Recenze: Ing. Radovan Bureš, CSc

Hutnické listy č.1/2010, roč. LXIII ISSN 0018-8069

Neželezné kovy a slitiny Non-ferrous Metals and Alloys

Methods of Preparation of TiNb Based Alloys Metody přípravy slitin na bázi TiNb Ing. Petr Štěpán, Doc. Dr. Ing. Monika Losertová, Ing. Daniel Petlák, Prof. Ing. Drápala Jaromír, CSc., Vysoká škola báňská – Technická univerzita Ostrava, Fakulta metalurgie a materiálového inženýrství

Two methods were used for preparation of TiNb alloy. The first method consisted of vacuum electron beam floating zone melting of Ti rod wrapped around with a Nb wire. Solution heat treatment at 900°C for 1 hour in flowing Ar gas, water quenching and precipitation hardening at different temperatures (200, 400 and 600°C) for 1 hour were performed. Plasma furnace melting was the second method for preparation of the experimental alloys with nominal composition of Ti-22Nb and Ti-25Nb (at.%) using TiNb (55/45 wt.%) master alloy and Ti pieces. Heat treatment at 1100°C for 12 hours in flowing Ar gas and water quenching of prepared samples was realised. Metallographic observation, micro-hardness measurement and general micro-analysis of the prepared alloys were performed in asmelted and heat treated conditions. The effect of heat treatment and Nb content on the micro-structure and microhardness was determined. Bylo provedeno studium možnosti přípravy slitin na bázi TiNb dvěma metodami, a to elektronovým zonálním a plazmovým tavením. První technologií byla v elektronové peci pro přípravu slitiny Ti-25 Nb (at.%) přetavena Ti tyč o průměru 10 mm a délce 300 mm, na kterou byl navinut tenký Nb pásek. Odebrané vzorky byly homogenizačně žíhány při 900°C po dobu 1 hodiny v ochranné atmosféře Ar a poté byly precipitačně vytvrzeny při různých teplotách (200, 400 a 600°C) po dobu 1 hodiny v průtoku Ar. Druhou metodou, pomocí které byla předslitina TiNb (55/45 hm.%) dolegována kusovým titanem a přetavena v plazmové peci, byly připraveny experimentální slitiny o nominálním složení Ti-22Nb a Ti-25Nb (at.%). Slitiny byly pouze homogenizačně žíhány 12 hodin při 1100°C v Ar atmosféře a zakaleny do vody. Na slitinách připravených oběma metodami byl proveden metalografický rozbor, měření mikrotvrdosti a fázová mikroanalýza pro litý i žíhaný stav. Slitina připravená elektronovým zonálním tavením s průměrným složením 73,89 at.% Ti a 26,11 at.% Nb vykazovala dvojfázovou strukturu (α+β). Mikrotvrdost naměřená pro různé stavy tepelného zpracování dosahovala nejvyšší hodnoty (292 HV) po precipitačním vytvrzení při 400°C, což odpovídalo precipitaci fází α a ω. Mikrostruktura slitin Ti-22Nb a Ti-25Nb připravených plazmovým tavením s průměrným obsahem 21,57 a 27,13 at.% Nb byla dendritická a po homogenizačním žíhání přešla na lamelární (α+β). Hodnoty mikrotvrdosti byly ve stavu po plazmovém tavení srovnatelné s výsledky po tavení metodou EBFZM, po žíhání se však mikrotvrdost výrazně zvýšila. Byl rovněž prokázán vliv rostoucího obsahu Nb na pokles mikrotvrdosti, což souvisí s množstvím a morfologií matrice β a precipitující fáze α.

1. Introduction For many years, titanium based alloys were applied in different bio-compatible applications. Shape memory alloys on the base of TiNi have been widely used for specific implant materials such as orthodontic archwire and orthopaedic implants due to their unique properties. New Ti-based alloys with bio-compatible elements, such as Nb, Zr, Ta, Sn, have been developed in order to solve the problems with toxicity of some additional elements, such as V, Al and Ni. Recently, β-titanium alloys, Ti Nb, Ti–Ta and Ti–Zr based alloy systems were studied and found to display both lower elastic module and higher tensile strengths that are preferable for bio-compatible metals and alloys. Binary TiNb-based alloys with shape memory effect and good bio-compatibility are investigated as Ni free

