The correlation of damping capacity with grain-boundary precipitates in

Materials Science and Engineering A366(2004)

45–49

The correlation of damping capacity with grain-boundary precipitates in

Fe–Cr-based damping alloys annealed at high temperature

Weiguo Wang a ,?,Bangxin Zhou b

a

Faculty of Mechanical Engineering,Shandong University of Technology,Zibo 255012,Shandong,China

b School of Materials Science and Engineering,Shanghai University,Shanghai 200072,China

Received 7May 2003;received in revised form 19August 2003

Abstract

We report the damping capacity measurements and grain-boundary (GB)precipitates observations on a set of four Fe–Cr-based ferromagnetic damping alloys annealed at 1473K.The alloys were prepared by vacuum induction melt furnace of which each cast ingot weighed 15kg.The technique of inverted torsion pendulum was employed to measure the damping capacity,and a ?eld emission scanning electron microscope (FESEM)with a X-ray energy dispersive spectrometer (EDS)attachment was used to observe the grain-boundary precipitates.The results indicate the damping capacity of the alloys is de?nitely correlated with the amount of the grain-boundary precipitates as well as with the chromium concentration in the precipitates.Among the alloys investigated,Fe–Cr–2Al–Si exhibits the highest damping capacity but the least amount of grain-boundary precipitates with the lowest chromium concentration,and vice versa to that Fe–Cr–Al–Si(L)does.Though chromium concentration in the grain-boundary precipitates are very close,the amount of grain-boundary precipitates in Fe–Cr–Al is obviously less than that in Fe–Cr–Si,and the damping capacity of the former is apparently higher than that of the latter.Further discussion pointed out such correlation stems from the effects of alloying on the diffusion behavior of solute atoms as well as on the chemical potential of Cr in the matrix,with which correlates the magnetic properties especially the energy density of domain walls (DWs)which has a signi?cant impact on the damping capacity of Fe–Cr-based damping alloys annealed at high temperature.?2003Elsevier B.V .All rights reserved.

Keywords:Fe–Cr-based damping alloy;Damping capacity;Grain-boundary precipitates

1.Introduction

Fe–Cr-based ferromagnetic damping alloys possess not only excellent damping capacity but also fairly good cor-rosion behavior,mechanical strength and processing appli-cability [1–3].As a new type of structural and functional materials for depressing noise and vibrations,such alloys have been drawing the great interest of the scientists and engineers across the world [4,5].In the know-how of im-proving the damping capacity of Fe–Cr-based damping alloys,an increase in the damping capacity with an in-crease in annealing temperature has been reported [6],and the signi?cant impact of alloying on the damping capacity of such alloys has also been noted elsewhere [7,8].Re-garding the damping mechanisms of ferromagnetic alloys,the vibration energy dissipation related to the irreversible

?

Corresponding author.Tel.:+86-533-2760017.

E-mail address:wangwgnpic@https://www.360docs.net/doc/881228090.html, (W.Wang).

movements of 90?domain walls has been recognized early in 1950s [18],the local internal stress model which was proposed by Smith and Birchak [13,14]has been commonly accepted [15],and the question of what is the local inter-nal stress source left open by Smith and Birchak has also been settled recently [16,17].However,the mechanisms of alloying elements’in?uencing on the damping capacity still remain unknown.In order to understand the roles that alloying elements play in changing the damping behavior,the relationship between damping capacity and some mag-netic properties,concerning several Fe–Cr-based damping alloys,has been investigated by the authors and satisfactory results are obtained [9].In the mean time,it has been no-ticed that the grain-boundary (GB)precipitates are formed in the alloys annealed at high temperatures and the amount of such precipitates vary with alloying elements.So,it will be no doubt fruitful to investigate the correlation of damping capacity with grain-boundary precipitates in some Fe–Cr-based damping alloys annealed at high temperature.

0921-5093/$–see front matter ?2003Elsevier B.V .All rights reserved.doi:10.1016/j.msea.2003.08.065

46W.Wang,B.Zhou /Materials Science and Engineering A366(2004)45–49

Table 1

Chemical composition of the alloys investigated (mass fraction,%)No.Alloys

Cr Al Si C N S P Mn Fe 1Fe–Cr–Si

12.64– 2.800.0180.00800.00690.0150.041Bal.2Fe–Cr–Al–Si(H)12.98 3.07 3.010.0160.00830.00610.0170.035Bal.3Fe–Cr–Al–Si(L)12.92 3.00 1.000.0150.00630.00400.0150.020Bal.4Fe–Cr–2Al–Si 12.78 1.89 1.050.0130.00660.00560.0140.024Bal.5

Fe–Cr–Al

12.85

2.99

0.15

0.012

0.0076

0.0080

0.015

0.014

Bal.

