Testing strength and fracture toughness of hard second phases at micron scale

Local mechanical properties LMP 2015, November 12-14, 2014, Stará Lesná, Slovakia. Many alloys and composites combine a ductile matrix with discrete reinforcing phases, such as the ceramic fibers or particles that are used in metal matrix composites. It is well understood that the quality of the particles strongly influences strength and fracture toughness of the composite material; however, the strength of the particulate reinforcing phases is seldom quantified because it is challenging to measure. We present two newly developed methods that probe, respectively, the local strength and the fracture toughness of such microscopic phases using NextelTM 610 nanocrystalline alumina fibers produced by 3MTM, used here as testbench material. The strength measurement method involves FIB machining of a wide rectangular notch within the fiber that is afterwards tested in compression via nanoindenter. This produces high bending tensile stresses opposite to the notch end, which eventually cause failure. The fracture toughness measurement method consists of carving microscopic cantilever beams within the fiber, which contain a triangular shaped chevron-notch, using focused ion beam micromilling. For such a beam, when loaded via nanoindenter in bending, a crack initiates at the apex of the triangle, and subsequently propagates stably in Mode I until it reaches a critical length at which unstable crack-growth begins. Both test methods are complemented with three-dimensional finite element simulation of each tested sample.

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

Probing 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

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.

2015

Measuring the local strength of alumina reinforcements

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

Ceramic particles in metal matrix composites, or hard second phases in alloys, often serve as reinforcements, raising its stiffness, hardness and strength. Their efficiency depends on their strength, a quantity that is statistically distributed over microscopic volumes and seldom quantified because it is, with the exception of long fibres, difficult to measure. We combine focused ion beam (FIB) milling methods with micro-testing techniques and finite element simulation to probe the strength of alumina fibres or particles that are used to reinforce aluminium. We present results from microscopic in situ tests conducted on Nextel ™ 610 nanocrystalline alumina fibres produced by 3M and used to reinforce continuous aluminium matrix composite wires. After exposing alumina fibres from the composite wire, we first use FIB milling to machine a wide rectangular notch in the fibre together with a line of load application along its top surface. Then, the notched fibre is loaded until failure by applying compressive force via a nanoindenter equipped with a flat tip: tensile stresses are thus created in the material, by bending under compressive loading. The deformation of each notched fibre sample is modelled using second-order elastic finite element analysis in order to obtain the three-dimensional stress state in the bent ligament prior to failure. The resulting strength distribution data are then used, knowing the local fracture toughness of the material, to estimate the nature and size distribution of local defects that govern the microscopic in-situ strength of the ceramic reinforcement. It is found that this defect population differs from that which governs the macroscopic tensile strength distribution of the fibres. We conclude with a discussion of the transposition of this methodology towards quantification of the microscopic strength of irregularly shaped ceramic particles or second phases that have potential for the reinforcement of aluminium.

2014

Measuring the local strength of alumina reinforcements

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

2014