Vol. 17 No. 1 Journal of Wuhan University of Technology - Mater. Sci. Ed. Mar. 2002

Hardness Measurement of (TiB2-TiAI)/TiAI Symmetrically

Function Gradient Materials

MEI Bign-chu 1 CHEN Yan-lin 1 HE Li-ping 2 Yoshinari Miyamoto 3

l) Wuhan University of Technology 2) City University of Hong Kong 3~oining and Welding Research Ins t i tu te , Osaka University (Japan)

( Received: June 6,2001 ) Abstract: ( TiB2-TiA1) / TiA1 syrrurtetrieally function gradient materials (FGM) were prepared by spark

plasma sintering (SPS) . Owing to the difference of the thermal expansion coefficients between TiB2 and TiAl, a

compressive surface stress was introduced to the FGM by the thermal expansion mismatch. The hardness values of

the uniform mateda]z and the FGM were tested, respectively. For the FGM with a compressive surface stress,

hardness is obviously superior to that of the uniform material. When the FGM was subjected to heat treatment, the

hardness decreased due to a partial relaxation of the compressive surface stress.

Key words: hardness measurement; function gradient materials; composite

1 Introduction

The materials based on TiA1 intermetallic compound

are prospective ones for applications [I" 3]. Introducing the

ceramic phase as TiB2 or TiC into the TiA1 structure en-

hances the hardness and wearability. It is well known that

the hardness of TiB2 particle is higher than that of TiC.

And also Some literatures I4'51 reported that there was a

chemical reaction between TiC and TiA1 (2TiA1 + TiC --~

Ti2A1C + TiAI). So it is advisable that TiB2 is selected as

the hard particle. The advantages of graded microstruc-

tures to the strength and toughness of materials have been

manifested [6'73 . In the case of surface compressive stress,

especially, these advantages are more outstanding. For

instance, J. S. Lin I7] reported that the fracture toughness

of (A12Os-WC)/TiC/Ni FGM at the surface was nearly

twice as high as that of the A12Oa-WC uniform composite

when a compressive surface stress in (A1203-WC)/TiC/

Ni FGM was introduced by the thermal expansion mis-

match between the outer and inner layers of the material.

In this work, (TiB2-TiA1)/TiAI/(TiB2-TiA1) sym-

metrically FGM was prepared by SPS [33 . Owing to the dif-

ference of the thermal expansion coefficients between TiB2

and TiA1, a compressive surface stress in the FGM was in-

troduced by the thermal expansion mismatch. The main

objective of this work is to examine the hardness of (TiB2-

MEI Bign-chu(~'J~@) :Born in 1962; Prof. ; State Key Iab of Advanced Technology for Materials Synthesis and Processing, Wuhan Univesity of Technology, Wuhan 430070, China

* Funded by the Natural Science Foundation of China(No. 50172O37)

TiA1)/TiA1/(TiB2-TiA1) symmetrically FGM when a

compressive surface stress was introduced.

2 Experimental TiB2 and TIM powders were used as raw materials.

Their particle sizes were about 5tan and 50wn, respectively.

A chemical analysis (by weight ) showed that the comtx~ition

of TiM was Ti 63.7wt%, A1 35.6wt%, Si 370ppm, Fe

180ppm, O 6200ppm and N 150ppm. TiB2 has a purity of

99.5%. The powders were well mixed in TiM with 5 and

10vol% TiB2, respectively. The samples were designed into

( 10vol % TiB2-TiAI) / ( 5vol % TiB2-TiA1)/TiAI/(5vol % TiB2-

TIM)/(10vol%TiB2-TiA1) symmetrically graded layer and

then subjected to SPS at 1473K in vacuum at 2Pa for 6 min-

utes. The pressure in sintering was 30 MPa. The SPS appara-

tus (Model SPS-1050, Sumitolno Coal Mining Co. Ltd. ) is

schen~tically shown in Fig. 1 . Some of products were an-

nealed at 1573K for 2hr in vacuum. To make a c o , son

between the hardness of the syrm~trically FGM and that of

the uniform material, the latter was also fabricated under the

same conditions. The relative densities were tested by means

of the buoyancy method. The results indicated that the rela-

tive density of the samples prepared under those conditions

was about 98.5% of theoretical one. The microstructures of

materials were analyzed by means of EPMA.

