EFFECTS OF TEMPERATURE GRADIENT ON LAMEL- LAR ORIENTATIONS OF DIRECTIONAL SOLIDIFIED TiAl BASED ALLOY

Transcript

1 Ù 46 Ù 10 Vol.46 No Å 10» Ù ß ACTA METALLURGICA SINICA Oct. 010 pp Ä¾ TiAl Ä ËÂ Ï Ê ( ¹Â Â Å ² Û ØÑ, ¹ ) Ï Æ Ñ ¾ 1580 É 1650 Ti 47Al Cr Nb Ì ² ÆÑ ¾ Ô. ÜÅ, Ì Á À» β À, À, ¾ β Ð À β À Ç, 1 ÆÑ, À 40 K/cm Í 160 K/cm, ß «ÓÝÁ Ð»ÅÓÝÁ. ¾ Å ß ÃÑ², γ ÆÑ ²., ¾ 1 mm/min Ñ Æ,» 40 K/cm (» 1580 ), Æ 74 Ñ Ó Å Æ¹ 45 Ñ ; ³ Í 160 K/cm (» 1650 ), Æ 74 Ñ Ó Æ¹ 90 Ñ. ÆÜ, ¾ Ù β Á ºÆ Õ 110 β Æ, ³ β Á Õ 110 β Æ µ, 001 β Ô ÆÁ ½À. ² TiAl ÑÌ, Æ, Æ, TG13.3 Á ½ A (010) EFFECTS OF TEMPERATURE GRADIENT ON LAMEL- LAR ORIENTATIONS OF DIRECTIONAL SOLIDIFIED TiAl BASED ALLOY XIAO Zhixia, ZHENG Lijing, YANG Lili, YAN Jie, ZHANG Hu Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing, Correspondent: ZHANG Hu, professor, Tel: (010) , zhanghu@buaa.edu.cn Manuscript received , in revised form ABSTRACT Directional solidification experiments under heating temperatures of 1580 and 1650 were performed on Ti 47Al Cr Nb alloy in order to obtain the evolution of lamellar grains. From microstructural analysis in the mushy zone of directional solidified ingots, β phase was firstly solidified, then phase was formed through peritectic reaction; phase depended on the pre existed β phase on which it nucleated, and only one of the 1 orientation variants was selected during solid state β transformations. As the average temperature gradient of the mushy zone was increased from 40 K/cm to 160 K/cm, the solidification interface morphologies were changed from columnar dendrite to celluar dendrite. Eliminating the influence of cutting plane to the γ lamella orientation, it was shown that the columnar lamellar grains with an angle of approximately 74 to growth direction gradually overgrew the ones with the angle of nearly 45 to the growth direction at the withdrawal rate of 1 mm/min and temperature gradient of 40 K/cm. Increasing temperature gradient to 160 K/cm, the grains with the angle of about 74 progressively rejected other with the angle of nearly 90 to the growth