172 I N Ż Y N I E R I A M A T E R I A Ł O W A ROK XXXIV

Ph.D. Anna Korneva (a.korniewa@imim.pl) – Institute of Metallurgy and Materials Science, Polish Academy of Science, Krakow

ANNA KORNEVA

Effect of deformation temperature on the microstructure of hard magnetic

FeCr30Co8 alloy subjected to tension combined with torsion

INTRODUCTION

The development of new generation of high-speed electrical de-vices requires high strength characteristics of magnetic materi-als used. Most magnetic materials of today reveal high magnetic characteristics but are brittle and have low ultimate rupture strength. The highest level of mechanical properties is realized in magneti-cally hard alloys of Fe-Cr-Co system. Fe-Cr-Co based alloys belong to the deformable magnetic materials of the precipitation-hardening class. Due to their good ductility, excellent magnetic properties and low cost, they are used for the production of permanent magnets of various sizes and shapes, such as wire, tube, bar, strip magnets, etc [1÷3]. A high-coercive state is obtained by a magnetic treatment and multistage tempering. This leads to the decomposition of the solid solution into the isomorphous α1 and α2 phases, containing ordered and coherent precipitates [4, 5]. The formation of such structures, in which each precipitate of the α1 phase is a single magnetic domain, provides superior magnetic properties. However, internal stress fields, which originate from the formation of coherent boundaries between the precipitates of the α1 and α2 phases, cause a reduction in ductility and strength.

It is known, that the structure of material and its mechanical properties can be changed using severe deformation techniques [6, 7]. Complex loading by compression, strength and torsion at an elevated temperature is rather a new method of severe plastic deformation [8]. It ensures a substantially refined microstructure without changing the shape of the specimen. Depending on the mode of the deformation chosen, this method allows localizing strain in specific regions and ensures the formation of gradient mi-crostructure with different combination of magnetic and mechani-cal properties [9, 10].

The aim of the present work is to present the results of the mi-crostructure and hardness investigations of the FeCr30Co8 alloy deformed by tension combined with torsion under isothermal con-ditions at different temperatures.

EXPERIMENTAL

In order to obtain the α solid solution the alloy was subjected to homogenization and water-quench from 1150°С. The chemical composition of the examined alloy was (wt %) 27.6% Cr, 7.9% Co, 1.8% Si, 1.1% Ti, 0.2% V, balance Fe.

The FeCr30Co8 alloy was subjected to plastic deformation in a modernized “Instron” machine, which allowed deformation at single or simultaneous actions of axial loading and torsion under superplasticity conditions [8]. Cylindrical samples, 8 mm in diam-eter and 44 mm of length, were deformed in two separate stages: the tension applied to the upper part of sample and then the torsion applied to the bottom part of sample (Fig. 1). The samples were sub-jected to tension at the rate of 6·10–4 s–1 to obtain the deformation of 20%. Then, the samples were deformed by torsion at the rate of 8.5·10–4 s–1 in 9 rotations. The deformation temperature (750, 800,

850°C) as well as deformation rates corresponded to the superplas-ticity conditions of the examined alloy. The tensile strain and the strain in torsion reached 0.18 and 1.65, respectively.

The tensile strain was calculated basing on the formula (1) [11]:

=

⎛

⎝⎜

⎞

⎠⎟ln l

li0

(1)

where: l0, mm, is the sample length before the deformation and li, mm, is the sample length after the deformation.

The strain in torsion was calculated based on the formula (2) [11]:

= + ⋅⎛

⎝⎜

⎞

⎠⎟ln 1

2R

li

i (2)

where: φ, rad, is the angle of the torsion deformation and Ri, mm, is the radius of the deformed sample.

The microstructure was examined by means of the scanning electron microscope (SEM) XL 30 ESEM, Philips and the trans-mission electron microscope (TEM) Tecnai G2 F20 (200 kV). The maps of orientations were measured with the use of EBSD method for the analysis of grain boundaries. The crystallographic orienta-tion was determined by means of convergent beam electron diffrac-tion (CBED) analysis in the TEM (Philips CM 20). The hardness was measured with the Vickers method at the load of 49 N. The hardness measurements were carried out in the sample longitudinal-section along the perpendicular direction to the axis of tension, at ½ of the effective length of the sample. The step of measurement was 0.5 mm.

a) b)

Fig. 1. The scheme of deformation of the FeCr30Co8 alloy by tension (a) applied to the upper part of the sample and then torsion (b) applied to the bottom part of the sampleRys. 1. Schemat odkształcenia stopu FeCr30Co8 metodą złożonego obcią-żenia: w pierwszym etapie zachodzi rozciąganie próbek od strony górnej trawersy (a), w drugim etapie – skręcanie od strony dolnej trawersy (b)

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RESULTS AND DISCUSSION

The X-ray analysis of the FeCr30Co8 alloy before the deformation showed only the reflections of α solid solution. An example of the initial microstructure is presented in Figure 2. The grain size of α phase was about 70 µm.

