Experimental and Thermodynamic Studies of the Fe Si Binary System

Transcription

1 ISIJ International Vol. 52 (2012 No. 4 pp Experiental and Therodynaic Studie of the Si Binary Syte Ikuo OHNUMA Shinya ABE Shota SHIMENOUCHI Tohihiro OMORI Ryouke KAINUMA and Kiyohito ISHIDA Departent of Material Science Graduate School of Engineering Tohoku Univerity Sendai Japan. E-ail: ohnua@aterial.tohoku.ac.jp (Received on October ; accepted on Deceber Phae equilibria in the Si binary yte were invetigated experientally and therodynaic aeent wa carried out. The α (A2 + α 3Si (D0 3 two-phae icrotructure at 600 C and 650 C were obtained whoe grain ize were ufficiently coarened to be analyzed by FE-EPMA with a patial reolution below 0.5 μ under the condition of 6 kv accelerating voltage. α Si ( + α 3Si (D0 3 twophae equilibria above 700 C were detected for the firt tie and equilibriu copoition were deterined by the diffuion couple ethod. The horn-haped two-phae icibility gap extend fro the low teperature α + α 3Si equilibriu along the /D0 3 econd-order tranition boundary and cloe below C. Four-ublattice plit copound energy forali wa applied to calculate the Gibb energy of the phae A2(α (α Si and D0 3(α 3Si and the therodynaic paraeter in the Si binary yte were evaluated. Equilibriu relation in the binary yte were well reproduced epecially the effect of the and D0 3 ordering on the liquidu and olidu curve and the icibility gap between phae. Optiized therodynaic paraeter a well a the experiental reult are expected to be helpful for developing higher ulti-coponent yte for practical teel. KEY WORDS: phae diagra; CALPHAD; diffuion couple; icibility gap; order-diorder tranition; FE- EPMA. 1. Introduction Silicon i one of the indipenable alloying eleent of the latet high-perforance teel uch a TRIP teel etc. a well a of advanced agnetic aterial. To tudy and develop uch practical aterial phae diagra and therodynaic propertie of Si bae alloy are helpful and the calculation of phae diagra (CALPHAD technique ha been widely applied not only theoretically but alo in the indutrial world. Therefore therodynaic paraeter of the Si yte with high accuracy a well a appropriate therodynaic odel to calculate the Gibb energy of coponent phae are required. Figure 1(a how a phae diagra of the Si binary yte aeed by Kubachewki 1 which conit of liquid γ (A1 α (A2 α Si ( α 3Si (D0 3 and oe interetallic copound phae. Si tabilize the α (A2 to for a γ- loop and caue two-tep ordering fro A2 to and to D0 3 configuration a hown in Fig. 2 with a Gibb energy gain due to each ordering reaction. A icibility gap between the Fig. 1. Phae diagra of the Si binary yte aeed by Kubachewki ISIJ 540

2 ISIJ International Vol. 52 (2012 No. 4 and D0 3 phae jut inide the /D0 3 econd-order boundary at low teperature below 700 C wa reported by Schlatte et al. 2 and the theoretical calculation by Inden and co-reearcher proved it validity. 3 7 In contrat to the calculation however the hape of the icibility gap which cloe with a helet-like top around 700 C ee to be quite trange and further invetigation are required at higher teperature. Therodynaic calculation of the Si yte were carried out by Lacaze and Sundan. 8 Reult of their calculation are hown in Fig. 3(a and 3(b in which there are till oe debatable point. A hown in Fig. 3(a the calculation of the ordering of the phae i liited to the tructure in which not only the D0 3 ordering but alo the icibility gap between the phae cannot be taken into account. In addition an anoalou icibility gap with a lower critical olution teperature in liquid phae appeared due to the inappropriate therodynaic paraeter of the liquid. Theoretical calculation of the ordering and ei-epirical calculation of the whole phae diagra of the Si yte were carried out by Inden et al. 36 and Lee et al. 9 repectively. Even though both calculation are quite ophiticated