Ultra High Pressure Consolidation of Ball Milled Nanocrystalline TiTaNb Alloys

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1 Materials Transactions, Vol. 48, No. 5 (2007) pp. 909 to 914 Special Issue on New Developments and Analysis for Fabrication of Functional Nanostructures #2007 The Japan Institute of Metals Ultra High Pressure Consolidation of Ball Milled Nanocrystalline TiTaNb Alloys Jan Dutkiewicz 1;2; *, Wojciech Maziarz 1, Lucyna Jaworska 2 and Kinga Zapała 1 1 Institute of Metallurgy and Materials Science of the Polish Academy of Sciences, Kraków, Reymonta 25, Poland 2 Pedagogical Academy, Kraków, Podchorążych 2, Poland The effect of increased Ta and Nb additions on the structure of high energy ball milled Ti alloys was studied using X-ray diffraction and high resolution TEM. Ball milled powders were consolidated using ultra high pressure from 4 7 GPa at temperatures of C. The microstructure of the compacts consisted of ultra fine grains in the range of 20 nm of and phases. Micro-hardness measurements showed very high hardness of ball milled and compacted powders close to 7 GPa (slightly decreasing with the increase of alloying additions) and a decrease in the Young s modulus with the increase of Nb and Ta content i.e. the amount of phase. [doi: /matertrans ] (Received October 26, 2006; Accepted December 19, 2006; Published April 25, 2007) Keywords: mechanical alloying, nanocrystalline TiNbTa alloys, ultra high pressure consolidation, structure, mechanical properties 1. Introduction Nanocrystalline titanium base alloys exhibit attractive properties such as unusually high strength combined with reasonable ductility and toughness. 1 6) Nanocrystalline structure can be obtained in titanium alloys using rapid solidification, 6) severe plastic deformation (SPD), 1 5) ball milling (BM) and compacting 7 9) or hydrostatic extrusion. 10) Equal channel angular pressing (ECAP) in the temperature range C improves the workability of Ti 1 5) and allows significant refinement of the grain size, increasing the strength by 60 90%. 1 3) Hydrostatic extrusion has been recently used for grain refinement of commercially pure titanium down to nano-metric scale. 10) The preliminary experiments have shown the ability to decrease the grain size down to 100 nm. Structural evolution of titanium powder during ball milling (BM) under different atmospheres was studied in several papers. 7 9) Ball milling of the commercial purity titanium enables a decrease in crystal size down to 10 nm after 40 hours of milling in a high energy planetary mill. 10) Especially interesting are additions of bio-neutral elements like Ta or Nb increasing ductility and decreasing Young Modulus of titanium alloys 9) due to increase of the fraction of the phase. In the present paper high pressure consolidation of MA powders was used to prepare nanocrystalline TiNbTa alloys. 2. Experimental Powders of titanium (110 mm size and of purity > 99:9%), tantalum (150 mm size and of purity 99.98%) and niobium (10 mm size and of purity > 99:8%) were used as starting materials. The powders were initially blended to the desired compositions of Ti-5Ta5Nb and Ti-10Ta10Nb (numbers indicate at%) in a glove-box under argon atmosphere and subjected to ball milling up to 80 hrs in high energy planetary mill (Fritsch Pulverisette P5/4). Subsequent ultra high pressure consolidation of MA powders at 7 GPa and 650 C using Bridgeman method was applied. The structural changes during milling as well as of the consolidated samples were *Corresponding author: nmdutkie@imim-pan.krakow.pl studied in a Philips PW 1830 diffractometer using Cu K radiation and a Philips CM 20 transmission electron microscope (TEM) equipped with a Phoenix energy-dispersive X- ray analysis system or Technai G20 FEG for high resolution. Thin foils from hot pressed samples were prepared by dimpling and ion milling using Gatan equipment, whereas the milled powders were embedded in resin and cut by using Leica microtome. The dynamic microhardness test was performed on CSEM Mikro-Combi-Tester with load of 100 mn and compression tests using Instron machine. 3. Results and Discussion Figure 1 shows X-Ray diffraction patterns from the elementary powders of composition TiTa5Nb5 ball milled for 5, 10, 20, 40 and 80 hours. One can see broadening of the peaks due to gradual decrease of the grain size with a milling time. After 80 hours of milling only one broad peak exists due to structure changes approaching the amorphous state. Figure 2 shows bright and dark field micrographs taken from the Ti5Ta5Nb powder ball milled for 80 h. One can see clearly bright areas of size 5 20 nm indicating the existence of nanocrystals within the powder. The average crystal size approaches 10 nm. The electron diffraction pattern placed as Intensity [a.u.] 80h 40h 20h 10h 5h 0h Θ Fig. 1 X-ray diffraction pattern taken from the TiTa5Nb5 composition elemental powders ball milled for 0, 5, 10, 20, 40 and 80 hours.

