Lamellar orientation control in directionally solidified TiAl intermetallics

Transcription

1 Celebrating the CHINA FOUNDRY Lamellar orientation control in ally solidified TiAl intermetallics *Su Yanqing, Liu Tong, Li Xinzhong, Chen Ruirun, Ding Hongsheng, Guo Jingjie, and Fu Hengzhi National Key Laboratory for Precision Hot Processing of Metals, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin , China Abstract: TiAl-based alloys are potentially used as high-temperature structural materials with a high specific strength in the range of ~ 900 C. However, the mechanical properties of TiAl-based alloys are extremely anisotropic with respect to the lamellar orientation of the microstructures. A balance combination of room-temperature ductility and strength can be achieved when the lamellar orientation are aligned parallel to the tensile stress. Lamellar orientation control of TiAl-based alloys by al solidification technique has been widely studied in recent years. Two different al solidification processes can be used to modify the lamellar orientation. One is a seeding technique and the other is adjusting the solidification path. This paper reviews the principles of the two methods and their progress. The influence of alloy composition and solidification parameters on lamellar orientation control is also discussed. Key words: TiAl-based alloys; al solidification; crystal growth; lamellar orientation CLC numbers: TG Document code: A Article ID: (2014) H igh-temperature structural materials have been studied extensively in recent years due to the development of the aviation and aerospace industry. Whether there will be a thorough reform in aerospace propulsion system is determined by the development of the high temperature structural materials [1, 2]. Among these high temperature materials, advanced aluminides of transition metals such as titanium, iron, and nickel have received special attention due to their potential for high temperature applications. The research mainly focuses on A 3 and A typed compounds of Ti-Al, Ni-Al, Fe- Al, which possess lower densities, high melting points and considerable mechanical properties due to their ordered crystal structures. The advantages of nickel-aluminides are excellent oxidation, carburization resistance and wear properties at high temperature because of the alumina layer formed in the surface of the materials, but the low ductility and fracture is unlikely to be overcome in the near future. The iron-aluminides, FeAl and Fe 3 Al, are notable for their low cost, and FeAl is characterized by good resistance to catalytic coking, carburization, and wear, but the application of FeAl alloys is reduced due to lower strength at temperatures higher than 600, with environmental embrittlement a particular concern, and Fe 3 Al alloys are still being tested for applying in the field of corrosion resistance and oxidation resistance [3-7]. The intermetallic alloys based on titanium, in particular, Ti 3 Al and TiAl aluminides, have been widely examined and developed for high temperature application in the past two decades [8-10]. Figures 1 and show the changes of specific strength and * Su Yanqing Dr, Professor, working at National Key Laboratory for Precision Hot Processing of Metals, School of Materials Science and Engineering, Harbin Institute of Technology, China. His research focus is mainly on the solidification processing of nonferrous alloys. He has published more than 300 papers and has been awarded 22 invention patents of China. He also has been granted four scientific awards. Suyq@hit.edu.cn Received: Accepted:

2 CHINA FOUNDRY Celebrating the Specific strength ( MPa M g ) -1 m Mg AI Ti alloys TiAl alloys Superalloys Single crystal superalloys Refractory metals Specific Young ' s Modulus ( GPa M g ) -1 m IMI 834 -TiAl IN 625 IN Temperature ( K) Temperature ( C) Fig. 1: Specific properties for structural alloys and selected intermetallics: specific strength versus temperature, specific modulus versus temperature [11] the specific elastic modulus with temperature for TiAlbased alloys in comparison with Ni-based superalloys and Ti-based alloys [11], which reflect the obvious advantages of TiAl alloys. Most of the Ti 3 Al based alloys are developed into a Ti-Al-Nb system, and the typical Nb content varies from 10at.% to 24at.