EFFECT OF YTTRIUM ADDITION ON MICROSTRUCTURE OF ALLOY Ti-47Al, PREPARED BY PLASMA MELTING AND VACUUM INDUCTION MELTING

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1 EFFECT OF YTTRIUM ADDITION ON MICROSTRUCTURE OF ALLOY Ti-47Al, PREPARED BY PLASMA MELTING AND VACUUM INDUCTION MELTING Tomáš ČEGAN, Miroslav KURSA VŠB-TU Ostrava, 17. listopadu 15, , Ostrava-Poruba, ČR, Abstract TiAl based alloys are very perspective material thanks to its outstanding properties, such as high specific strength, low density, high modulus of elasticity and resistance to creep. They might replace the currently used materials in many applications in aircraft and automotive industries, but also in other industries. The main reason why it happens only in very limited extent is their brittleness and complicated production of these alloys. Fabrication of TiAl alloys is difficult, because titanium and aluminium are reactive materials. That s why these alloys are usually prepared in an inert atmosphere. One of possibilities of enhancement of problematic properties of these alloys is influencing their micro-structure by alloying by elements, which influence grain size and reduce dispersion of precipitates (e.g. B, C, Si). Yttrium is one of the elements, which similarly influence micro-structure and properties of TiAl alloys. According to the work [1] it is possible by addition of yttrium up to 0.5 at.% to increase ductility, plasticity, formability at high temperatures and resistance to oxidation. TiAl alloys with nominal composition of Ti-47Al atomic percent (at.%) and with various additions of yttrium 0.1, 0.2, 0.3 and 0.4 (at.%) were prepared by different methods, namely by plasma melting and by vacuum induction melting. The applied procedures of preparation of alloys lead to creation of two different types of micro-structures. The samples were used for metallographic observation and for measurement of microhardness. Keywords: TiAl alloys; micro-structure; yttrium; porosity; micro-hardness 1. EXPERIMENT Plasma melting was chosen for the first stage of metallurgical preparation. Foundry alloys TiAl prepared also by plasma melting with composition of Ti-(5-20 at.% Al) were used for preparation, together with formed titanium with purity of 99.99%, bits of aluminium with purity of 99.9%, and bits of yttrium with purity of 99.9%. The charge was uniformly distributed in water cooled copper mould placed in the furnace. Melting was performed by quadruple passage through the zone (twice from each side), and inert atmosphere was ensured during melting by argon flowing through the furnace chamber. Rate of shift of the mould was 2 cm/min and current intensity was 500A. Products of plasma melting were ingots with mass of 600g of the given composition. Ingots were then cut to smaller parts in order to take the samples and to adjust dimensions of material for the next stage of preparation. It was found after investigation of taken samples, that composition of the alloy after plasma melting was not homogeneous in all ingot, which was evident not only from photos of micro-structure, but also from the results of measurement of micro-hardness [2]. For these reasons vacuum induction melting was chosen as another method for preparation. Ingots were re-melted in high-frequency vacuum induction furnace LEYBOLD HERAEUS IS1/FFF. Due to high reactivity of titanium a paint based on Y 2 O 3 was applied on the corundum melting crucible, which should have prevent contamination of the melt by particles of Al 2 O 3. After melting of the charge the melt was poured into pair of graphite moulds, onto which an Y 2 O 3 based paint was also applied to. Each of them made it possible to pour three samples next to each other in the form of sticks with diameter of 10mm and length of 100mm. Risers were then cut off from these sticks and the sticks were cut perpendicularly to their axis for 1

2 obtaining the samples for observation of optical microscope OLYMPUS GX51, equipped with digital camera OLYMPUS DP12. The risers, which remained after vacuum induction melting, were cut to smaller parts. They were then used as a charge for re-melting in plasma furnace into the form of oval shaped samples (Figures 1a, c, e). This melting was made for comparison of homogenised structure and distribution of the alloying element after plasma melting and vacuum induction melting. The melt was in molten state for 60s. Oval samples were then cut by vertical cut lead through their centres. Fig. 1 Macrostructures of samples (a) Ti-47Al after plasma melting; (b) Ti-47Al after vacuum induction melting; (c) Ti-47Al-0,1Y after plasma melting; (d) Ti-47Al-0,1Y after vacuum induction melting; (e) Ti- 47Al-0,4Y after plasma melting; (f) Ti-47Al-0,4Y after vacuum induction melting 2