2 910 J. Dutkiewicz, W. Maziarz, L. Jaworska and K. Zapała BF DF Fig. 2 TEM micrographs of a powder particle of the TiTa5Nb5 alloy milled for 80 hours showing an average crystal size of 10 nm. HREM IFFT a c FFT nm 2.34 Fig. 3 HREM image of Ti-10Ta10Nb alloy milled 80 hours and corresponding FFT and IFFT images. an insert shows diffused ring confirming the nanocrystalline state. The high resolution microscopy of milled powders has shown that depending on composition of powders the structure was composed of a mixture amorphous and or phases. Figure 3 shows HREM image of Ti-10Ta10Nb alloy milled 80 h and corresponding Fast Fourier Transform (FFT) and Inverse Fast Fourier Transform (IFFT) images. One can see in the IFFT image that two areas as the crystalline (c) or amorphous (a) of size of few nanometers exists in MA alloy. Between these areas the highly deformed close to amorphous structure can be seen, often characterized by a short range ordered regions. The FFT made from the crystalline area indicates that two most intense rings corresponding to nm and nm lattice plane spacings are in good agreement with (101) and (002) lattice planes of -Ti structure. The UHP consolidation at temperatures below 700 C allowed the retention of the nanocrystalline structure. In Fig. 4 are shown two X-Ray diffraction curves from Ti5Ta5Nb and Ti10Ta10Nb consolidated samples. One can see that TiTa5Nb5 alloy consists mostly only of the phase, whereas in the TiTa10Nb10 alloy a two phase þ structure forms. The broadening and relatively small intensities of diffracted peaks indicates, that the structure is of nanometric scale. This assumption is confirmed by the TEM observation. Figure 5 shows TEM bright and dark field images taken from the ultra high pressure compacted samples. One can see slight Intesity [a.u.] alfa Ti beta Ti Θ Fig. 4 X-ray diffraction patterns of the TiTa5Nb5 (bottom) and Ti- Ta10Nb10 (top) alloys consolidated by ultra high pressure. growth of the crystals (mostly ones) up to about 20 nm in the TiTa10Nb10 sample whereas the TiTa5Nb5 alloy has mostly only small crystals. The high resolution observation of TiTa10Nb10 hot pressed sample allowed to confirm the nanometric level of grain size. Figure 6 shows a set of high resolution images where one irregular -Ti grain of size of about 20 nm with [311] orientation attached to [100] oriented

3 Ultra High Pressure Consolidation of Ball Milled Nanocrystalline TiTaNb Alloys 911 BF DF TiTa5Nb5 SAPD 1-22 αti -113 αti 2-10 αti 0-12 αti -111 αti 002 αti BF DF TiTa10Nb10 SAPD 310 βti 004 αti 112 βti, 0-13 αti 2-10 αti 002 βti -111 αti 002 αti, 101 βti Fig. 5 TEM images of the TiTa5Nb5 and TiTa10Nb10 alloys ball milled and ultra high pressure compacted at 7 GPa and 650 C. -Ti grains i.e. forming high angle boundary. HREM observations allowed to identify the!-phase particles at [001] zone axis orientation (Fig. 7) observed in the grain of [111] zone axis orientation in TiTa10Nb10 hot pressed alloy. The same phase was described in 11) as formed during the! transformation in metastable -Ti alloys. The

4 912 J. Dutkiewicz, W. Maziarz, L. Jaworska and K. Zapała HREM IFFT FFT β-ti [100] β-ti [311] Fig. 6 HREM image of Ti-10Ta10Nb alloy ultra high pressure compacted at 7 GPa and 650 C corresponding FFT and IFFT images. mechanical properties of UHP alloys were determined using both, microhardness and a compression tests. Figure 8 shows the Depth-Load curves recorded during dynamic microhardness test of investigated alloys. One can see that in TiTa10Nb10 alloy the penetration depth is slightly larger than in TiTa5Nb5. This suggests that alloy with dominant structure is of higher plasticity. The Young Modulus measured during unloading process was calculated at 113 MPa and 104 MPa for TiTa10Nb10 and TiTa5Nb5 alloys respectively, corresponding to microhardness of about 6 GPa and 7 GPa. In Fig. 9 the results of compression test of investigated alloys are shown. They are in good agreement with the microhardness test. The TiTa10Nb10 alloy has a higher plasticity and reaches about 8% elongation, i.e. more than TiTa5Nb5 with maximum tensile deformation of about 6%. The compression strength for the Ti10Ta10Nb alloy containing structure is about 1420 MPa, higher than of the Ti5Ta5Nb alloy consisting only of the phase which reached UTS=1270 MPa. 4. Conclusions (1) Ball milling of elemental Ti powders containing 5 10 at% of Ta and Nb lead to nano-crystalline structures giving only a broad peak in X-ray diffraction. TEM allows the resolution of and crystals with an average size of 10 nm and microhardness above 10 GPa. (2) After ultra high pressure consolidation at 7 GPa and 650 C only minor grain growth is observed up to 20 nm. It is connected with a small hardness decrease. Increase of Ta and Nb content up to 10% causes small increase of hardness, tensile elongation and a tensile strength but causes a decrease of the Young modulus. (3) The phase composition of UHP compacted alloy TiTa5Nb5 consists mainly of the phase, while in the alloy TiTa10Nb10 forms significant fraction of the phase in addition to the phase. The! phase was identified using high resolution electron microscopy.

5 Ultra High Pressure Consolidation of Ball Milled Nanocrystalline TiTaNb Alloys 913 HREM [001] ω IFFT of ω [111] β-ti IFFT of β-ti Fig. 7 Set of HREM image of Ti-10Ta10Nb alloy ultra high pressure compacted at 7 GPa and 650 C showing! phase precipitates Ti10Ta10Nb Ti5Ta5Nb TTN L/mN TTN D/nm S [MPa] TTN Fig. 8 Depth-Load curves recorded during dynamic microhardness test of investigated alloys. Acknowledgements Financial support of the Research Project PBZ-KBN-096/ T08/2003 is gratefully acknowledged e [%] Fig. 9 " curves recorded during dynamic compression test of investigated alloys.

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