Mechanism of reaction of molten NiTi with EBM graphite crucible

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Mechanism of reaction of molten NiTi with. EBM graphite crucible. S. K. Sadrnezhaad*, E. Ahmadi and M. Malekzadeh. Ultra clean NiTi shape memory alloy was ...
Mechanism of reaction of molten NiTi with EBM graphite crucible S. K. Sadrnezhaad*, E. Ahmadi and M. Malekzadeh Ultra clean NiTi shape memory alloy was produced by electron beam melting of Ni rich vacuum inductionally melted butts together with pure Ti chunks in both condensed and electrographite crucibles. A hollow cathode discharge gun was used for heating up to 1623, 1653 and 1693 K and holding the charge materials under vacuum for 300, 600, 900 and 1200 s. Effects of temperature, time and compactness of the crucible on formation/disappearance of the hard compounds like Ni3Ti, Ti4Ni2O, Ti4Ni2C, Ti3Ni2OC and TiC were determined by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray analysis. A combination of the experimental results with the kinetic rate equations indicated that the reaction between NiTi and the crucible obeyed first order transfer kinetics with carbon intake activation energy of 225?8 kJ mol21 for the NiTi shape memory alloy melt. Keywords: NiTi, EBM, SMA, Transfer kinetics, Mechanism, Graphite crucible, Condensed graphite, Electrographite

Introduction The NiTi intermetallic compound displays shape memory effect, super elasticity, two way shape memory characteristics and all round shape memory effect together with biocompatibility, corrosion resistance, damping power, close to bone strength, water resistance, superior fatigue life and high specific electric resistance.1–7 These properties make NiTi an attractive biomaterial usable in smart devices of both medical and engineering applications. Cardiovascular incision, orthopedic surgery, orthodontia, surgical implants and separation systems of satellites are a few examples.7–11 During past two decades, extensive work has especially been carried out on NiTi alloys to produce NiTi shape memory alloys (SMAs) by powder metallurgy, selfpropagation high temperature synthesis,4–7,11–13 vacuum induction melting (VIM) and vacuum arc melting processes.14–19 Vacuum melting technology has, in the past, almost exclusively been used for NiTi alloys commercial mass production. Less attention has, however, been paid to the electron beam melting (EBM)20–25 as an alternative way for production of NiTi SMAs and other Ti containing alloys. Since 1950, EBM has continuously been developed for fusing of high melting point metals.26–28 This method is widely used even to an industrial scale for melting and/or remelting of the Ti alloys. A method is recently proposed to achieve a low carbon concentration during EBM processing of NiTi SMAs using the water cooled copper crucibles.20–23 The electron beam melting/ Materials and Energy Research Center, PO box 14155 4777, Tehran, Iran and Center of Excellence for Production of Advanced Materials, Department of Materials Science and Engineering, Sharif University of Technology, PO box 11365 9466, Tehran, Iran *Corresponding author, email [email protected]

ß 2009 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 15 February 2008; accepted 14 April 2008 DOI 10.1179/174328408X317075

casting technique is now accepted as a novel method for production of the Ti alloys.20–23,27,28 Desirable memory effects can, in general, be obtained with 49–50?7 at.-%Ti in the alloy. Binary alloys containing less than 49?4 at.-%Ti are, for example, extremely hard and brittle and cannot easily bear deformation processing to reach the eventual geometric shape.29–32 The binary Ti–Ni phase diagram3 (Fig. 1) demonstrates that deviation from stoichiometric concentration causes a considerable precipitation of brittle intermetallics in the NiTi matrix.3 Figure 2 represents the binary Ni–C and Ti–C systems and the isothermal section of the ternary Ni–Ti–C system at 1500 K.14 Formation of Ti2Ni, Ti2Ni3, TiNi3, Ti4Ni2 and Ti(N,C,O) compounds affects on the workability, ductility, strength, hardness and phase transformation temperatures of the material and its functional properties.30–35 Absorption of oxygen and nitrogen during heating, melting and annealing can also dramatically influence the mechanical properties and workability of TiNi samples.34 The EBM can decrease concentrations of the impurity elements such as N, O and H through their out-gassing. The decomposition of Ti4Ni2O and release of oxygen can partially occur during the electron beam remelting process.20,21,23–25 The EBM can, therefore, be assistive in remelting of the NiTi ingot butts produced even by other methods like VIM, helping to produce more workable TiNi SMA. During both melting and remelting, NiTi tends to react not only with oxygen but also with the crucible and mould surfaces. Zhang et al.19 applied an alternative VIM method for investigation of the reaction between the NiTi melt and the graphite crucible. The activation energy they obtained for TiC formation in VIM was 247¡34 kJ mol21. Otubo et al.20 showed that the carbon contamination of the NiTi produced by EBM in a water cooled crucible was 4 to 10 times lower than that of the

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1 Phase diagram of Ti–Ni system3

VIM process.20,21 When NiTi alloys were produced by induction and EBM in graphite crucibles, the carbon content of the NiTi alloys increased, however.20–23 The consequence was Ti carbide formation.