suitable substitution of TiNi-based alloys in bio-medical applications. It has been confirmed [1, 2, 3] for wide range of Nb content (16.7 - 50 wt.% Nb) that TiNb alloys exhibited at room temperature the shape memory effect and super-elastic behaviour that is related to stress induced martensite. Niobium as well as possibly other alloying elements (such as Mo, W, Fe, Si, B, Ta, Zr, Sn, V) is β stabilizer so that TiNb falls into β-Ti alloys, possibly into β rich (α+ β) alloys. Preparation of alloys with high content of Nb, which is a high melting point metal has difficulties with use of conventional methods. Powder metallurgy (PM), mechanical alloying (MA), electron beam floating zone melting (EBFZM), plasma melting or induction skull melting (ISM) are the suitable techniques for preparation of the TiNb-based alloys using elemental metals. In the present study, different methods were

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Neželezné kovy a slitiny Non-ferrous Metals and Alloys used for preparation of TiNb-based alloys. First, plasma melting of Ti and Nb pieces in water cooled copper crucible was realised, but homogeneous dissolution and distribution of Nb in the prepared TiNb alloy was not achieved. Mechanical alloying of pre-hydrogenated pieces of Ti and Nb needed long time of milling and even 240 hours of process did not lead to fragmentation of pieces, only one third of the amount was sufficiently fragmented, the rest remained compact and was low alloyed. Hence, because the plasma metallurgy or MA were not successful and ISM technique was not available, the two methods were used to produce the alloys with the required contents of Nb: vacuum EBFZM of Ti rod wrapped around with Nb ribbon, and plasma re-melting of TiNb master alloy with Ti pieces.

2. Experiment Experimental alloys based on TiNb with 22 at.% Nb and 25 at.% Nb were prepared by two different techniques. The first method consisted of vacuum electron beam floating zone melting in vacuum of 6.6x10-2 Pa, with anode current of 55 mA and accelerating voltage of 6 kV. The EB-gun travelled along the vertical long axis of specimen at a rate of 3 mm.min-1. The Ti rod of diameter of 10 mm and length of 300 mm with wrapped around rolled Nb ribbon of thickness of 1 mm and width of 4 mm was re-melted. The sliced specimens were submitted to the solution heat treatment at 900°C for 1 hour in flowing Ar gas and then they were water quenched. The precipitation hardening was realised at different temperatures: 200, 300 and 400°C for 1 hour in flowing Ar gas and then it was followed by water quenching. In the second method, the melting of experimental Ti25at.% Nb and Ti-22at.% Nb alloys was performed in plasma furnace with four passes (two passes in Ar and last two passes in Ar-5% H2) in order to obtain the homogeneous material. Heat treatment at 1100°C for 12 hours was realised in Linn HT1800 furnace in flowing Ar gas and it was followed by water quenching from 700°C. The samples for metallographic observation were polished and etched in Kroll´s reagent (8 HF:15 HNO3:77 H2O) for 20-60 s. The micro-structure was observed using metallographic microscope OLYMPUS DP GX51. Microhardness of the plasma melted specimens as well as of the specimens prepared by EBFZM technique was measured by means of the LECO AMH 2000 instrument with load of 0.025 kg and indentation step of 1 mm. Scanning electron microscope (SEM) JEOL JSM 6490LV equipped with EDS INCA X - ACT probe was used to determine chemical and phase compositions of micro-structure. X ray analysis was realised to determine the crystalline state.

