Gm Hughes

An examination of the linkage of cleavage cracks at grain boundaries

Abstract Grain boundaries resist the propagation of cleavage cracks in polycrystalline materials, and 3D geometrical models have been used to predict the accommodation required at a grain boundary as a crack propagates from grain to grain. This paper describes how focused ion beam (FIB) microscopy, which provides topographic and crystallographic contrast imaging and allows ion milling to be undertaken at selected areas of interest, can be used to investigate these local fracture events. Results of low temperature fracture of polycrystalline bcc Fe–3%Si and hcp zinc are presented. The interactions between these results and the geometrical modelling are briefly discussed.

Three-Dimensional Modelling of Fracture in Polycrystals

A great deal of research has been carried out on modelling the fracture of polycrystalline materials. However, this has been largely restricted to 2-D models and special cases of 3-D models, e.g. Smith et al. [1], Crocker et al. [2]. Using 2-D models, brittle cleavage cracks in adjacent grains meet at a point in their common grain boundary. A crack can therefore propagate from grain to grain without the necessity of any grain boundary failure. In real materials this is not the case. Cleavage cracks in adjacent grains do not in general meet in a line in their common grain boundary so that some grain boundary failure or an equivalent accommodation mechanism, such as multiple cleavage or ductile tearing, must occur. For this and many other reasons it is important to develop satisfactory 3-D models, Smith et al. [3]. The simplest that have been used are in the form of a body-centred cubic array of identical, regular tetrakaidecahedra (14-hedra). These have suggested that of the order of 30% of brittle fracture is in the form of accommodation grain-boundary failure. This does not include complete grain boundaries that fail because of some inherent weakness. The figure of 30% is much greater than most reported experimental values and therefore there is a need for more realistic 3-D models. One approach has been to construct prismatic grains on a random array of 2-D polygonal cells. This is a good approximation of the structure of columnar grains in weld metal and again suggests that in brittle fracture about 30% of accommodating grain boundary failure is needed.[1] However, as the prisms are effectively of infinite height, this figure gradually increases as fracture propagates.

The Brittle Fracture of Polycrystalline Zinc

In polycrystalline materials grain boundaries provide an important contribution to the resistance to the propagation of both brittle and ductile cracks. In this paper we describe experimental measurements of brittle cracks developed within both small punch and matchstick test specimens of polycrystalline hcp zinc. These specimens were tested over the temperature range 77 to 423K. Fractography undertaken using focussed ion beam imaging provides detail of the propagation from grain to grain and across {10-12} twins of (0001) basal and {10-10} prismatic cleavage cracks. The results are discussed by comparison with the predictions from previously described 3-D geometric modelling applied to this hcp polycrystalline material.

An experimental and modelling study of brittle cleavage crack propagation in transformable ferritic steel

Abstract Flaws that can be present within pressure vessels, pipework and other engineering structures are assessed using the principles of engineering fracture mechanics. It is necessary to support such an approach with an understanding of the underlying fracture mechanisms. Moreover, many of these components are fabricated using transformable steels. In the present paper, the authors describe the fracture of an A508 type steel, heat treated to produce a tempered bainitic microstructure, and subsequently impact tested at −196°C. In particular, focused ion beam microscopy has been used to produce high resolution fractography, combined with information relating to the underlying microstructure and crystallography. The results of cleavage crack propagation across prior austenite grain, lath packet and lath boundaries are described and then correlated with predictions from a three-dimensional geometric model of brittle cleavage fracture in polycrystalline steel. This model includes a consideration of a lath substructure developed within the grains and is based upon a Kurdjumov–Sachs orientation relationship with the parent austenite grain.

Preparation of location-specific thin foils from Fe-3% Si bi- and tri-crystals for examination in a FEG-STEM.

