Rapid Cellular Crystal Growth of TiAl-Based Intermetallic without

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Rapid cellular crystals growth of TiAl based intermetallic without peritectic reaction by melt-quenching in Ga-In liquid Shiqiu Liu, Hongsheng Ding, Jingjie Guo, Hailong Zhang, Zhanxing Chen, Qiang Wang, Ruirun Chen, and Hengzhi Fu Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b01691 • Publication Date (Web): 27 Feb 2017 Downloaded from http://pubs.acs.org on February 27, 2017

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Crystal Growth & Design

Rapid cellular crystals growth of TiAl based intermetallic without peritectic reaction by melt-quenching in Ga-In liquid Shiqiu Liu, Hongsheng Ding*, Jingjie Guo, Hailong Zhang, Zhanxing Chen, Qiang Wang, Ruirun Chen, Hengzhi Fu National Key Laboratory for Precision Hot Processing of Metals, School of Materials Science and Engineering, P. O. Box 434, Harbin Institute of Technology, Harbin 150001, China ABSTRACT: A rapid cellular microstructure of Ti-48Al-2Cr-2Nb (in at. %) intermetallic was grown without peritectic reaction by the method of melt-quenching in Ga-In liquid. After characterization to the microstructures and phase constituents, it’s observed that the cellular crystals mainly consist of α2 phase and directionally grew in the outer layer of the melt droplet, with the growth length and cellular spacing about 358~460µm and 0.68~3.6µm, respectively. Upon the detailed analysis of cellular growth process, it’s found that the formation of this characteristic microstructure is derived from the extremely rapid cooling effect of Ga-In liquid, mainly determined by the heat transfer process; the dominant heat transfer mechanism changes from heat convection to heat conduction at the growth distance about 70µm. By using the semireverse method, the dependence of cooling rate on cellular growth distance can be estimated accurately and conveniently, which ranges from 2.61×106 to 1.26×105 K/s. The nanoindentation examination proved that the rapid cellular microstructure possesses excellent micro-mechanical properties (8.457±0.336 GPa in nanohardness), with a significant improvement about 15~60 % than the general microstructures.

*Corresponding author: Hongsheng Ding Address: National Key Laboratory for Precision Hot Processing of Metals, School of Materials Science and Engineering, P. O. Box 434, Harbin Institute of Technology, Harbin 150001, China Tel: +86-451-8641 2394. E-mail: [email protected]

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Rapid cellular crystals growth of TiAl based intermetallic without peritectic reaction by meltquenching in Ga-In liquid AUTHOR NAMES: Shiqiu Liu, Hongsheng Ding*, Jingjie Guo, Hailong Zhang, Zhanxing Chen, Qiang Wang, Ruirun Chen, Hengzhi Fu AUTHOR ADDRESS: National Key Laboratory for Precision Hot Processing of Metals, School of Materials Science and Engineering, P. O. Box 434, Harbin Institute of Technology, Harbin 150001, China *E-mail address: [email protected].


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Rapid cellular crystals growth of TiAl based intermetallic without peritectic reaction by meltquenching in Ga-In liquid AUTHOR NAMES: Shiqiu Liu, Hongsheng Ding*, Jingjie Guo, Hailong Zhang, Zhanxing Chen, Qiang Wang, Ruirun Chen, Hengzhi Fu AUTHOR ADDRESS: National Key Laboratory for Precision Hot Processing of Metals, School of Materials Science and Engineering, P. O. Box 434, Harbin Institute of Technology, Harbin 150001, China KEYWORDS: Rapid cellular growth; TiAl based intermetallic; Melt-quenching; Rapid solidification; Nanoindentaion

ABSTRACT: A rapid cellular microstructure of Ti-48Al-2Cr-2Nb (in at. %) intermetallic was grown without peritectic reaction by the method of melt-quenching in Ga-In liquid. After characterization to the microstructures and phase constituents, it’s observed that the cellular crystals mainly consist of α2 phase and directionally grew in the outer layer of the melt droplet, with the growth length and cellular spacing about 358~460µm and 0.68~3.6µm, respectively. Upon the detailed analysis of cellular growth process, it’s found that the formation of this characteristic microstructure is derived from the extremely rapid cooling effect of Ga-In liquid, mainly determined by the heat transfer process; the dominant heat transfer mechanism changes from heat convection to heat conduction at the growth distance about 70µm. By using the semi-


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reverse method, the dependence of cooling rate on cellular growth distance can be estimated accurately and conveniently, which ranges from 2.61×106 to 1.26×105 K/s. The nanoindentation examination proved that the rapid cellular microstructure possesses excellent micro-mechanical properties (8.457±0.336 GPa in nanohardness), with a significant improvement about 15~60 % than the general microstructures.

