Pergamon Scripta Metallurgica et Materialia, Vol. 31, No. 9, pp. 1141-1144, 1994 Copyright ©1994 Elsevier Science Ltd Printed in the USA. All rights reserved

0956-716X/94 $6.00 + 00

COMPRESSION BEHAVIOR AND INTERFACES OF NiA1-TiB 2 I N S I T U COMPOSITES®

Z.P.Xing I, J.Y.Dai 2, J,T.Guo 3, G.Y.An I, Z.Q.Hu 3

1 Harbin Institute of Technology, Harbin 150001, P.R.China. 2 Laboratory of Atomic Imaging of Solids, Institute of Metal Research,

Chinese Academy of Sciences, Shenyang 110015, P.R.Cbina. Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110015, P.R.China.

( R e c e i v e d A p r i l 6, 1994) ( R e v i s e d June 9, 1994)

Introduction

The B2 crystal structure intermetallic NiA1 is one of the most attractive materials for high temperature structural applications due to its low density, high melting point and excellent oxidation resistance. Howev- er, the problems of its poor high temperature strength and low temperature ductility have not been over- come[l]. One possible approach to increase high temperature strength is to incorporate metallurgically sta- ble, uniformly dispersed, hard, fine second phase particles in sufficient amount in the alloy[2]. Recent works have shown that the elevated temperature strength of the NiAI can be effectively increased by dispersively distributed TiB 2 particles[3,5]. Such composites were fabricated by XD TM synthesis and hot pressed to full density. The microstructure and interfaces of NiA1/TiB 2 have also been investigated by analytical electron microscopy and high-resolution electron microscopy (HREM)[4]. But coherent or semi-coherent interfaces between TiB: and NiAI were not observed via HREM, only one-dimensional lattice fringes in both the matrix and the particles were observed.

If pressure could be applied during exothermic synthesis while the product mass is still at elevated tem- peratures, then it might be possible to synthesize and densify the product simultaneously. This paper uses a hot pressing aided exothermic synthesis (termed HPES) technique to fabricate the Ni-50at .%Al matrix composites containing 0 to 20 vol.%TiB 2 particles. Their interfaces and the compression behavior from room temperature to 1000"C were studied.

Experimental procedures

The HPES technique was used to produce Ni-50 at.%Al in situ composites containing 0 to 20 vol.% TiB: particles. The reagents used in this research consisted of elemental powders of Ni (98%, < l/am), AI (98%, < 13#m), Ti (99%, <75~tm) and B (95%, < l#m). After HPES synthesis, product densities were de- termined by the water immersion technique. Compression specimens (4 x 4 x 6 ram) were prepared by electro-discharge machining and grinding where their length was parallel to the hot pressing direction. Compression tests at strain rates ranging from 2 x 10 .3 to 1 x 10-2s -1 were conducted in Gleeble 1500 test machine to ~ 10% strain in most cases. The HREM foils were prepared by conventional procedure which

O Correspondent: Professor J.T.Guo, Institute of Metal Rcscarch, Chinese Academy of Sciences, Shenyang 110015, P.R.China.

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1142 IN SITU COMPOSITES Vol. 31, No. 9

involved cutting discs with a diameter of 3.0 mm, mechanical polishing them to 50#m, and dimpling to 20 /am and finally ion milling. T E M and H R E M observations were carried out on J E M 2000EX 11 with a point to point resolution o f 0.21 nm. X - r a y diffraction techniques were also used to characterize the

microstructure of the composites.

Results and discussion

The NiA1 matrix composites contained 0, 10 and 20vol .%TiB 2 particles with relative densities of 100%, 98.4%, 96.3%, respectively. The X - r a y diffraction pattern (Fig . l ) revealed that fully reacted prod- ucts of NiA1 and TiB 2 as the final phases were formed.

G

FIG. 1

b

G

25

o • • • o •

40 55 70 8.5

20(degree) X - r a y diffraction pattern of NiA1 - T i B 2 composites. (a) 20 vol .% TiB2,

(b) 10 vol .% TiB 2, (c) 0 vol .% TiB2;

C) NiA1, • TiB 2.

The results of compressive tests from room temperature to 1000"C are presented by the yield strength (0.2%YS) data as functions of volume fraction of TiB 2 and strain rate in Fig.2. The yield strength increased with in- creasing volume fraction of TiB2 , as well as strain rate(Fig.2a,b). The composites exhibited much higher strength than the unreinforced

NiAI at all temperatures. Especially the yield strength of NiA1-20vol .% TiB 2 was approximately twice as strong as the unreinforced NiAI at all temperatures. At room temperature, the composites containing 0, 10 and 20 vol .%TiB 2 particles did not begin cracking until 4%, 8% and 9% strain, respectively. So the ductility at room tempera- ture of the N i A I - T i B 2 particle composites was superior to monoli thic NiAI.