3 2. MACRO- AND MICROSTRUCTURE OF ALLOYS Fig. 1 shows macro-structures of selected samples of this alloy. Standard methods of grinding and polishing were used for visualisation of macro-structures. Etching was made with use of the etching agent composed of 50ml H2O, 40ml HNO3 and 10ml HF applied for 3 minutes. It is evident from photos of macro-structures that orientation and grain size distinctly differs in the alloys of identical composition depending on method of melting and shape of the mould. Columnar grains after vacuum induction melting were smaller and they grew from the mould wall towards centre of the crosssection of the samples. Columnar grains after plasma melting were larger and they grew from the mould wall upwards. The shape and size of grains also differ in dependence on content of alloying element. Area of cut can be in the samples prepared by plasma melting divided in dependence on micro-structure into 3 areas. They are shown in Fig. 2a. These areas are the following ones: 1. Area of equiaxed columnar grains at the bottom part, which is formed by grains with fully lamellar micro-structure composed of alternating lamellas of the phases α 2 and γ (see Fig. 2b). 2. Area of columnar to equiaxed grains in central part, which is formed by transient cell-dendritic structure (see Fig. 2c) 3. Area of coarser equiaxed grains at the top part, which is formed by grains with developed dendritic structure (see Fig. 2d). Dendrites are formed by alternating lamellas α 2 and γ, and in inter-dendritic areas the phase γ was identified, and also the phase YAl 2 in the samples containing yttrium. Fig. 2 (a) Schematic representation of areas of cut of the sample after plasma melting; (b) Example of micro-structure in the area 1 Ti-47Al; (c) Example of micro-structure in the area 2 Ti-47Al; (d) Example of micro-structure in the area 3 Ti-47Al-0,2Y These different types of micro-structure in individual areas were created as a result of different conditions of cooling. In the area 1 heat removal was the highest thanks to the rapid cooling through the mould wall, and it was the lowest in the area 3. Evaluation of grain size was made by linear method in individual areas of the cut of the samples with use of the program analysis auto, and obtained results are shown in Fig. 3. It is apparent from the results that in the areas 1 and 3 grain size decreases with an increasing yttrium content, only in the sample with 0.1 at.% Y grain size increased in the area 3. Contrary to that in the area 2 grain size increases with an increasing content of yttrium. 3

4 Fig. 3 Grain size in individual areas of the samples after plasma melting in dependence on the content of Y Evaluation of grain size was made also in the samples after vacuum induction melting. Fig. 4 shows the results. It follows from the results that grain size decreases with an increasing content of yttrium, similarly as in the sample after plasma melting, an increase occurred only in case of the sample containing 0.1 at.% Y, which might have been caused by an increased content of the Y 2 O 3 particles, which got into the alloy during vacuum induction melting from the paint applied on melting crucible and from graphite mould, and quantity of these particles was the highest namely in the sample containing 0.1 at.% Y. This can be explained by the fact that this type of alloy was poured as the first into the mould with applied paint. These particles are shown in Fig. 5. Grain size [µm] ,72 251,31 164,13 151,38 155, Yttrium content [at. %] Fig. 4 Grain size after vacuum induction melting Fig. 5 Particles of Y 2 O 3 (electron microscope, mode BSE) Fig. 6 Representation of the phase rich in yttrium in the sample Ti-47Al-0.4Y prepared by plasma melting, electron microscope, mode BSE Micro-structure of the samples after vacuum induction melting was dendritic. Dendrites were formed by alternating lamellas of α 2 and γ, and in inter-dendritic areas the phase γ was identified, as well as the phase YAl 2 in the samples containing yttrium. Only on the edges of the cut a small layer of fully lamellar smaller grains was observed, which were formed as a result of rapid cooling of the melt by the mould wall. It may be concluded from comparison of microstructure of the central part of cross-section of the alloys after vacuum induction melting, that dendrites are finer with an increasing content of yttrium. This phenomenon was not observed in the samples after plasma melting. Dendritic micro-structure was very similar in all the samples after plasma melting, only in the alloys with higher content of yttrium a larger quantity of inter-dendritic phase was observed. In order to determine distribution of phases rich in Y, and to determined their composition and homogeneity of distribution of alloying element. Observation on electron microscope was performed, as well as EDAX analyses of composition of the observed phases. Figure 6 taken in the mode BSE on electron microscope shows distribution of the phases rich in yttrium in the sample Ti-47Al-0,4Y. The phases rich in yttrium, which are in this photo represented by light colour, were afterwards analysed by EDAX. It was established that the phase rich in yttrium occurs in the samples in 2 different types. One is represented in Figure 6 by white colour, and the other one by grey colour. Light particles have smaller dimensions than darker grey 4