direction. Calculation of the β dendrite preferred growth orientation indicated that β dendrites tend to grow along the 110 β orientation at the present solidification experiments. Increasing temperature gradient, the preferred growth tendency of β dendrite became more drastically, other orientations, such as the 001 β directional orientation, could rapidly be replaced. KEY WORDS TiAl based alloy, directional solidification, lamellar orientation, preferred growth * Õ Áµ : , Õ µ : : ÌÌ,, 1984 «, ± DOI: /SP.J

2 14 Ù Ù 46 TiAl ÒÍ ÖÝ Ê Û ÄÕ Ü, Ã Ð Ø± Ô ÖÐ Ô Ð Ò º [1 3], ÖÒ Ò ÓÏÕ. Å ÝÆ TiAl ÒÍ PST(polysynthetically twinned) ÒÒ Đ [4,5], µđ¼æêè Ç, Þ, Ò Ó ÏÕÛ ¼ 0; ÆµĐ¼Æ Ç, ÊÒ ÓÏÕ ÞÐÝÍ. Î Ü Ô¼ ĐÇ Æ ÝÆ, ÂĐÇÆ ÒÖ [4,6 9] Þ Ê [10 13], Ô± Ç Ç, ³ Æ±É TiAl ÒÍ ÒÒ. Kim Õ [1] ¼ TiAl Í Á±, ÅĐÇÆ β Á ĐÁ Ä, ÖÕ 001 β Ç, β ÁÒ (001) Ç ÁØ β ÁË Ä, Ê β ÁÄ ĐÒ Ç ((110) β /(0001) ) Ç Ç ÅÜ ¼ Đ ÐÒ Æ (HRS) ĐÇ Æ, Ti 44Al Í Đ ¼ 1600 ÀÇ ¼ 90 mm/h ÒÆ β Â ÝÆ, Ò γ Ç 0 45, Í Ò Ï. Saari Õ [13] Í Ti 47.5Al W 0.5Si Ê Ti 46Al Mo Nb ³ Ê Cu Ê Ò Æ ĐÇÆ Ú. Đ ¼ ÀÇ ¼ 1 1 mm/min Ò ÀÇº 140 mm, Í β Â Æ, Ô Ä Ç Ç 0 90 Ò, ¹Ô 0 Ò, ÆÔ ß 90 Ò ; ÇÒ Æ Ô Ò Ö, Æ Ç 90 Ò ÝÆÊÍ ÁØ«. Jung Õ [10] Ti 47Al W Í ĐÇÆ Ò, ÕÀÇ 30 mm/h ĐÎ 90 mm/h, β Â Ò Ç [001] β ¼ [111] β, Ç Ç, ¼ Ç , ÇÒ Æ Í µ, ÞØ Æ ( Ê ) Ò Ä. Æ¹Å Ç 45 Ò ÝÆ, Ö Ç, Õ Ç Ò Ñ, Ò Ý Æ, Ï Đ Ç ÇÒ. Î, Ti 47Al Cr Nb Í ( Ï¼ 47 ), Đ ÛÊ ĐÇÆ. Ó ÄÒ, Ò Å ÇÒ Ä, Â Ò ÇØ ÔÍ, ¼ Ç ½. 1 È º 1.1 Å É Í «( ß, %) ¼ Ti 47Al Cr Nb, ¼ Ë ¼ 99.76% Ò Ú Þ Ti 99.99% Ò Al 99.98% Ò Cr Ø 99.90% Ò Nb. ØÅ Ý Ð, ÛÀ ¼ 0.05% 0.06%( ß). È½¼ 13.5 mm Ò Đ¼ĐÇÆ Í. Ú¼ Y O 3 ² Ò Al O 3 Í, µ ½ 0 mm, ½ 15 mm, 40 mm. ĐÇÆ Ô ÒÛÀ ¼ 0.090% 0.095%. ĐÇÆ Ú Bridgman ĐÇÆ Ð ³, Ú À Î Pa, ½ Ë Ar Î 0.05 MPa. Î 1580 Ê 1650 TiAl Í 15 min, 1 mm/min Ò ÀÇ 10 mm, À Ô, Ã Ga In Sn Í Đ, /Đ Á ÝÆ. ¼ Ô Á ÝÆÒÂ Ý ÄĐ ¼ 1580 Ê 1650, Á Ò G º¼ 40 Ê 160 K/cm. Ô Ö Ç, ĐÇ ÔÜÒ,», 1 ml HF+3 ml HNO ml H O ÍÀĐ, Á ¹¼ (OM) Ô Ò ½Ã. ¼ Æ» Ç ÈÒ², ÖÊÈ Ô Ò Ç Ç½Ï, ¼ Á Ò ÁÈÍ, Û Ë³ Ô Ò Ç. ¼Ò Ç, ¼ JXA 8100 Ý (EPMA) Á Ò. Å Ô ĐÇ Ò¹ ÝÆ, Â Ç À Ç ÇÄ Ò Ê 10, Ò Ç Ê ÀÇ Ç. 1. β µæîí Å± /Đ Á ÝÆ Ã, Ti 47Al Cr Nb Í ĐÇÆ β Â Æ. β Â Á, Ê ÎÒ Þ Ê γ Á, Î Á,»Â β Â Ò