
Coercivity (Oe)
1200
1151 984
1000
989
843
800 600 400 200 0 1
10
100
1000
10000
100000
Annealing time [log (min)] Coercivity 400 deg C
Coercivity 500 deg C
Coercivity 600 deg C
Fig. 5. The magnetic age hardening behavior, HC [Oe] against annealing time [min] (log-scale), for isothermal annealing at 400 C, 500 C and 600 C (lines connecting the relevant data points for each data set are only meant to guide the eye of the reader).
in the magnetically overaged state with respect to the microstructural state associated with the peak coercivity. However, the minimum grain sizes remain the same for these two microstructural states. The microstructural changes give rise to significantly different values of coercivity for each of the annealing temperatures.
Intensity vs 2 Theta (400 deg C - 7days)
Intensity (arbi. units)
600 500 400 300 200
47.05 83.43
100
48.87 84.81
0 0
10
20
30
40
50
60
70
80
90
100
4. Discussion
2 Theta Intensity vs 2 Theta
Fig. 6. Example XRD pattern, Intensity against 2y for Fe–Pd annealed at 400 C for 7 days.
the analysis of a statistically sufficient number of grains, about 800 grains per isothermal annealing treatment, are summarized in Table 1. For annealing times significantly shorter than those required to attain peak coercivity at a given annealing temperature the microstructures were complex (e.g. [12]). A meaningful correlation of microstructural metrics, such as grain size, with properties is difficult for these complex, only partially transformed microstructures. Hence, here, only results of grain size analyses for the essentially fully CR-transformed microstructures are shown. It is clear from Table 1 that the average grain size and especially the maximum grain size in the population of CR transformed grains increases
Comparison of the BSE SEM micrographs obtained for the as-deformed and the annealed microstructural states of the Fe–Pd alloys studied here (e.g. Figs. 1b and 2) reveals much more diffuse contrast for the as-deformed state (Fig. 1b). The high defect density and the large amount of stored strain energy present in the as-deformed material may be responsible for this contrast effect. Conversely, a reduction in the level of internal stress associated with the overall defect content of the material during subsequent annealing would be consistent with the improved BSE image contrast of Figs. 2–4. The evolution of the coercivity for the various microstructural states developed during isothermal annealing of the cold-deformed Fe–Pd shown in Fig. 5 confirms previous work [6–9, 12]. Due to their similarity to the development of the mechanical hardness in precipitation strengthened alloys
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Table 1 Some pertinent results of the computer assisted grain size analyses for peak magnetic hardness and magnetically overaged conditions for the three different isothermal annealing temperatures Treatment temp. ( C)
Treatment time (min)
Coercivity (Oe)
Min grain size (mm) Dmin
Max grain size (mm) Dmax
Avg. grain size (mm) Davg
1
400 400
10080 11520
1389 1151
0.035 0.036
0.901 1.83
0.234 0.255
2
500 500
360 600
1251 958
0.05 0.05
2.18 2.38
0.407 0.601
3
600 600
180 480
984 705
0.05 0.08
1.99 6.36
0.493 1.13
for structural applications during annealing or aging treatments, i.e. mechanical age hardening curves, the three data sets depicted in Fig. 5 may be considered to represent ‘magnetic age hardening’ curves. During isothermal annealing or aging the coercivity initially increases (magnetic hardening) up to a maximum value (peak magnetic hardness) followed by a regime of decreasing coercivity with increasing annealing time (magnetic softening). In analogy to the mechanical age hardening curves commonly used to describe precipitation hardened alloys, the magnetic softening, i.e. the decrease in HC after the peak magnetic hardness (maximum HC ) has been reached, may be considered as magnetic overaging. With the aid of Fig. 5 the main effects of the isothermal annealing temperature in the range of 400–600 C on the magnetic age hardening behavior of CR-processed Fe–Pd can be identified. The effects of annealing temperature appear to be twofold. Firstly, the increase in annealing temperature results in an acceleration of the kinetics of the microstructural transformations responsible for the characteristic magnetic age hardening behavior (Fig. 5). Secondly, as the annealing temperature is lowered from 600 C to 500 C to 400 C, the maximum values of coercivity measured increase from 984 to 1251e to 1398 Oe, respectively, and occur after longer annealing times. The maximum value of coercivity obtained here for bulk equiatomic Fe–Pd by CR processing, HC ¼ 1389 Oe, is about four to five times larger than the coercivity typical of Fe–Pd with the PT-structure after conventional processing, HC ðPTÞ ¼ 2502350 Oe,
