Microstructural characterization of TiB in in situ synthesized titanium matrix composites prepared...

LJournal of Alloys and Compounds 327 (2001) 240–247www.elsevier.com/ locate / jallcom

Microstructural characterization of TiB in in situ synthesized titaniummatrix composites prepared by common casting technique

a,b , a,b a a c c*Weijie Lu , Di Zhang , Xiaonong Zhang , Renjie Wu , Taokao Sakata , Hirotaro MoriaState Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200030, PR China

bState Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiao Tong University, Xi’an 710049, PR ChinacResearch Center for Ultra-High Voltage Electron Microscopy, Osaka University, Yamadaoka, Suita, Osaka 565, Japan

Received 11 January 2001; received in revised form 24 April 2001; accepted 24 April 2001

Abstract

Titanium matrix composites reinforced with TiB and TiC were fabricated by a non-consumable arc-melting technology utilizingself-propagation high-temperature synthesis reaction between titanium and B C. Microstructural characterization of in situ synthesized4

TiB was observed by scanning electron microscope (SEM), transmission electron microscope (TEM) and high-resolution transmissionelectron microscope (HREM). The TiB shows a typical whisker shape and the crystallographic planes of the TiB at transverse

¯cross-section are always of the planes (100), (101) and (101). Stacking faults are observed in the TiB. The TiB forms in a way ofnucleation and growth. The growth morphologies and formation of the stacking faults are related to crystal structure of the TiB. Due to itsB27 structure, the TiB is likely to grow along [010] direction and form the whisker shape. The stacking faults are also likely to form inthe (100) plane. The formation of above morphologies and the stacking faults serves to minimize the lattice strain at the interface betweenthe TiB and the titanium matrix alloy. 2001 Elsevier Science B.V. All rights reserved.

Keywords: Casting; Titanium matrix composites; Microstructure; TiB

1. Introduction the shortcoming of traditional techniques. Moreover, selec-tion of a compatible reinforcement is also very important

Metal matrix composites (MMCs) offer considerable factor for developing particle reinforced Ti MMCs. Variouspotential for improvement in various mechanical properties reinforcements are used in Ti MMCs including TiB, TiC,over conventional monolithic alloys, particularly in stiff- TiB , SiC, Al O . Among these reinforcements, TiB and2 2 3

ness, strength and wear resistance. Particle reinforced TiC are potentially very useful reinforcements because ofMMCs have made rapid progress in the past decade along their thermal stability and relative chemical stability in a Tiwith continuously reinforced MMCs as they offer more matrix. Despite recent advances in the processing andisotropic properties and lower processing costs. Over the manufacturing technology of TiB and/or TiC reinforced Tipast few years particle reinforced Ti MMCs have been MMCs by in situ techniques, there is a lack of a systematicproduced using various techniques, particularly using understanding of the structural characterization of in situpowder metallurgy [1,2] and liquid phase (casting) [3] synthesized reinforcements. However, it is well known thatprocessing routes. In recent years, novel processing tech- the structural feature of the reinforcement affects theniques [4–15] based on the in situ production of MMCs mechanical and physical properties of in situ synthesizedhave emerged. In situ techniques are valuable to fabricate Ti MMCs. So it is necessary to investigate the structuralTi MMCs with better properties because they overcome characterization of in situ synthesized reinforcement.

In the present work, traditional ingot metallurgy plusself-propagation high-temperature synthesis (SHS) re-action between titanium and B C were used to produce4

TiB and TiC reinforced Ti MMCs. The structural charac-*Corresponding author. Tel.: 186-21-6293-3106; fax: 186-21-6282-terization of in situ synthesized TiB was investigated in2012.

E-mail address: [email protected] (W. Lu). particular.