to decribe the D0 3 configuration the therodynaic odel of their calculation were inadequate for extending practical ulticoponent yte. Taking into account the preent ituation a entioned above the purpoe of thi tudy wa to deterine the icibility gap between phae preciely and to coplete practical and thorough therodynaic aeent including D0 3 ordering in the Si binary yte. 2. Experiental Procedure 2.1. Equilibria in Two-phae Alloy Si binary alloy were prepared by induction elting fro electrolytic (99.95% and high-purity Si (99.999% under an Ar atophere. Obtained ingot were hot rolled at 1100 C to a thickne of 2. The rolled plate were then heat-treated at 1100 C for 10 in for hoogenization and quenched in iced water. After oxidized cale on the urface of quenched plate wa reoved by echanical grinding all piece of aple were cut fro the plate echanically polihed and cleaned in ethanol by an ultraonic waher. Each aple wa ealed in an evacuated quartz capule and heat-treated under the condition hown in Table 1 for equilibration. Quenched aple were ounted in conductive rein polihed by wet polihing and finihed by a vibratory poliher (Buehler: VibroMet 2 for 8 hour with 0.03 μ Al 2O 3 upenion. The icrotructure of the heattreated aple wa exained by an FE-EMPA (JEOL: JXA-8500F with an accelerating voltage of 15 kv. On the Table 1. Prepared aple heat treatent condition and reult of equilibriu copoition. Fig. 2. Atoic configuration of phae diordered A2 and ordered and D0 3. Reult Alloy at.% Si T/ C t/hour at.% Si/phae 1 at.% Si/phae ingle phae ingle phae ingle phae /A2 ( 13.8/D /A2 ( 13.4/D0 3 Fig. 3. Calculated phae diagra of the Si binary yte evaluated by Lacaze and Sundan ISIJ

3 ISIJ International Vol. 52 (2012 No. 4 aple exhibiting a two-phae icrotructure after heat treatent equilibriu copoition were deterined by the FE-EMPA with an accelerating voltage of 6 kv under which condition the patial reolution of the icro-probe analyi can be controlled below approxiately 0.5 μ. Intenity of the characteritic X-ray of the Kα of Si and Lα of wa eaured becaue the Kα of i hardly exited by 6 kv acceleration. Hoogenized 13 at.% Si alloy copoition of which wa confired by weighing the raw aterial and the obtained ingot before and after elting wa utilized a a tandard aple for the ZAF calibration Diffuion Couple Method Six illieter cubic aple were cut fro induction elted ingot. Pair of cubic aple with the different copoition lited in Table 2 and hown in Fig. 4 were prepared by echanical polihing and cleaning in the ae way a above. Epecially one ide of each cube wa thoroughly polihed to obtain a flat urface for preure bonding. A hown in Fig. 5 two cube were tacked wrapped with a Mo heet and tied with two nichroe wire. The tip of a Pt-PtRh thero-couple wa bonded on a cube by electric reitance welding. Then the pair of cube were preed together by a thero-echanical proce iulator (Fuji Electronic Ind.: Therec-Mater Z and diffuionbonded under preure of 10 Pa at 950 C for 1 hour. After being ground polihed and cleaned the prepared diffuion couple were ealed in evacuated quartz capule and heattreated under the condition lited in Table 2. Both icrotructure exaination and copoition analye acro inter-phae boundarie were carried out by the FE-EPMA in the ae condition a with the two-phae alloy. Step of the line analyi were 1 μ acro the inter-phae boundarie 10 μ far fro the boundarie and 2 or 5 μ in between the. 3. Reult of Experient 3.1. Equilibria in Two-phae Alloy Figure 6 how back-cattered electron (BSE iage of (a 13.5 at.% Si alloy heat-treated at 650 C for 180 h and (b 13 at.% Si alloy heat-treated at 600 C for h. During the heat treatent in the forer aple pinodal Fig. 5. Apparatu for preparation of diffuion couple. Table 2. Prepared diffuion couple heat treatent condition and reult of equilibriu copoition. Diffuion couple alloy1/alloy2 13%Si/23%Si T/ C Reult t/hour at.% Si (A2/ at.% Si/ at.% Si/D (/ D / 17.0/D0 3 8%Si/23%Si / 15.8/D / 14.4/D0 3 Fig. 4. Prepared aple and diffuion couple with their heattreatent teperature. Fig. 6. BSE iage of two-phae icrotructure ISIJ 542