The structural properties of NiTi SMAs whether fabricated by EBM or by VIM depend greatly on the chemical composition.14,15 Carbide formation during melting/solidification of the NiTi melt results in Ti

2 Isothermal (1500 K) ternary Ni–Ti–C diagram with three of its binaries14,15

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a

4 Schematic representation of experimental set-up

b 3 Photographs of a EBM and b HCDG system

reduction and increasing the Ni percentage. This results in a decrease in the phase transition temperatures which requires carbon pick-up minimisation.30–35 Only limited information exists on EBM remelting of NiTi in graphite crucibles. Thermodynamics and kinetics of the reactions between NiTi melts and graphite crucibles need to be elaborated and different carbide phases must be portrayed. One objective of the investigation is to clarify the carbon pickup mechanism. Another is to determine the cleaning effect of the EBM remelting system. Rigorous investigation of the thermodynamics and kinetics parameters affecting the EBM carbon pickup rate is thus essential to the understanding of many high temperature processing routes that involve solid graphite crucibles contacting with a highly reactive metallic melt.

Experimental procedure Vacuum inductionally melted NiTi ingots containing 55?34 wt-%Ni were remelted in a graphite crucible placed inside an EBM vacuum unit having a hallow cathode discharge gun (HCDG) made of tantalum (DLKD-800, China). Two types of low and high density graphite crucibles were used for holding the samples. Photographs of the EBM with the HCDG system used in this investigation are demonstrated in Fig. 3. A schematic representation of the experimental set-up is also illustrated in Fig. 4. Remelted samples were held in electrographite or dense graphite crucibles of the EBM system for 300, 600, 900, 1200 and 1500 s at three different temperatures of 1623,

1653 and 1693 K. An infrared pyrometer of type IRtec P2000 was used for determining the temperature of the bath. The carbon concentrations of all ingots were determined by a LECO analyser using the combustion infrared absorption method. After solidification, the specimens were separated from the graphite crucible and their surfaces were ground, up to a depth of 2 mm. All melted specimens were rubbed with silicon carbide emery papers and then polished with Al2O3 impregnated clothes. Particle induced X-ray emission and microstructural analyses using a scanning electron microscope (Cam Scan S360 Mv2300) equipped with an energy dispersive X-ray spectrometer (EDAX) was carried out to determine the chemical analysis of the produced samples. Xray diffraction (XRD) patterns of the EBM remelted samples were achieved at room temperature using powder metallurgy 9920/50, Philips, Holland with Cu Ka radiation with the wavelength of 0?15405 nm. After particle induced X-ray emission, EDAX and XRD analyses, all specimens were etched in a 10 mL HNO3, 2 mL HF and 10 mL CH3COOH solution to see the phases. Scanning electron microscopy was used to evaluate the microstructure of the EBM samples. The mechanism of the reaction between NiTi and graphite crucible was investigated.

Results and discussion Figure 5 shows the calibration curve of the pyrometer expressed in terms of the bath temperature versus EBM electric current. Figure 6 shows the carbon concentration Cs of the Ti–55?34 wt-%Ni melt as a function of temperature after a holding time of 1500 s. The carbon content of the melt Cs increases with the bath temperature as can be seen in the figure. Figure 7 illustrates the carbon concentration C(t) of the EBM samples measured after holding for 300, 600, 900, 1200 and 1500 s at different temperatures: 1623, 1653 and 1693 K. The enrichment rate of carbon in the melt can be described by the following phenomenological equation19 dC ðtÞ ~k½CS {C ðtÞ (1) dt The saturated carbon content is shown by Cs; the EBM melt concentration is illustrated by C(t); the mass

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5 Bath temperature measured v. EBM electric current specified in amperes

transfer coefficient of carbon into the melt is represented by k and the thermodynamic driving force for entrance of carbon into the melt is proportionate with Cs2C(t). Integration of equation (1) between initial carbon concentration C0 and C(t) results in ln

CS {C0 ~kt CS { C ðtÞ

(2)

Plots of the left portion of equation (2) versus time at three different temperatures are shown in Fig. 7b. Results show that the first order mass transfer kinetic assumption is justified. Initial concentration of NiTi samples was substituted into equation (2) to determine the rate constant for the carbon pick-up of the melt. Assuming that the temperature dependence of the rate constant is of an Arrhenius type   Ea k~k0 exp { (3) RT where k0 is a pre-exponential factor, Ea is the activation energy, R is the universal gas constant and T is the absolute temperature of the NiTi bath. The semilogarithmic plot of the mass transfer constant k with respect to the reciprocal temperature T21 is shown in Fig. 8. From the slope of the straight line shown in Fig. 8, an activation energy of Ea5225?8 kJ mol21 is obtained. Equations (1)–(3) can