% for stabilizing the cubic and ductile β phase, so as to improve the room temperature elongation and high temperature oxidation [12-13]. However, the density of high Nb-Ti 3 Al based alloys increases with the Nb content, so to satisfy higher efficiency and lighter weight requirements of next generation of automotive and aircraft engines, the most promising intermetallics for turbine applications are -TiAl based alloys which are composed of phase and 2 / lamellar. Their advantages lie in low density ( g cm -3 ), high specific yield strength, high specific stiffness, excellent oxidation resistance and good creep property. ut the practical application is restricted due to the low ductility at room temperature, and the studies in recent years mainly focus on this problem. Great progress has been made and new technologies have been proposed as solutions, such as hot forming and al solidification [9, 14-19]. Properties of TiAl-based alloys significantly depend on their microstructures. Four typical microstructures at room temperature are divided among the copious microstructures that can be formed in TiAl-based alloys, as shown in Fig. 2. The fully lamellar microstructures consisting of TiAl (-phase) and Ti 3 Al ( 2 -phase) exhibit a good combination of room temperature toughness and elevated temperature strength, and the proportion of phase and 2 lamellae can be adjusted depending on the composition [20-21]. However, the mechanical properties of TiAl-based alloys are extremely anisotropic with respect to the lamellar orientation of the microstructures. A balance combination of roomtemperature ductility and strength can be achieved when the lamellar orientation are aligned parallel to the tensile stress [21]. Methods of controlling the lamellar orientation of TiAl-based alloys have been widely studied in recent years. Two different al solidification processes were used to modify the lamellar orientation. One is a seeding technique and the other is adjusting the solidification path. This paper reviews the principles of the two methods and their progress. The influences of alloy composition and solidification parameters on lamellar orientation control are also discussed. 1 Influence of lamellar orientation on mechanical properties of TiAl based alloys According to the binary Ti-Al phase diagram, lamellar microstructure is not formed from the liquid but from solid-state phase transformation, which causes difficulty in microstructural control by al solidification [23]. As shown in Fig. 3, the primary phase will be β phase when alloy composition locates in C1. The resulting microstructure consists of columnar grains with the lamellar boundaries oriented parallel and 45 to the growth due to the fact that the primary phase and the final lamellar microstructure follows the lackburn orientation relationship: {110} β //{0001} //{111}, <111> β // < 1120 > //<110>. However, phase is initially precipitated when alloy composition lies in C2, the preferred growth of phase is parallel to the <0001>, and thus the lamellar microstructures' orientation of the ingots are all perpendicular to the heat flow in conventional al solidification. Hence, to control the room temperature lamellar orientation, the orientation of the high temperature phase (primary phase) must be controlled. Study on the lamellar microstructure by polysynthetic twinned (PST) crystal shows that the mechanical properties of TiAl-based alloys such as strength, fracture toughness and ductility are all extremely sensitive to the orientation of the lamellar boundaries with respect to the loading axis [22]. As shown Fig. 4, the tensile properties of PST 220

3 Celebrating the CHINA FOUNDRY (c) (d) Fig. 2: Typical microstructures of TiAl-based alloys: full-lamellar microstructure, near-lamellar microstructure, duplex microstructure (c), single -phase (d) [21] β β-phase(cc) [0001] C2 L <100> C C2 C1 -phase(hcp) Ti (TiAl) (0001)/(110)β Al (111)/(0001) or Grawth (111)/(0001)β (c) Fig. 3: Solidification procedure of binary TiAl alloy and resulting lamellar: binary Ti-Al phase diagram, primary phase is β, (c) primary phase is [23] crystals are anisotropic at different angles (φ) between the loading axis and the lamellar boundaries. A good balance of strength and ductility can be obtained when φ= 0, where strength is not as high as that for φ= 90, but tensile ductility is as high as 5%-20% at room temperature. When φ is in the range 30-60, yield stress is much lower and elongation is higher than that for φ= 0 and φ= 90 [22]. This trend remains unchanged almost up to 1000 C [24]. Such anisotropic properties suggest that for a columnar grain material with the lamellar boundaries aligned parallel to the 221

4 CHINA FOUNDRY Celebrating the Yield stress ( MPa ) A Group-1 in compression Group-2 in compression Group-1 in tension Group-2 in tension D C N Tensile elongation (%) A C D Group-1 Group-2 N θ ( ) θ ( ) Fig. 4: Variation of yield stress and elongation of PST crystals of TiAl alloy with lamellae orientations at room temperature [22] growth, the material is expected to exhibit a balance combination of strength and ductility in a wide temperature range [14]. Two typical microstructures may be obtained from al solidification, one is PST crystal with all columnar grains having the same orientation, the other is all columnar grain rotated along its longitudinal axis, as shown in Fig. 5. ut the performance of PST crystal may show strong anisotropy and decrease due to the PST crystal consisting of a single unial lamella which leads