D0I:10.13374/j.issn1001-053x.1989.06.031 北京科技大学学报 第11卷第6期 Vol.11 No.6 1989年11月 Journal of University of Science and Technology Beijing Nov.1989 The Microstructure of Graphite Spherulites in Cast Iron' Miao Baihe(苗柏和)**，Fang Keming(方克明)* 、 ABSTRACT:The microstructure of graphite spherulites (G.S.)in Ce addition ductile cast iron was carried out using transmission electron microscopy (TEM) techniques.The structral characterization of the graphites is as following:there are platelets growing along the periphery as well as fan-like structure formed from platelet aggregates in the diameter section of spherulite,in which most of the platelets are with sizes ranging from several to tens nm in radial;the [001] direction of the platelets would have a tend to parallel to radial of the G.S. and meanwhile,they also a small misorientation deviated from the radial among the platelets each other.It shows that the anisotropy of graphites has been restrained by adding Ce element in cast iron and the structural characterization of the G.S.is consistent with the Double's model of the conical helixes formed in helical growing and branching. KEY WORDS:cast iron,graphite spherulite,microstructure The optical microscopy and scanning electron microscopy (SEM)have been extensively used to invcstigate the morphologies and growth mechanism of gra- phites in cast irons before and at the same time many growing models of G.S. 1 were proposed -1,but there are only a few studies on the structure of G.S. using TEM techniquest.Therefore,there was little possibility to review the theoretical models with the TEM experimental results. Recently,it was because of use of ion thinning instrument and a new know- how I in sample preparation that it is possible to perfcrm TEM analysis on the microstructure and Ce distribution in single G.S.I8]And thus it can further deal with the effect of Ce addition on the structure of graphites as well as the growing mechanism of G.S. Manuscript Received April 4,1989 ..Dept.of Material Science and Enginecring ···Dept,of Physical--Chemistry 581
第 卷 第 期 北 京 科 技 大 学 学 报 乃 。 后 年 月 入 苗 柏 和 ， 俐 方 克明 、 、 · 住 一 ， 〔 〕 一 ， 习 。 。 ， ， ， 、 、 从 · · ‘ 一 ， · · ’ ’ 〕 ， · ， 任 丁 · · 〔 · ， ， · · ， · 一 DOI ：10．13374／j ．issn1001－053x．1989．06．031
1 Experimental The G.S.used for TEM analysis were extracted from the cast iron conta- ining4.2%C,1.07%Si,0.005%P,0.003%S,0.19%Mn and 0.32%Ce melted in a carbon-tube furnace under argon atmosphere. Foil specimen preparation:First,each G.S.were inlaid on a polished plane in single layer,then polished from the tcp and the bottom till thinned down to about 0.2um in thick,and then fixed in a 3mm in diameter Cu suppor- ting net.Finally,using the Gatan-600B ionic thinner the specimens were thinned to have thin area suitable for TEM observations. The TEM observations,centre dark field (CDF),bright field (BF)and selected area electron diffraction (SAD)were performed with a Hitachi-800 analytical electron microscope. 2 Results and Discussions The microstructure of G.S.is shown in Fig.1.There is a typical annular graphite in the well-developed G.S.sections at low magnifications.As the previous paper mentioned t8],most of G.S.have a nucleus composed of a group of particles of cerium oxide or cerium oxysulfide. The detailed studies of the structure of G.S.are shown in Fig.2 and Fig.3,respectively.From the Fig.2,there are platelets with about 8-60nm in radial (see Fig.2b),along the periphery of the G.S.without apparent zigzag.The [001]direction of the platelets oriented approximately radially. And they have a small misorientation each other based on 002*diffraction spot extended to an arc (see Fig.2a).Fig.3 shows the other structure with 120 and 140 chevron formed from platelet aggregates towards the centre of the G.S. Fig.1 The microstructure of a Fig.2 The microstructure and SAD annular graphite spherutites. patterns of the graphites. 1 582