6 Long term carbon concentration of melt Cs as function of temperature

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7 Carbon content of EBM melt at 1623, 1653 and 1693 K plotted against holding time of liquid charge: initial concentration C0 for all NiTi samples is 0?06 wt-%

be used to predict the carbon pick-up of the NiTi melt with initially low carbon content at any given temperature. The above results indicate that during the EBM of the NiTi ingots in a graphite crucible, Ti can react with graphite to form Ti carbides contacting the carbon saturated melt. As it is clearly seen in Fig. 2c, TiC may also be formed according to the reaction between the

8 Estimation of activation energy from slope of Arrhenius line: apparent activation energy obtained is Ea5225?8 kJ mol21

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dissolved carbon and the Ti content of the melt after saturation of the melt with the carbon intake. TiC formation at the crucible interface can be stated according to the following reaction Tiðl ÞzC ðsÞ?TiC ðsÞ

(4)

The standard Gibbs free energy DGu of this reaction in the temperature rang from 1155 to 2000 K is as follows19  DG 0 ~{186:6064z132:2144|10{4 T kJ mol{1 (5) Assuming both activities of graphite and TiC are equal to one, Gibbs free energy of the reaction (4) can then be obtained from DG~DG 0 {RTlnðaTi Þ

(6)

where aTi is activity of the Ti dissolved in the NiTi melt given by aTi ~cTi XTi

(7)

where XTi and cTi are mole fraction and activity coefficient of the Ti species of the NiTi melt respectively. Assuming a regular solution model, the activity coefficients of Ti and Ni components of the melt can be obtained from 2 2 , RTlncNi ~bXTi RTlncTi ~bXNi

(8)

The coefficient b of the correlations depends only on the temperature of the system. Ogasawara et al.22 have shown that there is a strong attractive interaction between Ti and Ni and that the activity coefficient of Ti in the NiTi melt is so small that the average value of the constant b becomes 2151¡13 kJ mol21 at T51773 K   b 2 (9) ð1{XTi Þ cTi ~exp RT The negative value of b suggests that the liquid Ni–Ti system exhibits negative deviation from ideality with an activity coefficient of 0?077. Taking this into account, one can calculate the Gibbs free energy for the reaction given in equation (4) and obtain a negative DG value of 2114 kJ mol21 for an equiatomic NiTi melt. This value shows that the attractive interaction of Ti and carbon in the melt is strong enough to promote the formation of TiC from the early stages of the remelting process. The resulting gradient in the surface tension generates a strong surface flow through the Maragoni effect and this flow becomes part of the bulk flow regime within the pool.24 The combination of the downstream mass flow and the orthogonal Maragoni flow results in the NiTi melt turbulence. Strong fluid flow driven by a mixed electromagnetic force, buoyancy, mass flow and Maragoni force leads to the strong stirring of the NiTi melt during EBM process.

9 Microstructure of VIM ingot showing Ti4Ni2(O,C) and primary TiC phases precipitated in NiTi matrix during cooling

TiC pieces may be disintegrated from the TiC layer formed on the interior of the graphite crucible by the vigorous agitation of the liquid phase and disperse into the melt. These floating TiC pieces may also dissociate to increase carbon and Ti contents of the melt TiCðsÞ ?Tiðl ÞzC ðl Þ

(10)

At C(t).Cs, the reverse of the above reaction can thermodynamically occur. Production of TiC and Ti4Ni2C molecules during solidification/cooling stages can cause TiC formation according to this reverse reaction. It can then lead to cross-shaped TiC precipitates grown on the appropriate nucleation/growth sites of the TiC floating particles. Figure 9 illustrates the microstructure of VIM ingots consisting of NiTi matrix, Ti4Ni2C (dark layer), Ti3Ni2OC (bright layer) and primary TiC phases precipitated during cooling of the melt. Table 1 illustrates the EDAX chemical analysis of the phases present in the VIM melted and EBM remelted NiTi samples. Regions highlighted with Ti4Ni2C and Ti3Ni2OC in Fig. 9 consist of two layers: the dark layer is a carbide precipitate close to Ti4Ni2C; while the neighbouring bright layer attributes to an oxicarbide phase close to Ti3Ni2OC, as illustrated in Table 1. The cross-shaped black precipitates are primary TiC phase. Electron beam melting remelted VIM samples were held at different temperatures in graphite crucible of EBM unit and were heated with HCDG to investigate the evaporation rates of different phases remained from

Table 1 Energy dispersive X-ray analysis of phases present in VIM and EBM remelted samples Phase

