Effects of microstructure on hardness and palmqvist fracture toughness of WC-Co alloys

Materials Science and Engineering, A105/106 (1988) 377-382

377

Effects of Microstructure on Hardness and Palmqvist Fracture Toughness of WC-Co Alloys* Y. SHINt and W. CAO

Materials &'ience and Engineering Department, North Carolina State University, Raleigh, NC 27695- 7907 (U.S.A.)

G. SARGENT

School of Engineering, University of Dayton, Dayton, OH 45469 (U.S.A.)

H. CONRAD

Materials &'ience and Engineering Department, North Carolina State University, Raleigh, NC 27695-7907 (U.S.A.) (Received March 29, 1988)

Abstract

The effects of microstructure, specimen preparation and test procedure on the hardness and indentation fracture of commercial WC-Co alloys (4.5-20wt. %Co)fabricated by two companies were investigated. The measured hardness was in good accord with that calculated from the Lee and Gurland relations. The Palmqvist fracture toughness GIs was proportional to the binder mean free spacing; moreover, Gls- 1 increased linearly with increasing hardness. By considering the variation in the bulk fracture toughness Gic (and of Gis) with hardness, the two are related through Gic = C exp( -/3

G i s - 1)

Only a little difference in the hardness and Gl~ values occurred for the two material sources, specimen preparation and test procedures considered. 1. Introduction

Of importance in the application of cemented WC-Co alloys is a knowledge of the effects of microstructure on their hardness and fracture toughness. A convenient method for obtaining both hardness and fracture toughness in WC-Co

alloys is that proposed by Palmqvist [1], which consists of conducting a Vickers hardness test and measuring as a function of applied load the impression diagonal and the total length ( L t = 4l) of the cracks occurring at the four corners of the impression. For carefully prepared specimens, L t has been found to increase linearly with increasing applied load [2], so that

Lt-

(1)

W

where W is a crack propagation resistance parameter [2] and can be considered [3] to be a surface fracture toughness parameter G~s, similar to GIc for the bulk. Pc is generally small compared with the values of P normally employed, and in many cases a plot of L t vs. P passes through the origin [2, 4], although this need not necessarily occur [3, 5, 6]. A number of correlations [7-11] have been proposed for the effects of microstructure on the hardness of WC-Co alloys. Of these, that by Lee and Gurland [10] has the firmest physical basis and is the least empirical. It gives for the Vickers hardness of an alloy Hv = Hwcfwc + Hco(1 - f w c C)

*Paper presented at the 3rd International Conference on the Science of Hard Materials, Nassau, The Bahamas, November 9-13, 1987. tPresent address: Korea Advanced Energy Research Institute, Taejun, Chungnow, South Korea. 0921-5093/88/$3.50

(2)

where Hwc and Hco are the in situ hardnesses of the WC and cobalt phases respectively, fwc is the volume fraction of the carbide grains and C is their contiguity. © Elsevier Sequoia/Printed in The Netherlands

378

The Palmqvist fracture toughness parameter W for cemented carbides has been found [4] to vary with hardness according to (3)

W -t =A(Hv-Hv°).

This led Viswanadham and Venables [4] to conclude that W was proportional to the bulk fracture toughness parameter Gic. Peters [12] found that W increases linearly with increasing cobalt mean free path 2 and deduced that W = a + bGic where a and b are constants. However, noting that Gic was related to Hv through In GIc = c - g H v

(4)

where c (= 10.3) and g (=3.3 × 10 -l° Pa) are constants, Perrott [3] showed the inadequacy of a simple linear relationship between W and G1c and derived a more appropriate, albeit more complex, relation between the two. Warren and Matzke [13] showed that, for a limited range of cobalt contents, W may be related to the stress intensity factor KI~ through K1s = Kl~ = 0.087(Hv W )1/2. The major objective of the present investigation was to obtain additional data on the effects of microstructure on the hardness and Palmqvist fracture toughness of WC-Co alloys and to evaluate further the correlations presented above. A secondary objective was to compare results on specimens obtained from two fabrication sources, and which were prepared and tested by two procedures. TABLE 1

Specimen

4.5F 6FF 6F 6M 6MM 6CR 9MM 10M 10MM 13M 6A 6B 6C 10E

2. Experimental details The WC-Co alloys used in this investigation were commercial materials containing 4.5-20wt.%Co (8-32vo1.%Co) provided by the Multi-Metals Division of Vermont American Corporation and by Kennametal Corporation. The microstructural parameters of the materials determined by the linear intercept method [10] are presented in Table 1. The alloys from MultiMetals have in general somewhat lower values for the WC grain contiguity factor C. In one series of tests (series A), the as-received specimens were first ground on a 45/~m metalbonded diamond wheel and then polished on cloth-covered wheels employing 30, 15, 6, 3 and 1/zm diamond sprays. After polishing, the specimens were annealed for 1 h at 900°C in a vacuum furnace. The Palmqvist tests were then carried out with a commercial Vickers hardness tester (136 ° diamond indenter) at loads P of 0.2-40 kgf. The length of the impression diagonal and the Palmqvist cracks (which were an extension of the diagonals) were measured with an optical microscope at a magnification of 200-500 ×. In the second series of tests (series B), the as-received specimens were polished on a commercial vibrating polishing machine, starting with 15 Izm and ending with 1 /~m diamond paste. The Palmqvist tests were then carried out without an anneal using a standard 136 ° diamond Vickers indenter mounted on an Instron testing