Ç È Ä. Î Jung Õ [10] ÄÒ, γ ÇÒ, Ý Ä fcc Ö ÇË γ ÁÒ ÇÊß (γ Á¼ fcc ); ½ TiAl Í L1 0 γ, hcp Ê bcc β ÁÄ {110} β /{0001} /{111} γ Ê 111 β / 11 0 / 110 γ Ç [10,11,14], fcc Ò 111 γ Ê 110 γ Ø bcc Ò 110 β ( / 111 γ ) Ê 111 β ( / 110 γ ) ¼ Ý½ Ç (β Á¼ bcc ),, fcc ÇË γ ÁÒ ÇÊß (γ) Ñ bcc (β), Æ bcc Ö ÇËÒ ÇÊß Ù¼ β Â Ò Ç. Ý Â : (1) Ö ĐÇ Ú Ò γ Ç ÇÒ θ, Ù γ Ò {111} γ ÇÒ, Ñ¼ γ Ò 111 γ Ç ÇÒ, Ù ϕ=90 θ, ¼ Ç [15]

3 Ù 10 ËË Ó : ÐÅ TiAl ÐË ÅÐ Â 15 cos ϕ = h 1 h + k 1 k + l 1 l (h 1 + k 1 + l 1 ) (h + k + l ) (1), h 1 k 1 l 1, h k l ¼ ÇÒ ÇÊß. h 1 k 1 l 1 ¼ 111 γ, h k l ¼ ¾ ÇÒ ÇÊ ß, ϕ ¼ ¹ ÇÒ, Ú. Ü, Ã h 1 k 1 l 1 Ê ϕ Ò³, (1) Ä h k l Ò ÇÊß, Ù γ Á fcc Ö ÇË Ò Ç Êß hkl γ ; () ¼ (1) Ý Ä hkl γ Ç γ ÁÒ fcc ¹½ Ç 111 γ Ê 110 γ Ç Ò σ 1 Ê σ ; (3) Î hkl β Đ¼ β Â Ò Ç, ¼ (1) Ý β ÁÒ bcc ¹½ Ç 110 β Ê 111 β ÇÒ ω 1 Ê ω ; (4) σ 1 Ê σ ω 1 Ê ω Á Ò σ 1 Ê σ, ω 1 Ê σ 1 Ø ω Ê σ, Å Ê Â Ë ÇÒ ( Ú ¼ 10 ), ¼ hkl β β Â Ò Ç. È³.1 µ¹ Å ± 1 ¼ 40 Ê 160 K/cm Đ ÇÆ Ô Û Ò ÝÆ. ÖÕ Ç ¼ 5 ¹ ±: ½ (I) Õ Ñ¼ Ô Ò (II) Ô ĐÇ (III) /Đ Á (IV) Ê ĐÁ (V). ĐÇÆ Ú, Ô µ Ã Ga In Sn Í Đ, (I ) Í Ò ÖÊÝÆ. II ÀÇ³½ Ê ÝÆ Ò, ½ Ò ÝÆ ¼³ ÇÒÕ, ÀÇ ÎÕ Ê ÈÒ, Ô. ³Ã III, ĐÒÆ, Ò Ô «Æ Å ÇÒ,, III Ë ( IV ), ±¾ Ì ÒÖ Ç Ò Ô. Á (IV) Ò Đ.. Å Å ÅÇ ± a e»ä G=40 K/cm, Ô ĐÇ Ò Á, ĐÇ III (± 1a È) Û 4 mm ¹Ü ³, ± Ë ÒßÙ Á γ ÇÒ (deg)., Ç 74 Ò Ô Õ ß³ Đ, Ç 47 Ò «Ê; ĐÇ Â ¹ Çº Ò (± a);» 4 mm Ò, ± a Ç 41 Ò Æ (± b); Þ ³ 4 mm, ± Û¹³ ÇÒ, Ç 3 Ò¹ Ô Ò Æ (± c);» º 96 mm Ò, ĐÇ ß Ç 74 Ð Ò Æ Å Çº 45 Ò (± e). ± f i»ä G=160 K/cm Ô ĐÇ Ò Á., Â Þ ¹³ ÇÒ, G=40 K/cm ³ Ò, ³± γ Ç Ê Ò, Â 0, Ä Ç 90 Ò (± f); ĐÇ 4 mm, Ç ÊÒ Ô (º 0 ) ÎÁ Æ, Û¹ Ò (± g). Þ, Ð Ç 74 Ò Ô µ Ç ÊÈÒ (Â 87 Ê 84 ), ¼ Ò (± h); ĐÇ º 7 mm, Ç 74 Ð Ò Ô Æ Å Ç º 90 Ò (± i). ± 3 Ä Ö± f Ç 0 Ò Ç ÒÔÜ ÁÝÆ., ĐÇÆ Ò ³, Î ÎÁÉ..3» TiAl Í ÒÆ Ä Æ Ñ½, ³Æ± Þ Ò γ Ç. ± 4 ¼³ Ô Á ÝÆ. G=40 K/cm, Á Ë Ï Ï Á ÊÈÒ β Â (± 4a), Á β Â ºÄ ÈÒ Á, Â ± 4c. G=160 K/cm, Ô Á β ÁÒ«Ô ÞÂ ÝÆ, ¹, Á Þ º β ÁÒ 1» 40 É 160 K/cm Ñ Æ Ó Ú ß ÜÅ Fig.1 Macrostructure of the directionally solidified ingots with temperatures gradients G of 40 K/cm (a) and 160 K/cm (b) (I unfused zone, II transition zone, III directionally solidification zone, IV solid/liquid dual phase zone, V liquid zone)

