Probing the local strength and toughness of microscopic hard second phases in alloys and composites

Many engineering alloys and composites combine a ductile matrix with discrete microscopic reinforcing phases. It is known that the mechanical properties of such alloys and composites are strongly influenced by those of those reinforcing phases; however, surprisingly little is known of the toughness, or of strength-limiting flaws within those phases. To investigate the strength and fracture toughness of such microscopic particles directly, we have developed test methods that combine focused ion beam (FIB) milling with microtesting and bespoke finite element simulation. Local strength is measured using a nanoindenter probing micromachined samples carved into individual reinforcing particles in such a way that tensile stresses are produced in a volume of material that is free of FIB and polishing artifacts. Fracture toughness is measured using the chevron-notch fracture test technique adapted for microscopic samples. Testing methods are established using nanocrystalline alumina fibers as the testbench material, so as to compare data thus obtained with known fiber properties from tensile tests on long sections of the fibers. These test methods are now being transposed towards the microscopic testing of more irregularly shaped and anisotropic brittle second phases, while other tests are being developed to probe phases the shape of which enables the design of other tailored testing procedures.

Probing the local strength and toughness of microscopic hard second phases in metals and composites

Marco Cantoni, Raphaël Charvet, Cyril Dénéréaz, Willy Dufour, Marta Fornabaio, Lionel Michelet, Andreas Mortensen, Martin Guillermo Mueller, Václav Pejchal, Goran Zagar

Many engineering alloys and composites combine a ductile matrix with discrete microscopic reinforcing phases, e.g. silicon particles in aluminium alloys, carbides in steels or ceramic particles in metal matrix composites. It is well understood that the mechanical properties of such alloys and composites are strongly influenced by those of their reinforcing particles; yet not much is known of the nature and size of strength-limiting flaws within such microscopic second phases. To investigate the strength and fracture toughness of these microscopic particles directly, we have developed test methods that combine focused ion beam (FIB) milling with microtesting techniques and bespoke finite element simulation. Local strength is probed by micromachining individual reinforcing particles in such a way that tensile stresses are produced in a volume of material free of FIB and polishing artifacts upon the application of a load via nanoindenter. Fracture toughness is measured by the chevron-notch fracture test technique adapted for microscopic samples. Testing methods are established using nanocrystalline alumina fibers as the testbench material, so as to compare data thus obtained with known fiber properties from tensile tests on long sections of the fibers. These test methods are currently being transposed towards the microscopic testing of more irregularly shaped and anisotropic brittle second phases, with the objective to measure their local mechanical properties and identify microstructural flaws that govern the strength of such particles, using in-depth electron microscopy characterization combined with microtesting.

2015

Measuring the local strength and toughness of microscopic hard second phases in alloys and composites

Marco Cantoni, Raphaël Charvet, Cyril Dénéréaz, Willy Dufour, Marta Fornabaio, Lionel Michelet, Andreas Mortensen, Martin Guillermo Mueller, Václav Pejchal, Goran Zagar

It is well known that the mechanical performance of many metallic alloys or composites is strongly influenced by the local strength of the various embedded second phases that they might contain; yet, surprisingly little hard quantitative data exist to document such local strength values. We have developed test methods that combine focused ion beam (FIB) milling with microtesting coupled with bespoke finite element simulation to measure the strength and fracture toughness of such microscopic particles directly. Local strength is measured by probing, using an instrumented precision nanoindenter, micromachined samples that have been carved into individual reinforcing particles using the FIB. The shape and micromachining processes are systematically designed so as to confine elevated tensile stresses in a volume of the material that has not been altered by FIB micromilling. Testing methods initially established using nanocrystalline alumina fibers as the testbench material are now being transposed towards the microscopic testing of more irregularly shaped and/or anisotropic brittle second phases. A different test procedure has also been developed to probe coarsened silicon in aluminium, as the flat shape of this specific second phase makes another microtesting procedure possible.

2016

Damage propagation and fracture in highly reinforced particulate metal matrix composites

Aude Hauert

This study contributes to our understanding of the damage evolution and fracture behaviour of two-phase materials that combine brittle particles with a ductile matrix. We focus on model composites roughly half-ceramic/half-metal that are produced in-house by infiltrating ceramic powder beds with liquid pure aluminium or aluminium alloy. Like many other materials, these composites exhibit two modes of tensile failure; they fail either by tensile instability, or alternatively they break prematurely, in a brittle fashion. A micromechanical model is established that links damage build-up and composite fracture. This analytical model predicts the tensile curve and the strength of particulate composites that undergo damage in the form of statistical particle fracture. Similarly to models for continuous-fibre reinforced composites, two extreme modes of load sharing (a fully local and a global load sharing mode) are accounted for, which yield a lower and an upper bound for the failure stress. Under local load sharing, the model predicts either of two different failure modes, i.e. brittle failure or failure by the onset of tensile instability, whereas global load sharing induces the latter failure mode only. We show that this model captures the transition from failure by tensile instability to brittle failure that occurs with an increase of matrix strength or with a decrease of particle strength. The local load redistribution upon particle fracture and avalanche-like growth of damage can thus explain the premature brittle failure that is observed in some composites. The direct assessment of the particle strength properties not being possible, these are "back-calculated" using the aforementioned model and tensile tests (with periodic unload-reload cycles to monitor the evolution of Young's modulus) conducted on composites made with different matrices. Using these particle properties, we show in particular that composite strength predicted under the assumption of local load sharing is in good agreement with experiment, and that fracture of particles embedded in a soft matrix is not only governed by the average particle stress, but also by the composite plastic strain. Moreover, tests of axisymmetric notched specimens indicate that damage possibly also depends on stress triaxiality.

EPFL2009

Damage propagation and fracture in highly reinforced particulate metal matrix composites

Aude Hauert

EPFL2009