TEXT:

1. Introduction Peritectic reaction can be found in extensive technologically important materials, including alloys such as steels, Fe–Ni and Ti–Al alloys as well as inorganic nonmetallic materials for instance superconducting materials YBCO.1,2 Despite its benefits in aspects of grain refinement and the growth of single crystal,3,4 peritectic reaction readily generates microsegregation in multi-component alloys resulting from its characteristic of extremely slow reaction kinetics. Whereas cellular growth maybe an effective method to solve this problem. For a positive temperature gradient, cellular growth is a very common phenomenon in the process of crystal growth. According to solidification theories,5,6 with the increase of growth velocity, the morphology of solid/liquid (S/L) interface during solidification varies from plane front to cells, to dendrites, to cells again and finally back to plane front, namely the cellular growth can appear in ranges of both relative low and high growth velocity, which can be defined as low-velocity and high-velocity (or rapid) cellular growth, respectively.7 Compared with dendritic growth, cellular growth results in less microsegregation and more homogeneous microstructures which are ideal morphologies to obtain excellent properties.


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At first, researchers experimentally studied the cellular growth behavior by in situ observation of transparent dilute organic alloys.6,8 Recent years, extensive researches about cellular growth have mostly performed in dilute alloys as a consequence of the velocity range for cellular growth being enlarged with decreasing concentration.6 Such as W. Xu et al.7,9 have researched the cellular growth of Zn-rich Zn-Ag alloys by rapid solidification methods in detail; D. Ma et al.1012

have studied the cellular growth behavior of Zn-rich Zn-Cu alloys by Bridgman solidification,

some other relative researches also have been completed in dilute eutectic Al-Fe,13-15 Al-Mg-Si,16 and peritectic Al-Ti alloy systems.2 The above researches mainly concentrated on determining the relationship between microstructural parameter (cellular spacing λ1) and solidification parameters (temperature gradient G, growth velocity V and cooling rate Ṫ), it has been shown that the microstructural parameter decreases as solidification processing parameters increase, for the constant concentration. Hunt,17 Kurz and Fisher18 and Trivedi19 have proposed detailed theoretical models to characterize cells and primary dendrite spacing during steady-state growth conditions, as a function of G, V and C0 (C0 represents the initial alloy concentration in this work),2 their expressions about cellular/dendritic spacing have a similar formation, all of them 0.25 -0.5 -0.25 can be expressed as λ1 =KC0 G V , in which K is a constant depending on different

theoretic models. Due to Ṫ=GV in directional solidification process, then the above expression 0.25 -0.25 & -0.25 can also be expressed as λ1 =KC0 G T , therefore λ1 is the power function of Ṫ with -0.25

exponent for constant G and C0. M. Gündüz et al.2 have experimentally investigated the relationship between λ1 and Ṫ during cellular growth of dilute Al-Ti alloys under steady growth conditions, it was found that λ1 could be expressed as a power function of Ṫ with exponents of 0.29 ~ -0.32, which is close to the above expressions from theoretical models. However, these expressions were found not to be applied to cellular growth under unsteady-state conditions yet.


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A. Garcia et al.13-16, 20-21 have systematically researched cellular growth behavior of various alloy systems during transient directional solidification, all of the researches indicated that λ1 is the power function of Ṫ with -0.55 exponent under unsteady heat flow conditions, which is much valuable to the further research about unsteady-state cellular growth of alloys. Considering the dilute alloys are lack of application prospect, then it’s more meaningful to deeply research the cellular growth behavior of engineering alloys, and the relative research results on dilute alloys have laid the significantly theoretical and experimental foundation for further researches. In order to obtain excellent mechanical properties, engineering alloys usually own high alloying elements addition. Whereas compared with the dilute alloys, high alloys are much more difficult to realize the cellular growth, with the required growth velocity much low or high, so that relative research reports are still scarce at present. For the low-velocity cellular growth of high alloys, G. Liu et al.22,23 have obtained shallow celluar crystal growth of Ti-46Al-8Nb and Ti-50Al-4Nb (all compositions are given in at. % except especially statement) alloys by Bridgman-type directional solidification method at an extremely low growth velocity (2 µm/s). X. B. Zhao et al.24 also have fabricated typical cellular microstructures in the DD407 nickel-base superalloy at a low growth velocity of 6µm/s by similar method. Undoubtedly, these cellular crystal microstructures were very coarse (λ1=80~200µm) due to the extremely slow growth velocity, moreover the cellular microstructures didn’t retain to room temperature resulting from the subsequent solid-state transformation. Consequently, although low-velocity cellular growth can obtain homogeneous microstructure, its resulting coarse microstructures are undesired to some extents. In comparison to low-velocity cellular growth, rapid cellular growth can generate finer and more homogeneous microstructures. However, rapid cellular growth for high alloys need extremely high cooling rate. E. L. Hall et al.25 and Y. Liu et al.26 have observed rapid


