Effect of a small addition of Cr on soft magnetic and mechanical properties of Fe–49Co–2V alloy

1、Journal of Alloys and Compounds 556 (2013) 5155Contents lists available at SciVerse ScienceDirectJournal of Alloys and Compoundsj o u r n a l h o m e p a g e : w w w . e l s ev i e r . c o m / l o c a t e / j a l c o mEffect of a small addition of Cr on soft magnetic and mechanical properties of Fe4

2、9Co2V alloyChongqiang Hou, Yijiao Shan, Haichen Wu, Xiaofang Bi Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University (BUAA), 37# Xueyuan Road, Beijing 100191, Chinaa r t i c l ei n f oArticle history:Received 1

3、0 September 2012Received in revised form 3 December 2012 Accepted 6 December 2012 Available online 20 December 2012Keywords:FeCoVCr alloysVacuum annealingMechanical and magnetic propertiesTEMPrecipitatesa b s t r a c tFe49Co2V0.5Cr and Fe49Co2V alloys were prepared by hot and cold rollings combined

4、with sub-sequent annealing at temperatures ranging from 550 to 850 LC. The Fe49Co2V0.5Cr alloy annealed under an appropriate condition shows low coercivity of 2.6 Oe as well as an enhanced tensile strength of 912 MPa and elongation of 10.9% as compared to Fe49Co2V. The improved mechanical properties

5、 are consistent with fracture morphologies which demonstrate a quasi-cleavage feature for the annealed Cr-added alloy, which is revealed to be relevant to a precipitation process that is facilitated by the Cr addition occurring prior along grain boundaries and even in the grain interiors when anneal

6、ed at higher temperatures. Transmission electron microscopy combined with electron probe micro-analyzer is applied to identify the precipitates and reveals that they are comprised by numerous small-sized parti-cles enriched with V and Cr, which suggests be beneficial for a good combination of magnet

7、ic and mechanical properties.2012 Elsevier B.V. All rights reserved.1. IntroductionFeCo based soft magnetic alloys have been attracting consider-able attention due to their large saturation inductions, high Curie temperature and soft magnetic properties 1,2. For their applica-tions in rotors and sta

8、tors which are requisite components for elec-trical generator, mechanical properties are particularly required, in addition to the excellent magnetic properties, for this class of al-loys. Cold rolling over a critical reduction is known to be very effec-tive for improving tensile strength of the all

9、oys mainly arising due to work-hardening, which, unfortunately, leads to a deterioration of the soft magnetic properties due to a large increase in coercivity with increasing the cold-rolled reduction caused by introduction of defects and stresses which result in pinning against domain wall movement

10、 3,4. In this respect, subsequent annealing treatments are usually subjected to the alloys to decrease the coercivity by removing the pinning sites or increasing grain size. However, based on binary FeCo phase diagram 5, a transition from disorder to order occurs around 730 LC for the alloys with at

11、omic ratios of Co varying from 0.30 to 0.70. The transition is accompanied with formation of brittle phase which does great harm to mechanical workability for the alloys 6.Up to date, a number of studies have demonstrated that addi-tion of alloying elements is one of the promising approaches in Corr

12、esponding author.E-mail address: (X. Bi).0925-8388/$ - see front matter 2012 Elsevier B.V. All rights reserved. /10.1016/j.jallcom.2012.12.044improving the mechanical properties for FeCo alloys 79. It is well known that a small addition of vanadium can increase the te

13、n-sile strength as well as the ductility of FeCo alloys without a large deterioration of their magnetic properties 3,10,11. For example, addition of 2 at.% of V in an equiatomic FeCo alloy was found to be preferred by the removal of interstitial impurities as well as the slowdown of the ordering tra

14、nsition 12,13, which in turn im-prove room temperature ductility and cold workability for the bin-ary FeCo alloy. However, the FeCo2V suffers sensitivity of ductility to temperature of annealing which is generally needed for trade-off between the magnetic and mechanical properties 10,1416. It is sho