and also exceeds the highest coercivity reported previously for bulk CR-processed Fe–Pd, HC ðCRÞE800 Oe [6,7]. Thus, after further optimization of the processing parameters it appears that CR processing may be a suitable method to fabricate bulk Fe–Pd alloys that exhibit even larger coercivity in the range of 2–3 kOe, which previously has been achieved only in nanostructured thin film or powder processed Fe–Pd (e.g. [10]). The characteristic magnetic age hardening behavior of CR-processed equiatomic Fe–Pd documented in the graphs of Fig. 5 can be correlated to the increasing degree of L10-order, the increase in the fraction CR-transformed, the evolution of the grain size and the microstructural morphology, including the defect content. Based on the microstructural analyses by XRD and SEM it can be concluded that the magnetic hardening up to the peak magnetic hardness documented in the graphs of Fig. 5 correlates directly with an increase in the degree of L10-order developing in the Fe–Pd alloys. The XRD data further indicated that the ordering transformation was essentially complete at about the time when the peak magnetic hardness was obtained for each of the three different annealing temperatures. The microstructures associated with the peak magnetic hardness consisted essentially entirely of equiaxed L10-ordered grains that contained at least annealing twins (Figs. 2–4) and exhibited average diameters in the sub-micron range, namely between 234 and 493 nm (Table 1). Hence, it appears reasonable to propose that the initial increase in
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the coercivity during isothermal annealing of the cold-rolled Fe–Pd, the magnetic hardening, can be attributed to the monotonic increase in the fraction of ordered material and correspondingly the increase in the degree of L10-order. This conclusion is consistent with a previous study [12]. Furthermore, it is interesting to note that the PT-structure produced by the conventional ordering mechanism of coherent nucleation of L10ordered precipitates and their strain-affected growth has not been observed for any of the CRprocessed Fe–Pd studied here. The transformation from the deformed disordered parent phase to the stable ordered intermetallic phase results only in monolithic L10-grains of equiaxed morphology (Figs. 2–4). A transformation mechanism suitable to produce such a microstructure in the CRprocessed Fe–Pd may involve the stored strain energy assisted heterogeneous nucleation of ordered grains and their growth by the migration of highly mobile, essentially incoherent boundaries, akin to a ‘massive’ ordering transformation [6,7,12]. This is the CR transformation, a process of concomitant ordering and annealing of the deformation induced defect structure by recovery and recrystallization [12]. The sites for the heterogeneous nucleation of grains that are stable with respect to growth during the CR transformation have been shown to be associated with regions of large strain-energy gradients, i.e. prior FCC-grain boundaries, shear or deformation bands and dislocation cell-wall junctions [12]. In this latter work by Deshpande et al. [12] a smaller amount of stored strain energy was imparted to equiatomic Fe–Pd than in this study. The maximum coercivity reported after annealing at 500 C, HC ¼ 523 Oe, was associated with a fully L10-ordered microstructure comprised of a minority fraction of CRtransformed equiaxed grains and a majority fraction of larger grains with the PT-structure [12]. Hence, it appears that the mechanistically different ordering processes, i.e. the conventional ordering process and the CR transformation, compete with each other kinetically. For the larger amount of strain energy imparted to the Fe–Pd alloys here by the large reduction in thickness by cold rolling the kinetics of the CR transformation appear to dominate. Thus, for the material studied
here, the ordering and the microstructural transformation are facilitated by the CR transformation and the magnetic hardening can be attributed to an increase in the fraction of CR-transformed material, which as a consequence increases the degree of long range L10-order. The data collated in Table 1 indicate that the value of the peak magnetic hardness or coercivity achieved for the three different isothermal annealing treatments is proportional to the average grain size of the equiaxed CR-transformed grains. For a smaller average grain size a larger peak coercivity value results (Table 1). Furthermore, the average grain size clearly increases during overaging when magnetic softening is observed (Table 1 and Fig. 5). The grain size increase is most apparent for the highest annealing temperature of 600 C (Table 1 and Fig. 4). The formation of the many octahedrally conjugated annealing twins during overaging of the CR-processed Fe–Pd alloys (e.g. Fig. 4) is consistent with grain growth or ‘secondary recrystallization’ in low stacking fault energy metals with FCC-derivative crystal structure [13]. Mechanisms suitable for the formation of annealing twins by moving grain boundaries in L10-ordered structures are related to those proposed for FCC metals and alloys [14,15]. Hence, the characteristic softening observed in the magnetic hardening curves during the stage of overaging appears to be related to the thermally activated and presumably largely curvature-driven grain growth in the fraction of CRtransformed grains. Interestingly, previous studies [7] have proposed that a grain size magnetic hardening contribution to the coercivity enhancement (DHC ) may exist in Fe–Pd, which would be associated with resistance to the nucleation of reverse magnetization domains that are stable with respect to growth during the demagnetization process. To a lesser degree, the pinning resistance from the frequent grain boundaries as planar obstacles to magnetic domain wall motion may also contribute to increased coercivity. Hence, magnetic grain size hardening represents a nucleation type coercivity mechanism [7,16] and the corresponding coercivity enhancement can be given approximately by DHC ¼ ðbgÞ=ðDMs Þ:
ð1Þ