4 u h 16 u 46 9 D o ~NX;5&"XW {m"h> Fig. Metallographes of the transverse sections with different distances d from the position denoted by dashed lines in Fig.1a and b of directionally solidified ingots at G=40 K/cm (a e) and G=160 K/cm (f i) (numbers shown in Fig. mean the angles (deg) between the grain orientation and growth direction) n < m, n γ Y B V ' #:O n k,\ < L, # { Y Blackburn :O== {0001} //{111} R h11 0i //h110, \ ' #:O n /γ 6 9. >Æ, 3 bv? 7j ` : On /γ a 6'z^Y<:, B} 3 β,\ + Y β (5m ^, # p\ d 1 `, Æk L Ed ` :O n. Singh q D 0, 3r\PqRIY< qq8 *hn5 [3 næ< A, In:O::!! "\Pn β I n:o, Æ 3 I <LnI= k. YÆ<m 1DK, 3d Bk A,, I!F " n β I\PY<, V!:Otjd β I:OnFZ, 3 \ ^ 1 ` % `. β J, <a, z^ \ d - a 6 ', Æ ' #:O n 6'. +γ γ [5] [16] D 3 qqo $ h \#X; NA 0 m+[m 5 N +*\m"h> Fig.3 Metallograph of the section cut along the grain lamellar which orientation aligned at 0 angle to the growth direction in Fig.h fp IyN (% 4b). V, kl3sxa`5 A Ti 47Al Cr Nb U#nÆ<m1kI#n, u</ Vd L L+β L+β+ β+ n F E. 5 8a, dæ< 'r, Y<\ Yd jpzj,\ 0 pzj,\ n. 3 H>0 3.1 BÆN #[ β J YIÆ< n TiAl U#, /V β m ^, { Y Burgers :O== {11 0} //{0001} R h111i //h11 0i, - β,\ + D g\ 1 ` ' #:O n ; R<^, Y +γ β β [10] F A )-8MÆTK! TiAl U#Æp, M! J Y β Ir q h001i OY<, 07 Burgers :O==, Dz^\ $Bk O B 0 k 45 n /γ 6yN, ebv E γ 6$B k On==<# JV d. Bk!5 RHO q Æ< D % β I n 5 Y < O. Henry q 3 fcc 9n Al U# OÆ< E, 3(5 R(Æ< qa, nu#.o _Æd_ ( < k *s " q ) Y, Dd J, n 5 Y < O [001] O 0 [110] O. Jung q 0 A e 6Gl n OÆ<bvn5 E, 35 ( 3. β [17 19] Al Al [10]

5 Ù 10 ËË Ó : ÐÅ TiAl ÐË ÅÐ Â 17 4» 40 K/cm É 160 K/cm Ñ À ÜÅ Fig.4 Microstructures of the specimens in IV zones shown in Fig.1 with G of 40 K/cm (a), (c) and 160 K/cm (b) 300 K/cm, ÀÇ 30 mm/h ĐÎ 90 mm/h, β Â Ò Ç [001] β ¼ [111] β., Â β ÁÒ Ç 001 β, Ò Ò Ç ³½ ÍËÜ. β Á Ç Đ Þ Ò γ Ç, Î, 1. Ò ± Ä ÒÛ¹ Ò, Ý β Á Ò Ç, Â 1 Ê. Ò ± e Ê i Ä Ò, γ ÇÒ θ È. 1 Ý Ä γ Á fcc Ç hkl γ 111 γ Ê 110 γ Ç Ò σ 1 Ê σ ;» Ý, Å β Â Ö 001 β Ø 110 β Ç, Ý ºØ 110 β Ê 111 β ÇÒ ω 1 Ê ω ; 1 Ä ω 1 Ê ω Á Ò σ 1 Ê σ, ω 1 Ê σ 1 Ø ω Ê σ, Ò ÉÞ Ê, Þ ÉÊ 7, Ó Â Ç ÀÇ Ç 0 10 Ò, ¼ ω 1 σ 1 Ø ω