Investigation of FePSi glassy alloys

Journal of Non-Crystalline Solids 110 (1989) 81-88 North-Holland, Amsterdam

81

INVESTIGATION OF F e - P - S i GLASSY ALLOYS Ben-Zheng FENG, Yi-Qun GAO, Yun ZHENG, Shi-Lin LI, Xiong LIU, Ga-Lu JIN and Qing-Lin TAN Institute of Precious Metals, Kunmin~ Yunnan, PR China Received 2 May 1988 Revised manuscript received 3 March 1989

Seventyfive F e - P - S i alloys of different composition in the region of Fe-FeaP-FeSi of the F e - P - S i phase diagram have been prepared by rapid quenching technique of the single roller. The structures of all alloys studied were examined by means of an X-ray diffractometer and transmission electron microscope. The results show that the glass-forming region of F e - P - S i ternary alloys is 14.5 at.% < P < 18.5 at.%, 1.5 at.% < Si < 6.5 at.% and the balance Fe. The crystallization temperatures and the stability are observed by a differential scanning calorimeter (DSC). The characteristics of both subeutectic and supereutectic F e - P - S i glassy alloys containing P and annealing at different temperatures have been measured and a long-time stability has been observed.

1. Introduction

Many iron-based metallic glassy alloys made by rapid quenching techniques have drawn an increasing interest because of their mechanical and soft magnetic properties [1,2]. Fe-based glassy alloys have high tensile strength, that is about 400 k g / m m 2 which is more than those of Co-based (about 300 k g / m m 2) and Ni-based (about 270 k g / m m 2) glassy alloys. In order to develop a new enhanced agent of ceramic material, Fe-P-Si alloys are studied. Part of the magnetic properties of Fe-P-Si glassy alloy has been studied [3]. The authors [4] have reported the ductile-brittle transition and its kinetics for Fe-P-Si glassy alloys. In this paper, the formation, stability and some properties of Fe-P-Si glassy alloys are reported.

quenching technique of the single roller. The structure of ribbons was examined by an X-ray diffractometer and a transmission electron microscope (TEM). The crystallization temperatures, Tp, corresponding to the exothermic peaks of ribbons of glassy alloys were determined at various scanning rates by a "Thermoflex" differential scanning calorimeter (DSC). The microhardnesses of the glassy alloys after ageing at various times and temperatures have been measured by means of the MVK-type (c) microhardness tester with a load of 100 g. The long-time stabilities of the several glassy alloys were observed.

3. Results and discussion

3.1. The composition region for glass formation of the Fe-P-Si alloy system 2. Experimental

All alloy ingots of the Fe-P-Si ternary system were prepared in an induction furnace in an A1203 crucible under a reducing atmosphere with mother alloys of Fe-P, Fe-Si and Fe of 99.99% purity. The ribbons of 75 alloys were made by rapid 0022-3093/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

According to the phase diagram of the Fe-P-Si ternary alloy system [5], 75 alloys of different compositions in the region of Fe-Fe3P-FeSi, using the perpendicular selection method, have been tested. From X-ray diffraction patterns, electron diffraction patterns and the ductilities of alloys,

B.-Z. Feng et aL

82

/

Investigation of F e - P - S i glassy alloys

we can see that there are three different regions I, II and III, as shown in fig. 1. Region I, looped by the inner dashed line, is the glass forming region of the F e - P - S i ternary alloy system, in which the composition is 14.5 at.% < P < 18.5 at.%, 1.5 at.% < Si < 6.5 at.% and the balance Fe. For this region the ductility of ribbons was better and the ductile-brittle transition temperatures were higher, e.g. 3 0 0 ° C for Fe77.2P18Si4. 8 glassy alloy [4]. That is, the degree of vitrification is higher, as shown by X-ray diffraction patterns of three F e - P - S i glassy alloys in fig. 2 (I). Region II, the shaded area, is a region of mixture of glassy state and microcrystalline state, in which the composition is 18.5 at.% < P < 19.0 at.%, 12.5 at.% < P < 14.5 at.%, 6.5 at.% < Si < 9.0 at.%, 1.0 at.% < Si < 1.5 at.% and the balance is Fe. For this region the ductilities of ribbons were less. There are a few weak peaks in X-ray diffraction patterns shown in fig. 2 (II). In region III, out of the outer dashed line, the microcrystalline structure was predominant and ribbons are very brittle, although long ribbons could also be ob-

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A group of DSC exothermal curves with a heating rate of 20 K / m i n is plotted in fig. 3. Tpl , Tp2 a n d Tp3 are the temperatures corresponding with the 1st, 2nd and 3rd crystalline peaks in the DSC exothermal curves, respectively. The error of all Tp is _+2 o C. It is shown in fig. 3 that there are two or three crystallization peaks in the DSC

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exothermal curves of the subeutectic glassy alloys containing P, i.e. in the range 12.5 at.% < P < 16.4 at.%. There is only one exothermal peak when the contents of P are more than 16.4 at.%. The other alloys studied in the glassforming region show a similar result. These results indicate that crystallization processes in sub- and super-eutectic glassy alloys containing P are different. Part of the micrographs and diffraction patterns of the structure transition observed by means of a JEM-7A type transmission electron microscope after annealing for different times (475 ° C for Fes0.TP14.sSi4.8, 500 ° C for Fe77.2P18Si4.8 glassy alloys) are shown in figs. 4 and 5. There are many small grains precipitated from the matrix of Fes0.TP14.sSi4.8 glassy alloy heated for 5 rain and then complete crystallization after heating for 10 rain. The crystalline phase, however, is directly precipitated from the Fe77.2P18Si4.8 glassy alloy after heating for 2 min, and precipitation is completed after heating for 10 rain. We have reported