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Here b is a geometrical term with a value between 1 and 5, g is the magnetic domain wall energy (17 erg/cm2), Ms is the saturation magnetization (1100 emu/cm3) and D is the grain size for L10-ordered Fe–Pd. Inspection of the data in Table 1 shows that for reasonable values of the geometrical factor, b; between 2.0 and 3.5 coercivity enhancements on the order of the values experimentally observed here are obtained for the average grain sizes measured for the three different isothermal annealing temperatures. Thus, it appears that a large fraction of the coercivity enhancement observed in the CR-processed samples studied here may be attributed to such a grain size effect on magnetic hardness. Upper and lower bounds for a grain size regime dominated by a grain size magnetic hardening related coercivity mechanism may be evaluated based on the minimum size of a domain that is stable to grow in terms of magnetostatic energy. For grains smaller than or those considerably (perhaps 3–10 times) larger than this critical dimension this nucleation-type coercivity mechanism should not provide significant contributions to the coercivity. For Fe–Pd this single domain size dimension has been predicted to lie in the range of approximately 0.25–0.45 mm [7,17]. Hence, a grain size effect on coercivity akin to a Hall–Petch effect in mechanical strengthening by grain size may exist for CRprocessed Fe–Pd in the range of average grain sizes of about 0.3 mm to perhaps about 4.0 mm or so. Of course, in addition to the apparent grain size contribution to the coercivity in the CR-processed Fe–Pd reported here, contributions from the defect content of the equiaxed CR-transformed grains are expected. A reduction in the planar defect content of the CR grains during overaging would also be suitable to rationalize the observed magnetic softening due to a diminished contribution from a pinning-type coercivity mechanism [6,7,16]. However, in order to assess the role of planar defects, such as antiphase boundaries, stacking faults and twins inside of the CRtransformed grains for the observed magnetic age hardening additional detailed TEM studies of the evolution of the defect structures, particularly in the overaged state, are required. Such experiments are currently underway and will be the topic of a
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future communication. Hence, uncertainty regarding the role of magnetic domain wall pinning contributions to the magnetic age hardening remains. However, it may be noted that both of these microstructural origins of the magnetic softening in CRP Fe–Pd, grain growth and defect density reduction, are essentially consequences of processes associated with the classic annealing phenomenon of ‘secondary recrystallization’ or simply grain growth. The motion of mobile grain boundary segments accomplishes grain growth together with a driving force derived from curvature. The grain boundary motion facilitates the absorption of line and planar defects present in the interiors of both growing and shrinking grains and the deposition of annealing twins into the growing grains. Thus, it appears reasonable to attribute the magnetic softening during overaging of CR-processed Fe–Pd essentially to grain growth in the fraction of CR-transformed grains.
5. Summary and conclusions Combinations of XRD, SEM and VSM have been used to study the microstructural and property evolution in cold-deformed bulk equiatomic Fe–Pd during isothermal annealing. The characteristic magnetic age hardening behavior of CR-processed Fe–Pd has been rationalized in terms of thermally induced microstructural changes in the solid state. The main conclusions of this experimental work can be summarized as follows: 1. The method of CR processing enables the synthesis of bulk Fe–Pd with nanoscale microstructures that are devoid of the PT-structure and exhibit considerably improved hard magnetic properties compared with conventionally processed bulk Fe–Pd alloys. 2. A maximum coercivity of about 1.4 kOe has been obtained by CR processing for a condition with the ordering reaction complete and an average grain size of 234 nm. 3. The coercivity increase (magnetic hardening) up to the peak (peak hardness) has been attributed to the increase in the degree of long
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range L10-order, which is facilitated mechanistically by the CR transformation. 4. The CR transformation has been described as an ordering transformation that involves heterogeneous nucleation and is assisted by the strain energy stored in the defect structures of the cold-deformed state. 5. The decrease in coercivity (magnetic softening) during overaging has been attributed to grain growth or ‘secondary recrystallization’ in the fraction of CR-transformed grains and its consequences for the defect content of these fully ordered equiaxed grains. 6. An apparent grain size effect on the coercivity of the CR-transformed Fe–Pd microstructures may exist for average grain sizes in the range of about 0.3–4.0 mm. Acknowledgements The materials presented are based upon work supported by the National Science Foundation, NSF, Grant Number DMR-Metals-0094213, with Dr. K.L. Murty as program manager. Any opinions, findings, and conclusions or recommendations expressed in this article are those of the authors and do not necessarily reflect the views of the National Science Foundation. References [1] D. Weller, A. Moser, IEEE Trans. Magn. 35 (1999) 4423. [2] L.M. Magat, A.S. Yermolenko, G.V. Ivanova, G.M. Makarova, Y.A.S. Shur, Fiz. Metal. Metalloved. 26 (1968) 511.
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