Paulínska 16, Trnava, Slovak Republic *

Transcription

1 EFFECT OF THE SOLIDIFICATION PARAMETERS ON THE DISTRIBUTION OF THE ALLOYING ELEMENTS DURING DIRECTIONAL SOLIDIFICATION OF THE INTERMETALLIC Ti-44-5Nb-0.2B-0.2C ALLOY a,b ena KLIMOVÁ *, a Zuzana GABALCOVÁ, a Jura LAPIN a Institute of Materials and Machine Mechanics, Slovak Acadey of Sciences, Račianska 75, Bratislava 3, Slovak Republic b Faculty of Materials Science and Technology in Trnava, Slovak University of Technology in Bratislava, Paulínska 16, Trnava, Slovak Republic * usakli@savba.sk Abstract Quench during directional solidification (QDS) experients and energy-dispersive spectroetry easureents were carried out to study the effect of growth rate V and teperature gradient in liquid at the solid-liquid interface G L on distribution of ain alloying eleents during solidification of the interetallic Ti- 44-5Nb-0.2B-0.2C (at%) alloy. Directional solidification was perfored in a odified Bridgan-type apparatus in dense cylindrical Y 2 O 3 oulds at nine cobinations of three constant V and three constant G L. After directional solidification to a constant length the saples where quenched by a rapid displaceent into the water-cooled crystallizer. For the qualitative and quantitative description of the distribution of the ain alloying eleents, the evolution of the concentration of Ti, and Nb as a function of the fraction of solid f s and segregation deviation paraeter were evaluated. The studied alloy solidifies with β priary solidification phase and undergoes L + β α peritectic transforation. The foration of peritectic α-phase affects backdiffusion of the alloying eleents. Key words: titaniu aluinides, Ti, solidification, energy-dispersive spectroetry, icrosegregation 1. INTRODUCTION Interetallic Ti based alloys due to their low density and attractive high teperature properties have a very good potential for applications in power engineering and aircraft industry, especially for processing of low pressure turbine blades for stationary gas turbines and aircraft engines. Coarse-grained icrostructure, casting texture and significant cheical inhoogenity of cast γ(ti)+α 2 (Ti 3 ) ingots are detriental to their echanical properties. However, the proper selection of alloying eleents leads to a significant grain refineent and hoogenization of the icrostructure. Solidification through the β-phase (Ti-based solid solution with cubic crystal structure) induced by the addition of about at.% of and 5-10 at.% of Nb results in the foration of different orientation variants of the α-phase (Ti-based solid solution with hexagonal crystal structure) fro β-phase. Boron increases the rate of heterogeneous nucleation of the α-phase during β α transforation and Nb decreases the growth rate of α grains. Nb, B and C increase the stability of α grains against their growth on passing through the α single-phase field during cooling [1]. Despite the fact that the effect of above entioned alloying eleents on the grain foration is already well known, distribution of these eleents during solidification at the different solidification conditions is still of large interest, especially in relation to the investigations of the colunar to equiaxed transition in these alloys. The ai of the present work is to study the effect of solidification paraeters such as growth rate V and teperature gradient G L on distribution of ain alloying eleents (Ti, and Nb) during directional solidification of Ti-44-5Nb-0.2B-0.2C (at%) alloy. Quench during directional solidification (QDS) experients were carried out to prepare saples at various V and G L.