σ, Ý Í. Ý Ã, Đ ¼ 1580, Ô ĐÇ Ë 001 β Ê 110 β ÇÒÂ. ± e Ã, Ö 001 β Ç ÒÂ ( Ç Çº 45 Ò ) Ò ³ÂÖ 110 β ÇÒÂ ( Ç Çº 74 Ò ) Ò Á. Đ, Đ ¼ 1580, 001 β ÇÁ, β Â ¾Á Ö 110 β Ç. Îµ, Ö 001 β Ç ÒÂ, Ê ÎÒ ½ Ç 0 Ò ÝÆ. ¼ Jung Õ [11] ÒÒ, 1580 ÒĐ Ò ±º 40 K/cm Ð, Ò Ç¼ 0 Ò ĐÒ. Æ, Đ Î 1650, Á Ò Đ, Þ½ Ç 0 Ò ÝÆ. Ã, Đ, Ô ĐÇ Ë β Â Ò Ç ¼ 110 β, ½Ä Ö 001 β Å Ç ÒÂ, ĐÇ Â Ù Ä Å ÇÒÂ ÞÎ Á Æ, Â± 3 Çº 0 Ò. Ý Ä TiAl Í Á Ò, Ä γ ÇÒÍ. TiAl Í γ ÁÊÂ β ÁÄ {110} β /{0001} /{111}γ Ê 111 β / 11 0 / 110 γ Ç [10,11,14], ÅÂ β ÁÒ Ç 001 β ¼ 110 β, Ú¼ γ Á β ÁÒ Đ, γ Ò ÇÞ Ä., ¼ 40 K/cm, Ô Ý Ö 001 β Ê 110 β ¹ Ç Ò β Â, Þ 110 β Ç, ÇÒ ¼ Ç 74 Ð Ò Ô «Æ Å Çº 45 Ò, ; Î 160 K/cm, Å ÇÒÂ ĐÇ Â ÎÁ Æ, Ô Ý Ö 110 β Ç Ò β Â, ÇÒ ¼ Ç

6 18 Ù Ù 46 1 ÆÊ γ ÀÑ Æ 111 γ É 110 γ Ñß Ü Table 1 Calculated angular relations between possible growth direction and the 111 γ, and 110 γ The grain with Angle between Possible orientation Set of angle Set of angle G different lamellar the growth index in between the growth between the growth K/cm orientations, θ direction and growth direction, direction and the direction and the the 111 γ, ϕ hkl γ 111 γ ( / 110 β ), σ γ ( / 111 β ), σ γ 15.79, 54.74, , 45, 76.37, γ 43.09, , 47.87, 63.44, γ 41.37, , 48.96, 66.80, γ 15.79, 54.74, , 45, 76.37, γ 10.03, 60.50, , 41.08, 81.33, γ 5.77, 64.76, , 38.43, 85.01, 90 ² β Á Ñ Æ Table Theoretical angles and estimated PGD (the preferred growth direction) of the β dendrites under different G The grain with Theoretical angle Theoretical angle Estimated PGD G different lamellar between PGD and between PGD and of the β K/cm orientations, θ the 110 β ( / 111 γ ), ω 1 the 111 β ( / 110 γ ), ω dendrite, hkl β grain β grain β grain β grain β grain β grain β 74 Ð Ò Ô µ Æ Å Çº 90 Ò Æ., β Â Ö 110 β Ç, 001 β Õ Å ÇÂ ¾Á Æ. Æ ÒÂ ÇÒ Đ Ò. ¼ Ê [0] : G v T 0 D L (), G ¼ /Đ ³ÖĐÁ, v ¼, D L ¼ĐÁÀ ÅÇ ß, T 0 ¼. Đ Á G, Á Ø /Đ ³ÖĐÁ G Þ Đ, Æ G v ( ÔÞÂ É, ÎÆ Ñ ¼«ÔÞÂ ), Ê ÇÒ Ä ± Â Ò Ç, ½ ºÇ ³ Ò 001 Ç (bcc ). Æ Õ [1] ¼ ĐÇÆ Ò Ò Tb- DyFe ÌÏÏ Í Ò Ç Ç ÉĐÒ È, ÀÇ Đ¼ 1 mm/min, G L 1000 K/cm Í «Ô, Ü Ç Ç¼ 110 Ç, Ö Îº 700 K/cm, «Â, Ü Ç Ç¼ 11 Ç, ³ µ Ö º 400 K/cm, ÒÞÂ, Ç 11 Ç Æ. β Á Ç Ò Ô Ý³ µ, Ú Ã, Å ½ß ³ÍÐ, β Á ³Ö 001 β Ç, Ò γ Ò Ç. Î, Ð Ò ½ßÂ ÊÀÇ, β ÁÖ 001 β Ç, Å Ç ¼ Ý. 4 ³¼ (1) Ti 47Al Cr Nb Í G ¼ 40 Ê 160 K/cm Ò ĐÇÆ Â Á¼ β Á, Á, β Ñ Á β Á È, 1 ± Đ Ç Ò. /Đ ÔÞÂ Ñ ¼«ÔÞÂ. () ¼ 40 K/cm, Ç 74 Ð Ò Ô «Æ Å Çº 45 Ò, ; Î 160 K/cm, Ç 74 Ð Ò Ô µ Æ Å Çº 90 Ò Æ. Â ÇÝ Đ, Ú