4 ISIJ International Vol. 52 (2012 No. 4 decopoition occurred inide forer α grain to for a fine laellae icrotructure. On the other hand along grain boundarie coare grain of two phae precipitated the ize of which wa large enough to be analyzed by the FE- EPMA quantitatively. A hown in Fig. 6(b even after a long period of heat-treatent for h at 600 C the laellae icrotructure wa too fine to be analyzed by the FE-EPMA. However further prolongation of heat-treatent to h caued Otwald ripening of the fine laellae up to 1 μ quare or ore a hown in Fig. 6(c. Equilibriu copoition at 600 C and 650 C were then eaured. The icrotructure after heat treatent at and 900 C did not exhibit decopoition i.e. their ingle phae icrotructure before the heat treatent reained a they had been. Obtained reult are uarized in Table 1 and plotted in Fig. 7. Atoic configuration of the obtained two phae were identified by electron diffraction. Figure 8(a how a dark field iage taken fro the (111 uper-lattice reflection hown in Fig. 8(c. A two-phae icrotructure which correpond to Fig. 6(b could be confired. Figure 8(b and 8(c repreent elected area diffraction (SAD taken fro the dark phae and the bright one in Fig. 8(a repectively. The uper lattice reflection (111 in Fig. 8(c a well a the icrotructure without anti-phae doain boundarie (APB ugget that the bright phae correpond to the D0 3 ordered α 3Si phae at 600 C. On the other hand the uper lattice reflection (100 in Fig. 8(b ee to be diffue which indicate that the A2 diordered phae at 600 C ight be ordered to the configuration during quenching. Further dicuion will be ade baed on the obtained phae diagra in ection Diffuion Couple Figure 9(a how a BSE iage of a 8 Si/23 Si (at.% diffuion couple heat-treated at 700 C for 840 h acro an inter-phae boundary. In addition to oe grain boundarie which can be ditinguihed due to the channeling effect an obviou iicible boundary could be confired a indicated by an arrow. Figure 9(b how a copoition profile of Si acro the boundary which clearly exhibit a icibility gap between the and D0 3 phae. Each copoition profile wa extrapolated to the inter-phae boundary and the equilibriu copoition were deterined to be 12.0 and 14.4 at.% Si for the and D0 3 phae repectively. According to the phae diagra hown in Fig. 7 the A2/ boundary at 700 C i located at about 11.7 at.% Si which ugget that the phae i equilibrated with the D0 3 phae although it thickne i etiated to be 1 μ or Fig. 7. Reult of preent experient on two-phae alloy and diffuion couple. Fig. 8. (a Dark field iage of two-phae icrotructure taken fro (111 uper-lattice reflection. SAD how (b and (c D0 3 uper-lattice tructure. Fig. 9. (a BSE iage and (b copoition profile of Si of 8Si/23Si diffuion couple heat-treated at 700 C ISIJ

5 ISIJ International Vol. 52 (2012 No. 4 le. Therefore the A2/ boundary could not be ditinguihed in the icrograph and copoition profile in Fig. 9. In addition the extrapolation of the copoition profile in the A2 phae wa ade intead of that in the phae becaue of it unrecognizable thickne which ight caue the underetiation of the Si content in the phae equilibrate with the D0 3 phae a the lope of the profile in the phae i teeper than that in the A2 phae a hown in Fig. 10(b. However the difference between extrapolated value fro the A2 and phae can be conidered to be all becaue the thickne of the phae i unrecognizably thin. Therefore the obtained value 12.0 at.% Si extrapolated fro the A2 phae wa accepted a the equilibriu copoition of the phae. Figure 10(a how a BSE iage of a 8Si/23Si (at.