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The Metallurgical Nutcracker: Probing at the Nanoscale the Structure and Properties of Hard Second Phases in Alloys and Composites

Final Report Summary - PHASENANOCRACKER (The Metallurgical Nutcracker: Probing at the Nanoscale the Structure and Properties of Hard Second Phases in Alloys and Composites)

Many metallic materials (cast iron, tool steel and aluminium casting alloys are examples) combine a ductile matrix of a metal such as aluminium or iron, with a significant volume fraction of hard "second" phases, which are often non-metallic carbides, oxides or silicon. These hard second phases serve to make the material stiffer, stronger, harder and/or more wear resistant. In alloys, such second phases are grown and shaped within the metal over the course of solidification, deformation and heat-treatment processes that underlie its fabrication. Alternatively, those second phases can be produced separately and then be introduced into the metal; the resulting material is then a metal matrix composite.
Typically, those hard second phases are tiny particles, roughly 1 to 100 µm wide, and can have a wide variety of irregular convex shapes. Although their intrinsic strength is a key ingredient in the mechanical behaviour of the metallic material that they reinforce, this strength is seldom measured because, given their size and convex shape, this is a difficult task. The goal of this project was to devise ways to measure the strength of such small, hard convex particles, and then draw lessons from results thus obtained that may drive progress in designing stronger or tougher two-phase metallic materials.
We have achieved our goals by combining focused ion beam micromachining with nanomechanical testing in ways that produce, within such small particles, micron-scale regions of material where it is subjected to elevated tensile stress. One challenge in such tests is to avoid testing material that has been affected by ion milling, as this is known to alter the material and introduce artefacts in test data.
Tests that we have developed or improved include (i) bend tests of beams or "C" shaped particles, (ii) crush tests conducted on entire spherical particles, (iii) nanohardness testing and (iv) the chevron-notched fracture toughness test. We have applied those tests on three brittle second phases that all three have engineering importance, namely (i) oxide particles for the reinforcement of metal composites; (ii) silicon in cast aluminium, and (iii) MC carbides in steel. Defects limiting the strength of these hard brittle phases have been identified, and include grain boundaries, pores, external surface irregularities as well as a defect specific to silicon in aluminium, namely deep near-cylindrical holes which we have shown are caused by alloy impurities.
When those defects are absent, we have shown that such brittle second phases, silicon in aluminium for example, can have very high strengths, on the order of 10 GPa, which approach the theoretical strength of strong solids. This, in turn, implies that, should pathways to eliminate the defects identified in the course of this work be found, then remarkably strong structural metallic materials might be produced.