to deviation between loading and lamellar interface. So the microstructures of the columnar grain rotated with its longitudinal axis [Fig. 5] are expected for this material, and the highest fracture toughness is obtained in ally solidified specimen with the lamellar orientation perpendicular to notch [25], as seen in Fig. 6. Thus, appropriate processing techniques have to be developed for aligning the lamellar boundaries parallel to the growth. The al solidification is developed to control the lamellar orientation in TiAl alloys like Ni-based superalloys. In addition, Liu J P et al. [26] controlled the angles between the lamellar interface and the sheet plane below 30 by the foil metallurgy method. However, the method of foil metallurgy can not control the lamellar orientation precisely, so al solidification is the key technique and an important development for TiAl-based alloys. Fig. 5: Schematic representation of ally solidified TiAl ingots with one orientation of columnar grains and rotation about longitudinal axis [1] 2 Directional solidification methods for lamellar orientation control in TiAl-based alloys Directional solidification (DS) methods are used to eliminate the horizontal grain boundary in cast alloys, and to dramatically enhance the strength, plasticity and creep resistance in the loading. For aligning the lamellar boundaries parallel to the growth in TiAl-based alloys, different al solidification methods are developed. The ram type PST PLD DS K IC (MPa m 1/2 ) K Q (MPa m 1/2 ) K Q (MPa m 1/2 ) Fig. 6: Schematic illustration of notch geometry with respect to lamellar orientation in bend test specimen for PST of binary TiAl alloy, PLD of Ti-43Al-3Si alloy and DS of Ti-43Al-3Si alloy [25] 222

5 Celebrating the CHINA FOUNDRY al solidification furnace is used first, as illustrated in Fig. 7. D. R. Johnson [27] aligned the TiAl/Ti 3 Al lamellar microstructures in Ti-47Al and Ti-46.5Al-3Nb-0.5Si by ram type al solidification furnace. However, this method is not used because of the low temperature gradient, and the temperature gradient decreases continuously with the distance between the melt and water cooling zone during the process of the ram type al solidification. The lower the temperature gradient, the coarser the grain obtained, leading to coarse microstructure after al solidification. Graphite heater Electrode Crucible Master ingot Seed Water cooled chill plate Fig. 7: Schematic of ram type al solidification furnace [27] Another commonly used method is optical floating zone technique for al solidification in TiAl based alloys, as shown in Fig. 8, which adopts emission ellipsoid lens focusing light emission from a halogen laser lamp or a xenon lamp as the source of heat. It is a potential method for growing rod-like crystals, and this method does not require the crucible; the melt is mainly supported by the surface tension. However, this method also brings some negative effects, such as serious segregation, small specimen diameter, simple specimen geometry and the difficulty in controlling crystal growth process, including growth rate and temperature gradient. In the early 1990s, Yamaguchi et al. [28-31] did a series of detailed studies and produced PST in TiAl alloys through al solidification by using an optical Reflector Xenon lamp Crucible Fig. 8: Diagram of al solidification process by optical floating zone technique [28] floating zone furnace in the laboratory, proposed a method of controlling lamellar orientation by seeding technique, and found that the Ti-43Al-3Si is a proper seeding material. Liu R H et al. [32] studied the al solidification process of Ti-43Al-3Si alloys in an optical floating zone furnace. The polycrystalline rods with appropriate microstructure are cut from cast ingot as seeds. The DS ingots with lamellar structure parallel to longitudinal axis are obtained at the growth rate of 5 mm h -1. Using a ridgman al solidification furnace with ceramic crucible, as shown in Fig. 9, is the most popular al solidification technique. The apparatus consists of a heating system (induction coil or resistor), a cooling system (liquid Ga-In-Sn alloy), an adiabatic zone which is located between the heater and the cooler and a mechanical transmission system. Fan J L et al. [33-35] successfully controlled the lamellar orientation parallel to the growth of the Ti-49Al and Ti-46Al-0.5W-0.5Si alloys in the ceramic crucible by ridgman type furnace under the solidification parameter of V = 20 μm s -1 and G = 12.1 K mm -1 (V is withdrawal rate and G is temperature gradient). Ding X F et al. [36-37] used the double DS technique to control the lamellar microstructure of ally solidified TiAl-Nb alloys by using the ridgman type apparatus. After the first DS step, the samples were turned 180 lengthwise and the DS was repeated under the same conditions as the first DS process. A well-aligned lamellar microstructure can be easily achieved if the alloy