Effect of TiB Whiskers Orientation on Mechanical Properties

in an In Situ TiB/Ti-1100 Composite

Ma Feng-cang1;*, Liu Ping1, Li Wei1, Liu Xin-kuan1, Chen Xiao-hong1 and Zhang Di2

1School of Materials Science and Engineering, University of Shanghai for Science and Technology, Shanghai 200093, P. R. China2State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200040, P. R. China

In this paper, Ti-1100 composite reinforced with TiB whiskers was fabricated using in situ technologies. Mechanical properties of thecomposite with randomly oriented and aligned TiB reinforcements are evaluated by tensile tests at 923K, and the failure process of thecomposite is observed by SEM. Strengthening efficiency of the differently oriented reinforcement is calculated. The failure mechanism andstrengthening efficiency of the TiB whiskers during tensile tests are investigated. The effect of the orientation of TiB whiskers on the failuremechanism and strengthening efficiency for the investigated composite is also discussed. [doi:10.2320/matertrans.M2010014]

(Received January 15, 2010; Accepted March 31, 2010; Published May 26, 2010)

Keywords: titanium matrix composite, mechanical properties, strengthening efficiency, failure process

1. Introduction

With the rapid development of technology in aerospaceand atomic energy, the requirement for materials for suchapplications is increasing. Titanium matrix composites(TMCs), reinforced with ceramic particles, which by itselfpossesses a high specific strength at room and moderatelyelevated temperatures, significantly improves its specificmodulus, specific strength and creep resistance.1–3) Thesecomposites produced via in situ techniques exhibit highspecific strength and modulus, as well as excellent oxidationand creep resistance. Among the candidate reinforcementsproduced via in situ techniques, TiB is favorable for somecharacters, such as: high elastic modulus, whisker-like crystalgrowth and ease of fabrication utilizing the reaction betweenTi and B or B4C. Previous researches have reportedpreferable mechanical properties of TMCs reinforced withTiB whiskers.4–7) On the other hand, it is well understood thatthe mechanical properties of the TMCs reinforced with TiBwhiskers anisotropic if TiB whiskers are aligned. S. Gorssereported the different mechanical properties of TiB/Ti-6Al-4V composite with randomly or aligned oriented TiBreinforcements.8)

In this paper, in situ Ti-1100 composites reinforced withrandomly or aligned oriented TiB reinforcements werefabricated. Strengthening efficiency and failure mechanismof the composite during tensile tests were studied, and theexperiment results were discussed. It will deepen the under-standing of the relationship between the strengtheningefficiency and TiB microstructure characters. Moreover, itwill be helpful to improve the mechanical properties of theinvested composites by these results and discussions.

2. Experimental Procedure

Ingots of the composite were prepared in a vacuum arcremelting (VAR) furnace. Because it was more availablecommercially and cheaper than boron powders, B4C powders(99.8mass%, average particle size 5–7 um) were used to

prepare TiB/Ti-1100 composites in this work. TiB whiskerswere synthesized through the reaction: Ti + B4C !4TiB + TiC. But above reaction was not finished completelyin this case. It was found that the max solubility of C in the Tialloy is about 0.28mass% in this work. Effect of solute Celement in the matrix was reported in our previous work.9)

The volume fraction of TiB whiskers is 4% in the composite.In order to investigate the effect of reinforcements on themechanical properties of the composite, Ti-1100 alloycontaining 0.28mass% C was also prepared in this work.The nominal alloy composition of Ti-1100 is Ti–6Al–2.75Sn–4Zr–0.4Mo–0.45Si. In order to ensure the chemicalhomogeneity of the samples, the ingots were melted at leastthree times. The forging of the composite was performedusing a hydraulic pressure machine. The amount of defor-mation is calculated as the total reduction in cross sectionA0/Ai, where A0 and Ai is the cross sectional area before orafter hot forging. The total reduction in cross section was 4 inthis work. Samples for optical microscopy (OM) were cutfrom the forged specimens. Then the samples were preparedusing conventional techniques of grinding and mechanicalpolishing. The tensile samples were a plate with a gaugethickness of 1.5mm and length of 40mm, as presented inFig. 1. They were machined from hot-forging rods with theplate parallel to or vertical towards the metal flowingdirection during forging. The tensile tests were performedusing a SHMADZU AG-100KNA servohydraulic structuraltest machine. The samples were heated to 923K in 5min in a

Fig. 1 Schematic diagram of tensile sample pieces.

*Corresponding author, E-mail: mafengcang@163.com

Materials Transactions, Vol. 51, No. 7 (2010) pp. 1277 to 1280#2010 The Japan Institute of Metals

furnace equipped on the tensile machine, and hold for 3min.Then the tensile tests started, and the average strain rate was5:0� 10�3 s�1. Three samples were tested for the inves-tigated composite and matrix metal, and the result was theaverage of the three values. Fractography of the compositesafter tensile tests was investigated by scanning electronmicroscopy (SEM) Philips SEM 515.

3. Results and Discussion

3.1 Mechanical properties of the compositeAccording to the orientation relationship between TiB

whiskers and the load direction, two types of tensile samplesare used in tests, i.e. sample A and sample B, as presented inFig. 2. For sample A, the orientation of TiB whiskers israndom towards the load during tensile tests. But for thesample B, most TiB whiskers were aligned along the metalflowing direction during forging. And the orientation of TiBwhiskers is almost parallel to the load during tensile tests.