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cellular growth microstructures in the process of investigating the rapid solidification behavior of γ-TiAl based alloys by melt spinning (Ṫ in 105~106 K/s)27 and laser remelting (Ṫ in 105~107 K/s)28 method, respectively. The obtained rapid cellular microstructures were extremely fine (λ1=0.6~10µm), nevertheless these microstructures were much small in scale (10~20 µm in length) and the researchers didn’t proceed the intensive research about the process of rapid cellular growth yet, as well as characterization to the interesting mechanical properties of rapid cellular microstructures. Conclusively, due to the huge potential of rapid cellular growth to achieve extremely fine and homogeneous microstructure, meanwhile relative researches on engineering alloys are in the initial stage, thus it’s highly necessary to carry out further researches about the rapid cellular growth behavior of engineering alloys. TiAl based intermetallics are considered as extremely promising candidates for aerospace and aircraft applications due to their excellent high temperature mechanical properties. Particularly, Chen et al.29 recently developed a seedless growth approach to manufacture polysynthetic twinned (PST) single crystals of a TiAl alloy with a superior combination of ductility and strength, as well as resistance to creep, which offers a significant contribution towards more widespread use of TiAl alloys at high temperatures. While a substantial amount of work remains to investigate metallurgical approaches to push their performance higher.30 It’s well-known that microsegregation derived from peritectic reaction is always a problem hindering the further improvement of TiAl alloys in mechanical properties, therefore it’s meaningful to seek a method to cancel the microsegregation problem faced by TiAl alloys. As the above stated, rapid cellular growth exactly has such potential. Therefore, the main aim of this research is to grow rapid cellular microstructure of Ti-48Al-2Cr-2Nb intermetallic with relative large scale and further carry out particular investigations to the microstructure (also including its mechanical properties)


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and the rapid cellular growth process. Based on the technical advantages, such as low melting point, pressure of saturated vapors and toxicity of vapors, Ga-In liquid is an excellent heat transfer agent,31 therefore rapid cellular growth of the investigated alloy was realized by meltquenching in Ga-In liquid, and for avoiding the contamination from ceramic crucible, the investigated alloy was melt using electromagnetic levitation cold crucible.

2. Experimental Section The master ingot with nominal composition of Ti-48Al-2Cr-2Nb was fabricated by induction skull melting furnace under argon atmosphere. The alloy was prepared from pure Ti (99.99 wt. %), pure Al (99.99 wt. %), Al-75wt%Nb master alloy and pure Cr (99.9 wt. %), and the exact chemical compositions of obtained ingot was Ti-48.53Al-2.02Cr-2.05Nb examined by the method of X-ray fluorescence spectrometry. The charge bars for the melt-quenching experiment were sectioned from the master ingot in diameter of 15mm by wire electrical discharge machining. The melt-quenching experimental apparatus is schematically shown in Figure 1a, which mainly consists of heating coil, electromagnetic levitation cold crucible (EMLCC), charge bar, temperature measuring equipment and the coolant liquid of Ga-In alloy. When installing the apparatus, the end of charge bar needs to be placed in center of the cold crucible at the height of the projecting part for an effective melting process, as shown in Figure 1a. Moreover, due to that the EMLCC can generate more outstanding electromagnetic levitation force on the alloy melt, then the alloy melt can be heated for a longer time before dropping into the Ga-In liquid in comparison to the one melted by conventional electromagnetic cold crucible (EMCC), which makes the alloy melt superheated to higher temperature. In order to reduce the contamination of oxygen to the minimum, the furnace chamber needs to be evacuated to 3 Pa, and inflated with


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high purity argon (99.99 %) for three times, eventually backfilled with argon to a pressure of 300 Pa. The charge bar was melted by the heating coil connected with a high frequency (50 kHz) AC power supply. The temperature of Ti-48Al-2Cr-2Nb melt was measured by (W-5Re)/(W-26Re) thermocouples for its ability to measuring high temperature of metal liquid (≤2573 K) and high accuracy (≤0.5%T, T represents the measured temperature). After turning on recycling water, heating power was successively switched on and increased by adjusting the heating voltage in the rate of 5 V/min. When the voltage was increased to 320 V, the Ti-48Al-2Cr-2Nb melt droplet fell off with a height of h=73.4mm from the Ga-In liquid surface and quenched into the alloy liquid coolant subsequently; meanwhile, the temperature of alloy melt was measured to be 1755 K, and the temperature of Ga-In liquid was identified to be room temperature (298 K). The temperature-heating voltage curve of the heating end of charge bar is shown in Figure 1b. The TiAl melt-quenching specimen was obtained with the diameter Φ=10mm and thickness δ=5mm in this work.