has a suitable composition. Inductor Al2O3crucible Graphite heater Adiabatic shield Water in Connecting link Thermocouple Ⅰ Thermocouple Ⅱ Thermocouple Ⅲ Water out Liquid Ga-In-Sn Drive unite Fig. 9: Schematic of ridgman type al solidification furnace [33] However, in previous processing in a ridgman type al solidification furnace, ceramic crucibles, such as Al 2 O 3 [38], CaO [23, 28] and Y 2 O 3 [34, 39] are used to hold the TiAl melt. Unfortunately, these refractory materials usually have a strong reaction with the molten TiAl-based alloys, leading to contamination of the melt. Among these ceramics, a Al 2 O 3 crucible is conventionally used for the DS of metals owing to its reasonable cost and relatively good service performance. However, the long-term interactions between alloy and crucible result in high oxygen pickup and ceramic inclusion contamination in the alloy matrix, leading to the deterioration of microstructures and mechanical properties of the resultant alloys, even when the process is carried out under ultra-high 223

6 Report CHINA FOUNDRY Special Celebrating the vacuum conditions. CaO has a strong hydration tendency which limits its practical applications although it is much cheaper and shows better thermo-chemical inertness. Y2O3 presents an excellent stability in contact with molten TiAl alloys. However, Y2O3 suffers from two drawbacks: inherently poor thermal shock resistance and high cost. Thus, common oxide crucibles/ moulds coated by an Y2O3 protective coating seem to be the most effective, stable and less expensive solution [40]. To avoid the contamination arousing from interactions between TiAl melt and ceramic crucibles, Fu H Z et al. [41, 42] in Harbin Institute of Technology put forward an effective method to fabricate highly reactive alloys with ally solidified structures on basis of a cold crucible technology, as shown in Fig. 10, which combines electromagnetic confinement of melt in an inductive water-cooling cold crucible. Ding H S et al. [43, 44] successfully fabricated DS Ti-Al and Ti-Al-Nb intermetallic ingots by electromagnetic confinement of melt within a water-cooling cold crucible. The studies on Ti-46Al-0.5W-0.5Si show that this alloy could be solidified completely via β phase precipitates preliminarily without the stabilizing when the growth rate is lower than 1.0 mm min-1, and exhibits a good combination of tensile properties at ambient temperature. Cold crucible Water Coil TiAl-based alloys is typical peritectic alloys, and the final microstructure of TiAl-based alloys is obtained by a solid-state transformation. So peritectic reaction and subsequent solidstate transformation play a very important role in the form of the lamellar orientation. Su Y Q et al[45] investigated the ally solidified TiAl microstructures near the peritectic reaction L+β in Ti-Al binary system, and found that there is competitive growth between the stable phase (β) and metastable phase (). They also calculated the phase selection map of TiAl alloys containing (44-50)at.% Al based on the criterion of the highest interface temperature, as shown in Fig. 11. Li X Z et al.[46] simulated the growth of peritectic phase along the primary phase using phase-field model for ally solidified Ti-Al alloy at a high value of G/V, and found that two typical microstructures of discrete β and island band are caused due to the difference of extending character of trijunction. The nucleation undercooling of peritectic phase and initial composition affect the extension of trijunction of ally solidified peritectic alloy and final microstructure. Ding X F et al. [47] calculated the peritectic reaction of primary β-phase in the al solidification process, also obtained a complete peritectic reaction when the alloy composition in hypoperitectic side and close to the peritectic composition point also at low temperature gradients and low growth rate. The complete peritectic reaction can eliminate the heterogeneity and solute segregation effectively and help obtain parallel lamellar orientation. Water Slit 1.40E+010 O 1.20E+010 Cross-section of cold crucible Fig. 10: Diagram of al solidification process by electromagnetic confinement of melt in cold crucible[44] 3 Difficulties in 2 + lamellar orientation controlling The efforts for controlling lamellar orientation were mainly focused on the basic conditions of equipment which ensure the macro process controllability and the maximization of temperature gradient. However, the lamellar microstructure is not formed from the liquid but from solid-state phase transformation, which causes main difficulty in microstructural control; and generally, the characteristics of the solidification path in al solidified TiAl alloy with respect to the high temperature alloys are neglected, such as peritectic reaction, the selection of primary phase and the competitive growth between preferred orientation and non-preferred orientation. Peritectic reaction has important influence on the solidification process and solidification path because 224 and structure βplanar 1.60E+010 G V-1 (Km-2 s) Water 1.00E+010 R N I P planar Q S H M L I 8.00E E+009 H G EF C D cellular/dendrite A 4.00E+009 βcellular/dendrite 2.00E E AI composition (at.