2.Experimental

The Fe–Cr-based damping alloys were prepared,using commercially pure iron,chromium,aluminum and highly pure (single crystal bar)silicon as raw materials,by a vacuum induction melt furnace of which each cast ingot weighed 15kg.The chemical composition of the alloys is listed in Table 1.After removing the skins,the lower half of the ingots were cut off to be forged at the temperatures from 1300to 1173K,cold drawn into ?1mm wire,and ?nally annealed at 1473K in vacuum for 1h followed by air-cooling and furnace-cooling,respectively.The damping capacity was measured by JN-1inverted torsion pendu-lum at room temperature,and the vibration frequency was 1Hz.The grain-boundary precipitates were observed under Amray-1886?eld emission scanning electron microscope (FESEM)and the chemical composition of GB precipitates were analyzed by X-ray energy dispersive spectrometer (EDS)attached to the FESEM.The crystal structure of GB precipitates was determined by JEM-200CX transmission electron microscope (TEM).

The specimen for FESEM observation,EDS analysis and TEM diffraction were prepared by mechanical abrasion and electrolytic polishing which was carried out under

the

Fig.1.Damping capacity vs.maximum shear strain-amplitude in Fe–Cr-based damping alloys annealed at 1473K for 1h:(a)air-cooled;(b)furnace-cooled.

voltage of 20V and the temperature around 15?C.The electrolyte is made up of 85vol.%glacial acetic acid and 15vol.%perchloric acid.

3.Results and discussion

Fig.1a and b are the measurement results of damping capacity of the Fe–Cr-based damping alloys as annealed at 1473K followed by air-cooling and furnace-cooling,respec-tively.Since the no.2alloy (see Table 1)is too brittle to be drawn into wire,Fig.1excludes the results of this alloy and the following discussion will not consider it either.

As shown in Fig.1,Fe–Cr–2Al–Si alloy has the high-est damping capacity while Fe–Cr–Al–Si(L)holds the lowest,and Fe–Cr–Al alloy has the damping capacity slightly higher than that Fe–Cr–Si does.For the same al-loy,the air-cooled specimen has higher damping capacity than the furnace-cooled one does,which is probably at-tributed to the negative effect of K state [10]formed during furnace-cooling.

Fig.2shows the FESEM morphology of grain-boundaries of the alloys as annealed followed by furnace-cooling.It can be seen that GB precipitates are formed in

W.Wang,B.Zhou/Materials Science and Engineering A366(2004)45–49

47

Fig.2.FESEM morphology of the grain-boundary precipitates in some Fe–Cr-based damping alloys annealed at1473K for1h followed by furnace-cooling: (a)Fe–Cr–Si;(b)Fe–Cr–Al–Si(L);(c)Fe–Cr–2Al–Si;(d)Fe–Cr–Al.

all the four alloys and qualitatively the relative amount of GB precipitates is100%for Fe–Cr–Al–Si(L),90%for Fe–Cr–Si,30%for Fe–Cr–Al and20%for Fe–Cr–2Al–Si. These results de?nitely correlate with the damping capacity as shown in Fig.1,which means the less amount of GB pre-cipitates,the better is the damping capacity,and vice versa. The specimen as annealed but followed air-cooling has the GB precipitates slightly less than furnace-cooled one does. This indicates that the GB precipitates are largely formed during the holding time of annealing.

Fig.3shows the EDS pro?les of the GB precipitates.We can see the GB precipitates of the alloys as annealed are all chemically composed of iron and chromium since no spe-ci?c peaks for silicon and aluminum were detected as EDS scanned ranging from0.5to10keV,and the only differ-ence is that Cr concentration in the GB precipitates changes from one alloy to another.The alloy sequence of Cr con-centration in GB precipitates varies from lower to higher,is Fe–Cr–2Al–Si,Fe–Cr–Si,Fe–Cr–Al and Fe–Cr–Al–Si(L). This result is generally in correlation with the damping ca-pacity which implies the lower the Cr concentration in GB precipitates,the higher is the damping capacity of the alloy, and vice versa.Of course,it can also be found,in Fig.3, that Cr concentration in the GB precipitates of Fe–Cr–Al is slightly higher than in that of Fe–Cr–Si,but the damping ca-pacity of the former is a little bit higher than that of the later.Such discrepancy might be attributed to the less amount of GB precipitates in Fe–Cr–Al.The quantitative result of Cr concentration in GB precipitates is given in Table2.