% diffuion couple heat-treated at 800 C for 336 h. The oberved difference in contrat of the iage correpond not to copoition difference uually een in the BSE iage but to the channeling effect due to grain orientation. According to the reult of copoition analyi hown in Fig. 10(b two kind of inter-phae boundarie were identified. One i the icibility gap between the and D0 3 phae and the other correpond to the A2/ econd-order tranition boundary. At the latter boundary an abrupt change of the lope of the profile without copoition difference o-called ingular point appeared. Thi wa caued by the difference of the interdiffuion coefficient due to a tep of the therodynaic factor between different ordered tructure The equilibriu copoition were then deterined to be 15.1 and 15.8 at.% Si for the and D0 3 phae repectively and the tranition copoition fro A2 to wa deterined to be 12.5 at.% Si a hown in Fig. 10(b. Figure 11(a and 11(b how copoition profile of 13Si/23Si (at.% diffuion couple heat-treated at 900 C and C for 168 h repectively. Only one icibility gap wa obtained at 900 C fro which the equilibriu copoition were deterined to be 16.5 and 17.0 at.% Si for the and D0 3 phae repectively. No ingular point wa oberved becaue the A2/ tranition boundary ee to be le than 13 at.% Si at 900 C i.e. outide the copoition range of the 13Si/23Si diffuion couple. At C two ingular point without a copoition gap were identified which ugget that the equilibria aong phae at C conit of A2/ at 15.2 at.% Si and /D0 3 at 18.4 at.% Si econd-order tranition without a icibility gap. Obtained reult are uarized in Table 2 and plotted in Fig Therodynaic Aeent 4.1. Pure Eleent and Interetallic Copound Phae The Gibb energie of the table and etatable tructure for the pure eleent are taken fro the SGTE databae copiled by Dindale. 12 In the Si yte three diordered olution phae the liquid α (A2 and γ (A1 two ordered phae α Si ( and α 3Si (D0 3 five Fig. 10. (a BSE iage and (b copoition profile of Si of 8Si/23Si diffuion couple heat-treated at 800 C. Fig. 11. Copoition profile of 13Si/23Si diffuion couple heat-treated at (a 900 C and (b C ISIJ 544

6 ISIJ International Vol. 52 (2012 No. 4 interetallic copound (IMC phae 2Si 5Si 3 (D8 8 Si (0 αsi 2 and βsi 2 and Si (A4 are conidered to contitute the equilibriu phae. Si wa conidered to have no olubility of. All the IMC phae were aued to be toichioetric copound without olubility range. Then the Gibb energy of thee IMC phae were decribed a G... (1 n G n Sin A2 n G A4 G f Si Sin +Δ + f where ΔG i the Gibb energy of foration per ole Sin of ato of the Si n copound and decribed by the following equation.... (2 The Gibb energy decription of the other five olution phae are given below and evaluated therodynaic paraeter are lited in Table 3. All the evaluation and calculation were carried out uing Thero-Calc oftware Diordered Solution Phae The liquid γ (A1 and α (A2 phae are odeled a ubtitutional olution and the Gibb energy without the agnetic contribution i decribed by the ub-regular olution approxiation. Table 3. Therodynaic and agnetic paraeter of the Si yte evaluated in thi tudy. Lattice tabilitie of pure eleent are taken fro reference 12 and the agnetic propertie of diordered olution phae γ (A1 and α (A2 are fro reference. 8 Therodynaic paraeter [J/ol] and agnetic paraeter [K] [μ B ] Liquid : ( Si 0 L γ (A1 : ( Si 1 (Va 1 α (A2 : ( Si 1 (Va 3 α ( : ( Si 0.5 ( Si 0.5 (Va 3 α (D0 3 : ( Si 0.25 ( Si 0.25 ( Si 0.25 ( Si 0.25 (Va 3 2Si: ( 0.667(Si Si 3: ( 0.625(Si T Si: ( 0.5(Si T α Si 2:( 0.333(Si f ΔG A+ BT Si T 1 L L Si n T 2 L L Si γ T 1 γ γ T γ C 201 x β γ 2.1 x 0 α T 1 α α T α 1043 x x x ( x x β α 2.22 x x x C Si Si ΔG Si ΔG 3Si ΔT csi ΔT c3si Si 200 Δβ Si f ΔG A+ BT Sin +65 Δβ 3Si G G x + GSi xsi + RT( x lnx + xsi ln xsi + x xsi LSi... (3 G G Si where and are the Gibb energie of pure and pure Si repectively in the correponding tructure of. x and x Si are the ole fraction of and Si repectively. L