83

[6] that there is a spinodal decomposition before crystallization in Fe77.2P18Si4.8 glassy alloy. The structures of both sub- and super-eutectic glassy alloys containing P were examined by X-ray diffraction after annealing the Fe77.2P18Si4.8 glass alloy at 5 0 0 ° C for 5 and 10 min, and the Fes0.TP14.sSi4.s glassy alloy at 450 ° C for 5 and 10 min. The data of X-ray diffraction show that the diffraction peaks corresponding to the a-Fe phase appear in the pattern of Fes0.7P14.sSi4.s glassy alloy after annealing for 5 min. This phase is then followed by Fe3P and FeSi after annealing for 10 rain. The diffraction peaks corresponding to Fe3P and FeSi only appear in the pattern of the Fe77.2P18Si4.8 glassy alloy. The results of X-ray diffraction are quite consistent with those of TEM. We attribute the first peak in the DSC exothermal curve of the subeutectic glassy alloys to precipitation of the a-Fe phase, the second peak (or the strongest peak) may be due to the Fe3P and FeSi phase, the third peak may correspond with the balance F%P, FeSi phase; however, there is one crystallization peak in the DSC exothermal curves for most of the supereutectic glassy alloys. This crystallization peak comes from the Fe 3P and FeSi phases precipitated directly from the matrix of glassy alloys. (Note: X-ray diffraction patterns made in Nov. 1987, DSC and TEM in 1980.)

3.3. The effect of isothermal annealing on crystallization transition The hardness of alloys can provide information about the bonding strength of atoms and the distribution. The microhardness, H v, with an error of + 10 k g f / m m 2, of both sub- and supereutectic F e - P - S i glassy alloys after isothermic annealing at different temperatures and times are shown in fig. 6. It is obvious from fig. 6 that the changes of the hardness contain two maximum values with the increase in time at each annealing temperature, Ta, with an error of + 1 ° C, when Ta is higher than 200 ° C [4]. The plots, therefore, of temperature T , - time corresponding to each maximum value of the hardness - transformation, which is called a T - T - T diagram, in both suband super-eutectic glassy alloys can be drawn, as shown in fig. 7. It is obvious that the transition

84

B.-Z. Feng et al. //lnoestigation of F e - P - S i glassy alloys

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behavour is correlated with the data from X-ray diffraction, TEM and DSC measurements.

3.4. The effect of composition on the crystallization temperature In order to observe the effect of alloy composition on the crystallization temperature, Tp, corre-

sponding with the strongest peak in DSC exothermal curves, part of the data of DSC in region I of fig. 1 are listed in table 1. Figure 8 shows the plot of composition vs. Tp for three groups of F e - P - S i glassy alloys. It is shown from the above results that the composition dependence of Tp is linear when the Fe content in F e - P - S i glassy

B.-Z. Feng et al. / Investigation of Fe-P-Si glassy alloys

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Tp2 is c o r r e s p o n d i n g with the s t r o n g e s t p e a k of the D S C e x o t h e r m a l curves.

3.5. Observations of long-time stability T h e s a m p l e s e x a m i n e d have b e e n e x p o s e d in r o o m t e m p e r a t u r e from 1980 to 1987. T h e crys-

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tallization temperatures, Tpl and Tp2 , measured by DSC at 20 K / r a i n , are listed in table 2. We assume that the crystallization transition in F e - P - S i glassy alloys is described by the Kissinger equation in first approximation [7], d(ln f l / T p 2 ) d(1/Tp)

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glassy alloys. (a) Feso.oP20_xSix; (b) Fe95.2_xPxSi48; (c) Fe84.5 -.~ P] 5.5Si 4.

glassy alloy, prepared in January 1980. This result shows that the long-time stability of Fe77.2P18Si4. 8 glassy alloy is good.

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B.-Z. Feng et at / Inoestigation of Fe-P-Si glassy alloys

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(1) The glass-forming region of F e - P - S i ternary alloys has been determined, that is 14.5 at.% < P < 18.5 at.%, 1.5 at.% < Si < 6.5 at.% and the balance Fe. (2) There are two or three crystallization peaks in the DSC exothermal curve of each subeutectic F e - P - S i glassy alloy with P. The a-Fe phase is first precipitated from the matrix of F e - P - S i glassy alloy in the process of crystallization, and then followed by precipitation of Fe3P and FeSi phases; there is only one crystallization peak in the DSC exothermal curve for the supereutectic alloy with P. The Fe 3P and FeSi phases are directly precipitated from the matrix of F e - P - S i glassy alloys. (3) F e - P - S i glassy alloys are stable for long times. We would like to thank Mr. Ga-De Gun, of the Central Laboratory of Yunnan University, for the analysis of the X-ray diffraction. References

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[1] F.E. Luborsky, Amorphous Metallic Alloys (Butterworth, London, 1983). [2] A. Inoue, T. Masumoto, S. Arakawa and T. lwadachi, Rapidly Quenched Metals III, Vol. 1, ed. B. Cantor (Chameleon Press, London, 1978) p. 265. [3] T. Masumoto and H.S. Chen, Lecture Japan. Soc. Metals Sci. 4 (1979) 188. [4] Q.L. Tan, B.Z. Feng and Y.H. Zhu, Acta Metallurgica Sinica (in Chinese) 19 (5) (1983) 201. [5] V.R. Vogel and B. Giessen, Arch. Eisenhiittenwesen 30 (1) (1959) 619. [6] Y.Q. Gao, S.L. Li, B.Z. Feng, X. Liu, Y.S. Gao and W. Wang, Mater. Sci. Eng., in press. [7] H.S. Chen, J. Non-Cryst. Solids 27 (1978) 257.