Microstructural parameters of the WC-Co alloys

Binder content fc o (wt.%(vol.%))

4.5 6.0 6.0 6.0 6.0 6.0 9.0 10.5 10.5 13.0 6.4 6.0 6.3 10.0

(8) (10) (10) (10) (10) (10) (15) (17) (17) (21 ) ( 11 ) (10) (11) (16)

Mean WC grain size awe (/~m)

Mean Cobalt free patht

Contiguity C

~. (/tm)

6 (/~m)

1.0 0.9 1.1 1.6 1.8 2.6 1.6 1.7 1.8 1.6 0.7 1.0 3.3 1.3

0.08 0.09 0.12 0.18 0.20 0.29 0.28 0.35 0.37 0.43 0.08 0.11 0.35 0.26

0.16 0.16 0.20 0.35 0.43 0.72 0.43 0.51 0.53 0.57 0.18 0.37 0.93 0.95

0.48 0.36 0.28 0.49 0.53 0.60 0.35 0.32 0.30 0.26 0.55 0.69 0.63 0.67

Specimens 6A, 6B, 6C and 10E were provided by Kennametal Corporation; the remaining specimens were provided by MultiMetals. t 2 = dwcfco/(1 -fco): assumes presence of cobalt film between W C - W C interfaces. 6 = 2/(1 - c): assumes no cobalt film between W C - W C interfaces.

379

machine, which was operated at a cross-head speed of 2.1 x 10 -5 cm s -t to attain loads of 10-150 kgf. The lengths of the impression diagonals and associated cracks were measured as above. 3. Results

3.1. Measurements and fracture path Typical behavior for test series A is shown in Fig. 1. It should be noted that the average length 2 a of the impression diagonal increased with increasing p ~/2 over the entire load range, in accord with the Vickers hardness equation Hv=2Psin(O/2)/ (2a) 2, and that H v is independent of P. At a critical load P*, cracks developed at each of the four corners of the impression. The average crack length L = ( l I + l2 + l3 + 14)/4 of the four cracks of length li increased with increasing P more rapidly than did the diagonal length. A plot of L t = 4l i vs. P as a function of cobalt content is presented in Fig. 2, where it is seen that L~ increases linearly with P, giving a small intercept Pc ( = 2-3 kgf) on the load axis. The slope of the lines in Fig. 2 increases and the intercept decreases as the cobalt content decreases and the WC grain size becomes smaller. L t vs. P plots as functions of cobalt content and of WC grain size dwc for test series B are presented in Figs. 3 and 4 respectively. In view of the higher loads employed and the reduced scale for the load axis, the straight lines now appear to pass through the origin. The fracture path in all cases was mostly along the cobalt phase, with only an occasional transgranular fracture of a WC grain.

3.2. Hardness and fracture toughness The Vickers hardness values obtained for the two tests series are given in Table 2; those for test series A tend to be slightly higher (average, 3%) than those for series B. In Fig. 5, the measured hardness for both test series is plotted vs. that calculated from the microstructural parameters according to Lee and Gurland [10]. As found by others [13], rather good agreement exists between the measured and calculated values. The values of the fracture toughness G~.~= dP/ dL, obtained for the two test series are included in Table 2. The values for test series A tend to be

TABLE 2 Hardness and fracture toughness of the WC-Co alloy specimens Specimen

GI, (MJ m =)

Vickers hardness

( G N m -z)

4.5F 6FF 6F 6M 6MM 6C 9MM 10M 10MM 13M 6A 6B 6C 6D 6E 14M 20M

Series A

Series B

17.38 16.17 16.03 14.62 14.21 13.65 12.62

16.83 16.39 15.20 14.36 13.94 12.83 11.77 12.43 12.16 11.47 17.59 14.81 11.69 14.88 12.26 11.83 10.23

12.16 11.82

Series A

Series B

0.681 0.782

0.686 0.815 //.894 1.390 1.265 1.491 3.364 3.844 2.630 3.776 0.722 0.912 2.589 1.353 2.707 2.707 10.230

1.186 1.381 1.477 3.042 2.635 3.442

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4. Discussion slightly lower (average, 4%) than those for series B. Figure 6 shows that G~s-~ increases linearly with increasing hardness in accord with eqn. (3), giving A = 2 x 10 - 16 m 2 N -2 and Hv ° = 10.26 Pa; these values are in good accord with those reported by others [3, 4] for cemented carbides (A = 1.85 x 10 -16 m 2 N -2 and Hv ° = 10.6 GPa). Figure 7 shows that G~s is proportional to cobalt mean free path, giving (5)

with f l = 7.6 x 10 ~2 J m -z, which is in reasonable accord with the value of 5 x 1012 J m - 2 obtained by Peters [12] for W C - C o alloys.