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Figure 1. (a) Schematic diagram of TiAl alloy melt-quenching apparatus, in which “A-A” is the cross-section drawn of the assembly consisting of TiAl charge bar and thermocouple. (b) The temperature-heating voltage curve of the heating end of TiAl alloy charge bar. For the microstructure analysis, the as-quenched specimen was sectioned into two halves along the direction of the droplet falling down, and embedded into denture base acrylic resin, then polished using standard metallographic techniques. The polished surface was etched with Kroll’s agent (5 vol.% HNO3 + 3 vol.% HF +92 vol.% H2O). The cellular crystal microstructure was characterized by an Olympus optical microscopy (OM) and a Quanta 200FEG field-emission scanning electron microscopy (SEM) equipped with an EDX detector (EDAX). The phase constitution of the specimen was identified by X-ray diffraction (XRD) with Cu Kα radiation using a Philip X-ray diffractometer, and the sample was scanned in a 2θ range of 20 ~ 100° with a speed of 6°min−1. Cellular and dendritic spacing at different growth distance were measured by Gatan Microscopy Suite (GMS) software. For the accurate mensuration, at least 4 adjacent pieces of cellular/dendritic crystals were chosen to calculate the average value, which was adopted as the cellular/dendritic spacing. The nanoindentation tests were performed using a high-precision nanohardness scratch tester (Nano Indenter G200, Agilent Technologies) with a Berkovich-type indenter at room temperature, the loading depth and hold time were 1.5 µm and 2 s, respectively. In order to obtain more accurate experimental data, three different cellular crystal growth regions were selected to proceed examinations, in each region eight tests were carried out along the growth direction of cellular crystals with the test spacing of 50 µm.

3. Results and Discussion 3.1. Microstructures and phase constituents


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Figure 2. The rapidly solidified metallographic microstructures of Ti-48Al-2Cr-2Nb alloy meltquenching specimen: (a), (c) longitudinal section images of the outermost layer located at different positions of the melt-quenching specimen; (b), (e), (f) are the magnification images obtained from zone 1, 2, 3 marked by boxes; the arrow in (b) indicates the growth direction of


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cellular crystals; (d) the transversal section image of cellular growth microstructure at the position of zone 3. As shown in Figure 2a, c, there are evident differences in solidification microstructure between the outer and inner layer of the melt-quenching specimen, so that an outstanding interface is presented between the two parts, the thickness of the outer layer is around 358~460 µm. According to Figure 2b, in comparison to the inner part, the outer layer microstructures are finer, more homogeneous and oriented, their growth direction is almost perpendicular to the surface profile of the melt-quenching specimen, exactly inverse to the heat flow direction. Figure 2d, f are the magnifying images of the transversal and longitudinal section microstructures obtained from the zone 3 in Figure 2c, respectively. The microstructure morphologies present uniformly cellular and stripe-like in transversal and longitudinal section, respectively, which is the typical characteristic of cellular crystal microstructure. Consequently, it is identified that the growth mode of the outer microstructures is cellular interface growth. According to the above stated, there are two kinds of cellular crystal microstructures, namely the low-velocity and rapid cellular crystal microstructure, respectively. The most obvious distinction in morphology between them is the cellular spacing (λ1), the latter is much less than the former (approximately 10-6 vs. 10-4 m in order of magnitude). The λ1 are measured about 2µm in the Figure 2d, f, then it can be concluded that the process of melt-quenching in Ga-In liquid makes the outermost layer of specimen undergoing a rapid solidification process, which results in a rapid cellular growth with relative large scale. Figure 2e shows the magnification microstructures obtained from zone 2, which is located at the interface between the outer and inner layer of the melt-quenching specimen. It’s observed that there is an evident transition in microstructural morphology at two sides of the interface, and


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cellular crystals turn into dendritic ones at once when going across the interface. According to the fundamentals of solidification,5 in the process of rapid cellular growth, there is a critical growth velocity Vc, when V< Vc, the crystal growth mode will change from cellular into dendritic since the interface instability derived from constitutional supercooling is promoted with the decrease of growth velocity. In the directional solidification process, the cooling rate Ṫ = GV in value. Assume the G is a constant during the stable growth process of cellular crystals, then the growth velocity V is in direct proportion to the cooling rate Ṫ. Consequently, it can be inferred that during the melt-quenching of the TiAl alloy, the cooling rate Ṫ decreased gradually with the cellular crystals growing inwards due to the change of heat transfer conditions, then the growth velocity V decreased accordingly, as the growth velocity decreased below the critical value Vc , the cellular growth mode changed into dendritic growth. As the dendritic lines marked in Figure 2e, the secondary dendrite arms keep an approximate 60° angle with the dendrite trunk, which indicates the dendritic crystals have a hexagonal close packed (h.c.p) crystal structure, based on the solidification characteristics of TiAl based alloys, it can be ascertained that the primary dendritic crystals consist of α phase. Because of the continuity of crystal growth, the primary cellular crystals can be deduced as α phase, too. Considering the primary phase of Ti-48Al-2Cr-2Nb alloy is β phase in equilibrium solidification, thus it’s indicated that the rapid solidification process resulting from the melt-quenching in Ga-In liquid changed the primary phase from β into α. Furthermore, there is no lamellar microstructure presenting in the cellular crystals, thus it can be inferred that the primary α phase straightly transformed into α2 phase subsequently at a low temperature. Conclusively, the non-equilibrium solidification and transformation path of Ti-48Al-2Cr-2Nb can be recognized as follows L → L + α → α + γseg → α2 + γseg under rapid solidification


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conditions by the method of melt-quenching in Ga-In liquid. In other words, the outermost layer of Ti-48Al-2Cr-2Nb droplet solidified without peritectic reaction and with a primary α phase straightly transforming into α2 phase under the extremely rapid cooling effect of Ga-In liquid, which is consistent to the results of C. Kenel et al.32 by in-situ observation method in one aspect.