The microstructure analysis by SEM on the longitudinal-section of the deformed samples showed the formation of microstructure of gradient type with the largest grain refinement in the surface layer, where the deformation was the greatest. The highest grain refine-ment of microstructure was observed at 800°C. It should be noted that only at this temperature the grains of α phase were globular at the surface layer of the sample, while at 750 and 850°C the grains of α phase were elongated in the direction perpendicular to the tension axis [12]. An example of the microstructure of the sample deformed at 800°C is shown in Figure 3. The composition contrast makes vis-ible the subgrain microstructure formed during the deformation, as a result of dynamic recovery (Fig. 3b). Precipitations of the σ phase (Fe-Cr) with the grain sizes of about 5 μm (bright color in Fig. 3) are observed also for each temperature of deformation. The pres-ence of the σ phase was also confirmed by the X-ray analysis (Fig. 4b). The σ phase precipitated generally in the grain boundaries of the α phase. It confirms the diffusion character of its precipitation. The maximum precipitation was observed in the areas of intense deformation. For example, according to the EBSD measurements, the fraction of the σ phase at the surface layer (location a, Fig. 3c) was 12, 43, 22% for 750, 800 and 800°C, respectively, while in the middle part of the samples (location b) it only reached 2, 12, 10 (Tab. 1). It should be noted that the σ phase for the examined alloy, according to its stability equilibrium range, can occur at the annealing temperatures of 560÷800°C. However, the process of its precipitation is very time-consuming (it can require several days of annealing [12]). So the deformation activated the precipitations of the σ phase. The difference in σ phase precipitation between the samples deformed at 800°C, in which the maximum was observed, and 750°C was caused not only by the deformation but also by a stronger thermal activation of diffusion processes. Then, the de-crease in the amount of precipitation at 850°C was due to the fact that the σ phase (according to the equilibrium phase diagram) was no longer stable at this temperature. A similar situation occurred when the alloy FeCr30Co8 was deformed by torsion and compres-sion [10].

It is probable that the σ phase precipitation caused an additional refinement of the microstructure. The σ phase usually precipitated at grain boundaries of the α phase. Therefore, the movement of dis-locations and the transfer of deformation from one grain to another became more difficult. Consequently, the density of dislocations

Fig. 2. Initial microstructure of the α-solid solution after homogeniza-

tion at 1150°CRys. 2. Wyjściowa mikrostruktura roztworu stałego α po homogenizacji w 1150°C

a)

b)

c) Fig. 3. The microstructure at the surface (location a) and in the center (location b) of the sample deformed at 800°C. The microstructure ob-servations carried out on the longitudinal-section of the sample in lo-cation a and b marked in the scheme (c)Rys. 3. Mikrostruktura przy powierzchni (miejsce a) oraz w środku (miej-sce b) próbki odkształconej w 800°C. Obserwację mikrostruktury wyko-nano w przekroju podłużnym

Table 1. The fraction of phases and grain boundaries in the samples of FeCr30Co8 alloy deformed at different temperatureTabela 1. Udział faz i granic ziaren w próbkach stopu FeCr30Co8 od-kształconego w różnej temperaturze

Temperature of deformation, °C

Place of EbSD

measuring

Fraction of phases%

Fraction of boundaries

α σ LAGb HAGb

750location a 88 12 63 37

location b 98 2 84 16

800location a 57 43 29 71

location b 88 12 47 53

850location a 78 22 57 43

location b 90 10 64 36

a)

b)

c)

174 I N Ż Y N I E R I A M A T E R I A Ł O W A ROK XXXIV

inside the α grains increased and brought about their refinement. The grains size of the α phase after deformation at the surface of samples was about 35 μm in length for 750 and 850°C, and about 8 μm for 800°C. The thickness of the fine grained surface layers were about 2, 3 and 2,5 mm to 7 mm of the thickness of the whole samples deformed at 750, 800°С and 850°C, respectively.

The detailed analysis of the grain boundaries by EBSD in SEM showed that the fraction of high angle grain boundaries (HAGB) in the surface layer of the each deformed sample was always greater than that in the volume of material (Tab. 1). The maximum amount of high angle grain boundaries was observed in the sample de-

formed at 800°C. In the volume of all deformed samples the strong development of subgrain microstructure of the α phase was also noted (Fig. 4). The maximum amount of low angle grain boundaries (LAGB) was detected in the sample deformed at the lowest tem-perature.