The present results indicate reasonable agreement between the results for the various conditions considered here, namely two suppliers, two surface preparation procedures and two test methods. The slightly higher hardness and slightly lower Gts values obtained in the series A tests compared with the series B tests probably result from a higher compressive stress existing in the WC grains (tensile in the cobalt) at the surface following the annealing treatment at 900°C, which was employed in the former series and not in the latter. Exner [2] has shown that WC-Co specimens which had been given an annealing treatment exhibited slightly larger Palmqvist crack lengths (lower Gis ) than those which had

381

been carefully polished. Moreover, the crack length was shown to increase with an increase in compressive stress in the WC grains (tensile in the cobalt) at the surface. Of special interest is the relationship between G~ and the bulk fracture toughness parameter Glc. Taking the data of Leuth [14] and Murray [15], Perrott [3] has shown that G~c decreases with increasing hardness according to eqn. (4) given above. Combining this relation with that between hardness and Gts (eqn. (3)), we obtain In G,c = a

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(6)

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(6a)

where C = e ~, a = (c - g H v °) and fl = g/A. A plot of log Glc (data of Leuth [14] and Murray [15]) vs. GI~ obtained in the present tests for the same hardness value is given in Fig. 8. It should be noted that reasonably good correlation is obtained, yielding C = 8 . 7 0 x 1 0 2 J m -2 and /3 = 1.86 x 106 J m -2. In comparison, employing the values of the constants c, g, A and Hv ° derived by Perrott [3], we obtain C = 8.97 x 102 J m-2 a n d f l = 1.80x 106 j m 2

5. Summary and conclusions The effects of microstructure, specimen preparation and test procedure on the hardness and indentation fracture of commercial WC-Co alloys (4.5-20wt.%Co) fabricated by two companies were investigated. The following is a summary of the results obtained and the conclusions derived therefrom.

(1) The effects of microstructure on hardness were in good accord with that calculated from the Lee and Gurland [10] relations. (2) The Palmqvist fracture toughness parameter Gis = dP/dL was proportional to the binder free spacing and to the reciprocal of the hardness. (3) The bulk fracture toughness G~c was related to the Palmqvist parameter Gis through Glc = Cexp(-flG1s-l), where C and fl are constants. (4) Slightly higher hardness and lower GI~ values were obtained for specimens which had been annealed at 900 °C prior to testing compared with those which had not been annealed. This was attributed to a higher compressive stress in the WC grains in the annealed specimens.

Acknowledgments The authors acknowledge and appreciate support of test series A of this research by the University of Kentucky Institute for Mining and Minerals Research and of test series B by the U.S. Department of Energy under Grant DE-FG0584ER45115. They also wish to express their appreciation to Multi-Metals, a Division of Vermont American Corporation, and to Kennametal Corporation for providing the WC-Co alloys.

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382

References 1 S. Palmqvist, Jernkonterets Ann., 141 (1957) 300; Arch. Eisenhiittenwes., 33 (1962) 629. 2 H.E. Exner, Trans. Metall. Soc. AIME, 245 (1970) 677. 3 C.M. Perrott, Wear, 45(1977)293; 17(1978) 82. 4 R. W. Viswanadham and J. D. Venables, Metall. Trans. A8(1977) 187. 5 E O. Snell and E. P/irnama, Planseeber. Pulvermetall., 21 (1973) 17. 6 E. A. Almond and B. Roebuck, in R. K. Viswanadham, D. J. Rowcliffe and J. Gurland (eds.), Science of Hard Materials, Plenum, New York, 1983, p. 597. 7 J. L. Chermant, A. Deschanvres, G. Hautier, A. Iost and G. Manier, Phys. Status Solidi, 15 (1973) 149.

8 G. Grathwohl and R. Warren, Mater. Sci. Eng., 14 (1974) 55. 9 R. Warren and M. B. Waldron, Powder Metall. Int., 7 (1975) 18. 10 H. C. Lee and J. Gurland, Mater. Sci. Eng., 33 (1978) 125. 11 M.T. Langier, Acta Metall., 33 ( 1985 ) 2093. 12 C.T. Peters, J. Mater. Sci., 14 (1979) 1619. 13 R. Warren and H. Matzke, in R. K. Viswanadham, D. J. Rowcliffe and J. Gurland (eds.) Science of Hard Materials, Plenum, New York, 1983, p. 563. 14 R.C. Leuth, in R. C. Brandt, D. E H. Hasselman and F. E Lange (eds.) Fracture Mechanics of Ceramics, VoL 2, Plenum, New York, 1974, p. 791. 15 M. J. Murray, Proc. R. Soc. London, Ser. A, 356 (1977) 483.