Si repreent the interaction paraeter which ha a copoition dependence in the for of the Redlich-Kiter (RK polynoial decribed by the following equation:.... (4 Teperature dependent coefficient n L Si are evaluated on the bai of the experiental data on phae boundarie and thero-cheical propertie. In addition to the Gibb energy decribed by Eq. (3 the agnetic contribution to the Gibb energy G ag which wa originally uggeted by Inden and odified by Hillert and Jarl i appended in the fcc phae and phae A2 and D0 3. Thero-cheical propertie of diordered olution phae and other phae boundarie liquidu and olidu 1920 γ-loop 2122 and IMC phae 2324 were alo taken into account to evaluate the therodynaic paraeter hown in Eq. (2 and ( Ordering Contribution to the Gibb Energy After the Gibb energy paraeter of a diordered A2 phae G decribed in the regular olution approxiation are evaluated the ordering contribution due to the ( x tructure i ipleented by the plit copound energy forali (-CEF 25 a follow: A2 (1 (2 (1 (2 G G ( x + G ( y y G ( y x y x... (5 (1 (2 where G i the Gibb energy decribed in the twoublattice copound energy odel and G ( y x y x ( y y (1 (2 (1 (2 i a diordered coponent of G ( y y. x i the ole ( fraction of an eleent and y n i the ite fraction of the eleent on the n-th ublattice. In thi forula only the ordering contribution i extracted fro the Gibb energy of (1 (2 the two-ublattice decription G ( y y by ubtracting that of the diordered tate n n L ( x x L Si Si Si n (1 (2 G ( y x y x. In thi paper the ordering contribution of the tructure ΔG Si i uppleented o a not to have the diordered coponent (1 (2 i.e. under the condition of G y x y x a ( 0 follow: G :... ( ( (6.3 A unified forula can then be applied to decribe the Gibb energie of both A2 and in the -CEF a follow: G 0 Si:Si G G ΔG :Si Si: Si L L L LSi:Si ΔG Si 0 :Si Si: Si:Si β Si 2: ( 0.3(Si ISIJ

7 ISIJ International Vol. 52 (2012 No. 4 G G x + GSi xsi 1 + x x { L + L ( x x + L ( x x } 2 2 Si Si Si Si Si Si 2 1 ( n ( n ( n ( n RT ( y lny ysi ln ysi 2 n 1 (1 (2 y yt G:t t (1 (1 (2 (2 (2 (1 y ysi yt LSi:t + y ysi y L:Si t Gag (7 where and t repreent either the or the Si ato Four-ublattice Decription The four-ublattice decription i applied to uppleent the ordering contribution of the D0 3 and tructure to the Gibb energy of the le ordered phae. Two-ublattice paraeter hown in Eq. (6.1 (6.3 were converted to equivalent four-ublattice paraeter by ubtituting (1 I II (2 III IV y and and the paraeter ( z + z /2 yt ( zt + zt /2 of the D0 3 ordering contribution and and ΔG 3Si ΔG Si3 thoe of the tructure ΔG 2Si2 were uppleented o that thee contribution diappear in the configuration I II (1 where z z y and III IV (2 z z y a follow: G ::: t... ( (8.2 G G G 1 :::Si ::Si: :Si:: GSi::: Si+ 3Si 4 ΔG ΔG... (8.3 t G 0 Si:Si:Si:Si G G ΔG ::Si:Si Si:Si:: t Si G G :Si:Si:Si Si::Si:Si G 1 Si:Si::Si GSi:Si:Si: Si + 4 ΔG ΔG Si3... (8.4 G G :Si::Si :Si:Si: G Si:::Si GSi::Si: L L L ΔG2Si2... (8.5 Si::: :Si:: ::Si: :::Si LSi:Si:: LSi:Si:: L::Si:Si L::Si:Si 1 ΔGSi 4 ΔG 3Si... (8.6 L L L L Si:::Si :Si::Si :Si:Si: :Si::Si LSi::Si: L:Si:Si: LSi::Si: LSi:::Si LSi:Si::Si LSi:Si::Si L:Si:Si:Si L:Si:Si:Si LSi:Si:Si: LSi:Si:Si: LSi:F e:si:si LSi::Si:Si 1 1 B ΔGSi ΔG 32 2Si (8.8 Then the Gibb energie of the phae in the Si yte are decribed in the -CEF with the ordering contribution of the and D0 3 tructure a follow: G G x + GSi xsi 1 + x x { L + L ( x 2 2 Si Si Si Si Si Si IV 1 n n n n RT ( z lnz zsi ln zsi 4 n I I II III IV zz tzvzw G:: t v: w t v w I I II III IV zzsi zt zv zw LSi: t: v: w t v w II II I III IV zzsi zzv zw L:Si: v: w v w z z z z z L III III I II IV Si t w ::Si: t w t w IV IV I II III + z zsi zztzv L:: t v:si t v +Gag... (9 where t v and w repreent either the or the Si ato and z n repreent the ite fraction of an eleent on the n-th ublattice. Paraeter of the Curie teperature