Figure 3. (a) X-ray diffraction patterns of as-cast and melt-quenching Ti-48Al-2Cr-2Nb alloy. (b) Schematic illustration about the formation mechanism of primary α phase during the meltquenching process of Ti-48Al-2Cr-2Nb alloy, where C0 is equal to the Al-equivalent of the investigated alloy calculated according to Ref. (33), ∆T is the degree of supercooling suffered by the investigated alloy during the cellular growth, ∆T1 and ∆T2 are the temperature differences of the melting point with T0,

L→α

and T0,

L→β

at the composition of C0, respectively. (c) Back-


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scattered electronic (BSE) picture of rapid cellular microstructure of Ti-48Al-2Cr-2Nb alloy. (d) The EDS analysis result at the location of little cross in (c). Figure 3a presents the X-ray diffraction (XRD) results of as-cast and melt-quenching Ti-48Al2Cr-2Nb alloy. Apparently, both the as-cast and melt-quenching Ti-48Al-2Cr-2Nb alloys have the same phase constitutions, primarily consisting of α2 and γ phase. In spite of this, there exist obvious differences in dominant peak and the intensity of diffraction peaks between the two states, which indicates that the melt-quenching process made a transformation of the main constituent phase in Ti-48Al-2Cr-2Nb alloy. As revealed in the as-cast XRD pattern, the dominant diffraction peak belongs to the (111) γ crystal planes, and there are also some other diffraction peaks of γ phase detected in different angles, which illustrates that the main constituent phase in as-cast Ti-48Al-2Cr-2Nb is γ phase; while there is only one relative strong diffraction peak of α2 phase detected in the as-cast alloy, the diffraction peak comes from the (0002) α2 crystal planes and has a good match with the diffraction peak of (111) γ crystal planes, which results from the coherent relationship between (0002) α2 and (111) γ, corresponding with the following orientation relationship: (0002)α2 ∥ (111) γ,

1210 α 2

∥

110 γ

,34 then it can be concluded that the α2 phase principally exists in

lamellae aligned with γ phase in the as-cast alloy. In comparison to the as-cast XRD pattern, the dominant diffraction peak of the melt-quenching pattern turns into the

( 2021) α

2

rather than the

common (0002) α2 in lamellae, and there are many other diffraction peaks of α2 phase appearing. On the contrary, all the intensity of γ phase diffraction peaks weaken and even some of the peaks disappear. So the following three points can be demonstrated in the melt-quenching process. At first, the main constituent phase of Ti-48Al-2Cr-2Nb alloy changes from γ into α2; at second, the


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dominating α2 phase is originated from the straight transformation of the primary phase α without accompanied by γ phase; at last, the formation of γ phase is outstandingly restrained. The XRD results further verify the fact that the rapid cellular crystals grow in α phase which straightly transforms into α2 subsequently at a low temperature. On the condition of non-equilibrium solidification, some researches32, 35 have revealed that the primary phase of TiAl alloys with specific compositions could change from β into α phase, this phenomenon can be reasonably explained by T0 temperature, just as Figure 3b illustrated. The so-called T0 temperature is the temperature at which the Gibbs free energies of the two phases are equal with the same composition, for example, T0, A-B indicates the T0 temperature between A and B phase, in which A is the low temperature phase, then when the temperature rapidly cools under the T0,

A-B,

the A phase will be formed instead of B phase, because GA< GB at the

moment.35 In this research, in order to make the binary TiAl phase diagram applied to the multicomponent TiAl alloy, the Al-equivalent method is introduced, according to the Ref. (33), the Al-equivalent of Ti-48Al-2Cr-2Nb is calculated to be 47.2 at.%, which is slightly more than the peritectic point (46.7 at.% Al). The T0, L→α and T0, L→β dashed lines in Figure 3b are drew referring to relative literatures.35,36 In the conventional casting situation of Ti-48Al-2Cr-2Nb, the degree of supercooling of alloy melt is very small during the solidification process, similar to the equilibrium solidification, then the primary phase is β phase according to the equilibrium binary phase diagram. However, on the condition of this research, the Ti-48Al-2Cr-2Nb alloy droplet (~1755K) was quenched in the Ga-In liquid (298K), due to their enormous temperature difference and the superior heat transfer effect of Ga-In liquid, the outermost layer of the alloy droplet suffered a rapid solidification process and obtained a large degree of supercooling ∆T prior to nucleation, when ∆T10, then T3 can be solved as follows by substituting equation 15 into this equation,