Strengthening efficiency of the reinforcements is intro-duced to evaluate the strengthening effect, which is calcu-lated by the following formula:

S ¼�MMC

�mc

ð1Þ

In above formula, �MMC and �mc is the strength of thecomposite and Ti-1100 alloy containing 0.28mass% C,respectively. The effect of carbon on strength of thecomposite was reported in our previous paper,9) and it isnot discussed in this work. Mechanical properties of thecomposite and homogeneous Ti-1100 alloy are presented inTable 1. It can be seen from Table 1 that the compositeimprove the strength of the homogeneous Ti-1100 alloy.However, as presented in Table 1, compared with sample A,there is a noticeable improvement in strength for sample B.Calculated strengthening efficiency of the reinforcements forsample B is greater than that of sample A of course.

3.2 Failure process of the compositeThe SEM micrograph of the sample A near the fracture

is presented in Fig. 3. As presented in Fig. 3, many TiBwhiskers were broken during tensile process, and some TiBwhiskers were broken many times. The evolution of TiB

Fig. 2 (a) Sample A, the orientation of TiB whiskers is random towards the load direction; (b) sample B, the orientation of TiB whiskers

is almost parallel to the load direction.

Table 1 Mechanical properties of the investigated composite and homo-

genous matrix alloy during tensile tests at 923K.

Sample No. �mc (MPa) �MMC (MPa) � (%) S

A 437 476 12.4 1.09

B 440 540 10.6 1.23

Fig. 3 Breaking of TiB whiskers with different amounts of deformation during tensile test at 923K: (a) small deformed, (b) large

deformed.

1278 M. Feng-cang et al.

whiskers breaking during the composite failure process wasstudied in this work.

The failure process of the sample A during tensile tests canbe explained using schematic mechanism process in Fig. 4.When the composites deformed slightly, some longer TiBwhiskers are broken and cracks form near breakpoint, butshorter ones will not, as presented in Fig. 4(b). As thedeformation increased, shorter TiB whiskers are broken, andthe primary longer TiB whiskers are broken once again. Onthe other hand, the cracks also form in the matrix metal,which is presented in Fig. 4(c). As presented in Fig. 4(d),cracks formed by the breaking of TiB whiskers and thoseformed in the matrix grow and incorporate into big ones.It leads to the failure of the composite finally.

3.3 Effect of TiB whisker on strengthening efficiencyDuring the composite tensile tests, the tensile load transfer

from the matrix to the TiB whiskers via the interface shearstress between the whisker and the matrix. On the other hand,the mechanical properties depend on the length of thewhiskers. Based on the hysteresis deformation method, amodel for stress as a function of the whiskers length wassuggested.7,10) In this model, it is assumed that only shearstress can be transferred by the matrix, but tensile stress can’tbe transferred. So the interface shear stress along the whiskeris constant, and the value of the shear stress is equal to thatof the matrix shear yield strength (�i)

�s ¼ 2�i �l

dð2Þ

In above formula, �s, �i, l and d is the shear stress in TiBwhisker, interface shear strength between TiB whisker andthe matrix, the length and the diameter of the whisker,respectively. From above formula it can be seen that theaspect ratio is a very important parameter for the composite.The increase in the aspect ratio will be helpful to improve thestress in TiB whisker.

If the shear strength of interface (�i) is high enough, i.e.no interface debonding occurs during tensile process. Then,the maximum value of shear strength of interface (�i) islimited by the shear strength of matrix. Accordingly, forthe composite reinforced by single TiB whisker, the strengthof the composite can be predicted by the followingfunction.11,12)

�cy ¼ �my

1

2�vf

l

dþ 2

� �þ 1� vf

� �ð3Þ

In above formula, �cy, �my, vf ,ldand � is the strength of the

composite, the strength of the homogenous matrix, thevolume fraction of the whisker (vf ¼ 0:04), the ratio of lengthto diameter and the reinforced coefficient of the whiskerdepended on its orientation. When the TiB whisker orienta-tion is random in the composite, � ¼ 0:357, as presentedin sample A. When all TiB whiskers orientation is alignedalong the tensile direction, � ¼ 1, as presented in sample B.In the present work, the averaged value of l

dis 12.0 measured

by the authors using metallographic analysis. The predictedyield strengths of the different samples based on formula (3)are listed in Table 2. From Table 2, it can be seen that thestrengthening efficiency of TiB whisker improves about 20%for sample B compared with sample A.

4. Summary

In this work, Ti-1100 composite reinforced with TiBwhiskers was fabricated using in situ technologies. Mechan-ical properties of the composite were evaluated by tensiletests at 923K. There is a noticeable improvement instrengthening efficiency for the composite with aligned TiBwhiskers than that with randomly oriented TiB whiskers. Thefailure process of the composite was observed. TiB whiskersthose are longer than the critical length are broken firstly.Based on these results and discussions, it is stressed that theorientation and the aspect ratio of TiB whiskers have obviouseffect on the strengthening efficiency of TiB whiskers for thecomposite.

Acknowledgments

The authors would like to acknowledge the financialsupports provided by the National Research Fund of Scienceand Technology Commission of Shanghai Municipalityunder Grant No. 09ZR1422100 and the innovation ResearchFund of education of Shanghai Municipality under GrantNo. 10YZ94.

Fig. 4 Schematic of the TiB whiskers breaking in different failure stages of the composite during tensile test at 923K.

Table 2 Calculated strength and strengthening efficiency of the inves-

tigated composite according to formula (3).

Sample

No.�

l

d�cy S

A 0.357 12.0 1:06�my 1.06

B 1.0 12.0 1:24�my 1.24

Effect of TiB Whiskers Orientation on Mechanical Properties in an In Situ TiB/Ti-1100 Composite 1279

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