and the agnetic oent can alo be forulated in -CEF which conit of the diordered coponent and the four-ublattice ordering contribution in the ae way a with the Gibb energy. The deviation of the Curie teperature and the agnetic oent due to the ordering fro thoe of the A2 tructure were taken into account by introducing ΔT csi and and and which correpond to ΔG Si Δβ Si and ΔT c2si 2 Δβ 2Si 2 ΔG 2Si2 in the above equation repectively. L x + L ( x x } L L L Si::Si:Si :Si:Si:Si Si:Si:Si: Si:Si::Si 0 LSi:Si:Si:Si L Si:Si:Si:Si LSi:Si:Si:Si LSi:Si:Si:Si 1 ΔGSi 4 ΔG Si 3... (8.7 L Fig. 12. Effect of and D0 3 ordering on the Currie teperature of Si alloy ISIJ 546

8 ISIJ International Vol. 52 (2012 No. 4 Fig. 13. Calculated phae diagra of the Si binary yte evaluated in thi tudy. The deviation of thoe agnetic propertie of two kind of D0 3 tructure fro thoe of the tructure can be alo introduced by ΔT c3si Δβ 3Si ΔT csi 3 and Δβ Si3 which alo correpond to ΔG 3Si and ΔG Si 3. The agnetic contribution to the Gibb energy G ag i given a a function of teperature the Curie teperature and the agnetic oent of an alloy and in the ordinary odel both the Curie teperature and the agnetic oent are forulated a function of the copoition of alloy. Therefore once a copoition and a teperature are given the agnetic contribution to the Gibb energy can be calculated uniquely. On the contrary the Curie teperature and the agnetic oent which are decribed in the four-ublattice decription in the preent tudy depend not only on the copoition but alo on the degree of ordering in the cae of ΔT c 0 and Δβ 0. Thi ean that G ag varie depending on the degree of ordering even in the ae copoition. A a reult the interaction between the cheical and the agnetic ordering occur Aeent of Paraeter of the Ordering Contribution Since paraeter of the A2 tructure decribed in the regular olution approxiation have already been deterined the other paraeter of the ordering contribution of α Si ( and α 3Si (D0 3 copound ΔG Si ΔT csi Δβ Si ΔG 3Si ΔT c3si and Δβ 3Si need to be evaluated baed on the experiental reult and data in the literature. Other ordering contribution due to 2Si 2 ( and Si 3 (D0 3 were aued to be 0 becaue no inforation on the tability of thee phae i available. ΔG Si and ΔG 3Si were evaluated to reproduce the phae diagra hown in Fig. 7 and other experiental data. The effect of and D0 3 ordering on the Curie teperature of the phae wa evaluated experientally a hown in Fig. 12 in which olid circle and double circle repreent the Curie teperature of the table (D0 3 and eta-table (A2 & configuration repectively. The Curie teperature of the A2 A2 T c and T c of the 16%Si and 24%Si (at.% alloy in which the A2 and are eta-table at each condition were etiated by extrapolating the T c A2 and T c of the Co Si ternary alloy to thee binary Si alloy. 26 and were evaluated to reproduce the devia- ΔT csi ΔT c3si Δβ Si tion of the Curie teperature hown in Fig. 12. and Δβ 3Si were alo taken into account by auing thee value deviate by the ae ratio a the Curie teperature. All thee paraeter are uarized in Table 3. Figure 13 how calculated phae diagra of the Si binary yte in which not only the but alo the D0 3 ordered tructure were taken into account. Abnoral icibility gap in the liquid phae at high teperature which wa obtained in the previou therodynaic aeent 8 diappear by optiizing the paraeter of liquid appropriately. At high teperature both the A2/ and /D0 3 econd-order boundarie eet the olidu curve. The lope of the olidu a well a liquidu curve then change gradually. Such an anoalou hift of the equilibriu copoition i alo confired in the experiental phae diagra hown in Fig. 1 which i caued by the ordering contribution due to the and D0 3 tructure and explained quantitatively by therodynaic calculation. 