Fatigue crack growth processes in novel alumina particulate reinforced titanium MMCs

Binns, Andrew John (1999) Fatigue crack growth processes in novel alumina particulate reinforced titanium MMCs. University of Southampton, Doctoral Thesis.

Abstract

The development of functionally graded materials (FGMs) has demonstrated that such systems have the potential to enjoy a wide range of thermal and structural applications, including thermal gradient structures, wear and corrosion resistant surfaces and metal/ceramic joining. The capability of being able to tailor an FGMs properties to suit a given function may be beneficial to the medical devices industry particularly in relation to producing wear resistant alumina rich surfaces on load bearing components.

Whilst a number of detailed continuum mechanics analysis exist in the literature regarding stress redistribution behaviour at cracks in theoretical functionally graded layers, there is a clear deficiency of detailed experimental investigations of failure in real FGM systems. Two fabrication routes were investigated to manufacture FGMs for study.

Mechanical alloying of powder Ti-6A1-4V and A1₂0₃ produced two novel alumina/titanium metal matrix composites(MMCs)(30 and 70 vol % A1₂0₃). Whilst milling of the 30 vol % material was complete, producing a very fine distribution of reinforcement, milling of the 70 vol % material was incomplete. Consolidation of the 30 and 70 vol % precursor powders to form bulk materials via hot isostatic pressing (HTPing) was not complete and metallographic evidence showed reaction phases had formed between the matrix and reinforcement particles. The hardness of the MMCs was much greater than for unreinforced Ti-6A1-4V and fracture toughness of the composites was between that of alumina and unreinforced Ti-6AL-4V. Both the composites failed in a brittle manner during monotonic loading. The brittle nature of the materials is attributed to the formation of intermetallic materials during the extremes of HIPing.

Powder blending was used satisfactorily to produce MMCs with a less fine microstructure than the mechanically alloyed material. The coarse microstructure is thought to account for the limited particle-matrix interactions observed in the powder blended material, with Ti₃Al being the principle reaction product. Fatigue studies showed that the 30 vol % MMC displayed very rapid crack growth at low stress intensity levels with an exponent of ~15 in the Paris regime of crack growth.

The 15 vol % MMC exhibited some preferential fatigue crack growth behaviour over Cp titanium at low ΔK levels. The behaviour was attributed to crack tip shielding (rougness induced closure) and high matrix flow stress.

Cracks were seen to grow to the interface between bulk materials, followed by delamination and subsequent rapid re-initiation and growth into the substrate material. Investigation into growth through multi-layered material (30/15/Cp Ti) indicated that growth was dominated by monotonic failure of brittle 30 vol % MMC layer.

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