2 2. EXPERIMENTAL PROCEDURE The interetallic alloy with the noinal coposition Ti-44-5Nb-0.2B-0.2C (at.%) and oxygen content of about 500 wtpp was supplied in the for of vacuu arc reelted conical ingot with a diaeter changing fro 35 to 60 and length of 310. The ingot was cut to sall rectangular blocks with diensions of 11x11x150 3 by electro spark achining. Cylindrical saples for QDS experients with a diaeter of 10 were lathe achined fro the blocks. Directional solidification was perfored in dense cylindrical Y 2 O 3 oulds with inside/outside diaeter of 10/15 at nine cobinations of three constant growth rates V and three constant teperature gradients in liquid at the solid-liquid interface G L, as suarized in Table 1. Directional solidification was perfored in a odified Bridgan-type apparatus described elsewhere [2]. After directional solidification to a constant length of 80 the saples where quenched by a rapid displaceent of the ould into the water-cooled crystallizer at a cooling rate of 50 Ks -1. Microstructural investigations were perfored by optical icroscopy (OM) and backscattered scanning electron icroscopy (BSEM). As the ost suitable ethod for the evaluation of the cheical coposition and distribution of alloying eleents was selected the energy-dispersive spectroetry (EDS). OM, BSEM and EDS saples were prepared using standard grinding and polishing etallographic techniques. After echanical polishing the saples for optical icroscopy were cheically etched in a reagent of 100 l H 2 O, 10 l HNO 3 and 3 l HF. For EDS area and point analyses along a line, JSM-7600F scanning electron icroscope with EDS detector was used. Before each easureent the quant optiization on Ti and standardization using a standard with noinal coposition Ti-46-8Nb (at.%) were perfored. The preparation of hoogeneous standard for calibration of EDS equipent was described elsewhere [3]. Accelerating voltage during all easureents was 10 kv. According to Monte Carlo siulation of electron traectory in the studied alloy the predicted signal depth at 10 kv is around n. Table 1. Paraeters of the directional solidification Maxiu elt Saple teperature No. [ C] Growth rate V [10-5 s -1 ] Teperature gradient G L [10 3 K -1 ] RESULTS AN DISCUSSION 3.1 Distribution of ain alloying eleents in the as-cast alloy Microstructure of the as-cast alloy Ti-44-5Nb-0.2B-0.2C (at.%) consists of colunar and equiaxed grains (Fig. 1), which are fored by α 2 (Ti 3 ) and γ(ti) laellae, ribbon-like boride particles and isles of γ-phase at the grain boundaries and in the interdendritic regions. Detailed icrostructure description of the ingot is published elsewhere [4]. The cheical coposition of the studied alloy was easured by the EDS area analysis. Despite the liitation of the EDS ethod concerning easureents of sall aounts (0.2 at.%) of light eleents (B, C), the average content of Ti, and Nb in the studied ingot is in good agreeent with the average cheical coposition easured by the wet cheical analysis (WCHA), see Tables 2 and 3. It should be noted that in

3 the case of the interetallic Ti based alloys, EDS easureents correspond to WCHA only after the standardization of the EDS equipent using the standard with the cheical coposition close to the easured alloy [3]. Fig. 1. Macrostructure of the ingot: (a) top part and (b) botto part. C - colunar grains, E - equiaxed grains. Table 2. Cheical coposition of the as-cast alloy easured by EDS Ti Nb wt.% ± ± ± 0.26 at.% Table 3. Cheical coposition of the as-cast alloy easured by WCHA Ti Nb B C wt.% ± ± ± at.% Distribution of Ti, and Nb across the transversal sections in the top part (TP) and botto part (BP) of the ingot is shown in Fig. 2. EDS easureents were carried out fro the surface to the center of the ingot. Each point in the figure represents the average cheical coposition easured on the area of 0.5x The inhoogeneity of the cheical coposition is ore pronounced in the top wider part of the ingot, where slightly higher aount of is segregated in the colunar grains and Nb reaches its axiu values in the central part at the expense of decrease of content. Fig. 2. Distribution of alloying eleents: (a), (b) Ti and (c) Nb. TP - top part of the ingot, BP - botto part of the ingot. 3.2 Distribution of ain alloying eleents in the QDS saples Distribution of the ain alloying eleents during solidification was investigated on the longitudinal sections of the QDS saples at four positions with different distance fro dendrite tip, arked as I, II, III and IV in Fig. 3a. EDS profiles consist of 100 individual easureents positioned by 20 μ of each other along a line (Figs. 3b-e). To describe ore detailed distribution of eleents, the additional easureents along the line with a length ranging fro 20 to 30 μ were perfored within the interdendritic region and dendrites (Figs. 3f and 3g).