%) Fig. 11: Phase selection map of Ti-(44-50)at.% Al alloys[45] Actually, there are three basic s in the crystal growth of al solidification, as shown in Fig. 12: the heat flow ( of maximum temperature gradient), the crystallographic preferred orientation in crystal growth and the crystal growth. The growth of crystal is constrained by the heat flow and preferred growth in al solidification, because the different parameters of al solidification such as growth rate, and the growth is changed between the of heat flow and preferred crystal growth. Also, the interface morphology can transit with increasing growth rate in the sequence: planar cellular cellular/

7 Celebrating the CHINA FOUNDRY Heat flow Growth θ Preferred growth Heat flow Growth Preferred growth ( b) Heat flow Growth Preferred growth (Solid) Fig. 12: A relationship among growth, preferred growth and heat flow [48] dendrite dendrite refine cellular banding planar at a given temperature gradient (G). Solid-liquid interface evolution process has significance in the al solidification, and the researchers obtain the solidification mechanisms by explore the solid-liquid interface. The interface morphology plays an important role in the process of al solidification. The influence of the interface morphology (planar cellular dendrite) on growth of primary phase has been studied in ally solidified Ti-46Al-0.5W-0.5Si (at.%) alloy [33]. When the solidliquid interface is a planar interface, the growth of -phase is affected by temperature gradient primarily. When the solid-liquid interface is mixed with the planar/dendritic cell, the growth of phase is controlled by the preferred orientation of phase and temperature gradient, respectively. When the solid-liquid interface is developed into dendritic morphology, the growth of phase is mainly controlled by preferred orientation. The result is owning to the competitive growth in different dendrite and anisotropic interface, as illustrated in Fig. 13. The formation of competitive growth mainly due to a dendrite is eliminated or blocked by new dendrites through dendrite branches. When the solid-liquid interface morphology transforms to cellular growth, most studies suggest that the cell () c Growth ( d) Heat flow Preferred growth Heat flow Preferred growth Growth Fig. 13: Relationship of growth of crystal, heat flow and preferred growth in different solid-liquid interface morphologies: planar, cellular, dendritic (c), three s are same (d) [49] crystal growth is mainly influenced by the heat flow. Dendrite growth is mainly affected by the crystal orientation, and it is widely accepted that the grain competition growth model is Walton-Chalmers model [50], as shown in Fig. 14. The growth of preferred orientation grains A 1 can block the non-preferred orientation of crystal grains at the interface in the convergence area, and Zhou et al. [51] obtained results that the non-preferred orientation can block the growth of preferred orientation A 1 due to the interaction of solute field in the growth convergence zone, as shown in Fig. 14. Fan J L et al. [52] studied the competitive growth of preferred orientation <0001> and non-preferred orientation of phase in Ti-46Al-0.5W-0.5Si by self-seeding technique, and found the preferred orientation <0001> grain expands continuously when the solidification conditions are in favor of the <0001> grain, then finally eliminates the non-preferred orientation grain. The competition and elimination in different orientation grains is completed by lateral movement of the grain boundary; the Gradient Gradient Liquidus Liquidus Z A Z Z A Z A Z Z A A 1 A 2 A 1 A 2 Fig. 14: Schematic diagrams showing grain competitive growth in al solidification process (grains A and are favourably and unfavourably oriented, respectively): Walton-Chalmers model [50], summary of experimental by results of Zhou et al. [51] 225

8 CHINA FOUNDRY Celebrating the elimination rate and competitive growth between preferred and non-preferred orientation in grain become more serious with the increase of growth rate. 4 Directional solidification of TiAl-based alloys by adjusting solidification path It is relatively easier to control the lamellar orientation by adjusting the solidification path because the primary β phase precipitation is the only requirement. Normally, we can get mixed lamellae including parallel or inclined 45 to the DS growth when β is the primary phase as shown in Fig. 3 [23]. To obtain a stable β phase growth, the general control of the alloy composition is Ti-(44, 45)at.