Probing the Strength and Fracture Toughness of Hard Second Phases at the Micron Scale

It is known that the mechanical properties of composite materials and alloys are influenced by the intrinsic mechanical properties of their reinforcements or second phases. It is however challenging to determine such local properties, given their small size and irregular shape. Here we present two newly developed methods to probe the local strength and fracture toughness combining focused ion beam (FIB) milling with microtesting techniques and finite element (FE) simulation. To probe local strength Nextel™ 610 nanocrystalline alumina fibers are used as a test bench material. Deep-etching is carried out to expose the fibres over lengths of a few tens of μm from an aluminium matrix. FIB milling is then used on selected fibres to machine a wide notch oriented perpendicular to the axis of the fibre. A nanoindenter with a flat tip is then used to apply a compressive force along the notched fibre's axis, which produces a bending moment in the ligament opposite to the notch leading to failure. The measured response coupled with the FE model of each notched fibre is used to calculate the stress state in the ligament prior to failure. The strength distribution across a series of such tests is statistically analyzed and fractographic analysis with a SEM is performed. To probe local fracture toughness we produce thin chevron-notched cantilever beams by FIB micromachining. These beams are then loaded using a nanoindenter. The method is successfully applied once the procedure to generate sufficiently thin triangular ligaments with the FIB is mastered, which is crucial to ensure that instability occurs after some stable crack growth, leading to meaningful toughness measurements which are minimally influenced by FIB-induced defects. Test data are interpreted using compliance calibration curves determined by FE simulation of each beam. The method is demonstrated on small volumes of fused quartz and alumina fibres and produces reproducible results consistent with expected values.

Microscopic chevron-notch fracture test of hard second phases

Marco Cantoni, Andreas Mortensen, Martin Guillermo Mueller, Václav Pejchal, Aparna Singh, Goran Zagar

It is well understood that the intrinsic mechanical properties of reinforcements and second phases strongly influence those of composite materials and alloys; however, these properties are challenging to determine due to the small size and irregular shape of the particles. We present a method for the measurement of the fracture toughness of micron-sized specimens based on Focused Ion Beam (FIB) milling, combined with micro-mechanical testing techniques and Finite Element (FE) simulation. Microscopic chevron-notched cantilever beams are first machined. These beams are subsequently loaded using a nanoindentation testing apparatus. It is found that the crack initiates at the chevron notch tip under a load that is in most tests lower than that corresponding to the onset of crack instability. Some degree of stable crack growth can thus be observed and meaningful toughness measurements gleaned on these small tested volumes. The measurements are furthermore minimally influenced by milling-induced defects given that, at the onset of unstable growth, most of the crack front is situated away from the FIB-machined surface. Test data are interpreted using compliance calibration curves determined by three‑dimensional FE simulation of each beam, after measurement of its dimensions using electron microscopy. The method is applied to nanocrystalline Nextel ™ 610 alumina fibres, which are used in aluminium-matrix composites. Results are consistent with expected values for the material at hand, suggesting that the technique is reliable despite the small specimen size, and that it can be transposed to other reinforcements. The influence on the test of environmentally induced subcritical crack growth, known to be operative with alumina in air, is also examined.

2014

Fracture toughness testing of nanocrystalline alumina and fused quartz using chevron-notched microbeams

Marco Cantoni, Andreas Mortensen, Martin Guillermo Mueller, Václav Pejchal, Goran Zagar

We show that chevron-notched samples offer an attractive approach to the measurement of fracture toughness in micron-scale samples of brittle materials and use the method to characterize quartz and nanocrystalline alumina. Focused ion beam milling is used to carve bend bars of rectangular cross-section a few micrometres wide and containing a notch with a triangular ligament. Load-controlled testing is conducted using a nanoindentation apparatus. If the notch is appropriately machined, cracks nucleate and propagate in a stable fashion before becoming unstable. Sample dimensions are measured using a scanning electron microscope, and are used as input in finite element simulations of the bars' elastic deformation for various crack lengths. The calculated compliance calibration curve and the measured peak load then give the local fracture toughness of the material. Advantages of the method include a low sensitivity to environmental subcritical crack growth, and the fact that it measures toughness at the tip of a sharp crack situated in material unaffected by ion-milling. The approach is demonstrated on two materials, namely, monolithic fused quartz and nanocrystalline alumina Nexter (TM) 610 fibres; results for the latter give the intrinsic grain boundary toughness of alumina, free of grain bridging effects. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd.