0925-8388/01/$ – see front matter 2001 Elsevier Science B.V. All rights reserved.PI I : S0925-8388( 01 )01445-1

W. Lu et al. / Journal of Alloys and Compounds 327 (2001) 240 –247 241

2. Experimental procedure

The materials used in this study were fabricated byvacuum melting in a non-consumable vacuum arc remelt-ing (VAR) furnace utilizing the SHS reaction betweentitanium and B C. The details of processing technique and4

chemical composition were described in Ref. [16]. Sam-ples for SEM observation were prepared following conven-tional metallographic techniques. The samples were cutdirectly from ingots and were polished and etched with asolution of water, nitric acid, and hydrofluoric acid inproportion of 85:10:5. The microstructures of the sampleswere characterized using a Philips SEM 515 scanning

Fig. 1. Typical SEM morphology of TiB in the titanium matrix compos-electron microscope (SEM) equipped with EDS analysisites.

system. Samples for TEM observation were also cut fromthe cast ingots. Then they were thinned to 100 mm bypolishing with SiC paper. The samples were punched todiscs with a diameter of 3.0 mm. The disc samples werethen dimple ground to 20 mm in thickness. The final stage the [010] axis of the B27 cell. Analysis of several selectedof thinning was performed by argon ion milling (Gatan) area diffraction patterns of TiB indicated that the crystallo-with an incident angle of 108 until perforation occurred. graphic planes in these TiB whiskers are always of the

¯Conventional TEM observation was performed using an type (100), (101) and (101). The angle between the (100)¯H-800 transmission electron microscope equipped with a plane and the (101) plane is 126.68, which is in good

Gatan666 PEELS spectrometer. The microscope was oper- agreement with the theoretical value of 126.78. From aboveated at 200 kV. HREM observations were carried out on an TEM results, it can be concluded that the growth directionH-9000 transmission electron microscope operated at 300 of TiB is [010].kV. Fig. 3 shows another bright field TEM image of TiB and

respective selected area diffraction pattern. From thefigure, it can be seen that there are stacking faults in TiB

3. Results and discussion and the plane of stacking faults is (100). Moreover, thefigure indicates that although the interfaces of TiB at the

3.1. Microstructural characterization of in situ transverse cross-section are not flat, the interfaces still¯synthesized TiB consist of the planes (100), (101) and (101).

Fig. 4 is an HREM image showing a TiB embedded inAs discussed in the former paper [16], TiB and TiC the titanium matrix alloy. The lattice image also reveals

reinforced titanium matrix composites can be fabricated by that the interfaces of TiB exhibit pronounced planes whichthe common casting technique utilizing the SHS reaction correspond to the close-packed planes, i.e., TiB(100),

¯between titanium and B C. In situ synthesized rein- TiB(101) planes. As shown in the image, the TiB is4

forcements are distributed uniformly in the titanium matrix precipitated with the definite crystallographic relationship¯alloy. TiB grows in whisker shape while TiC grows in of [010] i[0110] . The parallel crystal planes areTiB Ti

¯ ¯ ¯ ¯dendritic shape and equiaxed or near-equiaxed shape. Fig. (100) i(2110) , (001) i(0002) , (101) i(4221) . ItTiB Ti TiB Ti TiB Ti

1 shows a typical SEM morphology of TiB that has been also shows that there are stacking faults in TiB parallel todeeply etched. TiB shows whisker shape and the interfaces the (100) plane. The atoms in the vicinity of the stackingof TiB are flat, sharp. It also shows that the transverse faults shift their positions along the (100) plane. Fig. 5 iscross-section of TiB is a hexagonal shape. another typical HREM image showing a TiB embedded in

Fig. 2a,b shows bright field TEM images of TiB at the titanium matrix alloy. The following crystallographictransverse and longitudinal cross-section, respectively. Fig. relationships can be determined by Fig. 5:

¯ ¯2c,d shows selected area diffraction patterns in Fig. 2a and [001] i[0110] , (010) i(2110) , (200) i(0002) .TiB Ti TiB Ti TiB Ti

b, respectively. Fig. 2e shows an EELS analysis result ofTiB whisker, exhibiting an atomic composition of TiB.The mole ratio between Ti atom and B atom is 1:1. The 3.2. The formation mechanism of TiB reinforcementsTEM observations revealed that TiB whiskers are indexingto the orthorhombic structure TiB (B27) with lattice Fig. 6 shows projection of the B–C–Ti liquidus surfacesparameters a50.612 nm, b50.306 nm, and c50.456 nm. [17]. Because the compositions of the composites lie in theIn all cases, the whisker axis was found to be parallel to Ti-rich corner, it can be concluded from the projection that