% Al in Ti-Al binary alloys. However, in that case, the volume fraction of 2 phase is more than the phase in lamellar microstructure due to the low content of Al, and results in relatively lower high temperature properties such as oxidation resistance and strength. To obtain a high volume fraction ratio of / 2, the addition of β phase stabilizing elements such as Mo, RE, W, Nb, V, Cr, Ta etc pushes the β-phase region to Al rich side usually, also results in the liquid++β phase coexistence region appearing in the phase diagram [23, 53], as shown in Fig. 15. However, the solidification path becomes more complex with the adding of β phase stabilizing elements. So the primary phase being β phase during the solidification process is the key guarantee in lamellar orientation controlling, meanwhile, when phase nucleates from liquid, lamellae perpendicular to the growth may be obtained. Aligned lamellar microstructure can be produced by controlling the solidification path of TiAl±Mo alloys with the addition of a small amount of boron [14, 54]. The two reactions L Ti 2 +, L+ +Ti 2 could have occurred as shown in Fig. 16. oride particles nucleate between the primary β dendrites before the nucleation of phase, when phase continuous nucleates adhere to boride surface, then grains will accumulate after a very short growth distance and there will be competitive growth between the different oriented grains, as shown in Fig. 16. Then the grains growth parallel to <101-0> may be possible behind the columnar β dendritic front, resulting in the growth of columnar β phase followed by phase with growth parallel to <101-0> before the next solid phase transformation. However, the narrow process window of this approach and the high temperature gradient may also lead to the competing growth of -phase in the form of columnar with beta dendrites. Ding Xiangfei [14, 54] found that the addition of may bring difficulty in controlling lamellar orientation of the Ti-45Al-6Nb-0.3 alloy. L+β L L+β L++β L β L+ β L+ +β + L+ +β + L+ Fig. 15: Schematic phase diagrams of binary Ti-Al and ternary Ti-Al-M systems (M is a stabilizing element) [23] 15. β -dendrites < 1010 > ( at. %) Ti AI ( at. %) Columnar/equiaxed oride particles Fig.16: Schematic view of columnar growth of β and competitive growth of oriented equiaxed phase in Ti-46.5Al-1.5Mo-( ) alloy [54] 226

9 Celebrating the CHINA FOUNDRY Furthermore, crystal growth is restricted by heat flow, crystal preferred, and the morphology of solid-liquid interface in al solidification. Thus, with the change of the al solidification process parameters, the growth of the primary phase of TiAl-based alloys changes, resulting in a complex lamellar structure orientation. The values of primary dendritic arm spacing, secondary dendritic arm spacing and lamellar spacing decrease with the increase in growth rate are proved in the ally solidified Ti-46Al-2Cr-2Nb-0.2 alloy [56]. Jung et al [53] studied the microstructure of Ti-47Al-2W (at.%) alloy by al solidification, showing that the addition of W shifts the β phase field to the high Al composition side at liquid-solid temperatures. When the bcc β phase grows in the <001>, the {110}β planes have six variants of orientations as shown in Fig. 17 and. Two of them are parallel to the growth and four of them are inclined at an angle of 45 to the growth. Therefore, the probability that the lamellar orientation in the grains is parallel to the growth (A ) is 1/3, and the probability that the lamellar orientation in the grains is inclined with an angle of 45 to the growth ( orientation) is 2/3. However, the growth of A at the beginning stage of the DS ingots is annihilated gradually, then oriented grains have a dominant position. Eventually, a sample of the final solidified structure is entirely composed of crystal grains orientation, as illustrated in Fig. 18 (black lines indicate the lamellar orientation) [53]. Also, when the growth rate is increased from 30 mm h-1 to 90 mm h-1, the preferred growth orientation <001> of β-phase becomes non-preferred orientation <111>. Xiao Z X et al. [57, 58] studied the evolution of al solidified lamellar grains of Ti-47Al-2Cr-2Nb at several different temperature gradients. The morphologies of grains at solidification interface change from columnar dendrite to celluar dendrite as the temperature gradient of the mushy zone increases from 40 K cm -1 to 160 K cm -1. The growth orientation of β phase changes from <001> to <110>, resulting in the angle between the growth of the lamellar not only being 0 and 45, but also 74, 80 and 84. Hence, to obtain steady growth of lamellar orientation, parameters [100]β {110}β A [100]β 1 mm {110}β (c) 2 cm 1 mm 45 A 0 45 Fig. 17: {110}β plane variants in A orientation and orientation of bcc β phase and schematic microstructure of ally solidified Ti47Al-2W alloy (c) [53] 1 mm Fig. 18: Macrograph and micrographs of a part, b part and c part (c) taken from ally solidified Ti-47Al-2W alloy[53] 227