4 Fig. 3. EDS easureents of distribution of alloying eleents: (a) OM icrograph of the longitudinal section of the QDS saple; (b) position I, SEM; (c) position II, SEM; (d) position III, SEM; (e) position IV, SEM; (f) distribution of, Ti and Nb within interdendritic region; (g) distribution of, Ti and Nb within dendrite. For the qualitative and quantitative analysis of the distribution of the ain alloying eleents, dependence of the concentration of Ti, and Nb on fraction of solid f s and segregation deviation paraeter were selected. Data fro EDS easureents at the positions I, II, III and IV were sorted using single-eleent sorting schee with as the ain sorting eleent [5]. The fraction of solid f s (i) was assigned to each easured point i. Since absolute axiu and iniu concentrations cannot be guaranteed for the saple using the rando sapling, we assigned f (i ) (R 0.5) / N, where R i is the rank nuber and N s is the total nuber of points, so f s (i) continuously varies fro 0 to 1 (0 < f s < 1). The typical evolutions of the concentration of Ti, and Nb with f s at all four positions were shown elsewhere [6]. Segregation deviation paraeter was calculated for the positions I, II, III and IV for each alloying eleent where N 1 Ci C0 (1) NC 0 i 1 C 0 is the average concentration of the eleent, i Ci is the concentration of the eleent at the point i and N is the total nuber of the analysis points for one analysed position [7]. Segregation deviation paraeter is considered as the ost accurate and suitable for the description of the severity of icrosegregation, because it is calculated using all data easured at the analysed position. This paraeter has no liitation of other icrosegregation paraeters such as segregation range and segregation coefficient, which are based only on the iniu and axiu solute concentrations [6]. 3.3 Effect of the solidification paraeters V and G L on the distribution of, Ti and Nb As is shown in the Fig. 3, positions I and II are located in the ushy zone and positions III and IV below the ushy zone. Position I is situated close to the dendrite tip and position III close to the position of the peritectic transforation which leads to a strong back-diffusion of alloying eleents [4]. The growth rate V and teperature gradient G L affect the distribution of alloying eleents differently within and below the ushy zone. Fig. 4 shows the variations of segregation deviation paraeter with the growth rate and

5 Ti Nb teperature gradient in all four positions for. The evolutions of and show siilar variations with V and G L as evolutions of presented in Fig. 4. Fig. 4. Variations of the segregation deviation paraeter with the growth rate V in the positions I, II, II and IV at three constant teperature gradients G L. The teperature gradients are indicated in the figure. Severity of icrosegregation decreases with increasing growth rate in the ushy zone (positions I and II) and only slightly increases below the ushy zone (positions III and IV) at all three studied teperature gradients. Decrease of with the increasing V in the ushy zone is in a good agreeent with the odel based on the easured teperature curves proposed by Martorano and Capocchi [7]. In this odel, siilar dependency is predicted also for the region below the ushy zone. However, lower at lower growth rates in positions III and IV are observed in our case. This phenoenon can be explained by the ore extended hoogenization during solid state transforations when the growth rate is reduced. This is in agreeent with icrosegregation odel of Brody and Fleing [8] and also with the odel based on a constant average cooling rate of Martorano and Capocchi [7]. Teperature gradient affects the distribution of alloying eleents in the sae way in positions II, III and IV, where oderate decrease of with the increase of G L was observed at all three studied growth rates (Figs. 4b, 4c and 4d). On the other hand, there is an inverse tendency in the position I, as shown in Fig. 4a. According to Martorano and Capocchi [7], the value of is initially zero because the liquid was assued to be hoogeneous before the beginning of the solidification of the peritectic alloys and this value increases to a axiu due to solute partitioning during solidification. But there is no evident influence of different cooling rates or teperature gradients on predicted for early beginnings of the solidification [7]. The observed increase of with the increase of G L in the position I in the present work leads to the assuption, that the increase of at the beginning of the solidification is slightly faster at higher G L in the studied alloy.