5. Dicuion on Micibility Gap between Bcc Phae The icibility gap between phae in the Si binary yte wa deterined by two-phae alloy below 650 C and diffuion couple above 700 C. A lited in Table 1 and 2 even though the 15.5 and 17. at.%si alloy heat-treated at 700 C and 800 C repectively are located with in the + D0 3 two-phae region deterined by the diffuion couple ethod no two-phae tructure could be obtained. It wa uggeted by Schlatte and Pitch 7 that the driving force for the precipitation of the econd phae in thi + D0 3 equilibriu i quite all epecially at higher teperature which ight caue the retardation of the decopoition between the and D0 3 phae at thee teperature. In the diffuion couple ethod an inter-phae boundary with a copoition difference exit ince two alloy were initially bonded. Therefore the decopoition reaction during the heat treatent i not necearily inevitable. According to the reult of the electron diffraction hown in Fig. 8 the obtained two phae were identified to be the and D0 3 uper-lattice tructure. However the experiental and calculated phae diagra hown in Fig. 7 and 13(b repectively ugget that the A2/ tranition boundary eet the + D0 3 icibility gap jut below 700 C. The two-phae equilibriu then change to the A2 + D0 3 one below that ISIJ

9 ISIJ International Vol. 52 (2012 No. 4 teperature. Therefore the oberved uper-lattice reflection of the (100 hown in Fig. 8(b ee to be caued by the configuration ordered during quenching of the A2 + D0 3 two-phae alloy. 6. Concluion The icibility gap between phae α (A2 + α 3Si (D0 3 at 600 C and 650 C and α Si ( + α 3Si (D0 3 at 700 C 800 C and 900 C were deterined by two-phae alloy and diffuion couple repectively by FE-EPMA whoe patial reolution of quantitative analyi i le than 0.5 μ at an accelerating voltage of 6 kv. Contrary to the exiting phae diagra the horn-haped + D0 3 two-phae region extend above 700 C along the / D0 3 econd-order tranition boundary and cloe below C equilibriu relation of which are conitent with the conventional BWG calculation. The A2/ and /D0 3 econd-order tranition boundary wa alo deterined fro the ingular point of the diffuion couple which agree well with the reported phae diagra. Four-ublattice plit copound energy forali (-CEF wa applied to calculate the Gibb energy of the phae α (A2 α Si ( and α 3Si (D0 3 and copletely new therodynaic paraeter of the Si binary yte were evaluated. Equilibriu relation in the binary yte were well reproduced epecially the effect of the and D0 3 ordering on the liquidu and olidu curve and the icibility gap between phae. REFERENCES 1 O. Kubachewki: Phae Diagra of Binary Iron Alloy ed. by H. Okaoto ASM International Ohio (1993 p G. Schlatte and W. Pitch: Z. Metallkd. 66 ( G. Inden and W. Pitch: Z. Metallkd. 62 ( G. Inden and W. Pitch: Z. Metallkd. 63 ( H. H. Ettwig and W. Pepperhoff: Z. Metallkd. 63 ( G. Schlatte G. Inden and W. Pitch: Z. Metallkd. 65 ( G. Schlatte and W. Pitch: Z. Metallkd. 67 ( J. Lacaze and B. Sundan: Metall. Tran. A 22A ( B. J. Lee S. K. Lee and D. N. Lee: CALPHAD 11 ( C. G. Schön and G. Inden: Acta Mater. 46 ( R. Kainua I. Ohnua and K. Ihida: Method for Phae Diagra Deterination ed. by J.-C. Zhao Elevier Oxford UK ( A. T. Dindale: CALPHAD 15 ( B. Sundan B. Janon and J. O. Anderon: CALPHAD 9 ( F. Woolley and J. F. Elliott: Tran. Metall. Soc. AIME 239 ( H. Sakao and J. F. Elliott: Metall. Tran. A 6A ( Y. M. Gertan and P. V. Gel d: Ru. J. Phy. Che. 36 ( K. El Khaan K. Abdel -Aziz A. A. Vertan and A. M. Saarin: Ru. Metall. 3 ( E. C. Fillipov: Izv. Vyh. Uchebn. Zaved. Che. Metall. 5 ( E. Übelacker: C.R. Acad. Sci. 261 ( E. Schurann and U. Hengen: Arch. Eienhüttenwe. 51 ( W. A. Ficher and R. Uberoi: Arch. Eienhüttenwe. 33 ( E. Übelacker: Me. Sci. Rev. Metall. 64 ( W. Köter and T. Gödecke: Z. Metallkd. 59 ( E. Wachtel and T. Mager: Z. Metallkd. 61 ( I. Anara N. Dupin and B. Sundan: CALPHAD 21 ( N. Kaiya I. Ohnua R. Kainua and K. Ihida: unpublihed work ISIJ 548