T3 =

& 2 − λ br + 298hr λT2 + c ρTr hr + λ

(16)

Then substitute the expression of T3 into equation 15, G(r) can be express in function of T2 as follows,

G (r ) =

& − hbr − 298h hT2 − c ρTr hr + λ

(17)

In the end, combined equation 17 and 12, the cooling rate in actual situation can be expressed as follows,

& − hbr  hT2 − 298h − c ρTr  − G (r + dr )    hr + λ   c ρ dr + c1 ρ∆H m G (r + dr )

λ T& =

(18)

Compared with equation 13 obtained by TC method, there are three differences which can be found in the equation 18. The first one is that the G(r) here is less; the second one is that G(r+dr) is also less than the invariable G value when r is large; the last one is that the thickness of mushy zone dr is becoming larger here when r increases to some extent. Obviously, these differences tend to zero when r is very little, thus the cooling rate can be calculated accurately by equation 13 when r→0. However, with the increase of r, these differences enlarge gradually, which makes the calculated value away from practical situation further and further. Based on the characteristics of equation 13 and 18, With the r value increased, both the first and last difference will make the actual value of cooling rate less than the calculation by equation 13, on the contrary, the second difference will make the actual value relatively higher. Actually, because


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& the proportion of the two items that c ρTr +hbr and c ρ dr in corresponding numerator and denominator of equation 18 are relatively little, then their differences between equation 13 and (18) have little influence on the calculation of cooling rate, it’s the second difference, namely the adoption of invariable G=1.6×107 K/m in equation 13, that mainly leads to the calculated value being less than the true value more and more with the increase of r. In addition, by a simple calculation of kinematics, it can be known that approximately 2.1×10-3 s later after the TiAl melt droplet contacts with the Ga-In liquid, the relative motion velocity between TiAl droplet and GaIn liquid will decrease gradually. According to equation 4 and 5, the h ∝ v , then with the 1/ 2

decrease of v, the convective heat-transfer coefficient of Ga-In liquid will reduce accordingly. Due to

V t = r (λ1 ) V , is the average value of cellular growth velocity, then by taking advantage

of the data of λ1 and Ṫ at different r in Figure 4a and Figure 6a, the growth time t at different r can be calculated, further take them compared with the value of 2.1×10-3 s, it can be easily known that the cellular growth distance is about 200 µm after the cellular crystals grow up for 2.1×10-3 s, therefore, after r>200 µm, the decrease of h is another factor which makes the difference between the calculated value by equation 13 and actual value becoming larger and larger. Conclusively, in order to further employ the TC method in more precise estimating the cooling rate at different growth distance, it’s necessary to correct the invariable G in equation 13 to a function of G1(r), after r>200 µm, the constant h also needs to be corrected accordingly. In conclusion, compared with the TC method, the SR method is more accurate and simpler in estimating cooling rate during the rapid cellular growth process. This method is based on the experimental results, and the assumption deviation has relatively less effect on the accuracy of calculated value, therefore, the SR method can be readily used to simply estimate the cooling rate distribution of rapid cellular growth instead of relatively complicated numerical simulation.


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3.4. Micro-mechanical properties of the rapid cellular microstructure

Figure 7. Micro-mechanical properties of the rapid cellular microstructure of Ti-48Al-2Cr-2Nb alloy fabricated by melt-quenching in Ga-In liquid: (a) the linear regression analysis of nanohardness with

λ1-1/2 ; (b) the load-displacement curves of rapid cellular microstructures with

three typical cellular spacing; (c) the frequency distribution histogram of the nanohardness measured values; (d) the frequency distribution histogram of the modulus measured values. (The indenter is Berkovich style in this work.) Because the cellular microstructure of Ti-48Al-2Cr-2Nb alloy is very fine and uniform, and there are few literatures about its mechanical properties, then it’s interesting to investigate the


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mechanical properties of this kind microstructure of Ti-48Al-2Cr-2Nb alloy, considering the size of the fabricated cellular microstructure, only the micro-mechanical properties can be examined, thus the nanoindentation test has been carried out. Figure 7 is the investigated results about the miro-mechanical properties of the rapid cellular microstructure of Ti-48Al-2Cr-2Nb alloy in this work. Figure 7a shows the statistical results on the nanohardness of cellular microstructure with different

λ1-1/2 as well as their linear regression

analysis. As Figure 7a shown, there is a linear relationship between the nanohardness with the

λ1-1/2 , namely the dependence of nanohardness of the rapid cellular microstructure on the

cellular spacing meets the Hall-Petch relationship, their relational expression is as follows, H N = 7.559 + 1.198λ1−1/ 2 R 2 = 0.967

(19)

