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Hardness profile measurements in functionally graded WC–Co composites
Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, The Institute of Technology.
Linköping University, Department of Mechanical Engineering, Engineering Materials. Linköping University, The Institute of Technology.ORCID iD: 0000-0002-2286-5588
2004 (English)In: Materials Science & Engineering: A, ISSN 0921-5093, E-ISSN 1873-4936, Vol. 382, no 1-2, 141-149 p.Article in journal (Refereed) Published
Abstract [en]

Micro- and nanoindentation were used to determine hardness as a function of depth in two functionally graded WC–Co composites. The gradients were continuous (extended over ∼70 and ∼40 μm, respectively) and consisted of varying WC and Co phase volume fractions. Five comparable homogeneous samples with different Co contents and different average WC grain sizes were also used for direct comparison. A relationship between hardness and Co content was established for both the graded and the homogeneous samples wherein the hardness decreased with increasing Co content. The magnitude of the hardness was the same (for a given Co content) for the functionally graded and the homogeneous materials. The hardness measurements were also correlated with X-ray diffraction studies of thermal residual stresses and the absence of any major influence explained. It is generally concluded that the hardness values are dominated by the local Co content. Additionally, the examination of surface cracks around indents suggests that compositional gradients in WC–Co composites offer increased toughness.

Place, publisher, year, edition, pages
2004. Vol. 382, no 1-2, 141-149 p.
Keyword [en]
hardness, functionally graded composites, WC-Co, nanoindentation, microindentation, residual stress
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:liu:diva-46174DOI: 10.1016/j.msea.2004.04.065OAI: oai:DiVA.org:liu-46174DiVA: diva2:267070
Available from: 2009-10-11 Created: 2009-10-11 Last updated: 2017-12-13
In thesis
1. Determination of residual stresses and mechanical properties using neutron, X-ray diffraction, micro- and nanoindentation techniques
Open this publication in new window or tab >>Determination of residual stresses and mechanical properties using neutron, X-ray diffraction, micro- and nanoindentation techniques
2003 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The existence of residual stresses in engineering materials can significantly affect subsequent lifetime by augmenting or impeding failure. Consequently, for an accurate assessment of engineering lifetimes, there is a need to quantify residual stresses. Furthermore, knowledge of the origin of these stresses in conjunction with mechanical properties such as hardness and fracture toughness, among others, can be used to improve functionality by tailoring the microstructure through processing. In this work, neutron, x-ray diffraction, micro- and nanoindentation techniques were used for residual stress determination and mechanical characterization of WC-Co functionally graded composites, a Co-based Haynes® 25 alloy weld, compressed steel and compacted Fe-brass powders. The neutron and x-ray diffraction techniques were used to assess residual strains and stresses while the instrurnented indentation techniques were used to determine hardness, fracture toughness and elastic modulus. In each of these engineering materials, valuable insight relating to the overall mechanical performance was obtained.

X-ray diffraction was used to determine thermal residual stresses that develop in a functionally graded WC-Co composite, commonly used as tool bits. Microstresses in the graded zone were attributed to the thermal mismatch between WC and the Co phase. The compressive macrostresses were determined to be a result of the compositional gradient. Micro- and nanoindentation experiments were used to determine hardness as a function of depth in two WC-Co functionally graded materials (FGMs). A relationship between hardness and Co phase content was established and explained for the two graded and five homogeneous samples.

An experimental and simulation study of residual stresses was made in the vicinity of a gas tungsten arc weld in a Co-based Haynes® 25 alloy used in a satellite component. The experimental measurements were made by neutron diffraction on the recently commissioned Spectrometer for Materials Research at Temperature and Stress (SMARTS) at Los Alamos National Laboratory, USA and the simulation used the implicit Marc finite element code. Comparison between experiment and theory showed general agreement.

Strain pole figures representative of residual intergranular strains were determined from an -2.98 % uniaxially compressed austenitic stainless steel sample. The neutron diffraction measurements were made on SMARTS, using an Euler cradle to obtain spectra over a range of sample orientations. The measurements were compared with predictions from an elasto-plastic self-consistent model and found to be in reasonable agreement. The model was also used to assess the sensitivity of the strain distribution in the deformed sample to the initial texture.

Neutron diffraction was used to measure residual stresses in a powder metallurgical green body manufactured by high speed compaction from Fe and 15 wt.% brass powders. The tests were performed on SMARTS with the aid of radial collimators configured to measure spatially resolved strains in the axial and radial directions in a cylindrical specimen. Furthermore, sharp (Berkovich) and spherical (Hertzian) indenters were used for instrumented indentation experiments to determine the hardness and elastic modulus.

Place, publisher, year, edition, pages
Linköping: Linköpings universitet, 2003. 24 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 856
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-30056 (URN)15516 (Local ID)91-7373-786-0 (ISBN)15516 (Archive number)15516 (OAI)
Public defence
2003-12-19, Kvalitetstekniks seminarierum, Linköpings Universitet, Linköping, 10:15 (Swedish)
Opponent
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2013-01-07
2. Thermal residual stresses, hardness and microstructural characterisation of functionally graded WC-Co composites
Open this publication in new window or tab >>Thermal residual stresses, hardness and microstructural characterisation of functionally graded WC-Co composites
2001 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Functionally graded materials (FGMs) incorporate gradual transitions in their microstructure and/or composition with distance. The gradients provide means to favourably engineer their thermomechanical properties by changing e.g., the local modulus, yield strength, state of residual stress etc. However, prior to optimising such gradients, an understanding of their effect on thermomechanical properties is necessary. WC-Co composites are commonly used as tool bits, where the presence of Co improves the toughness of the wear resistant WC. In this thesis, emphasis has been placed on investigating the thermal residual stresses, hardness, and toughness of functionally graded WC-Co.

The first paper in this thesis reviews existing literature on the subject while introducing FGMs and WC-Co composites. Additional topics addressed include: fabrication of FGMs, mechanical properties of WC-Co and estimates of residual stresses.

In the second paper, X-ray diffraction was used to determine thermal residual stresses that develop in a functionally graded WC-Co composite. The gradient was continuous during a distance of about 40 Jlm below the surface and the stresses were measured in both WC and Co phases at various depths. Due to the difference in coefficient of thermal expansion between the two phases, tensile residual stresses where found in the Co phase and balancing compressive stresses in WC. Pole figures were obtained in order to determine optimal sample orientations that provided adequate intensity for measurements in the Co phase. Microstresses in the graded zone were attributed to the thermal mismatch between WC and the Co phase. The compressive macrostresses were determined to be a result of the compositional gradient.

The third paper reports micro- and nanoindentation experiments to determine hardness as a function of depth in two different WC-Co FGMs. A relationship between hardness and Co phase content was established for the two graded and five homogeneous samples wherein the hardness decreases with increasing Co phase content. For a given Co phase content, no significant differences were observed between the FGMs and homogeneous samples. The measured thermal residual residual stresses did not appear to influence the hardness. Additionally, examination of indents using a Vickers indenter with different loads suggested that larger loads were required to initiate cracks in the FGMs compared to the homogeneous materials. While the investigated WC-Co FGMs did not offer advantages in hardness properties (which seem to be dominated by the local Co content in the graded zone), they appear to offer tribological advantages through increased toughness in terms of crack suppression.

Place, publisher, year, edition, pages
Linköping: Linköpings universitet, 2001. 12 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 884
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-30048 (URN)15508 (Local ID)91-7373-026-2 (ISBN)15508 (Archive number)15508 (OAI)
Available from: 2009-10-09 Created: 2009-10-09 Last updated: 2013-11-28

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