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Fig.3 The microstructure of the other G.S.and SAD patterns. (a)BF imagc of ithe fan-like arca.(b)-(f).SAD diffractions from Ictters A,B,C,D and I positions in (a). In order to examine the structure and crystallgraphic orientation of the platelets in the G.S.a series of SAD at the interesting positions marking A,B,C,D and I were carried ont as shown in Fig.3a and Fig.3b-3f,respectively.At the both sides of ihe interface marking letter I,the C001]direction of the platelets are approximateiy parallel to the radial of the G.S.though there are small mi- sorientation among the platelets each other (see Fig.3b,002*diffraction spot extended to an arc). The formation mechanism of G.S.and the effect of Ce addition on growing of graphites could be proposed from present results by TEM. The {001}planes of graphite are closest packing planes,[001]is not a close packing direction.Reviewing the morphologies and the size of the platelets,it is clear that the growing rate of (001)plane is much bigger than the 001]direction (see Fig.2b and Fig.3a).It implies that the preferred growing of the graphites do still influence the morphologies of G.S.As a matter of fact,{001}planes are quite developed than the others (see Fig.2b).It was only due to Ce-addition that the anisotropy of the graphites was restrained during the growth of G.S.In addition,as Hunter and Chadwick ,a)suggested that the first growth is in flake form or as the present observations it is adherence to a nucleus of cerium oxide or cerium Oxysulfide if possible.Furthermore,it showed that growth branching of graphites has paid an important role in formation of G.S.otherwise the graphites in cast irons could not grow up a spherulitc.Finally,from the morpho- logies and crystallgraphic characterization of thre G.S.as shown in Fig.3,they are the projection of two reciprocal pyramids formed in helical growing or conical helixes in the both sides of the interface I,as is proposed by Double and Hallawellt.And the {001 crystallgraphic plancs of a graphile pyramid have 583
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got a fixed conical apex angle relative the pyramid symmetrical axis,thus the C001)of the platelets oriented approximately radially,while the (100 planes observed were nearly parallel to the electron beam.Moreover,if the symmetrical axis of the pyramid is parallel to the electron beam,the C001]zone axis diffraction pattern of the graphite would be got.These results are consistent with Double's modelt 11 of conical helixes and the previous SEM observations by Chent 51. 3 Conclusions (1)The microstructure of G.S.consists of platelets of the graphites.The platelets are with sizes ranging from several to tens nm in radial and orientate themselves approximately radially though they have a small misorientation. (2)A fan-like aggregate of the platelets is a helical growing reciprocal pyramid or a conical helixes,towards the centre of the G.S. (3)The G.S.in Ce-addition cast iron is composed of pyramids or conical helixes formed in helical growing and branching. (4)As a spheroidization element,the main effect of Ce-addition cn the formation of G.S.is to restrain the anisotropy of the graphites. Acknowlegemeat The authers are grateful to Feng Huiping fopreparing TEM specimen by ionic thinner and to Prof.Liu Guoxun for revicwing this paper and many helps.This work has been supported The Agency of Rare-carth Metals of The Minister of Metallurgical Industry and University of Science and Technology Beijing. REFERENCES 1 Double DD,Hellawell A.The Metallurgy of Cast Iron Eds.B Lux, Switzerland,1975 2 Hunter M J,Chadwick G A.J.Iron Steel Inst.,London,1972;210:707 3 Hunter M J.J.Iron Steel Inst.,London,1973;211:85 4 Minkoff I.The Physical Metallurgy of Cast Iron,John Wiley and Sons Ltd.Chic!ester,1983 5 Chen Sysen,Wang Zulun,ct al.Acta Mechanical Engineering,in Chi- nese,1982;18(1):7 6 Song Weixi,Yang Wenying.Acta Metallurgy SINICA.1987:23:B324 7 Fang Keming.Patent,G01 N 1/28,or CN 85 105786,1985 8 Fang Keming,Shen Ansin,Yuan Guilian.Giesserei Forschung,1986: 38:55 584
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