7 Ù 10 ËË Ó : ÐÅ TiAl ÐË ÅÐ Â 19, β Â»Ç Ö 110 β Ç, β Â Ö 110 β Ç, 001 β Õ Å ÇÂ ¾Á Æ. Á [1] Yamaguchi M, Inui H, Ito K. Acta Mater, 000; 48: 307 [] Dimiduk D M. Mater Sci Eng, 1999; A63: 81 [3] Kim Y W. JOM, 1995; 47: 39 [4] Yamaguchi M, Johnson D R, Lee H N, Inui H. Intermetallics, 000; 8: 511 [5] Kishida K, Johnson D R, Masuda Y, Umeda H, Inui H, Yamaguchi M. Intermetallics, 1998; 6: 679 [6] Johnson D R, Inui H, Muto S, Omiya Y, Yamanaka T. Acta Mater, 006; 54: 1077 [7] Lee H N, Johnson D R, Inui H, Oh M H, Wee D M, Yamaguchi M. Mater Sci Eng, 00; A39 331: 19 [8] Takeyama M, Yamamoto Y, Morishima H, Koike K, Chang S Y, Matsuo T. Mater Sci Eng, 00; A39 331: 7 [9] Lee H N, Johnson D R, Inui H, Oh M H, Wee D M, Yamaguchi M. Acta Mater, 000; 48: 31 [10] Jung I S, Oh M H, Park N J, Kumar K S, Wee D M. Met Mater Int, 007; 13: 455 [11] Jung I S, Jang H S, Oh M H, Lee J H, Wee D M. Acta Mater, 00; : 13 [1] Kim M C, Oh M H, Lee J H, Inui H, Yamaguchi M, Wee D M. Mater Sci Eng, 1997; A39 40: 570 [13] Saari H, Beddoes J, Seo D Y, Zhao L. Intermetallics, 005; 13: 937 [14] Sastry S M L, Lipsitt H A. Metall Trans, 1977; 8A: 99 [15] Pan J S, Tong J M, Tian M B. Fundamentals of Materials Science. 3rd Ed. Beijing: Tsinghua University Press, 00: 40 (, Ð, Ð. ¹ ÑÇ, Ú 3. º: ¼ Ã Í, 00: 40) [16] Singh A K, Muraleedharan K, Banerjee D. Scr Mater, 003; 48: 767 [17] Henry S, Jarry P, Rappaz M. Metall Trans, 1998; A9: 807 [18] Henry S, Jarry P, Rappza M. Metall Trans, 1997; A8: 07 [19] Henry S, Minghetti T, Rappaz M. Acta Mater, 1998; 46: 6431 [0] Hu H Q. The Principle of Metal Solidification. nd Ed., Beijing: China Machine Press, 000: 119 ( Á. Ú µì. Ú. º: Ó Ã Í, 000: 119) [1] Jiang C B, Zhou S Z, Zhang M C, Wang R. Acta Metall Sin, 1998; 34: 164 ( Æ,, Ù», Ä. Ú, 1998: 34: 164)