MICROSTRUCTURAL ANALYSIS OF CEMENTED TUNGSTEN

CARBIDE USING ORIENTATION IMAGING

MICROSCOPY (OIM)

by

Vineet Kumar

A thesis submitted to the faculty of The University of Utah

in partial fulfillment of the requirements for the degree of

Master of Science

Department of Metallurgical Engineering

The University of Utah

May 2008

Copyright © Vineet Kumar 2008

All Rights Reserved

THE UNIVERSITY OF UTAH GRADUATE SCHOOL

SUPERVISORY COMMITTEE APPROVAL

of a thesis submitted by

Vineet Kumar

This thesis has been read by each member of the following supervisory committee and by majority vote has been found to be satisfactory.

-

Chair: Zhigang Zak Fan! -

Ravi Chandran

Dinesh K. Shetty

THE UNIVERSITY OF UTAH GRADUATE SCHOOL

FINAL READING APPROVAL

To the Graduate Council of the University of Utah:

I have read the thesis of Vineet Kumar in its final form and have found that (1) its fonnat, citations, and bibliographic style are consistent and acceptable; (2) its illustrative materials including figures, tables, and charts are in place; and (3) the fmal manuscript is satisfactory to the supervisory committee and is ready for

submission to The Graduate School.

>/-v6-lo�- DatI I Zhigang Zak Fang

Chair: Supervisory Committee

Approved for the Major Department

{J 1. D. Mille;'C

Chair

Approved for the Graduate Council

David S. Chapl\1an Dean of The Graduate School

{

ABSTRACT

Cemented tungsten carbide is one of the most widely produced powder

metallurgy products. For the past 75 years cemented tungsten carbide tools have been

performing at an increasingly popular rate. Fine-grain, especially nano-grain, cemented

tungsten carbides make it possible to achieve a new range of properties that are improved

from their present counterparts. Developments in powder processing enable us to produce

true nano-size tungsten carbide powder « 30 nm). Producing true nano-grain cemented

carbide compacts remains a challenge. The conventional grain growth inhibitors are not

able to inhibit grain growth of nano-grain carbides, and liquid phase sintering is not

suitable for nano-grain carbides due to rapid grain growth. The only possible way to

sinter of nano-grain size cemented tungsten carbide is solid state sintering. Several

studies have focused on the sintering behavior of nanocrystalline WC-Co. The majority

of densification and grain growth in the specimen occurs in solid state. Hence in order to

achieve fully dense nano-grain size WC-Co, it is necessary to understand the underlying

mechanism of densification and grain growth. In this study, a comprehensive

microstructural analysis was carried out during solid state and liquid phase sintering on

micron grade samples using Orientation Imaging Microscopy (OIM). Several

microstructural parameters were analyzed to investigate the grain growth mechanisms. A

comprehensive grain boundary analysis was also done to investigate grain boundary

evolution during sintering, especially for the preferred misorientation. Since cemented

carbide showed preferred prism shape in the microstructure, a faceting analysis was also

carried out. OIM software does not provide tools for all of the analysis that were carried

out in this study, so an algorithm for faceting analysis was generated and implemented.

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TABLE OF CONTENTS

ABSTRACT . .... . .... ....... .. ............ . . . . . _ .... . ... .. .. . . . . .. . ......... . . . .............. iv

ACKNOWLEDGMENTS . . .. . ..... . .............. . .. ............... .. . . . . ........... . .... ix

1. INTRODUCTION .. .. .. . .... . ......... . . . . .... ............ ... ........... .. ........ . .... .. I

1. 1 Scope ............. . ... . .. ... ... . .. ......... . ... . . . .. . .... . ...... ................... 2

2. BACKGROUND ............. . .... .. ... . ................... . ................... . . . ... . ..... 5

2.1 Sintering of Fine-Grain Cemented Carbides . .. .. ............ ..... ........... 5 2.1. 1 Submicron-Grain Cemented Carbides . . . .. .............. . . . .. . .. . ..... 6 2.1.2 Ultrafine-Grain Cemented Carbides ....... . .. . .. .. .... ... ....... ..... . . 7 2.1.3 Nanocrystalline Cemented Carbides ................. .. .... . . . ... . .... 7 2.1.4 Grain Growth ........ . .............. .... .. .. ...... ... ... ..... .... ....... .......... . 8

2. 1.4.1 Grain Size Distribution . . . . .... ... ..................... .. ....... 10 2.2 Electron Backscattered Diffraction ........................... . .. ... . . .. . . .. ... 13

2.2.1 Electron Diffraction .... . ............ . . . . . .. ...... . . .... ................. . 13 2.2 .2 Formation of Kikuchi Patterns .. ... .............. . ..... ... .. .... . ... .... 14 2.2.3 Identification and Indexing of Kikuchi Patterns ... . ........ . .. ..... ... 15 2.2.4 Indexing the Patterns ... .... . . . .. ......... . .......... .. ...... ..... ..... ........ ..... 17