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3. Results and discussion The micro-structure observed in as-melted samples after EBFZM had duplex and fine precipitate character, as it is seen in Fig. 1 and 2. Based on average results of micro-analysis performed on two specimens (Table 1), the Ti and Nb contents show presence of α and β phases. The results obtained from the micro-hardness measurement are analogous to those for Ti-25 at.% Nb published in [4]. The micro-hardness values showed an obvious relation with heat treatment and aging. The specimens in the as-melted state and aged at 600°C showed similar micro-hardness (273 HV and 271 HV, respectively, in Table 2) related with α phase precipitation. The highest micro-hardness value of 292 HV for aging at 400°C is due to α and ω phase precipitation as it was proved in [4]. Conversely, aging at 200°C caused the decrease of the micro-hardness to 244 HV. The X-ray analysis proved that TiNb rod was prepared as a single crystal. Hence, no grain boundaries were observed in the specimens. Single crystal growing of inter-metallic alloys is difficult, so the preparation of TiNb alloy in a single crystal state could be considered as extraordinary achievement that at present cannot be explained using physical or metallurgical theories. The phase analysis of the prepared TiNb alloy was realised in an austenitic state, martensite phase was not observed. Generally, the properties of TiNb alloys are strongly affected by content and fraction of α, β and ω phases, but the metallographic observation in the present work was limited by microscopy resolution, so the next analyses by TEM of the aged specimens are needed for confirmation of this effect on microstructure. The micro-structure of Ti-22Nb and Ti-25Nb samples after plasma melting was dendritic (Fig. 3, 4, 7 and 8) with very fine laths. After heat treatment at 1100°C β phase with coarse or fine laths of α phase were found (Fig. 5, 6, 9 and 10). Table 3 summarises the results of phase-analysis and general micro-analysis of both alloy compositions. Therefore, the Ti-22Nb and Ti-25Nb alloys were prepared with 21.57 at.% and 27.13 at.% of Nb content, respectively. The results of microhardness measured for both heat treatment conditions of Ti-22Nb (272 and 621 HV) and Ti-25Nb (235 and 409 HV) alloys as seen in Table 4 showed that annealing increased micro-hardness because of modification of dendritic micro-structure to lamellar one. Furthermore, the higher Nb content in Ti 25Nb alloy decreased micro-hardness of the samples in as-plasma melted as well as annealed conditions due to lamellar character of micro-structure.



Tab. 1 The average values of general micro-analysis performed on two as-melted EBFZM specimens Tab. 1 Průměrné hodnoty plošné mikroanalýzy dvou vzorků po elektronovém zonálním tavení

Ti Specimen 1 2 Average values

wt. % 60.27 58.42 59.35

Nb at. % 74.63 73.15 73.89

wt. % 39.73 41.58 40.65

at. % 25.37 26.85 26.11

Tab. 2 The average values of HV micro-hardness measurements of EBFZM specimens after different heat treatment Tab. 2 Průměrné hodnoty mikrotvrdosti HV pro vzorky po elektronovém zonálním tavení a různém tepelném zpracování

heat treatment micro-hardness

as-melted 273 ± 16

200°C 244 ± 8

Fig. 1 Micro-structure of Ti-25Nb after EBFZM. Obr.1 Mikrostruktura Ti-25Nb po elektronovém zonálním tavení.

400°C 292 ± 25

600°C 271 ± 27

Fig.2

Micro-structure of Ti-25Nb with α and β phases after EBFZM (Detail of Fig. 1). Obr.2 Mikrostruktura Ti-25Nb s fázemi α a β po elektronovém zonálním tavení (Detail Obr.1).

Tab. 3 Results of phase micro-analysis of Ti-22Nb and Ti-25Nb after plasma melting and heat treatment. Tab. 3 Výsledky fázové mikroanalýzy Ti-22Nb a Ti-25Nb po plazmovém tavení a tepelném zpracování.

alloy treatment plasma melted (Fig. 4 and 8) heat treated (Fig. 6 and 10 )

micro-structure element white dendrite (1) dark inter-dendritic space (2) general analysis of alloy dark lath (1) bright lath (2) fine grain (1) large grain (2)

Ti-22Nb [at.%] Ti Nb 76.46 78.81 78.43 94.45 52.81 ---

23.54 21.19 21.57 05.55 47.19 ---

Ti-25Nb [at.%] Ti Nb 71.04 75.21 72.87 --73.23 58.21

28.96 24.79 27.13 --26.77 41.79

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1 2

Fig. 3 Dendritic micro-structure of Ti-22Nb sample after plasma melting. Obr. 3 Dendritická mikrostruktura Ti-22Nb po plazmovém tavení.

Fig. 4

SEM micro-graph of Ti-22Nb sample after plasma melting. Detail of dendritic micro-structure in Fig. 3. with analysed spots. Obr. 4 SEM mikrostruktura Ti-22Nb po plazmovém tavení. Detail dendritické mikrostruktury na Obr.3 s vyznačenými body mikroanalýzy.

1 2

Fig. 5 Large grains with lamellar (α+β) micro-structure of Ti22Nb after annealing (12h at 1100°C). Obr.5 Velká zrna Ti-22Nb s lamelární mikrostrukturou (α+β) po žíhání (12h při 1100°C).