The sintered samples were cut into strip specimens with

3 x 5 x 20 nrn. The specimens were polished to a diamond

surface finish of 3wn before testing. The hardness of the

specimens was tested by using a Vickers hardness testing

machine with an indentation load of 196N and the method for

measurement was schematically shown in Fig.2.

2 Journal of Wuhan University of Technology- Mater. Sci. Ed. Mar. 2002

Fig. 1 Schematic diagram of SPS sintering system Fig. 2 Diagram of measurement method for hardness of sample

(a) before heat treatment Fig.3

3 Results and Discussion

Fig. 3 shows the microstructure of the prepared mate-

rials. A graded structure was observed, but in every layer

a uniform structure was shown. In the central layer, there

was the pure TiA1 in which a full lamellar structure was

observed after heat treatment.

The hardness of the obtained materials was tested.

For the sample with a compressive surface stress, as

shown in Figs. 4 and 5, the average hardness of the

pressed surface of the sample before heat treatment was

about 5GPa (Fig. 4 ) , while after heat treatment the aver-

age hardness decreased by 1.2GPa to about 3.8GPa

(Fig.5) . The hardness of the side surface of the sample

is shown in Fig. 6. The hardness of the sample varied

with the measurement direction in parabola. By compari-

son with Fig. 6, it is shown that the hardness shown in

Fig. 7 is obviously inferior to that in Fig. 6, which also

means that the hardness of the sample also decreased after

heat treatment.

To make a comparison between the hardness of the

sample with a compressive surface stress and that of the, u -

(b) after heat treatment Micrographs of the graded materials

niform sample, Figs. 8 and 9 indicate the hardness of the

uniform sample before and after heat treatment. It can be

seen that the hardness hardly varies with heat treatment

(3.5 - 3.6GPa).

Fig.4 Vicker's hardness of the pressed surface of the graded materials before heat treatment

According to Fig. 4 and Fig. 8, it can be found that

the former is 40% higher than the latter. The reason for

this result is probably that a compressive surface stress

was introduced by the thermal expansion mismatch be-

tween the outer and inner layers of the material. The in-

troduction of a compressive surface stress results in the in-

Vol. 17 No. 1 MEI Bign-chu et al: Hardness Measurement of (TiB2-TiA1)/TiA1... 3

5.5

5.0

i 4, ~ 4.0

~ 3.0

2.5 0

Fig. 5

i ' , . , . , . , �9 , ' i

FG M 10voI%TiB2-TiAI/5voI%TiB2-TiAt/ i TiAI/5voI%TiB2-TiAI/10voI%TiB2-TiAI Treated at 1573k after sintered at 14"/3K

�9 ; -,,%,-,-,_,.~ \ ,~"~

I I I I i I I I

2 4 6 8 10 12 14 16 Distance from Edge/nun

Vieker' s hardness of the pressed surface of the graded

materials after heat treatment

6.0

~ 5.5

5.0

~ 4 . 5

.~ 4.0

3.5

3.0

t . . . . t , . L : �9 ' , : t .

i :

1OvoI%TiB~ 5voI%TiB=-:: Ti-48AI layer 5vol%TiB2~ IOvoI%TiB 2" :Ti-48AI tayerTi-48AIlay~r Ti~,SAIlayEtr Ti~8AI lay@

Sin'lered at 1473K for6mins i F G M s 10vo l%TiB2-T iAI / 5vo l%TiB2-T iAt /T iAI I 5vo l%TiS2-T iAI / 10vo l%TiB2-T iAI