2.2.4.1 Confidence Index . . ........ .. ...... ...... .. .... .. .. .............. 19 2.2.4.2 Fit. ....................... . .. .. .... . .. .. . .. .. . . . . ... . .. . . .. . . .. . ... .. 19 2.2.4.3 d-spacing Fit. . .. .. . .. .. . . . .. . ... . . . ........ . ........ . ............. 19 2.2.4.4 Image Quality . . .... . ..... .. .. ... .. .... . ........ . .. . ..... .. ....... . 19

2.2.5 Phase Identification ....... . ..... .... .. .. .. ..... . .......................... 20 2.2.6 Orientation Determination .. . ................... . .. . .. . .. .. . . . . ..... .. ... 20 2.2.7 Data Collection . .. ....... ............... ... .. ... . .......................... 20 2.2.8 OIM Analysis . ... . ........ . ... .. .... .................... . . . ......... . ... . .. 21

2.2.8.1 Grain Size Analysis . .. . ............... .. .. ... ... . .... . .. .. ....... 21 2.2.8.2 Orientation Analysis and Representation . ...... ..... . ... ... . . 21

2.2.8.2.1 Crystal direction map ........... ... . ... .. ........... . .. 22 2.2.8.2 .2 Texture index . . ........ . .. .. .. .. ... ... .. ...... ....... .. .. 22

2.2.9 Misorientation Analysis .. .. . .......... . .. ... .......... . . . . ... .. . ... .. . .. . . 22 2.2.9.1 Misorientation Angle Chart .... ............... . .... . . .. ......... 23 2.2.9.2 Misorientation Distribution Function (MODF) ........ ...... . 23 2.2.9.3 Misorientation Texture Index . ......... . .. . . . ............ .. .. ... 24 2.2.9.4 Faceting Analysis .. ... . . ... . . . ........... . ..... . ... . ............. . 24

3. EXPERIMENTAL PROCEDURES .. .. ...... .. ........ .. ..... .................. .. .............. ... . 25

3.1 Sample Preparation ............... . ..... .. .. .. ............ .. . ....... . . . .. .... .... 25 3.1 .1 Powder Preparation .. . . .. ............ . ........... ... . . ........ . ......... ... 25 3.1.2 Compaction and Dewaxing . ....... .. . . ............ . . ........... .... .. . ... 25

3.2 Sintering ........ .. ... . ... .. ..... . .... . .. . .. .. .... .... . . ..... . ... .. .. . . ... . ......... 26 3.2. 1 Cutting, Mounting, and Polishing . ............. . ..... ... . ... ... .. . ....... 26 3.2 .2 SEM and OIM Data Collection .. .. . . .................... ... . . ..... . . .. .. 26

3.3 OIM Analysis . .......... . .. .. ....... . ........ .. , . . .. . .. . .. . . .... .................. 28 3.3.1 Faceting Analysis . .............. . . ...... . ..... .... ... ...... . ..... . . . ... .... 28

3.3.1.1 Data Cleaning .. ... . . . ................. . ......... .. . .... . . . .. . . .. .. 29 3.3.1.2 Reconstructed Boundaries . . .................... . . . ..... ... . .. .. 30 3.3.1.3 Angles with Low Energy Planes ........ ........ , ....... . ,. " .. 31 3.3.1.4 Quantitative Analysis of Faceted Boundaries .... .... .. .... .. ..... 31

4. RESULTS AND DiSCUSSION ... ... .... ........... .. .. . .... . ..... .. .. .. .... ... . ..... . 32

4.1 Microstructural Analysis ........................... . . ............ . ........... ... 32 4.1.1 Morphology ... . . .. . .. .... ....... ..... . .... . .... , . ... ... . ............ .. ...... 33 4.1.2 Orientation .. . . . . ... . . .. . .... .. .. .......... ..... .. .. . ...... . ... . ... . ......... 58 4.1.3 Qualitative Faceting Analysis .... . .... ..... .... .. .. . ........ .. .. ... . . .. .. 59

4.2 Faceting Analysis . ................... ........................... .. .. . .. . ......... 59 4.3 Misorientation of WC-WC Boundaries .. .. ................... ... ..... . . . .. ... 65 4.4 Area Fraction of Tungsten Carbide in Microstructure ..................... . 73 4.5 Grain Size and Grain Size Distribution . . .... . .. . ................... ... . . .. .. . 73

4.5.1 Comparison of Grain Growth with Existing Models ........ .. . . . .. ... 76 4.6 Proposed Grain Growth Mechanism . ........... .. ... .... . . ............. . ..... 80

5. CONCLUSIONS ......... ......... ................. .. ..... ...... .. .. . .. ........... . . .. .. .. 82

APPENDIX: FACETING ANALYSIS CODES .. ... ... .. ..... ... .. ..... .. .. . . .. ...... 83

REFERENCES ..... .. .. . . , . .. . ..... .... .. .... .... ....... . . ... ....... ....... ........ . . ..... ... 91

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