Fatigue crack growth in a particulate TiB 2 -reinforced powder metallurgy iron-based composite
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8/8/03
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Page 2017
Fatigue Crack Growth in a Particulate TiB2-Reinforced Powder Metallurgy Iron-Based Composite N. YANG and I. SINCLAIR Fatigue crack growth behavior has been examined in a particulate titanium diboride (TiB2)–reinforced iron-based composite that had been produced via a mechanical alloying process. Comparison with equivalent unreinforced material indicated that fatigue crack growth resistance in the composite was superior to monolithic matrix material in the near-threshold regime. The composite exhibited relatively low crack closure levels at threshold, indicative of a high intrinsic (effective) threshold growth resistance compared to the unreinforced iron. The lower closure levels of the composite were consistent with reduced fracture surface asperity sizes, attributable to the reinforcement particles limiting the effective slip distance for stage I–type facet formation. The observed shielding behavior was rationalized in terms of recent finite-element analysis of crack closure in relation to the size of crack wake asperities and the crack-tip plastic zone. The different intrinsic fatigue thresholds of the composite and unreinforced iron were closely consistent with the influences of stiffness and yield strength on cyclic crack-tip opening displacements. Cracks in the composite were generally seen to avoid direct crack-tip–particle interaction.
I. INTRODUCTION
PARTICULATE metal matrix composites (PMMCs) have generated significant interest as high specific strength and stiffness materials suitable for a range of aerospace and automotive applications.[1] While specific strength and stiffness are of course critical in many aspects of load bearing structures for the transport industry, components may also be subject to critical size constraints (e.g. gears and drive-train parts), where intrinsically high stiffness and strength are necessary. Iron/steels offer the highest Young’s modules of common engineering alloys (210 GPa), with recent work on PMMCs of iron- and steel-based matrices containing particulate titanium diboride (TiB2) showing that materials may be produced with Young’s moduli of up to 285 GPa for a reinforcement content of 30 pct,[2,3,4] while reductions in density are of course also realized by the addition of the reinforcement phase. While failure characteristics are often identified as a limiting factor in the use of structural PMMCs, it has been shown that mechanically alloyed particulate TiB2/iron–based composites that exhibit clear improvements in strength and stiffness over monolithic material may retain reasonable ductility, notched tensile strength, and fracture toughness levels.[3] As a number of promising potential applications of such composites are in aerospace and automotive components subjected to severe cyclic loads, the fatigue properties are clearly of interest. Fatigue strengths have been reported previously and shown to be superior to equivalent unreinforced materials;[3] however, details of the micromechanisms of failure and the associated implications for
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