Predicting whether a material is ductile or brittle

In this paper we discuss the various models that have been used to predict whether a material will tend to be ductile or brittle. The most widely used is the Pugh ratio, G/K, but we also examine the Cauchy pressure as defined by Pettifor, a combined criterion proposed by Niu, the Rice and Thomson model, the Rice model, and the Zhou-Carlsson-Thomson model. We argue that no simple model that works on the basis of simple relations of bulk polycrystalline properties can represent the failure mode of different materials, particularly where geometric effects occur, such as small sample sizes. Instead the processes of flow and fracture must be considered in detail for each material structure, in particular the effects of crystal structure on these processes

Wavy cracks in drying colloidal films

Fracture mechanics successfully predicts when cracks will grow. Describing the path that cracks follow, however, has remained difficult. The study of crack paths has recently focused on a single experimental system, that of thermally quenched glass, where straight, wavy, helical, and branched cracks appear under different conditions. Several models of crack path prediction have been developed but none is generally accepted. Here we show that slowly oscillating wavy cracks can form during the drying of a colloidal dispersion. These drying films are subject to large stress gradients perpendicular to the mean direction of crack growth. Under these conditions existing models do not predict periodic paths. We show, instead, how to model crack paths by allowing a growing crack to curve towards the direction of maximum energy release rate. Not only does this explain wavy cracks in drying films, and correctly describe the wavelength dependence of our experiments, but it is generally applicable to predicting crack paths in spatially varying stress fields

Plasticity and fracture in drying colloidal films

Cracks in drying colloidal dispersions are typically modeled by elastic fracture mechanics, which assumes that all strains are linear, elastic, and reversible. We tested this assumption in films of a hard latex, by intermittently blocking evaporation over a drying film, thereby relieving the film stress. Here we show that although the deformation around a crack tip has some features of brittle fracture, only 20%-30% of the crack opening is relieved when it is unloaded. Atomic force micrographs of crack tips also show evidence of plastic deformation, such as microcracks and particle rearrangement. Finally, we present a simple scaling argument showing that the yield stress of a drying colloidal film is generally comparable to its maximum capillary pressure, and thus that the plastic strain around a crack will normally be significant. This also suggests that a film’s fracture toughness may be increased by decreasing the interparticle adhesion

Solidification and ordering during directional drying of a colloidal dispersion

During drying, colloidal dispersions undergo processes such as solidi cation, cracking, and the raining f interstitial pores. Here we show that the solidication of polystyrene and silica dispersions, during directional drying, occurs in two separate stages. These correspond to the initial ordering, and subsequent aggregation, of the colloidal particles. Transitions between these stages are observed as changes in transparency and color that propagate as distinct fronts along the drying layer. The dynamics of these fronts are shown to arise from a balance between compressive capillary forces, and the electrostatic and van der Waals forces described by DLVO theory. This suggests a simple method by which the maximum inter-particle repulsion between particles can be measured through the optical inspection of the dynamics of a drying dispersion, under a microscope

Examination of Ni-based superalloy/intermetallic diffusion couples containing multiphase regions

Increasing gas turbine operating temperatures are driving the development of novel coatings for niche applications. One such application is as an anchor phase material for use in the high-pressure turbine stage, for which NiAlTa alloys are a promising candidate. Extended exposure to the high temperatures of this environment can cause interdiffusion of elements between the coating and the underlying blade material. In this study, NiAlTa/CMSX-4 diffusion couples were investigated experimentally and computationally. The couples initially contained two two-phase regions (γ + γ′) and (β + τ1). After heat treatment at 1100 °C, interdiffusion had caused the τ1 Laves phase in the coating to transform to the τ2 Heusler phase, and TCP precipitation was observed in the CMSX-4. A CALPHAD-based model, using Thermo-Calc and DICTRA, developed for this system was able to predict the concentration profiles across the diffusion couple at 1000 °C, with the presence of the predicted phases in the interdiffusion zone verified by x-ray diffraction. However, due to the limited diffusion data for intermetallic phases available in the kinetic database, the model predictions were poor at higher temperatures. In order for the development of intermetallic coatings to be aided by CALPHAD-based simulations, more kinetic data is needed for intermetallic phases than is available at present