242 W. Lu et al. / Journal of Alloys and Compounds 327 (2001) 240 –247

Fig. 2. Bright field TEM images of TiB in in situ synthesized titanium matrix composites: (a,b) transverse and longitudinal views of TiB; (c,d) selectedarea diffraction patterns in (a) and (b), respectively; (e) EELS pattern of TiB.

the reinforcements would undergo disintegration, dissolu- composites followed three stages. Firstly, primary phasetion, nucleation and growth. Namely, TiB and TiC can be would nucleate and grow from the liquid phase. Thensynthesized by the SHS reaction between the sponge binary eutectic would nucleate and grow simultaneouslytitanium and B C as the temperature increases during once the monovariant line was met. At last, invariant4

processing. TiB and TiC are molten thoroughly in a liquid ternary eutectic would disperse when ternary eutectic pointphase as the temperature increases continuously. During was met. The morphologies of the reinforcements arecooling, TiB and TiC precipitate and grow from the liquid related to the solidification path, i.e., nucleation andphase. The solidification paths of the titanium matrix growth have an effect on the morphologies of the re-


Fig. 3. Bright field TEM image of TiB showing the stacking faults parallel along (100) plane.

inforcements. As discussed in Ref. [18], the effect of should be faster than [001] direction. So TiB is likely tosolidification paths on the growth morphologies of TiC is grow along [010] direction and form whisker shape. Thevery evident. However, the effect of solidification paths on symmetry axis is parallel to the [010] B27 direction.the growth morphologies of TiB has not been completely According to the rule that crystals are usually bounded byunderstood. The different growth morphologies between the slowest growth planes, when they grow from melt, theTiB and TiC are related to their different crystal structures. (001) plane will disappear because growing velocity of the

The crystal structure of the TiB is B27 structure [19] (as (001) plane may be faster than that of the (100) plane inshown in Fig. 7a). The boron atoms form a zigzagging the transverse cross-section of TiB whisker. Moreover, the

¯chain of bonds along the [010] direction. The crystal planes (101) and (101) are low-index planes and the¯structure of TiB shows very high asymmetry, indicating growing velocity of (101) and (101) planes seems to be

that TiB will grow in anisotropy and form crystal with slower than (001) plane. So the planes in the transverse¯angular, planar facets. The projects of TiB along [010] and cross-section consist of (100), (101) and (101).

[001] directions are shown in Fig. 7b and c, respectively. The probability theory is also used to analysis theThey also show that configuration of boron and titanium growth morphologies. If we assume that the probability toatoms are different between (010) and (001) planes. On occupy a, b, a and b site is equal, then the probability tothe (010) and (001) planes, the atom ratio between occupy a, b, a and b is 1 /4. The probability (P) to formtitanium and boron is 1:1. However, in the (100) plane, the normal sequences along the [100], [010] and [001]boron atom and titanium atom occupy the sites alternating direction can be expressed as following:planes. The growth normal to planes containing both metal

811 1 1 1 1 1 1 1and B atoms in the same stoichiometry as the crystal ]S DP 5 C C C C C C C C 5a 4 4 4 4 4 4 4 4 4should be faster than growth along directions involving1alternating planes of metal and B atoms. So the growth

]]5 for the [100] directionnormal to planes (010) and (001) should be faster than the 65 536growth normal to plane (100). Moreover, the stacking

21 11 1period of atomic planes in [010] and [001] directions are ] ]S DP 5 C C 5 5 for the [010] directionb 4 4 4 16abab . . . , aa9bb9aa9bb9 . . . , respectively. The stackingperiod of atomic plane in [010] direction is shorter than 41 11 1 1 1 ] ]S Dthat in [001] direction. The growth along [010] direction P 5 C C C C 5 5 for the [001] directionc 4 4 4 4 4 256


¯Fig. 4. HREM image of the TiB/Ti interface in (TiB1TiC) /Ti composites along [010] zone axis. The facets of TiB correspond to the (100) and (101)planes that are the close-packed planes. The stacking faults form in the (100) plane.