15、wn that addition of 4.5 at.% of Ni decreases the adverse effect of heat treatment on mechanical properties 17,18. Several other alloying elements were also reported to be beneficial more or less in the improvement of the mechanical properties. Additions of B and C increases ductility due to grain re

16、finement attributed to formation of fine carbides 12. The de-crease of grain size was also seen for the alloy with Nb addition, leading to an increase in tensile strength 19. Addition of either W or Mo is shown to increase tensile strength as well as elonga-tion, which is probably attributed to the

17、formation of precipitates 9. For example, adding 2 at.% of W brings about a tensile strength of 2000 MPa and an elongation of 9% for FeCo alloys.Nevertheless, most of the researches have been focused on either soft magnetic property or combination of the mechanical property. In this work, we report

18、that addition of Cr improves ten-sile strength and ductility with only a little increase in coercivity for Fe49Co2V, which is revealed to be closely related to the52C. Hou et al. / Journal of Alloys and Compounds 556 (2013) 5155precipitation enhanced by the Cr addition. The mechanism is dis-cussed c

19、ombined with microstructure analysis.2. ExperimentalThe raw materials with purity of 99.99 for Fe and 99.9 for Co, V and Cr were arc melted in an argon atmosphere. To ensure homogeneity, the ingots of approxi-mately 1 kg in mass were then repeatedly melted for five times. It was confirmed that the m

20、ass loss accompanied with the melting process was negligible, indicating that the chemical compositions were close to the nominal compositions, which was consistent with the result by electron probe micro-analyzer. The alloys are repre-sented as Fe49Co2V and Fe49Co2V0.5Cr, respectively, based on the

21、ir compo-sitions. The number indicates atomic percentage of each element. After being homogenized in vacuum at 1200 LC for 4 h and furnace-cooled, the ingots were forged into slabs of 6 mm in thickness at 1150 LC. The slabs were then processed to 2.6 mm in thickness by hot-rolling at 1120 LC. To rem

22、ove the ordered phase formed in the alloys, the hot-rolled samples were subjected to an isothermal annealing at 950 LC for 30 min and then to a quenching in an ice brine solution, which was followed by cold-rolling with reduction varying from 30% to 70%. The cold-rolled sheets were then subjected to

23、 isothermal annealing in vacuum for 2 h at temperatures ranging from 550 to 850 LC.Tensile tests were carried out at a nominal strain rate of 0.5 mm/min on the IN-STRON-8801 tensile test machine with its loading axis parallel to the rolling direc-tion. The samples used for the tensile tests were cut

24、 into slices of 70 mm 1.8 mm in size prior to the annealings. Good reproducibility was shown and the datum for each sample is an average of three tests. Coercive forces for the as-rolled and the annealed samples were measured using vibrating sample magnetometer and MATS-113 2010SD DC B-H loop tracer

25、, respectively. Fracture morphologies of sam-ples were observed by CS3400 scanning electron microscope (SEM). An electron probe micro-analyzer (JXA-8100 EPMA) and a transmission electron microscope (JEM 2100F TEM) were used for the observation of distribution of precipitates and characterization of

26、the precipitates combined with selected area electron dif-fraction pattern (SAED).3. Results and discussionFig. 1 shows effect of the cold-rolled reduction on coercivity, tensile strength and elongation for the Fe49Co2V0.5Cr alloy. As expected, the coercivity increases with increasing the reduction

27、due to introduction of lattice distortion and structural defects such as vacancies and dislocations during the cold-rolling. The micro-structural inhomogeneity inhibits domain wall movement for a sample while being magnetized. Meanwhile, proliferation of dislo-cations induced by plastic deformation

28、results in work hardening, reflecting an increase in tensile strength with the reduction for the cold-rolled samples, as shown in Fig. 1. It is seen that the tensile strength increases quickly from 1015 MPa for the hot-rolled to 1265 MPa when cold-rolled with the reduction of only 30%, fol-lowed by

29、a slight decrease with further increasing the reduction. It can be explained that when work hardening reaches to a certain level, the dislocation density will no longer increase. On the other hand, the increasing of reduction decreases the elongation for the alloy due to the work hardening. Neverthe

