Examination of fracture and deformation mechanisms in WC/Co hardmetals

November 8, 2011

The development and validation of a powerful technique for direct in-situ observation of fracture and deformation in WC/Co hardmetals was described in a paper from M Gee, K P Mingard, B Roebuck, J W Nunn and H Jones (National Physical Laboratory, UK)  at the Euro PM2011 Conference, held in Barcelona, 9-12 October.

Dr David Whittaker reports exclusively for ipmd.net on the group’s development of a new test system for in-situ controlled crack growth experiments in a high resolution SEM.

The system is shown schematically in Fig. 1. The double cantilever beam fracture specimens are placed between an anvil and 90° wedge made from alumina. Load is applied to the alumina wedge using a piezoelectric displacement transducer capable of extending by a nominal 60μm and supporting a load of 4.5 kN. The anvil is supported by a piezoelectric load cell (connected to a Charge Amplifier) to measure the applied load and is in turn positioned against a large bolt that is used to pre-load the specimen.


Fig. 1 In-situ test system schematic (top) and photograph (bottom) (From paper by M G Gee etal,  ‘Examination of Fracture and Deformation Mechanisms in WC/Co Hardmetals’ presented at Euro PM2011 Conference. Published in the Conference Proceedings by EPMA, UK)  



Fig. 2 Test specimen, a) view from top, b) underside of

sample showing groove  (From paper by M G Gee etal,

 ‘Examination of Fracture and Deformation Mechanisms

in WC/Co Hardmetals’ presented at Euro PM2011

Conference. Published in the Conference Proceedings

by EPMA, UK)  

In the experiments reported in this paper, 40mm x 12 mm x 1 mm thick samples (Fig. 2) were manufactured by EDM. The flat surfaces of the samples were both ground. The samples were then annealed in a vacuum furnace at 800°C for 30 minutes, and the top surface polished.

At one end of the sample a 5 mm wide throat was machined to accommodate the ceramic wedge. The part of the sample that contacted the ceramic wedge was rounded to ensure that there was a single point of contact and that the stress generated at this contact point was low enough to ensure that no damage occurred to the sample and wedge during an experiment. The back face of the sample was machined with a 0.5 mm deep groove to constrain crack growth to the centre line of the sample. A starter notch, 0.5 mm wide and 3 mm long, was EDM machined along the centre line of the sample.

When a crack initiates, depending on its size and speed, it may cause a drop in the compressive load, or a reduction in the rate of load increase. The developed control software is capable of using either of these conditions as triggers to halt the loading of the specimen so that the crack progression can be studied in detail under the SEM. In practice, it was found that using detection of reduced loading rate and keeping the excitation voltage constant gave the best results.


Fig. 3 Frames from video showing crack growing, 

a) start of sequence, b) 0.8 seconds into crack growth,

c) 1.6 seconds into crack growth (From paper by M G

Gee etal,  ‘Examination of Fracture and Deformation

Mechanisms in WC/Co Hardmetals’ presented at Euro

PM2011 Conference. Published in the Conference

Proceedings by EPMA, UK) 

Tests were carried out on a range of WC/Co hardmetals with different grain sizes and binder phase contents.

An example of the results obtained is shown in Fig. 3.  This figure shows a sequence of frames from a movie at the crack tip as the crack was grown by increasing the exciting voltage on the piezoelectric actuator. Fig. 3a shows the crack tip as it was seen after arresting the growth during a previous application of the load and immediately before the further growth seen in Figs. 3b and 3c. The controlled crack growth can be judged from the relatively slow rate of crack growth seen in this figure and the arrest of the crack on removal of load at the crack extension shown in Fig. 3c.

Characteristic features of the fracture process include the discontinuous crack growth with areas of the structure remaining bridging across the crack and the propagation of the next stage of the crack from a point further back from the crack tip (arrowed in Fig. 3b).