Here HN is the nanohardness (GPa) of the cellular microstructure, λ1 is in µm. According to Figure 7a, with the decrease of cellular spacing, the nanohardness of cellular microstructure increase gradually, when λ1=0.76 µm, the nanohardness value can reach up to 8.902 GPa, Combined with the nanohardness frequency distribution histogram of the cellular microstructure, shown in Figure 7c, it’s discovered that the nanohardness of the cellular microstructure is mainly distributed in the range from 8.0 to 8.9 GPa, especially in the range of 8.45~8.7 GPa, the frequency of occurrence is the highest. As above analysis, the cellular microstructure mainly consists of α2 phase, referring to relative literatures, it’s known that the measured nanohardness of α2 phase is 7.4±0.5 GPa in PST-TiAl alloy49 and 5.3 GPa on a globular microstructure with the alloy composition of Ti-43.5Al-4Nb-1Mo-0.1B (in at. %),50 then it’s indicated that the α2 phase with the rapid cellular microstructure has much better nanohardness than that with general microstructures, with a significant improvement about 15~60 %. The substantial improvement


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mainly results from the following two reasons, the first one is the solution strengthening effect, according to the analysis above, the elements segregation phenomena during the rapid cellular crystal growth is rather slight so that the cellular α2 phase basically keeps the alloy composition and is supersaturated seriously; the other one is the fine-grain strengthening effect, the rapid cellular microstructure is very fine and uniform (λ1 is about 0.68~3.6µm), the tiny cellular spacing makes the dislocations difficult to go across the grain boundaries, hence resulting in high nanohardness values. Figure 7b shows the typical load-displacement curves of rapid cellular microstructure with different cellular spacing, based on the identical loading depth of 1.5 µm, the maximum indentation load increases with the decrease of cellular spacing, accordingly, on the other hand the residual indention depth decreases, both of these phenomena is consistent with the fine-grain strengthening effect. As shown in Figure 7d, the elastic modulus of the rapid cellular microstructure mainly distributes in the range of 136~147 GPa, the distribution frequency is homogeneous in this range, the average modulus of 142 GPa determined for the cellular α2 phase is in good consistence with the calculated value of 141 GPa along the < 1120 > direction given by Ref. (51).

4. Conclusions A fine and homogeneous rapid cellular microstructure of Ti-48Al-2Cr-2Nb intermetallic was grown without peritectic reaction by the method of melt-quenching in Ga-In liquid. The cellular crystals, mainly consisting of α2 phase, directionally grew in the outer layer of the melt droplet with the growth length and cellular spacing about 358~460µm and 0.68~3.6µm, respectively. This cellular crystal microstructure of TiAl intermetallic is rarely observed in other rapid solidification methods, such as melt spinning, rotating electrode process etc. The formation of this characteristic microstructure is derived from the extremely rapid cooling effect of Ga-In


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liquid, mainly determined by the heat transfer process. The detailed analysis about the heat transfer process was carried out in two different ways, it was found that the dominant heat transfer mechanism changes from heat convection to heat conduction at the growth distance about 70µm. By using the semi-reverse method, the cooling rate at different cellular growth distance can be estimated accurately and conveniently, which ranges from 2.61×106 to 1.26×105 K/s; and the corresponding growth velocity for rapid cellular crystals is in the scope of 163~8 mm/s. The specific analysis focusing on the heat transfer behavior during the rapid cellular growth expands the theory of cellular growth, and lays the foundation for further researches about the rapid cellular crystal growth control. Considering the melt-quenching method can retain the primary solidifying microstructures of TiAl alloy to room temperature, then the detailed analysis in this work also makes a preparation for the further research about the solidification behavior influenced by melt structure transition of TiAl intermetallic by the meltquenching method. Besides, a brief characterization of micro-mechanical properties to the cellular crystals was carried out, the cellular crystals exhibited excellent micro-mechanical properties (8.457±0.336 GPa in nanohardness), with a significant improvement about 15~60 % than the general microstructures. This cellular crystal microstructure has extensive application prospects on the improvement of comprehensive mechanical properties of γ-TiAl base intermetallic.


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TABLES.

Table 1. Main parameters used for calculations Physical parameters of Ti- Physical parameters of Ga-In Other important parameters 48Al-2Cr-2Nb liquid liquid (at Tm=997K) Liquidus temperature K

T l,

Thermal conductivity W/mK

λ,

1755

Thermal conductivity W/mK

79.94 λL,

Solid ratio in 0.604 mushy zone c1

Specific heat c', 301 J/kgK

Temperature 16 difference between cellular tip and root ∆T' ,K

Specific heat c, 824 J/kgK

Density ρ', kg/m3

5806

Average 10-6 thickness of mushy zone dr, m

Density ρ, kg/m3

Thermal diffusion coefficient m2/s

4.57×10-5

Average 1.6×107 temperature gradient in mushy zone G, K /m

1.2×10-7

Spherical 6.25×10-3 surface radius R, m

11

3636

a,

Latent heat of 3.77×105 Kinematic fusion ∆Hm, J/kg viscosity ν, m2/s

In Table 1, dr = λ1 / 2 , λ1 is the average value of cellular spacing, equal to 2 µm here; G=∆T'/dr.