The damping capacity of the alloys correlates with the amount of GB precipitates as well as with the Cr concen-tration in GB precipitates must have its physical origin.It is not dif?cult to?nd,in Figs.2and3,that both silicon and aluminum has the effect of enhancing the diffusion of chromium and iron from matrix to grain-boundary.Such enhancement by silicon is greater than that by aluminum while the enhancement of Cr/Fe special diffusion by silicon is slightly weaker than that by aluminum.This is the reason why FeCr precipitates are formed in the grain-boundaries of the alloys and why Fe–Cr–Si has more GB precipitates with less Cr concentration than Fe–Cr–Al does.It is obvious,on the one hand,that the above-mentioned effect gives rise to a Table2

Quantitative results of Cr concentration in GB precipitates of the alloys as investigated

Alloys Cr concentration(mass%) Fe–Cr–Si47

Fe–Cr–Al–Si(L)65

Fe–Cr–2Al–Si43

Fe–Cr–Al49

48W.Wang,B.Zhou /Materials Science and Engineering A366(2004)

45–49

Fig.3.EDS pro?les of Fr-Cr-based damping alloys annealed at 1473K:(a)Fe–Cr–Si;(b)Fe–Cr–Al–Si(L);(c)Fe–Cr–2Al–Si;(d)Fe–Cr–Al.

net ?ow of Cr and Fe from matrix to grain-boundaries,and it probably results in the changes of some magnetic proper-ties,especially the energy density of domain walls (DWs)which has a signi?cant impact on the damping capacity [9].On the other hand,it re?ects that the chemical potential of Cr has been raised when certain amount of either silicon or aluminum or both added into Fe–Cr matrix and Cr has experience as uphill diffusion [12],which means the diffu-sion proceeds from lower concentration (matrix)to higher concentration (GBs)(see Tables 1and 2).Such increase of chemical potential of Cr might in?uence on the magnetic properties as well as on the damping capacity remarkably.It is easy to understand that 1%silicon addition to Fe–Cr–Al,which the new alloy Fe–Cr–Al–Si(L)is formed,makes the chemical potential of Cr in the matrix be raised sharply and a great deal of Cr must ?ow from the matrix to the GBs when the alloy is being annealed at high tem-perature.Such behavior results in not only the formation of the most amount of GB precipitates but also the occur-rence of the highest Cr concentration in GB precipitates in Fe–Cr–Al–Si(L)compared with the other alloys.Fur-thermore,it can be found that the energy density of DWs,which is proportional to the vibration energy dissipation [9],of Fe–Cr–Al–Si(L)is the lowest among the alloys in-vestigated [9],so its damping capacity is the lowest as well.However,1%silicon substitution for the same amount of Al in Fe–Cr–Al,which another new alloy Fe–Cr–2Al–Si is formed,results in the slightest raising of chemical potential of Cr in the matrix,so the least amount of GB precipitates with lowest Cr concentration is formed.And correspond-ingly,Fe–Cr–2Al–Si has the highest energy density of DWs [9],so its damping capacity reaches the highest.In the same way,the correlation of damping capacity with GB precipitates concerning the rest two alloys Fe–Cr–Al and Fe–Cr–Si can also be interpreted reasonably.

In brief,the amount of GB precipitates and its Cr con-centration,concerning Fe–Cr-based damping alloys,reveals the effects of alloying on the diffusion behaviors of the so-lute atoms as well as on the chemical potential of Cr in the matrix.It correlates with the energy density of DWs,which is the most important in?uential factor on the damping ca-pacity.Generally,the less the GB precipitates as well as the lower its Cr concentration,the better is the damping capac-ity of Fe–Cr-based damping alloys annealed at high temper-ature,and vice versa.Undoubtedly,this result is signi?cant for improving the damping capacity of such Fe–Cr-based damping alloys.

Fig.4a and b are the bright-?eld TEM morphology of GB precipitates in Fe–Cr–Al–Si(L)as annealed,and it can be seen that a lot of stacking faults exist in the precipitates (Fig.4b ).Fig.4c and d are the selected area diffraction (SAD)patterns obtained from the same area of GB precip-itate but with different incident angles of electron beams.It is determined that the GB precipitate is of body-centered tetragonal structure of which a =b =0.23nm and c =0.70nm.Obviously,such GB precipitate is different from the brittle tetragonal σphase (a =b =0.45nm,c =0.88nm)usually formed in some stainless steel [11].Of course,the GB precipitates in the rest three alloys as investigated are unlikely the σphase because the Cr concentration in the precipitates are low and the alloys have never been kept at 475–821?C for long time [11].Nevertheless,it is necessary

W.Wang,B.Zhou/Materials Science and Engineering A366(2004)45–49

49

Fig.4.TEM observation on the grain-boundary precipitate in Fe–Cr–Al–Si(L)alloy annealed at1473K:(a)and(b)morphology;(c)and(d)SAD patterns.

to make further investigation into the relationship between the characteristics of such GB precipitates and the compre-hensive performance of the alloys.

4.Conclusions

1.Grain-boundary precipitates,which are composed of Cr

and Fe,are formed in Fe–Cr-based damping alloys an-nealed at high temperature.

2.The damping capacity of Fe–Cr-based damping alloys

annealed at high temperature correlates with the amount of GB precipitates as well as with the Cr concentration in the precipitates,which implies that the less amount of GB precipitates and the lower the Cr concentration in the precipitates,the better is the damping capacity,and vice versa.

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