Production process

Ti, at.-%

Ni, at.-%

O, at.-%

C, at.-%

NiTi matrix Oxicarbide Ti3Ni2(O,C) Carbide Ti4Ni2C Primary carbide TiC Remelted NiTi

VIM VIM VIM VIM EBM

49.90 38.79 52.14 50.50 49.90

50.10 26.13 33.26 1.46 50.10

– 16.55 – – –

– 18.53 14.60 48.04 –

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b

c

d

10 Scanning electron microscopy structures of EBM remelted NiTi sample held at 1623 K for a 300 s, b 600 s, c 900 s and d 1200 s: note that primary and eutectic TiC precipitates have been augmented with EBM carbon intake

11 Scanning electron microscopy tructure of NiTi alloy produced by EBM in high density graphite crucible

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previous stage. Figure 10 shows the SEM microstructure of the ingots remelted in the lower density electrographite crucibles by EBM at 1623 K and holding times of 300, 600, 900 and 1200 s respectively. As is seen in the figure, the amount of TiC remarkably increases with the holding time of the samples in the EBM crucible. Results of EDAX analyses show reduction of undesirable hard phases except TiC which adds up with the carbon intake from graphite crucible (Fig. 10). The amount of the TiC that forms during melting and holding of the NiTi also depends on density of the graphite crucible. Figure 11 illustrates the SEM microstructure of the EBM remelted NiTi SMAs in a high density graphite crucible. The figure shows that the microstructure of the EBM ingot consists merely of NiTi phase without an appreciable content of brittle oxicarbide phase. To verify insignificant carbon intake, both EDAX and XRD microanalyses were taken. The results indicated that pick ups of carbon and other impurities were lowered (or even eliminated) when the higher density graphite crucible was selected. This led

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a

b

a electrographite; b high density graphite 12 Scanning electron microscopy structure of graphite samples

to clean NiTi samples free of undesirable intermetallic phases. The clean NiTi alloy produced by this way shows excellent mechanical and shape memory properties and can be used for manufacturing of many smart biomedical and micromechanical devices.36 Scanning electron microscopy structure of both electrographite and high density graphite samples are compared in Fig. 12. A lower pore size and porosity fraction is generally expected for the latter. It seems, therefore, that the rate of the reaction between the NiTi melt and the carbon of the higher density crucible is small enough to be reasonably neglected. The carbon content of the NiTi melt can, therefore, significantly reduce when dense graphite crucible is used with the electron beam remelting process. Production of the NiTi shape memory alloy in a high density graphite crucible embedded in an EBM unit seems, therefore, a technically feasible process resulting in a high purity SMA.

Conclusions Electron beam remelting is a novel homogenisation/ impurity reduction method usable for NiTi alloy ingots with the following specifications. 1. EBM can simply reduce the amount of contaminants and help the emergence of excellent chemical homogeneity and cleanliness in the NiTi SMA. 2. The solid–liquid reaction between the graphite crucible and the liquid melt is thermodynamically feasible, but can be suppressed by substitution electrographite with a dense graphite crucible. 3. The carbon intake of the melt obeys a first order rate equation attributed to carbon transfer from TiC saturation positions into the liquid phase. 4. The activation energy of the carbon intake is 225?8 kJ mol21 corresponding to a physical transport process. 5. The carbide formation reaction indicates that the formation of TiC at the surface of the graphite crucible and its decomposition at further distances are thermodynamically feasible resulting in the eventual carbon pick-up of the melt. 6. If TiC forms from direct contact between the melt and the crucible, the predominant mechanism for

carbon content enhancement would be mass transfer from saturated layers covering dissociating TiC particles. 7. Presence of newfangled oxy carbide phases like Ti3Ni2OC indicates feasibility of formation of rare semistable phases besides well known Ti4Ni2O and Ti4Ni2C precipitates at VIM and/or EBM conditions. 8. Fluid flow driven by a mixed electromagnetic force, buoyancy, mass flow and Maragoni forces lead to the strong stirring of the NiTi melt during EBM process and bring the dissolved Ti and carbon to the reaction sites for TiC, Ti4Ni2C and Ti3Ni2CO formation. 9. Utilisation of a high density graphite crucible with high vacuum EBM, not only reduces the TiC content, but also suppresses the presence of all other undesirable precipitates like Ti4Ni2O, Ti3Ni2CO, Ti4Ni2C, Ti2Ni and TiNi3. 10. The carbon content of the NiTi melt can substantially decrease below carbon saturation level when dense graphite crucible is used with electron beam remelting of the NiTi SMA.

Acknowledgements The authors wish to express their appreciation to Dr A.J. Novinrooz and Mr H. Sayedi from Nuclear Research Center for Agriculture and Medicine of Karaj, AEOI for their assistance in SEM and EDAX analyses. The authors also acknowledge the help of Ms S. Vatankhah for chemical analyses of the samples.

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