10 CHINA FOUNDRY Celebrating the stability must be first controlled for the growth of β phase. In all, it is a relatively easier way to control the lamellar orientation by adjusting the solidification path due to the simple operation of this method and suitablility for industrial production. The addition of β phase stablizing elements such as: Mo, W, RE, etc, brings difficulties for the melting and the solidification process because these elements are refractory with high melting temperature. 5 Directional solidification of TiAlbased alloys with seed materials Normally, TiAl-based alloys with an aligned lamellar microstructure have a good combination of strength and ductility over a wide temperature range. The different lamellar orientation can be obtained from different types of primary phase in TiAlbased alloys, and the primary phases are determined by the composition and solidification conditions of alloys according to the phase diagram of the binary Ti-Al alloys. The ideal lamellar orientation can be achieved when the primary phase growth is perpendicular to the preferred or parallel to <112-0>. However, the <112-0> is unstable in thermodynamics. Directional solidification of TiAl-based alloys with seed materials is that controlling the phase is the primary phase which solidify from the liquid, and the primary phase must be seeded by a seed grain. So the orientation of the phase is determined, and then that of the lamellar microstructure, since the lamellar microstructure forms with the (111) //(0001) orientation relationship, as shown in Fig. 19. and the 2 phase simply disordered to the phase. (3) The phase is thermodynamically stable and the volume fraction of phase increases by thickening lamellae while not by nucleating new lamellae at the given temperature. (4) The process is reversed and the original orientation of the lamellar microstructure is restored during cooling. These requirements cannot be met in the binary system due to the relative position of the and phase fields in the Ti-Al phase diagram. The above four requirements are found to be met in the Ti-Al-Si system for a composition near Ti-43Al-3Si [60], as shown in Fig. 20. For this composition, the silicide phase Ti 5 Si 3 is present in the microstructure. Upon heating or cooling, the lamellar structures are found to be stable when entering the + silicide region. Thus, seeding is possible, since phase is stable up to the melting temperature. The Ti-43Al-3Si seed was cut from an ingot cast in a metal mould [25] (Fig. 21). Lamellar with orientation aligned parallel to the growth was obtained in Ti-47Al alloy by using Ti-43Al-3Si alloy as seed [27]. Ti Ti 5 Si 3 + Ti 5 Si 3 + β Si +Ti 5 Si Composition range of seed material Fig. 20: Liquidus surface projected onto 1,100 C isothermal section [60] 10 0 Al (0001) Fig. 19: Schematic diagram of seeding and al solidification technique for microstructure control of TiAl-based alloys [59] However, the main problem of this approach is how to find a suitable seed material where the orientation of the phase can be maintained during heating. According to the studies of D. R. Johnson [60], the seed material must meet the following four requirements: (1) The phase is the primary solidification phase. (2) When heating above the 2 + eutectoid temperature, the lamellar microstructure is stable (1120) Seed Solid metal Liquid < 0001> Seed Fig. 21: Schematic illustration of columnar grains and their lamellar microstructure in a Ti-43Al-3Si ingot cast in a metal mould [25] The original orientation of the lamellar microstructure in seeding material can be restored after heating to the single phase -region and cooling from the region, so seeding of the alloys with primary phase is possible. Also, the lamellar microstructures of Ti-45at.%Al to Ti-49at.%Al can be successfully aligned in al solidification by using appropriately oriented Ti-43Al-3Si seeds, though β phase is the primary phase in Ti-45at.%Al to Ti-49at.%Al from the Ti-Al phase diagram [28]. An example of seeding growth of a Ti-47Al (at.%) ingot is shown in Fig. 22 [31]. The lamellar microstructure is aligned parallel to the growth and continuous with the seed [Fig. 22(c)]. Silicide particles in this ingot were 228

11 Celebrating the CHINA FOUNDRY (c) Fig. 22: Microstructures taken from a ally solidified binary Ti-47Al ingot grown from a (b,c) Ti-43Al-3Si seed (SEM back scattered electron images)[31] observed only for the first centimeter of processed material [Fig. 22] with the rest of the ingot being free from such particles [Fig. 22]. Therefore, the orientation of the high-temperature -phase in the Ti-47Al alloy was successfully seeded, though the growth of β-phase dendrites would be expected for a Ti47Al alloy from the phase diagram. The exact compositional range for which the high temperature -phase can be seeded is dependent upon the required undercooling needed to nucleate the β phase. As the undercooling needed for β nucleation decreases, the compositional window for seeding narrows and shifts towards higher Al concentrations. However, the growth velocity is limited due to the potential to nucleate β grains in the constitutionally cooled liquid ahead of the growing phase front [28]. The lamellar orientation is changed with the parameters of al solidification such as temperature gradient and growth rate. Luo W Z et al.