30、less, it is noted thatthe elongation is still larger than 15% with the reduction of 30%. Taking into account the variations of cold-rolled reductions on coercivity and the mechanical properties, the alloys with reduction of 30% were subjected to vacuum annealing treatments at different temperatures

31、in the following, with an aim of improving the mechanical properties effectively while minimizing the deteriora-tion of soft magnetic properties.Fig. 2(a)(c) presents variations of the coercivity and mechanical properties with annealing temperatures for the Fe49Co2V0.5Cr alloy. The results of Fe49Co

32、2V are also presented for comparison. It is seen that the values of coercivity for the two alloys continue toFig. 1. Variations of coercivity, tensile strength and elongation for the Fe49CoFig. 2. Dependence of the magnetic and mechanical properties for the Cr-added Fe2V0.5Cr alloy with reductions o

33、f cold-rolling.49Co2V alloy on annealing temperatures ranging from 550 to 850 LC.C. Hou et al. / Journal of Alloys and Compounds 556 (2013) 515553decline with increasing the temperatures up to 760 and 800 LC, which is obviously attributed to the gradual elimination of the cold-rolling induced micros

34、tructure-inhomogeneity due to recov-ery and/or recrystallization during the annealing processes. After reaching the minima of 0.45 and 1.97 Oe for Fe49Co2V and Fe 49Co2V0.5Cr, respectively, the values of coercivity start to rise with further increasing temperature. The rebound of coercivity for Fe49

35、Co2V alloy in the high temperature range is often explained by a decrease in the grain size 20. It can be assumed that the dependence of coercivity on the annealing for the Cr-added sample may also be related to the variation of grain size at higher temper-atures, which will be discussed below. It s

36、hould be mentioned that despite the Cr addition, the coercivity of Fe49Co2V0.5Cr is still very low at 760 LC. As shown in Fig. 2(b), the tensile strength for the Fe49Co2V shows a large and monotonic decrease from 1204 MPa of the cold-rolled to less than 500 MPa with increasing the annealing temperat

37、ures to 800 LC, which is easily understood by a gradual decrease in structural defects with annealing temper-ature. By contrast, dependence of the tensile strength on the tem-perature for the Cr added alloy is characterized by a critical behavior. Following a gradual decrease to the minimum point of

38、 824 MPa at 760 LC, the strength starts to increase with further increasing the temperature. The result is found to be owing to the formation of precipitates facilitated by the Cr addition as discussed below. Effect of the Cr addition is also observed on the variations of elongation for the alloys a

39、nnealed above the 600 LC at which elon-gations of the alloys both exhibit minimal values, as shown in Fig. 2(c). The decrease of elongation with increasing annealing tem-perature below 600 LC is associated with the orderdisorder transi-tion of FeCo alloys. The transition temperature varies in the ra

40、nge of 650730 LC, depending on the content of vanadium in the equi-atomic FeCo alloys. When subjected to isothermal annealings below 650 LC, the alloys undergo consecutively ordering transition, result-ing in an increase in degree of ordering with increasing the anneal-ing temperatures. The increasi

41、ng order phase is known to enhance brittleness of FeCo alloys 2123, giving a good explanation on the decreases in the elongation as well as in the tensile strengths with increasing the temperatures. As the annealing temperature further increases in the range above 650 LC, the elongation of Fe49Co2V

42、turns to increase until 670 LC and tends to stabilize in the range above 760 LC after a slight decrease. This is consistent with previous researches as reported in the literature 17,18. It isclaimed that the temperature dependent elongation above the crit-ical temperature arises mainly from a gradua

43、l elimination of the de-fects like internal stress and dislocation density. The stabilization of elongation at high temperatures can be attributed to stabilization of the grain size. By contrast, the elongation for the Cr-added alloy in-creases monotonically, which already reaches 9.8% at 760 LC and

44、 17.8% at 850 LC, respectively. The monotonic rise in elongation may be also attributed to the elimination of defects during recovery and/or recrystallization. However, it is noted that, in the tempera-ture range above 760 LC, the temperature dependent elongation for the Fe49Co2V0.5Cr alloy shows a