T E M examinat ion of as-fabr icated com- posites indicates that the TiB 2 particles, with brighter contrast, were present in the form of hexagonal and blocky crystals in most cases (Fig.3a). However , in some cases, especially in

the low volume fraction material (Fig.3b), many needle-l ike TiB 2 particles were formed. These TiB 2 par- ticles with the particle size of 0.5-- 3#m, tended to be uniformly dispersed in the matrix. There was little dis- location in the TiB 2 particles while considerable number of dislocations existed in the NiA1 matrix, espec- ially a high dislocation density was observed near the particle / matrix interface. Fig.4 is a T E M micrograph of as- fabr ica ted 20 vol .% TiB 2 / N i A I composites at a high magnification. Strain fields existed at the inter- face, as indicated by the arrows. This is probably due to (i) the noncoherent N i A I / T i B 2 interface pro- duced by in situ formation of TiB2 particles and (ii) the differences in thermal expansion between the NiA1 and TiB 2 . The presence of small TiB2 particles in the matrix along with the dense interfacial dislocation substructure and the interfacial strain fields may be responsible for the strength enhancement of

NiA1-TiB 2 composites.

In general, no consistent crystallographic orientation relationship exists between the NiAI matrix and TiB 2 particles. However , in some cases, the N i A 1 / T i B 2 interlaces can be tilted on their edge with both

Vol. 31, No. 9 IN SITU COMPOSITES 1143

sides of the interfaces oriented in a low- index zone axes, which is suitable for direct observation of the in- terface. Fo r instance , Fig.5 shows a H R E M image of semicoherent N i A 1 / T i B 2 interface viewed along [Zll]niAj// [0001]Tia 2 , with the {'II0}NiA1 planes parallel to the (01]'0}Tin 2 planes. The periodically ar- ranged misfit dislocations at the interfaces ,indicated by the arrows,were spaced five (110)ninJ planes. The distance between misfit dislocations is calculated by d = d t d 2 / (dl-d2) , where dt(0.2624 nm) and d2(0.2037 nm) are the interplanar distances for TiB 2 and NiAI, respectively[4]. The calculated d = 0.9106 nm, which is just about five times as large as the interplanar distance of (110)NiA 1 planes. This fits well with the observed results. In most cases, the TiB 2 particles bonded well to NiAI matrix and NiA1 / TiB 2 interfaces were very c lean, free from any interfacial phase. However , there was some evidence to suggest that an interfacial layer might exist at some TiB 2 / NiA1 interlaces. More work should be done in order to clarify the presence of an interfacial phase and find its relationship to the mechanical properties of the composites.

1000

800 &.

600 O} C

400 " 0

>- 200

FIG.2

o 0 vol.% -~ 10 vol.%

(a) 0 . . . . . 0

0 200 400 600 800 1000 1200 0 Temperature, °C

1000

800

600 c

400

200 (b) I I i I i

200 400 600 800 1000 1200 Temperature, °C

The yield strength of NiA1 matrix composites containing 0 to 20vol .%TiB 2 as a function of tem- perature. (a) strain rate 2 x 10-3s-t,(b) strain rate 1 x 10-2s -I.

FIG.3 T E M micrograph of as - fabr ica ted NiA1-TiB 2 composites. (a) 20 vol.%TiB2, (b) 10 vol .%TiB 2.

1144 1N SITU COMPOSITES Vol. 31, No. 9

Conclusion

With Ni, AI, Ti and B elemental powders, Ni-50 at.%A1 matrix composites have been successfully fab- ricated by HPES technique. The TiB 2 particles with the particle size of 0.5 ~ 3#m tended to be uniformly dispersed in the matrix. The composites exhibited much higher strength than the unreinforced NiA1 at all temperatures. The ambient ductility of the composites was also superior to monolithic NiA1. The presence of small TiB 2 particles in the matrix along with the dense interfacial dislocation substructure and the clean interface are beneficial to the strength of NiA1 / TiB 2 composites.

FIG.4 Interfacial strain fields between NiAI matrix and TiB 2 particle.

FIG.5 HREM image of the NiA1/TiB 2 in- terface.

Acknowledgments

This work was partly supported by the National Natural Science Foundation of China and National Advanced Materials Committee of China.

References

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