The effect of boron on the refinement of microstructure in cast cobalt alloys
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Stuart D. McDonald Defence Material Technology Centre, School of Mechanical and Mining Engineering, The University of Queensland, Brisbane, QLD 4072 Australia; and CAST Cooperative Research Centre, School of Mechanical and Mining Engineering, The University of Queensland, Brisbane, QLD 4072 Australia
David H. StJohn Defence Material Technology Centre, School of Mechanical and Mining Engineering, The University of Queensland, Brisbane, QLD 4072 Australia; and CAST Cooperative Research Centre, School of Mechanical and Mining Engineering, The University of Queensland, Brisbane, QLD 4072 Australia; and Centre for Advanced Materials Processing and Manufacturing, School of Mechanical and Mining Engineering, The University of Queensland, Brisbane, QLD 4072 Australia
Matthew S. Dargusch Defence Material Technology Centre, School of Mechanical and Mining Engineering, The University of Queensland, Brisbane, QLD 4072 Australia; and CAST Cooperative Research Centre, School of Mechanical and Mining Engineering, The University of Queensland, Brisbane, QLD 4072 Australia; and Centre for Advanced Materials Processing and Manufacturing, School of Mechanical and Mining Engineering, The University of Queensland, Brisbane, QLD 4072 Australia (Received 15 November 2010; accepted 10 January 2011)
Controlling the grain size and morphology of cast cobalt-based components is important for optimizing a component’s in-service properties. This work investigates the role of boron on the grain size of binary cobalt–boron alloys by application of contemporary grain refinement theory. Boron solute is found to refine the width of the columnar grains but fails to promote the columnar to equiaxed transition. The lack of equiaxed grains is attributed to the thermal solidification conditions and a lack of potent nucleant particles. The refinement of the columnar grains with boron solute may be due to a growth restriction mechanism.
I. INTRODUCTION
Cobalt-based alloys offer superior oxidation, corrosion, and wear resistance compared to many conventional materials. These properties make investment cast cobaltbased alloys suitable for a variety of applications including turbine vanes1 and surgical prosthesis.2 The grain size of cobalt-based alloys (in bulk alloys and surface coatings) is reported to influence a variety of mechanical properties including strength, ductility, hardness/wear resistance, and fatigue resistance3–8 as well as the alloy’s susceptibility to martensite formation.9 For superalloys in general, component properties improve with decreasing grain size at room temperature, but coarse grains improve the high temperature properties (especially creep resistance).10 The final application of the component will determine the need for fine equiaxed grains or coarse a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.21 J. Mater. Res., Vol. 26, No. 7, Apr 14, 2011
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columnar or single crystal grains. In any case, it is apparent that understanding
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