AUTHOR INFORMATION

Corresponding Author *Tel: +86-451-8641 2394. E-mail: [email protected].

Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT The authors would like to thank the National Nature Science Foundation of China (Grant No.51471062, No. 51671072) for the financial support. REFERENCES (1) Luo, L. S.; Su, Y. Q.; Guo, J. J.; Li, X. Z.; Li, S. M.; Zhong, H.; Liu, L.; Fu, H. Z. J. Alloys Comp. 2008, 461, 121-127. (2) Gündüz, M.; Kaya, H.; Çadırlı, E.; Maraşlı, N.; Keşlioğlu, K.; Saatçi, B. J. Alloys Comp.

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For Table of Contents Use Only

Rapid cellular crystals growth of TiAl based intermetallic without peritectic reaction by meltquenching in Ga-In liquid AUTHOR NAMES: Shiqiu Liu, Hongsheng Ding*, Jingjie Guo, Hailong Zhang, Zhanxing Chen, Qiang Wang, Ruirun Chen, Hengzhi Fu

A rapid cellular microstructure of Ti-48Al-2Cr-2Nb (in at. %) intermetallic was grown without peritectic reaction by melt-quenching in Ga-In liquid; the cooling rates during cellular growth were accurately calculated to be 2.61×106 to 1.26×105 K/s by a semi-reverse method; the cellular microstructure exhibits more excellent nanohardness (8.457±0.336 GPa) than general microstructure of TiAl intermetallic, improved by 15~60 %.


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FIGURES

Figure 1. (a) Schematic diagram of TiAl alloy melt-quenching apparatus, in which “A-A” is the cross-section drawn of the assembly consisting of TiAl charge bar and thermocouple. (b) The temperature-heating voltage curve of the heating end of TiAl alloy charge bar.


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Figure 2. The rapidly solidified metallographic microstructures of Ti-48Al-2Cr-2Nb alloy meltquenching specimen: (a), (c) longitudinal section images of the outermost layer located at different positions of the melt-quenching specimen; (b), (e), (f) are the magnification images obtained from zone 1, 2, 3 marked by boxes; the arrow in (b) indicates the growth direction of


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cellular crystals; (d) the transversal section image of cellular growth microstructure at the position of zone 3.

Figure 3. (a) X-ray diffraction patterns of as-cast and melt-quenching Ti-48Al-2Cr-2Nb alloy. (b) Schematic illustration about the formation mechanism of primary α phase during the meltquenching process of Ti-48Al-2Cr-2Nb alloy, where C0 is the Al-equivalent of the investigated alloy calculated according to Ref. (33), ΔT is the degree of supercooling suffered by the investigated alloy during the cellular growth, ΔT1 and ΔT2 are the temperature differences of the melting point with T0, L→α and T0, L→β at the composition of C0, respectively. (c) Back-scattered electronic (BSE) picture of rapid cellular microstructure of Ti-48Al-2Cr-2Nb alloy. (d) The EDS analysis result at the location of little cross in (c).


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Figure 4. (a) Dependence of cellular/dendritic spacing λ1 on the cellular crystal growth distance r. (b) Schematic diagram of the rapid cellular growth process, in which r=0 indicates the initial moment of this process, and r>0 indicates the stage that the cellular crystals having grown to a certain distance, v is the descent velocity of TiAl melt droplet when contacting with the Ga-In liquid, R is the lower surface radius of the TiAl melt droplet, T1, T2 and T3 are indicative of the corresponding interface temperatures, respectively.


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Figure 5. Backscattered electron (BSE) pictures of rapid cellular microstructure in longitudinal section during different growth stages: (a) initial growth stage, (b) stable growth stage, (c) transition growth stage. (d) is dendritic growth microstructure in the same section, the crystal growth direction is upwards.


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Figure 6. (a) Calculated results of the dependence of cooling rate T on the cellular growth distance r by semi-reverse (SR) and theoretical calculation (TC) method, respectively. (b) The dependence of cellular/dendritic spacing on growth velocity.


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Figure 7. Micro-mechanical properties of the rapid cellular microstructure of Ti-48Al-2Cr-2Nb alloy fabricated by melt-quenching in Ga-In liquid: (a) the linear regression analysis of -1/2 nanohardness with 1 ; (b) the load-displacement curves of rapid cellular microstructures with

three typical cellular spacing; (c) the frequency distribution histogram of the nanohardness measured values; (d) the frequency distribution histogram of the modulus measured values. (The indenter is Berkovich style in this work.)


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Rapid Cellular Crystal Growth of TiAl-Based Intermetallic without

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