Strength and fracture of silicon second-phase particles in aluminium casting alloys

Abstract

The mechanical behaviour of metallic materials that have a microstructure composed of a brittle phase embedded in a ductile matrix is dictated by a complex interplay of factors such as local phase properties, cohesive properties, geometrical characteristics, and specific damage mechanisms. An important example of such two-phase materials are Al-Si casting alloys, which are widely used in automotive applications. In the micromechanics of these alloys, fracture of the particulate brittle phase plays a dominant role. Namely, when the alloy is deformed, the brittle silicon particles within the aluminium matrix start fracturing, leading ultimately to fracture of the alloy. In this thesis, micromechanical methods to measure local fracture toughness or strength of individual brittle microscopic particles within alloys and metal matrix composites (MMCs) are developed. The methods are based on coupling experimental techniques such as Scanning Electron Microscopy (SEM), Focused Ion Beam (FIB) milling and nanoindentation with finite element modelling (FEM). Special attention is put in probing portions of the particles that are left unaffected by the FIB micromachining process, and are thus representative of the particles’ intrinsic properties. These novel methods are also used to study the fracture of silicon particles within Al-Si casting alloys. To measure fracture toughness at a small scale, a microscopic chevron notch test is developed and demonstrated on benchmark materials. The main advantage of this method with respect to most existing small-scale fracture toughness test methods is that in chevron-notched samples the crack growth resistance is measured on a real crack instead of a pre-notch. The main difficulty, on the other hand, is achieving crack initiation at applied loads low enough to allow for subsequent stable crack growth. This was found to be particularly challenging in silicon. Local strength measurements on individual microscopic silicon particles within Al-Si alloys were achieved through two different, novel, methods. The first is a microscopic 3-point bending test by which the large facets of plate-like particles extracted from an Al-Si alloy can be probed. In the second approach, particles that are only partially exposed by deep-etching from an Al-Si sample are carved by FIB milling into such a shape that by compressing its top, bending is produced on a well-defined portion of the particle. The main finding of the local strength measurements is that silicon particles within Al-Si alloys can achieve extremely high strength values, yet fracture early when an Al-Si alloy is deformed because most of them feature stress-concentrating defects on their surfaces. The most important stress-limiting defects are found to be grooved interfaces between contacting silicon crystals, followed by surface pinholes, this being a defect identified here. Using FIB cross-sectioning and EDX examination it was revealed that these pinholes originate from alloy impurities. The insights gained on the intrinsic strength of silicon particles unveil the great potential of silicon as a reinforcing phase in Al-Si alloys. This, together with the acknowledgement of particle strength-limiting defects, may be used to devise strategies that, through the avoidance of those defects, should lead to improved alloy mechanical properties.