Elsevier2015

Multiaxial Yield and Fracture of Replicated Microcellular Aluminium

Etienne Combaz

This work addresses the behaviour of replicated microcellular pure aluminium under multiaxial stress states and in the presence of stress and strain localization sites. Processing of the foam was conducted in-house, using the replication process. The main processing steps are: (i) infiltrating a bed of sieved sodium chloride (NaCl) particles with pure molten aluminium under Argon pressure, (ii) solidifying the casting directionally, (iii) machining samples in the solidified composite obtained, and (iv) dissolving the salt in water containing chromate corrosion inhibitors. The main advantage of this process is its great versatility; notably, it allows tailoring independently both the size of the final pores in the replicated foam (by sieving the initial particles), and the relative density of the obtained foam material (by compacting the salt particles before infiltration). The first part of this work is dedicated to the study of the multiaxial yield behaviour of replicated foams. For this purpose a new triaxial loading frame was built to test cubical specimens. In Chapter 3 preliminary tests on pipe specimens are presented together with the results obtained on the triaxial device for 75 µm pore size foam cubes under axisymmetric and biaxial stresses. These show that the yield surface depends on all three stress tensor invariants, a dependence that has been suggested recently by theory and finite element simulation, but hitherto not shown in microcellular metal. A simple empirical expression taking the influence of the third stress tensor invariant into account is proposed based on the data. Experiments on 400 µm pore size foams under axisymmetric and biaxial stress and in three different deviatoric II-planes (planes in stress space perpendicular to the hydrostatic axis) confirm the dependence of the yield surface on the third stress tensor invariant, the yield surface becoming trilobal at elevated hydrostatic stress (tensile or compressive) in the II-planes, while being circular at zero hydrostatic stress, a feature that is captured by the empirical expression proposed on the basis of the present experiments. The second part of this thesis addresses foam deformation and fracture in the presence of cracks, notches or holes. The fracture toughness of replicated 400 µm pore microcellular aluminium is explored using disc-shaped compact tension testing broadly along the lines of the ASTM E1820-08a standard. The material shows marked R-curve behaviour, and the presence of bridging ligaments in the crack wake. Fractography reveals that the crack propagates via the rupture of struts normal to the crack plane, this being accompanied by a degree of plastic deformation of the struts near the crack plane that is made visible by slip markings along the strut surface and that increases in extent with Vm. Measured crack initiation and steady state propagation J values vary roughly as Vm3; this is a stronger dependence than has been observed in commercial (closed-cell) metal foams. A simple model is proposed to describe the initiation toughness of ductile open-cell foams, based on an estimate of the crack tip opening displacement at the moment when a strut aligned in the loading direction fails by ductile rupture just ahead of the crack tip. The model agrees well with the data up to a relative density of 20%, accounting in particular for the observed scaling of the toughness of replicated foams on the third power of the relative density. Finally the tensile flow and failure of flat dog-bone specimens containing a cylindrical hole, and of cylindrical notched samples of microcellular aluminium is studied. Both 400 µm and 75 µm pore size foams exhibit a notch strengthening effect, i.e., the peak failure stress increases as the depth of notches in cylindrical samples increases. In dogbone samples, the presence of a hole ranging from 0 to 4mm in diameter (in a sample 9mm wide) does not affect the net section peak failure stress of the 75 µm foam while the 400 µm pore size foam exhibits a slight increase in net section failure stress as the hole diameter is increased to 2 mm. Plastic flow curves for notched and hole-containing samples are well predicted by a finite element simulation based on the Deshpand – Fleck flow law, showing that the observed trends in the data are predominantly mechanical in nature, and strongly linked with the presence of triaxiality at the center of the notched samples.

EPFL2010

Multiaxial Yield and Fracture of Replicated Microcellular Aluminium

Etienne Combaz

EPFL2010