B. Roebuck

Recrystallisation of single crystal superalloy CMSX–4

Abstract A series of experiments investigating the recrystallisation of single crystal superalloy CMSX-4 have been carried out. Indentation atroom temperature has been used to study the effects of annealing time and temperature, and it has been found that a very strong dependence upon temperature is evident. Annealing above the γ′ solvus temperature results in very rapid growth of recrystallised grains whereas annealing below the γ′ solvus greatly suppresses the advancing grain boundaries. Additionally experiments have been carried out using an electrothermal mechanical test (ETMT) machine, to study the effects of degree of plastic strain and the temperature at which the strain is introduced. The strain threshold for recrystallisation under various annealing conditions has been determined and it has been found that recrystallisation occurs more readily if strain is introduced above 950°C. Finally, apparent activation energies for recrystallisation have been determined by measuring the change in resistivity that occurs during recrystallisation.

Extending the application areas for titanium carbonitride cermets

Abstract Titanium carbonitride cermets have been largely developed as cutting tool materials. However, their properties indicate that there is potential for extending the application areas to other tools. This report examines the industrial possibilities and indicates a number of alternative products which might benefit from the use of cermets.

Terminology, testing, properties, imaging and models for fine grained hardmetals

The interest in hardmetals with fine grain sizes derives from an understanding that hardness generally increases with a decrease in WC grain size. WC powders with very fine sizes are increasingly becoming available. The use of non-standard terms for these materials can be confusing and benefits could accrue from a rationalisation of the terminology for their description. Their use is also putting new demands on the requirements for testing and characterisation of sintered structures. These issues are considered in detail in this paper together with a review of several models for predicting hardness and magnetic properties from grain sizes. For example, the use of the Hall-Petch model for predicting the relation between the grain size and hardness of hardmetals is discussed. The model appears to be satisfactory for the range of materials currently commercially available. However, extrapolation beyond that range will require some care. Coercivity measurements provide an indirect method for estimating grain size. There are a number of empirical equations available in the literature relating coercivity to grain size. These are also critically examined and it is shown that there is a need for standardisation of the grain size measurement method before agreement can be reached on the most appropriate empirical equation.

Magnetic moment (saturation) measurements on hardmetals

This note summarises the background to the use of appropriate units for the measurement of magnetic moment (saturation) in hardmetals. It also discusses the methods used for correlating composition/structure with magnetic properties and indicates the theoretical ranges likely to be obtained in typical industrial products. It includes comparative data obtained on two different measurement systems on both hardmetals and synthetic Co, Co-Ni and Ni alloys containing W and C in solution.

Grain size measurement by EBSD in complex hot deformed metal alloy microstructures

The measurement of grain size by EBSD has been studied to enable representative quantification of the microstructure of hot deformed metal alloys with a wide grain size distributions. Variation in measured grain size as a function of EBSD step size and noise reduction techniques has been assessed. Increasing the EBSD step size from 5% to 20% of the approximate mean grain size results in a change in calculated arithmetic mean grain size of approximately 15% and standard noise reduction techniques can produce a further change in reported size of up to 20%. The distribution of measured grain size is found not to be log‐normal, with a long tail of very small sizes in agreement with a computer simulation of linear intercept and areal grain size measurements through randomly oriented grains. Comparison of EBSD with optical measurements of grain size on the same samples shows that, because of the ability of EBSD to distinguish twins and resolve much smaller grains a difference of up to 50% in measured grain size results.

Effects of microstructure on the thermo-mechanical fatigue response of hardmetals using a new miniaturized testing rig

Abstract High thermo-mechanical fatigue (TMF) resistance is essential for hardmetal materials used for milling, interrupted-cutting or even mining and construction applications. The National Physical Laboratory has developed a miniaturized electrothermo-mechanical test rig which offers considerable potential over conventional testing frames for thermo-mechanical testing of hardmetals. The present study has investigated the effects of microstructural parameters on the thermo-mechanical fatigue behavior of WC–Co hardmetals. Clearly identifiable trends were observed as each microstructural features were changed. Regardless of the temperature/loading scheme used, the fatigue resistance was found to increase when the Co mean free path, WC grain size, cobalt content or carbon content were decreased, or when the solid solution cubic content was increased. Incidentally, the rig potential both in terms of discrimination capacity when testing parameters or microstructural features are changed, and in terms of degree of reproducibility between repeated tests were explored. The discrimination potential was remarkable and the reproducibility was adequate for fatigue testing.

Mapping microstructure inhomogeneity using electron backscatter diffraction in 316L stainless steel subjected to hot plane strain compression tests

Abstract The microstructure inhomogeneity in 316L stainless steel subjected to hot plane strain compression tests has been assessed using electron backscatter diffraction (EBSD). Two variables were investigated: the effect of strain rate and the effect of friction at the tool/specimen interface. Tests were performed isothermally at 950°C at nominal equivalent tensile strain rates of 0·01 and 1 s−1. Low and high friction conditions have been simulated by applying both a glass based lubricant and a boron nitride spray respectively. Results suggest that friction causes a variation in microstructure from the surface to the midplane of the deformed specimen. Several methods used to quantify and represent this inhomogeneity are presented in the present paper. Electron backscatter diffraction measurement issues are discussed. A grain size mapping method using a two-dimensional moving average has been developed to overcome the difficulties associated with the visualisation of measurement results over large areas on EBSD maps. It has proved to be a powerful tool for the spatial statistics of large quantity data obtained by EBSD.

Mechanical test discriminability for WC hardmetals

Abstract How good are the commonly used mechanical property tests for discriminating between materials of different types? In a preliminary exercise to examine this question, two experimental 6 wt % Co/WC hardmetals were prepared with small differences in particle size distribution. Although manufactured from the same WC powders, one specimen contained a distribution of small Co regions, several micrometres in diameter, while the other specimen was structured with the more conventional uniform distribution of Co and WC. These two materials were tested at room temperature using six different methods: hardness, bend strength, notched bend strength, plane strain fracture toughness, indentation fracture toughness and abrasion wear. The results of the tests are described in detail and it is shown that of the six tests only two, notched bend and abrasion wear were able to discriminate between the two experimental materials.

Notched bend tests on WC-Co hardmetals

Notched bend tests have been performed on a range of WC-Co hardmetals. The notch stress concentration factors were derived from conventional notch stress analysis and by a finite-element method. The introduction of the notch suppressed the initiation of failures from macroscopic defects such as pores and inclusions. The strength values obtained were thus representative of the underlying hardmetal microstructure.

Dependence of oxidation rate of WC powder on particle size

Isothermal oxidation experiments on WC powders revealed a systematic dependence of oxidation rate on powder particle size. Oxidation was followed by measuring the change in mass of the WC powder as WC is converted to WO3. Fine powders oxidized more quickly than coarse powders because for the same initial mass the fine powder had a larger surface area. Measurement of the change in mass with time were shown to resolve differences in mean size of 0.1 μm, and possibly less, between separate batches of powder. A theoretical expression for the change in mass with time of spherical particles has been derived which compares well with experimental measurements and which can also be used with appropriate assumptions to calculate the initial powder-size distribution.