[61] proposed that the orientation of lamellar microstructure is no longer parallel to the DS with increasing growth rate because of the variations of constitutional undercooling and temperature gradient, which cause β phases precipitate between the dendrites, so the lamellar is changed. Li X Z et al [33] successfully controlled the lamellar orientation to parallel to the growth and studied the influence of growth rate on lamellar orientation in detail. They concluded that the orientations of lamellar are not always perpendicular to the growth, even if phase is the primary phase. With increasing growth rate, the variation of preferred growth of phase is as follows: <0001> <1120> <2243>. Seeding al solidification can result in fully lamellar microstructure completely parallel to the growth in TiAl based alloys, and significantly improves the performance of TiAl-based alloys at room and high temperature [60]. However, the seed preparation is relatively complicated, which limits its application in large-scale industrial production. Therefore self-seeding technology (SST) is suggested for simplifying the process [33, 62], as illustrated in Fig. 23. The lamellar orientation of Ti-46Al-0.5W-0.5Si alloy is successfully aligned parallel to the growth using the SST at V = 20 μm s-1 and G = 12.1 K mm-1. Lamellar orientations in the unmelted region are parallel to the growth (seed) and suitable solidification parameters are the basis for successfully controlling the lamellar orientation by SST. Suitable solidification parameters are the key factors for ensuring the phase grows along the <1120> and makes the parallel lamellar structures grow continuously until the end of solidification. Also, the efficiency of lamellar orientation control by SST is greatly increased due to the elimination of the composition transition region between the seeding material and master ingot. This method gets rid of the cutting and fixing of the seed and simplifies the processing of controlling the lamellar orientation of TiAl alloys. Therefore, it can promote the engineering applications of the lamellar orientation control of TiAl-based alloys. <1120> <0001> Liquid S/L interface Singlephase DS region As seed Fig. 23: Schematic diagram of preparing specimens for lamellar orientation control by SST: structures of master ingot, structures of seeding specimen, solidification processing of SST (c)[62] 229

12 CHINA FOUNDRY Celebrating the 6 Conclusions This paper reviewed the al solidification of TiAl-based alloys in recent years, as lamellar orientation control can improve the properties of TiAl-based alloys significantly. Lamellar orientation of TiAl-based alloys can be controlled by two different al solidification processes. One is the seeding technique and the other is to adjust the solidification path. (1) It is relatively easier to control the lamellar orientation by adjusting the solidification path due to the simple operation of this method and suitability for large scale industrial production. However, the addition of β phase stablizing elements bring difficulties for alloy melting and the complexity of the solidification process, making control of lamellar orientation in TiAl-based alloys more difficult. (2) Seeding al solidification can obtain fully lamellar orientation parallel to the growth completely, and self-seeding technology can simplify the solidification process. It can promote the engineering applications of the lamellar orientation control of TiAl-based alloys. Therefore, seeding technique is the of future development for lamellar orientation control of TiAl-based alloys. (3) Lamellar orientation changes with the parameters of al solidification such as temperature gradient and growth rate. In order to control the growth precisely and implement large scale production of TiAl castings, it is necessary to establish the laws between lamellar orientation and solidification parameters because of the deficiency of clear understanding of the regularity in the microstructure related to the complex solidification path. References [1] Yamaguchi M, Inui H, Ito K. High-temperature structural intermetallics. Acta Materialia, 2000, 48(1): [2] Lasalmonie A. Intermetallics: Why is it so difficult to introduce them in gas turbine engines? Intermetallics, 2006, 14(10 11): [3] Deevi S C. 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Materials Science and Technology, 2014, 30(2): This study was financially supported by the 973 project (2011C610406, 2011C605504), NSFC project ( ) and Heilongjiang project (JC201209). 231