Fig. 6

SEM micrograph of Ti-22Nb sample with lamellar (α+β) micro-structure after annealing. Detail of Fig. 5. with the analysed spots.

Obr.6 SEM snímek Ti-22Nb s lamelární mikrostrukturou (α+β) po žíhání. Detail Obr.5. s vyznačenými body mikroanalýzy.

1

2

Fig. 7 Dendritic micro-structure of Ti-25Nb sample after plasma melting. Obr.7 Dendritická mikrostruktura Ti-22Nb po plazmovém tavení.

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Fig. 8 SEM micro-graph of Ti-25Nb sample after plasma melting. Detail of dendritic micro-structure in Fig.7 with the analysed spots. Obr.8 SEM snímek Ti-22Nb po plazmovém tavení. Detail dendritické mikrostruktury na Obr.7 s vyznačenými body mikroanalýzy.



1 A

2

Fig. 9 Lamellar (α+β) micro-structure of Ti-25Nb after annealing (12 h at 1100°C ). Obr. 9 Lamelární mikrostruktura (α+β) v Ti-25Nb po žíhání (12h při 1100°C).

Fig. 10 SEM micro-graph of Ti-25Nb sample with fine grains. Detail of A region in Fig. 9 with the analysed spots. Obr. 10 SEM mikrostruktura Ti-25Nb s jemnými zrny. Detail oblasti A na Obr.9 s vyznačenými body mikroanalýzy.

Tab. 4 Comparison of the results of EDS micro-analysis and micro-hardness measurement in as-melted and heat treated (HT) conditions of Ti22Nb and Ti-25Nb alloys prepared using two different methods: plasma melting and electron beam floating zone melting (EBFZM). Tab. 4 Srovnání výsledků mikroanalýzy a mikrotvrdosti v Ti-22Nb a Ti-25Nb slitinách tavených a tepelně zpracovaných připravených dvěma metodami: plazmovým tavením a elektronově zonálním tavením.

Alloy

Preparation

Ti-22Nb

Plasma melting HT at 1100°C

Ti-25Nb

Plasma melting HT at 1100°C EBFZM HT at 200°C HT at 400°C HT at 600°C

Nominal content [at.%]

Measured content [at.%]

Ti 80

Nb 20

Ti 78.43

Nb 21.57

75

25

72.87

27.13

75

25

73.89

26.11

Microhardness HV

272 621

235 410 273 244 292 271

HT – heat treatment; EBFZM – electron beam floating zone melting;

4. Conclusions Metallographic observation and chemical microanalysis of the alloys prepared by both methods proved the composition homogeneity. The micro-structure after EBFZM method, as well as after heat treatment was duplex with α and β phases. After solid solution annealing of plasma melted Ti-22Nb and Ti-25Nb alloys with dendritic micro-structure the presence of β phase with coarse or fine laths of α phase in the microstructure was observed. The martensite phase was not found in the prepared TiNb alloy. Micro-hardness values increased with heat treatment and decreased with higher Nb content. Explanation of the Nb effect and heat treatment on micro-hardness need further phase analyses that will be the subject of future research work.

Acknowledgement The research work was realised within the frame of the project MSM 6198910013 „Processes of preparation and properties of highly pure and structurally defined special materials “. Literature [1] WANG, Y. B. , ZHENG, Y. F.: The micro-structure and shape memory effect of Ti–16 at.% Nb alloy. Materials Letters, 62, 2008, 269–272. [2] KIM, H. Y. et al.: Effect of thermo-mechanical treatment on mechanical properties and shape memory behavior of Ti–(26– 28) at.% Nb alloys. Materials Science and Engineering, A 438– 440, 2006, 839–843. [3] LI, S. J. et al.: Phase transformation during aging and resulting mechanical properties of two Ti–Nb–Ta–Zr alloys. Materials Science and Technology, 2005, 21, 6, 678-686. [4] MANTANI, Y., TAJIMA, M.: Phase transformation of quenched martensite by aging in Ti - Nb alloys. Mat.Sci. and Engineering. 2006, A 438 - 440, 315-319.

Recenze: Ing. Radovan Bureš, CSc.

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Recenze: Ing. Radovan Bureš, CSc

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