The application of indentation techniques to the evaluation of fracture toughness is examined critically, in two parts. In this first part, attention is focused on an approach which involves direct measurement of Vickers-produced radial cracks as a function of indentation load. A theoretical basis for the method is first established, in terms of elastic/plastic indentation fracture mechanics. It is thereby asserted that the key to the radial crack response lies in the residual component of the contact field. This residual term has important implications concerning the crack evolution, including the possibility of post indentation slow growth under environment-sensitive conditions. Fractographic observations of cracks in selected “reference” materials are used to determine the magnitude of this effect and to investigate other potential complications associated with departures from ideal indentation fracture behavior. The data from these observations provide a convenient calibration of the Indentation toughness equations for general application to other well-behaved ceramics. The technique is uniquely simple in procedure and economic in its use of material.

A Critical Evaluation of Indentation Techniques for Measuring Fracture Toughness: I, Direct Crack Measurements

A Critical evaluation of indentation techniques for measuring fracture toughness

1. Introduction 2. The physical metallurgy of nickel and its alloys 3. Single crystal superalloys for blade applications 4. Superalloys for turbine disc applications 5. Environmental degradation: the role of coatings 6. Summary and future trends.

The Superalloys: Fundamentals and Applications

A review of the observations of indentation-induced fracture suggests that there is no simple generalization which may be made concerning crack initiation sequences. Here, we investigate the material dependence of the initiation sequence of indentation cracks (cone, radial, median, half-penny, and lateral) using an inverted tester allowing simultaneous viewing of the fracture process and measurement of the indeter load and displacement during contact. Two normal glasses, two anomalous glasses, and seven crystalline materials are examined. Key results include (i) direct evidence that the surface traces of cracks observed at indentation contacts are those of radial cracks, rather than median-nucleated half-penny cracks (at least for peak contact loads <40 N) and (ii) that, in crystalline materials, radial cracks form almost immediately on loading of the indenter, in anomalous glasses at somewhat greater loads, but in normal glasses during unloading. A detailed consideration of the stress fields arising during indentation contact predicts material-dependent initiation sequences, in agreement with observations, particularly those of radial crack formation on loading for materials with large modulus-to-hardness ratios. In addition, a new, unexplored crack system is demonstrated, the shallow lateral cracks, which appear to be responsible for material removal at sharp contacts.

Direct Observation and Analysis of Indentation Cracking in Glasses and Ceramics

An experimental and numerical study of deformation in metal-ceramic composites

Deformation and fracture processes have been examined in WC–Co tool materials, still the most commonly used hardmetal. The relevant microstructural parameters are described in detail and an assessment is made of their effects on strength and fracture tests at ambient temperature, for the constituent phases separately and in combination. The results of properties more relevant to service behaviour, such as fatigue and creep, are summarised. Finally, recommendations are made with regard to the need for further fundamental research.

Deformation and fracture processes and the physical metallurgy of WC–Co hardmetals

Wear mechanisms in abrasion and erosion of WC/Co and related hardmetals

Sensitivity of stress corrosion cracking of stainless steel to surface machining and grinding procedure

The possibility has been investigated of using measurements of the mechanical properties of alloys with binder phase compositions to identify binder phases that will provide high hardness and toughness in WC hard metals based on cobalt, nickel, CoNi, NiAl, NiCrMo and NiCrMoAl. The results showed that, in WCCo and WC(CoNi) hard metals with constant grain size and binder phase content, an optimum grain size and binder phase content, an optimum combination of fracture toughness and compressive strain could be obtained at particular tungsten and carbon contents which could be determined from measurements of magnetic saturation and lattice parameters. As predicted from tests on NiWC alloys, it was found that WCNi hard metals had lower toughness and strength than WCCo hard metals do. Additions of aluminium to nickel binder phases to form γ′ precipitates raised the strength and conferred creep resistance but decreased the fracture toughness. Solid solution strengthening by chromium and molybdenum raised the hardness without reducing toughness as measured by Palmqvist tests and gave properties which matched those of WCCo hard metals. By combining γ′ hardening and solid solution hardening by chromium and molybdenum in nickel-based binder phases, better values of hardness and toughness were obtained than those of WCCo hard metals. The new compositions also offered the possibilities of enhanced resistance to creep and corrosion, and properties that could be varied by heat treatments to meet specific requirements.

Identification of optimum binder phase compositions for improved WC hard metals

Accurate measurements of grain size are required to underpin development of models to predict properties, particularly of hybrid and multiphase materials. The conventional method for WC grain size determination in WC/Co is to use linear intercept measurements from light microscope or scanning electron microscopic images, usually requiring lengthy manual measurements, and subjective interpretation of grain boundary positions according to the level of etching and change in contrast. The possibilities offered by electron backscattering diffraction (EBSD) for improving the characterisation of both the WC and Co phase size, and contiguity between them, has been investigated. Samples covering a range of grain sizes have been examined by both EBSD and conventional methods to determine mean values and size distributions, with a special emphasis on the measurement of small grains. Diameters determined by both area (to give a circle equivalent diameter) and linear intercept from EBSD data are compared with the SEM image linear intercept method. It is shown that although the EBSD method is automated, great care must be taken in data analysis to identify and handle consistently small size values. If care is taken in the data handling then a good correlation can be shown between the EBSD and optical techniques.

Bryan Roebuck

Papers

Deformation and fracture processes and the physical metallurgy of WC–Co hardmetals

Wear mechanisms in abrasion and erosion of WC/Co and related hardmetals

Sensitivity of stress corrosion cracking of stainless steel to surface machining and grinding procedure

Identification of optimum binder phase compositions for improved WC hard metals

Comparison of EBSD and conventional methods of grain size measurement of hardmetals