Softening non-metallic crystals by inhomogeneous elasticity

High temperature structural materials must be resistant to cracking and oxidation. However, most oxidation resistant materials are brittle and a significant reduction in their yield stress is required if they are to be resistant to cracking. It is shown, using density functional theory, that if a crystal's unit cell elastically deforms in an inhomogeneous manner, the yield stress is greatly reduced, consistent with observations in layered compounds, such as Ti₃SiC₂, Nb₂Co₇, W₂B₅, Ta₂C and Ta₄C₃. The mechanism by which elastic inhomogeneity reduces the yield stress is explained and the effect demonstrated in a complex metallic alloy, even though the electronegativity differences within the unit cell are less than in the layered compounds. Substantial changes appear possible, suggesting this is a first step in developing a simple way of controlling plastic flow in non-metallic crystals, enabling materials with a greater oxidation resistance and hence a higher temperature capability to be used.The work was supported by the EPSRC/Rolls-Royce Strategic Partnership (EP/M005607/1)

Compressive deformation and failure of CrAlN/Si3N4 nanocomposite coatings

The deformation and failure mechanisms of CrAlN/Si3N4 coatings containing grains a few nanometres in size have been compared with those of conventional CrN-based coatings. It is shown that the addition of amorphous Si3N4 phase increased the yield stress and hardness of the coating material, but did not change their ratio. This is consistent with theoretical predictions using existing models. However, cracking in conventional CrN-based coatings was catastrophic, whereas that in the fine-grained CrAlN/Si3N4 structure was much more benign, suggesting that the improved performance of these materials is associated with their fracture behaviours.This research was funded by A*STAR, Singapore and the Engineering and Physical Sciences Research Council (EPSRC) and Rolls-Royce Strategic Partnership “Structural Metallic Systems for Advanced Gas Turbine Applications” (EP/H500375/1).This is the accepted version of an article published in Applied Physics Letters. The final version is available online at http://scitation.aip.org/content/aip/journal/apl/104/8/10.1063/1.4867017. © 2014 AIP Publishing LL

Plastic flow at the theoretical yield stress in ceramic films

Using fine-grained ceramic films based on chromium nitride, and suppressing fracture by using microcompression, it is shown that plastic flow at the theoretical yield stress can be obtained in brittle materials, with shear yield stresses of ~ G/24 at room temperature, which extrapolate to ~ G/19 at 0 K. Surprisingly, it is also found that the rate of deformation, and hence the hardness and the yield stress, are determined not by the soft, glassy grain boundary phase in the fine-grained materials, but by the harder crystal phase.This research was funded by A*STAR, Singapore and the Engineering and Physical Sciences Research Council (EPSRC) and Rolls-Royce Strategic Partnership (EP/H500375/1)

Deformation of lamellar TiAl alloys by longitudinal twinning

© 2016 Elsevier Ltd. The occurrence of longitudinal twinning in the engineering alloy Ti-45Al-2Nb-2Mn (at.%)-0.8 vol.% TiB2 has been studied by measuring the changes in crystallographic orientation within individual lamellae during microcompression. Twinning in this alloy appeared to be a nucleation-limited process with the twins growing from lamellar boundaries at resolved shear stresses as low as 100 MPa, consistent with observations elsewhere. However, instead of forming twins ∼ 10-200 nm in thickness, as in polysynthetically twinned crystals, the longitudinal twins in this alloy were initiated at a lamellar boundary and then spread through the whole lamella.The work was supported by the EPSRC / Rolls-Royce Strategic Partnership (EP/H500375/1). Alberto Palomares Garcia, Claire Davis and Robert Jones are acknowledged for discussions and help with the TEM respectively

Stable Speckle Patterns for Nano-scale Strain Mapping up to 700 °C

The digital image correlation (DIC) of speckle patterns obtained by vapour-assisted gold remodelling at 200 – 350 °C has already been used to map plastic strains with submicron resolution. However, it has not so far proved possible to use such patterns for testing at high temperatures. Here we demonstrate how a gold speckle pattern can be made that is stable at 700 °C, to study deformation in a commercial TiAl alloy (Ti-45Al-2Nb- 2Mn(at%)-0.8 vol% TiB$_2$). The pattern is made up of a uniformly sized random array of Au islands as small as 15 nm in diameter, depending on reconstruction parameters, with a sufficiently small spacing to be suitable for nano-scale, nDIC, strain mapping at a subset size of 60 × 60 nm$^2$ . It can be used at temperatures up to 700 °C for many hours, for high cycle fatigue testing for instance. There is good particle attachment to the substrate. It can withstand ultra-sound cleaning, is thermally stable and has a high atomic number contrast for topography-free backscatter electron imaging.EPSRC / Rolls-Royce Strategic Partnership (EP/M005607/1