The microstructure analysis of thin foil which was prepared of the surface layer of the sample deformed at 800°C showed struc-ture with globular grains of α and σ phases with average size of about 3 μm (Fig. 5a). The grains of σ phase generally precipitated at triple points, which served as nucleation places for a new σ phase precipitates. The grains of σ phase did not contain dislocations

a) c) e)

b) d) f) Fig. 4. Maps of phase distribution obtained at surface (a, c, e) and in the centre (b, d, f) of samples deformed at 750 (a, b), 800 (c, d) and 850°C (e, f). The boundaries of high (disorientation angle >13.5°) and low (from 1.5 to 13.5°) angles are marked with thick black and thin light contours, respectively. The α grains are colored in grey, σ phase grains in whiteRys. 4. Mapy rozmieszczenia faz uzyskane przy powierzchni (a, c, e) i ze środka (b, d, f) próbek odkształconych w 750 (a, b), 800 (c, d) i 850°C (e, f). Na mapie rozmieszczenia faz granice dużego kąta (kąt dezorientacji >13,5°) i małego kąta (o kącie dezorientacji od 1,5 do 13,5°) oznaczono odpowiednio grubymi czarnymi i cienkimi szarymi liniami. Ziarna fazy α są zaznaczone na szaro, ziarna fazy σ na jasno

a) b)

a) b) Fig. 5. TEM image of the surface layer (a) and STEM image of the volume (b) of the FeCr30Co8 alloy deformed at the 800°C

Rys. 5. Mikrostruktura przy powierzchni (a – TEM) oraz mikrostruktura w objętości próbki (b – STEM) stopu FeCr30Co8 odkształconego w 800°C

a) b)

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(dark color on the Figure 5a) and their fraction reached about 40%. The microstructure analysis of thin foil which was prepared of the surface layer of the sample deformed at 800°C showed structure with globular grains of α and σ phases with average size of about 3 μm (Fig. 5a). The grains of σ phase generally precipitated at triple points, which served as nucleation places for a new σ phase pre-cipitates. The grains of σ phase did not contain dislocations (dark color on the Figure 5a) and their fraction reached about 40%. The grains of α phase were characterized by much lower dislocation density in comparison with the α grains from the volume of mate-rial. Therefore, it is possible that the formation of σ phase resulted in the reduction in the total dislocation density. In such conditions, the formation of σ phase may be thermodynamically advantageous. Figure 5b presents the microstructure in the volume of this sample taken with the STEM mode. In this mode the σ grains look bright, while the α grains are dark. It can be clearly seen that the disloca-tions in the elongated α grains first accumulated at the dislocation walls and then, the low-angle grain boundaries were formed.

The measurement of hardness showed a considerable increase of hardness at the surface of the samples. The maximum hardness was observed in the sample deformed at 800°C. In this sample the increase of hardness at the surface of material was about 30% in comparison with its interior, and about 40% in comparison with its initial state. A considerable increase of the hardness at the surface was caused by the microstructure refinement and the presence of the hard σ phase in the material.

It is also worth to note that it is possible to decrease the amount of the nonmagnetic σ phase by a prior heating of the alloy at tem-perature 900°C [9] before the magnetic treatment.

The investigations of the influence of the deformed microstruc-ture on magnetic and mechanic properties will be carried out in fur-ther investigations.

SUMMARY

The deformation by tension combined with torsion of the FeCr30Co8 alloy results in a formation of a gradient microstructure with the minimum grain size in the surface layer of the material. The advantage of such a gradient microstructure is a gradual change in the properties of the material.

A dynamic recovery is accompanied by a formation of a subgrain microstructure developed during the deformation.

The deformation stimulates the precipitation of the intermetallic σ phase. The highest precipitation was observed in the most de-formed surface layer. The precipitation of σ phase grains promotes the refinement of the α grains.

The temperature 800°C is an effective temperature for both the intensive grain refinement of the microstructure and the precipita-tion of σ phase in the surface layer of the sample.

The process can be applied for the surface treatment (hardening) of bulk specimens, e.g. in the case of their performance in the fric-tion conditions (the σ phase increases the abrasion resistance).

ACKNOWLEDGEMENTS

This work has been supported by the Ministry of Science and Higher Education of Poland within the Project No. 507 530539. The SEM and TEM studies were performed in the Accredited Testing

Fig. 6. The hardness graphs of the samples deformed at 750, 800 and 850°C. The hardness of the initial state is marked by a dotted lineRys.6. Wykresy twardości próbek odkształconych w 750, 800 i 850°C. Twardość w stanie wyjściowym oznaczono liną przerywaną

Laboratories at the Institute of Metallurgy and Materials Science of the Polish Academy of Sciences.

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