6 4. CONCLUSIONS 1. Quench during directional solidification (QDS) experients were carried out to prepare saples at various V and G L with the ai to study their effect on the distribution of the ain alloying eleents during directional solidification of the interetallic Ti-44-5Nb-0.2B-0.2C (at%) alloy. 2. The distribution of ain alloying eleents in the as-cast alloy was easured by EDS area analyses. The inhoogeneity of the cheical coposition is ore pronounced in the top part of the ingot, where slightly higher aount of is segregated in the colunar grains and Nb reaches its axiu values in the central part at the expense of decrease of content. 3. EDS point analyses along the line at four positions on the longitudinal sections of the QDS saples were perfored. Dependence of the concentration of Ti, and Nb on fraction of solid f s and segregation deviation paraeter were evaluated for all positions at the studied cobinations of V and G L. 4. Severity of icrosegregation decreases with increasing V in the ushy zone and only slightly increases below the ushy zone at the studied G L. This phenoenon can be explained by the ore extended hoogenization during solid state transforations when the growth rate is reduced. Moderate decrease of with the increase of G L was observed at the studied growth rates in all easured positions except the position I located close to the dendrite tip, where the inverse tendency was noticed. It leads to the assuption, that the increase of at the beginning of the solidification is slightly faster at higher G L in the studied alloy. The evolutions of Ti and Nb show siilar variations with V and G L as evolutions of. ACKNOWLEDGEMENTS This work is the result of the proect Copetence center for new aterials, advanced technologies and energy ITMS , supported by the Research and Developent Operational Progra funded by the European Regional Developent Fund. The financial support of the Slovak Research and Developent Agency under the contract APVV is acknowledged. The authors would like to thank to Ing. Martin Nosko, PhD. for the help with EDS analyses. REFERENCES [1] IMAYEV, R.M., IMAYEV, V.M., OEHRING, M., APPEL, F. loy design concepts for refined gaa titaniu aluinide based alloys. Interetallics, 2007, vol. 15, pp [2] LAPIN, J., ONDRÚŠ, Ľ., NAZMY, M. Directional solidification of interetallic Ti-46-2W-0.5Si alloy in aluina oulds. Interetallics, 2002, vol. 10, pp [3] KLIMOVÁ, A. Distribution of ain alloying eleents during directional solidification of Ti-44-5Nb-0.2B-0.2C alloy. In 7th Seinar of Central European PhD Students - Research in Materials Science, , Trnava. [4] KLIMOVÁ, A., LAPIN, J. Microsegregation behaviour of the alloying eleents in directionally solidified interetallic Ti-44-5Nb-0.2B-0.2C alloy. In METAL 2012 : International Conference on Metallurgy and Materials. Brno: TANGER, ISBN [5] GANESAN, M., DYE, D., LEE, P. D. A technique for characterizing icrosegregation in ulticoponent alloys and its application to single-crystal superalloy castings. Metallurgical and Materials Transactions A, 2005, vol. 36A, pp [6] KLIMOVÁ, A., LAPIN, J. On distribution of alloying eleents during solidification of peritectic Ti based alloy. In International Doctoral Seinar: Proceedings. - Trnava: unipress, 2012, pp ISBN [7] MARTORANO, M. A., CAPOCCHI, J. D. T. Effects of processing variables on the icrosegregation of directionally cast saples. Metallurgical and Materials Transactions A, 2000, vol. 31A, pp [8] BRODY, H.D., FLEMINGS, M.C. Solute Redistribution in Dendritic Solidification, Transactions of the Metallurgical Society of AIME, 1966, vol. 236, pp