5 particles, and composition determined by EDAX analysis also shows differences. Majority of the phase YAl 2 occurs along grain boundaries, where they form, especially in the alloys with higher content of yttrium (0.3, 0.4Y), kind of mesh composed of discontinuous ellipsoid or elongated particles. In the alloys with smaller contents of yttrium the phase YAl 2 is present, predominantly in the form of ellipsoid particles. 3. MICRO-HARDNESS OF ALLOYS Measurement of micro-hardness according to Vickers was performed with use of the micro-hardometer FM- ARS The measurement was performed with slightly etched samples under constant load of 100g for 7s. More than 60 measurements were made on each sample with an arrangement of the points of measurement into a matrix 10x8 in the samples after plasma melting, and 8x8 in the samples after vacuum induction melting, with spacing of 0.5mm between individual points of measurement. The matrix was situated in such a manner, that its centre is maximally identical with the centre of the cross-section of the samples. Always 3 the softest and 3 the hardest indents were documented. In the samples after plasma melting was evaluated not only an overall micro-hardness, but also micro-hardness of the areas 1 and 3. Diagram in Figure 7 summarises the results. It follows from this diagram that micro-hardness decreases with an increasing content of yttrium both in the samples after plasma melting and after vacuum induction melting. However, the cast samples have higher micro-hardness. Decrease of micro-hardness may be caused by an increasing content of inter-dendritic γ-tial and YAl 2 phases, which may be confirmed also by small drop of micro-hardness in the area 1, where structure was not dendritic, as well as by higher decrease of micro-hardness in the area 3, where the dendrites were developed the most (see Fig. 8). 4. POROSITY OF ALLOYS Fig. 7 Effect of Y content on microhardness Porosity of the alloy was investigated on optical microscope OLYMPUS GX51, equipped with digital camera OLYMPUS DP12. Identification of the pores was performed with use of the program analysis auto. Distribution of pores was evaluated by statistic methods. Altogether exactly 20 pictures were taken from the given sample, on which the pores were identified. Fig. 8 Effect of Y content on microhardness in regions 1 and 3 after plasma melting 5 Evaluation of distribution of pores depends on the used magnification of microscope. Possibility of detection of small pores increases with an increasing magnification. For our measurement a hundredfold magnification was chosen. With use of this magnification the pores with dimensions from 2µm to 20µm were identified. Identified pores were then ordered according to their diameter and divided into classes. It is evident from results of this measurement that both in the samples prepared by plasma melting, and in the samples prepared by vacuum induction melting the number of identified pores increases with an increasing content of yttrium. The only exception is drop in number of identified pores in the sample containing 0.4 at.% Y, prepared by vacuum induction melting. Total number of identified pores in the samples is shown in diagram in Fig. 9.Apart from evaluation of number of pores in the classes according to their diameters, also porosity of the alloys was evaluated by phase analysis. Results of phase analysis of content of area of pores from the overall surface of taken photos made by program analysis auto are

6 summarised in diagram in Fig. 10. Apart from evaluation of total number of identified pores and porosity percentage also the influence of yttrium alloying was determined by phase analysis fro number of pores larger than 10µm. The results were summarised in the diagram in Fig. 11. Fig. 9 Dependence of Y content and number of identified pores Fig. 10 Dependence of Y content on porosity Fig. 11 Dependence of Y content on number of pores larger than 10µm 5. CONCLUSIONS Ti-Al-Y alloys were prepared by plasma melting and vacuum induction melting. Area of cross-section of the samples after plasma melting can be divided according to micro-structure onto 3 different areas, namely area 1 with fully lamellar equiaxed fine grains, area 2 with predominantly columnar grains with transient cell-dendritic structure, and area 3 with equiaxed grains formed by developed dendritic structure. Dendrites after vacuum induction melting and also after plasma melting are formed by alternating lamellas α 2 and γ. In inter-dendritic areas the phase γ is present, and the phase YAl 2 is present in the samples containing yttrium. It was determined by measurement of grain size that yttrium influences solidification, and that with an increasing content of this alloying element in the alloy grain size decreases in the areas 1 and 3 in the alloys after plasma melting, with the exception of the samples containing 0.1 at.% Y, and of the cast alloy after vacuum induction melting. Influencing of the grain size was attributed to the phase YAl 2, which limits growth of grains [1]. It was determined that majority of the phase YAl 2 is present along the grain boundaries, where it forms, particularly in the alloys with higher content of yttrium, kind of mesh composed on discontinuous ellipsoid or elongated particles. The phase YAl 2 occurs in the alloys with lower content of yttrium, predominantly in the form of ellipsoid particles. It was observed that number of pores in the TiAl system increases with an increasing content of yttrium. However, number of pores, diameter of which was larger than 10µm, showed a decreasing tendency at an increasing content of yttrium up to 0.3 at.% in the samples prepared by plasma melting, and up to 0.2 at.% in the samples prepared by vacuum induction melting. It was established by measurement of micro-hardness of the alloy Ti-47Al that micro-hardness decreases with an increasing content of yttrium in the samples. This may be caused by higher share of inter-dendritic area with γ phase in the samples with higher content of alloying element. 6

7 ACKNOWLEDGEMENT This paper was created in the project No. CZ.1.05/2.1.00/ "Regional Materials Science and Technology Centre" within the frame of the operation programme "Research and Development for Innovations" financed by the Structural Funds and from the state budget of the Czech Republic. REFERENCES [1.] LI, B., KONG, F., CHEN, Y. Effect of Yttrium Addition on Microstructures and Room Temperature Tensile Properties of Ti-47Al Alloy. Journal of Rare Earths 24, 2006, [2.] ČEGAN, T., KURSA, M. Structural characteristics of alloy TiAl - Y, prepared by plasma melting. Den doktorandů Ostrava