Bi-crystals and tri-crystals of a nominal Fe-3% Si (wt%) of well-defined orientations have been grown using a floating-zone technique with optical heating. The manufacture of these unique crystals and the preparation technique involved in harvesting thin foils from specific locations for transmission electron microscopy are described in detail. In particular, the grain boundary triple junction has been extracted from the tri-crystal and examined in high-resolution aberration-corrected FEG-STEM instruments. To achieve the necessary resolution, the foils have to be uniformly thin, in the range 50-100 nm over large areas of the specimen. For ferromagnetic materials, there are further challenges arising from the magnetic field interaction, with the electron beam placing significant demands on the aberration correction system. One way to minimise this interaction is to reduce the total mass of magnetic material. To achieve this, an in situ focused ion beam lift-out technique has been combined with an additional precision ion-polishing stage to reproducibly provide thin-foil specimens suitable for high-resolution EELS and EDX analysis. Examination of the foils reveals that the final precision ion-polishing stage removes residual damage arising from the use of focused ion beam milling procedures.

Temperature dependence of mechanical properties of zinc and Zircaloy measured using miniaturised disc tests

Abstract A miniaturised, small punch disc test method has been used to measure the load–displacement curves for a pure polycrystalline zinc and a Zircaloy 4 zirconium based alloy. The latter has a more complex microstructure than the zinc, comprising α phase (hcp) precipitates in a β phase (bcc) matrix. The load–displacement curves have been measured over a range of temperature that spans the ductile to brittle transition, with both yield stress and fracture energy measured from these data. The temperature dependence of the yield strength for each material has been compared with the corresponding bulk value. The temperature dependence of the total fracture energy obtained from the disc specimens provides a measure of the ductile to brittle transition temperature. Values obtained using Charpy impact test energies have been compared with these data empirically. The results demonstrate the benefits of undertaking tests on small specimens to evaluate yield strength, fracture energy, the ductile to brittle transition temperature and to characterise the variation of fracture mode with temperature.

An Overview of 3-D Geometric Models to Describe Brittle Fracture in Polycrystalline Ferritic Steels

Flaws that can be present within pressure vessels, pipework and other engineering components are assessed using the principles of fracture mechanics. In general, failure avoidance methodologies are used for assessments. In these, mechanical properties and fracture toughness of the particular materials, taking account of the thermo-mechanical history, are important input parameters. In addition, however, it is necessary to support these approaches with an understanding of the underlying fracture mechanisms. In the case of lower shelf brittle fracture of ferritic materials, various approaches have been adopted to provide an understanding of the initiation and growth of these cracks. Certainly polycrystalline ferritic steels are widely used for construction of engineering components. Although a truly multiscale problem, this overview describes the key features of 3D geometric models that can be used to consider brittle fracture of a variety of polycrystalline materials, including bcc materials, in the intermediate microscale range of 10-6 to 10-3 m. Results of the modelling applied to the brittle fracture of bcc polycrystalline ferritic materials are presented, particularly with respect to predicted crack paths as an initiated crack propagates across the model. In this situation, brittle cleavage and intergranular brittle fracture are addressed. One particular application is the effect of prior grain boundary creep damage, arising from plant service history, on the subsequent lower shelf brittle fracture of ferritic steel components. The implications of this 3D modelling of polycrystalline material are discussed. Copyright

The Role of Sub-Boundaries in the Brittle Fracture of Polycrystalline Materials

Grain boundaries are resistant to the propagation of cleavage cracks in polycrystalline materials. Indeed, the importance of grain boundary orientation on the propagation of cracks, and in particular brittle cracks has been recognised by various workers [1,2]. In a polycrystalline material, the effective cleavage planes and therefore cracks in adjacent grains do not meet each other in a line in the common boundary, except in special circumstances. Therefore if the polycrystal is to separate into two parts some additional failure at the grain boundary must occur; this can take the form of multiple cleavage, brittle intergranular failure or ductile fracture [3]. However, in different metals and alloys, a range of other boundaries and interfaces are encountered, which can modify the propagation of brittle cracks, and in particular cleavage. Examples are twins observed in bcc and hcp metals, martensite and bainite in ferritic steels and more generally, interphase boundaries. Although the importance of these boundaries on the initiation and propagation of cracks has been recognised for many years, there has been little attempt to address the interactions involved.