Material design and processing of a new class of titanium boride cermets with tough metallic phases and mechanical prope
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ARTICLE Material design and processing of a new class of titanium boride cermets with tough metallic phases and mechanical properties Alexander Lark, Jun Du, and K.S. Ravi Chandrana) Department of Metallurgical Engineering, The University of Utah, Salt Lake City, Utah 84112, USA (Received 27 June 2018; accepted 14 September 2018)
The design and the processing of a new class of titanium boride (TiB)-based bulk cermets containing a metallic phase (b-Ti phase) for toughening is presented. The general approach is rapid reaction and densification, using starting powders of Ti, TiB2, Fe, and Mo, by electric-fieldactivated sintering. The cermets consist of two-phase microstructures in which the boride phase formed as a networked structure of TiB whiskers that were created in situ upon the reaction between the powders. Hardness, flexural strength, and fracture toughness measurements of these materials revealed that they possess an interesting set of properties up to: hardness values of 1090 kg/mm2, flexural strength values of 953 MPa, and fracture toughness values of 18 MPa m1/2. A remarkable finding is that although the metallic phase fractured by microscopic cleavage, the cermets showed good fracture toughness values. The present study not only illustrates the process details and microstructure leading to these properties but also provides a broad powder metallurgical approach to design and synthesize cermets that may yield further improved properties.
I. INTRODUCTION
Cermets are composites consisting of a ceramic phase that provides hardness and strength and a metallic phase that provides ductility and toughness while structurally ‘binding’ the ceramic phase together. The metallic phase is usually limited to ,0.3 Vf (Vf 5 volume fraction) and is distributed discontinuously within the ceramic phase. Some well-known examples are tungsten carbide (WC) cermets liquid-phase sintered with Co or other metal phases,1,2 titanium carbide (TiC) sintered with Fe or Febased intermetallics,3–5 and titanium carbonitride (TiCN) cermets, sintered with compatible metallic phases.6 Among these, the microstructure and mechanical properties of WC-Co cermets have been extensively investigated7–9 and this class represents the most widely manufactured material for tools in cutting and drilling applications. In general, the cermet processing is done by liquid phase infiltration or sintering at very high temperatures, where the wettability of the carbide(s) and/or their reaction with the metal phase10,11 are critical aspects controlling the mechanical quality of the cermets. We present here a new direction in the processing of cermets, where the titanium boride (TiB)-based cermet is created in situ by reaction sintering of powders under the heating induced by the electric field. The in situ reaction produces a two-phase mixture of TiB and b-Ti phase (largely made of Ti and solid solution-strengthened by Fe and Mo). It is shown that the design space of these a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jm
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