The probability (P) to form the normal sequences along the TiB is nearly twice as large as that of the (0002) planethe [100], [010] and [001] direction is different. The in the a-Ti (d (TiB)¯2d (a-Ti)), the lattice mismatch001 0002

probability to form the normal sequence along the [010] is between inter-planar distances is 23.59%. The inter-planarthe largest, so the growing velocity along the [010] distance of the (010) plane in the TiB is nearly twice as

¯direction is much quicker than that of the other directions. large as that of the (2110) plane in the a-Ti (d (TiB)¯010

The growing velocity along [100] is the slowest. So the 2d (a-Ti)), the lattice mismatch between inter-planar2̄110

TiB is likely to grow along [010] direction and form distances is 23.59%. The lattice mismatch between TiBwhisker shape. In fact, the differences in probability to and the titanium matrix alloy is small. So it can beform the normal sequences should be larger owing to the concluded that the formation of above morphologies ofshortage of boron atom in the melt. TiB serves to minimize the lattice strain at the interface

Based on above crystallographic relationship and the between TiB and the titanium matrix alloy.lattice spacings of the TiB and the titanium matrix alloy, The stacking sequence of TiB along the [100] directionthe relationships between inter-planar distances of the TiB is abb9a9ABB9A9. The stacking sequence of TiB along theand the titanium matrix alloy have been deduced. The [100] direction is more complex than the stacking se-inter-planar distance of the (100) plane in the TiB is nearly quences of TiB along the [010] and [001] directions. The

¯four times as large as that of the (2110) plane in the a-Ti stacking disfigurement is likely to form along [100](d (TiB)¯4d (a-Ti)), the lattice mismatch between direction. Moreover, TiB forms in a way of nucleation and¯100 2110

¯the (100) plane in the TiB and the (2110) plane in the a-Ti growth. The diffusion of atoms affects the growth. Theis 2.76%. The inter-planar distance of the (001) plane in state that there is lack of B atom is likely to occur because


Fig. 5. HREM image of the TiB/Ti interface in (TiB1TiC) /Ti composites along [001] zone axis.

the amount of B atom in the melt is much less than that ofTi atom. So there are not enough B atoms to occupy thesites of a, a9, A and A9. It will lead to the formation ofstacking faults in the (100) plane. Because the stackingfaults form due to scarcity of boron atoms, namely theboron atoms on the sites of a, a9, A and A9 are absent, thestacking faults are intrinsic fault. Moreover, the inter-planar distance of the (100) plane in the TiB is longer than

¯four times distance of the (2110) plane in the a-Ti a little,so the formation intrinsic stacking faults in the (100) planeserves to minimize the lattice strain between the TiB andthe titanium matrix alloy. The shift of atom sites at (100)along [001] direction will increase the inter-planar distanceof the (001) plane in the TiB, which will also serve todecrease the lattice strain between the TiB and the titaniummatrix alloy. In all, the formation of stacking faults in the(100) plane is related to the crystal structure of the TiB.Moreover, it serves to minimize the lattice strain between

Fig. 6. Projection of the Ti–B–C liquidus surfaces. the TiB and the titanium matrix alloy.


Fig. 7. (a) Crystal structure of TiB and projects along the (b) [010] and (c) [001] direction.

4. Conclusion China under Grant No. 59631080. We are also grateful forthe financial support of the New Materials Research Center

In the present work, a common casting technique plus of Shanghai and Visiting Scholar Foundation of Key Lab.SHS reaction between the sponge titanium and B C in University.4

powder was used to fabricate TiB and TiC reinforcedtitanium matrix composites. The microstructural charac-terization of TiB was investigated, the following results

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Microstructural characterization of TiB in in situ synthesized titanium matrix composites prepared...

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