The results of Electron Back Scattered Diffraction (EBSD) analysis of areas around the crack tip are shown in Fig. 4 (in this case for WC only). In the Inverse Pole Figure (IPF) maps (Figs. 4a and 4c), only a few grains show a variation in shading or colour across a grain indicative of strain in the grain. This is even true for grains that have fractured, including a grain where a ligament of material remains connecting the two halves of the grain.


Fig. 4 EBSD analysis of regions around cracks – a) and c) are inverse pole figures maps, b) and d) are local misorientation maps (From paper by M G Gee etal,  ‘Examination of Fracture and Deformation Mechanisms in WC/Co Hardmetals’ presented at Euro PM2011 Conference. Published in the Conference Proceedings by EPMA, UK)  



Fig. 5 Digital image correlation of crack region,

a) image before crack extension, b) image

after crack extension, c) map of y displacement

(perpendicular to crack) from correlation of images

in a) and b). Note that in c) the colour scale runs from

blue with a y displacement of -1.5 micron to red with a

y displacement of 0.5 micron.(From paper by M G

Gee etal,  ‘Examination of Fracture and Deformation

Mechanisms in WC/Co Hardmetals’ presented at Euro

PM2011 Conference. Published in the Conference

Proceedings by EPMA, UK)   

Digital image correlation techniques have also been applied to track the movement of specific points in the image relative to another position (Fig. 5). The overall displacement field can be seen where the surface below the crack has moved relative to the surface above the crack (blue and yellow colouration in the displacement map in Fig. 5c). The displacement field  ahead of the crack tip can also be seen. Three small areas of very high displacement have been observed at the edge of the crack, but, at present, it is not clear how these relate to local microstructural features.

The group is continuing to refine this test system further. A current drawback with the system is that the loading across the notch that is causing the crack to grow is not well defined. A wedge loading system has been employed for reasons of space constraint, but friction at the contact point between the wedge and sample means that the crack opening load is lower than the load applied by the wedge.

To overcome this drawback, the test system is being extended by adding potential drop crack growth measurement and a clip gauge to measure the crack opening displacement. When the use of these techniques is combined with calculation of the compliance of the test specimen, the true load applied across the notch will be defined so that parameters such as the fracture toughness of materials tested can be measured and related to microstructural observations.

Author: Dr David Whittaker is a consultant to the Powder Metallurgy and associated industries. Contact +44 1902 338498 email: [email protected]  


News | Articles | Market reviews | Search directory | Subscribe to e-newsletter



November 8, 2011

In the latest issue of PM Review…

Download PDF

Extensive Powder Metallurgy industry news coverage, and the following exclusive deep-dive articles and reports:

  • From powder modification to rejuvenation: Fluidised Bed Reactors in metal powder production and Additive Manufacturing
  • Retech: Enabling the atomisation of reactive and refractory alloys at substantially higher levels of productivity and lower cost
  • Sustainability in Powder Metallurgy: Highlights from the 41st Hagen Symposium
  • Innovations from Japan’s Powder Metallurgy industry: award winners highlight novel automotive and healthcare applications

The latest news from the world of metal powders, delivered to your inbox

Don't miss any new issue of PM Review, and get the latest industry news. Sign up to our weekly newsletter.

Sign up

Join our community

Discover our magazine archive…

The free-to-access PM Review magazine archive offers unparalleled insight into the world of Powder Metallurgy from a commercial and technological perspective through:

  • Reports on visits to leading PM part manufacturers, metal powder manufacturers and industry suppliers
  • Articles on technology and application trends
  • Information on materials developments
  • Reviews of key technical presentations from the international conference circuit
  • International industry news

All past issues are available to download as free PDFs or view in your browser.


Browse the archive


Looking for PM production equipment, metal powders, R&D support and more?

Discover suppliers of these and more in our
advertisers’ index and buyer’s guide, available in the back of PM Review magazine.

  • Powders & materials
  • Powder process, classification & analysis
  • PM products
  • Atomisers & powder production technology
  • Compaction presses, tooling & ancillaries
  • Sintering equipment & ancillaries
  • Post-processing
  • Consulting & toll sintering
View online
Share via
Copy link