I I , , , 1: I

0 2 4 6 Distance from Edge of Sample/nun

Fig. 6 Vicker' s hardness of the side surface of the graded ma-

terials before heat treatment

3.8

3.6

3.4

3.2

3.0

2.8

2.6

2.4

F G M IOvo I%T iB2 -T iA I /5vo I%T iB2 -T iAL t 5voI%TiB2-TiAI/10voI%TiB2-TiAI Treated at 1573K alter sintered at 1473K for 6rnins

I i I , I i . i . i . i , i , i , I . / �9 i . i . i

0 1 2 3 4 5 6 Distance from Edge of Sample/mm

Fig.7 Vicker' s hardness of the pressed surface of the graded

materials after heat treatment

crease of the hardness. Through Figs. 4 ,5 ,6 and 7, the hardness of the sam-

ple not subjected to the heat treatment is always superior

to that of the sample subjected to heat treatment, which

suggests that the compressive surface stress is partially re-

laxed by means of heat treatment. For the uniform materi-

als, as shown in Figs. 8 and 9, the hardness hardly varies

with heat treatment, which confirms further that

deduction.

4 Conclusions

a) Introduction of a compressive surface stress re-

sults in an increase of the hardness of the materials.

4.2

3.8 m ~ 9

.~'~ 3.6 " ~ 3 4 ~ -

~ 2 . 8

2.6 0

Fig. 8

4.2

~ 4.0

3.8

~ 3.4

3.0

~ 2 . 8

2.6

0

�9 t , , . , . , ' i . i ' i

==i for 6 m i n s �9

I \ ~ , �9 , . I t . _ i t . _ l t , ,

i, 7

21 41 l I , i i i 6 8 10 12 14

Distance from Edge of Sample/ram

Vicker' s hardness of the pressed surface of the uniform

materials before heat treatment

2 4 6 g 10 12 14 Distance from Edge of Sample/ram

Fig. 9 Vicker's hardness of the pressed surface of the uniform

materials after heat treatment

b) For the sample with a compressive surface stress,

the heat treatment leads to a decrease of the hardness of

the materials owing to the relaxation of the compressive

surface stress.

References

1 C T Liu and J 0 Stiegler. Ordered Intermetallic Alloys. Mater.

Eng. , 1984,100(5) : 29 - 33

2 M L Adams, S L Kampe, and L Christodoulou. Characterization

of Rapidly Solidified Ceramic-titanium Aluminide Powders. The

International J. Powd. Metall., 1990,26(2) : 105 - 113"

3 Bingchu Mei and Y Miyamoto. Preparation of Ti - A1 Intermetal-

lic Compounds by Spark Plasma Sintering. Metall. and Mater.

Trans. A, 2001,32A(March) : 843 - 846

4 R Ramaseshan, S K Senshadri and N G Nair et al. Micmstruc-

ture and Properties of y - TiAI/Ti2A1C Composites Produced by

Reactive Processing from Elemental Powders. J. Japan Soc.

Powd. and Powd. Metall. , 1998,45(4) :330- 335

5 Bingchu Mei Y Miyamoto.Fabricafion of Dense TiA1/Ti2A1C Com-

posites by Pulsed-electric Current Sintering and Their Mechanical

Properties./~ans of JWR/(Japan) ,1999,28(1) :27 - 33

6 Y Miyamoto,K Tznihala,Z Li,Y S Kang a d.In:Procea//ngs of the

8th CIM- TEC Con['erence on A ~ in Science and Technology.

Fxlited by P.Vincentini(Techr~, Florence), 1995,4:387

7 J S Lin, Y Miyamoto, K. Tanihata. Toughening Effects of WC/Co

Particles and Compressive Surface Stress on (A1203-WC/Co)/

TiC/Ni Graded Materials. J. Mater. Sci. , 1998,33 : 869 - 876