45、similar variation to the ten-sile strength. The interesting observation seems to be puzzled be-cause it is generally expected that tensile strength and elongation exhibit different variations. In order to make a better understanding of the result, variations of microstructures have been in detail in

46、ves-tigated through SEM and HRTEM observations.Fracture morphologies were firstly investigated for the Cr-added alloy annealed in the temperature range from 600 to 850 LC for 2 h, as demonstrated in Fig. 3. The cold-rolled alloy with the elongation of 8.7% is also shown for comparison, which is char

47、-acterized by ductile fracture with tiny ductile dimples. When an-nealed at 600 and 670 LC, the samples exhibit a cleavage fracture along with river patterns (arrowed), reflecting the brittleness char-acterization for the alloy due to introduction of the order phase. The observation is consistent wi

48、th the lower tensile strength and the elongation for the alloy annealed at the temperature range as compared to the as-cold one. By contrast, the samples annealed at 760, 800 and 850 LC exhibit a mixed-type fracture morphology which is comprised of cleavage fracture and tearing ridges/dimple bands.

49、The cleavage fracture becomes even less obvious while tear-ing ridge images more featured with increasing the annealing tem-perature. It can be assumed that the 850 LC annealed sample goes through a discontinuous fracture process. It is generally believed that formation of large numbers of tearing r

50、idges/dimple bands is attributed to the connection of hidden cracks with one another which tends to occur during a large plastic deformation process. The observations coincide with the improvement of the elongation with increasing the annealing temperatures above 600 LC and of the tensile strength a

51、bove 760 LC. The annealing temperature dependent fracture characterization is considered to be related to the precipitates existing in the annealed samples, as observed in Fig. 4. As clearly seen in Fig. 4(a)(c), there is no precipitateFig. 3. Fracture morphologies for the as-rolled and vacuum annea

52、led Fe49Co2V0.5Cr samples.54C. Hou et al. / Journal of Alloys and Compounds 556 (2013) 5155Fig. 4. Backscatter electron images for the Fe49Co2V alloy annealed at (a) 760 LC, (b) 800 LC, (c) 850 LC, and for the Fe49Co2V0.5Cr annealed at (d) 760 LC, (e) 800 LC, (f) 850 LC, respectively.Table 1Average

53、contents of Fe, Co, V and Cr corresponding to the grain interior and grain boundary, respectively, for the Fe49Co2V0.5Cr alloy annealed at 800 LC for 2 h. (at.%).FeCoVCrGrain interior49.1448.451.930.47Grain boundary40.9849.666.452.92appearing in the Fe49Co2V alloy even when annealed up to 850 LC. Fu

54、rthermore, the absence of precipitates leads in a large in-crease in grain size with increasing the annealing temperature. By contrast, precipitation is clearly observed for the Fe49Co2V 0.5Cr, as demonstrated in Fig. 4(d)(f). When annealed at 760 LC, the sample exhibits a few precipitates formed al

55、ong prior grain boundaries. The precipitates increase with increasing the anneal-ing temperature. At 800 LC, the grains are almost surrounded by the precipitates. As the temperature increases to 850 LC, a consid-erable fraction of precipitates can be seen not only along the grain boundaries but also

56、 in the intragranular region where they pre-sumably nucleated on prior dislocation lines. The volume fraction of the precipitates can be roughly estimated to be 9%, 12% and 20%, respectively, for the samples annealed at 760, 800 and850 LC. It is noted that the average grain size shows a tendency of

57、reduction with increasing the temperature due to the increasing precipitates. The rebound in coercivity above 760 LC for the alloy, as shown above, is evidently attributed to the decreases of the grain size.In order to identify the precipitates, composition measurement has been made on the grain int

58、erior and grain boundary to distin-guish the composition of the precipitates from that of the matrix. Table 1 lists the average contents of each element at the interior and boundary zones, respectively, obtained from at least three measurements for the 800 LC annealed sample. It is seen that the composition co