Microstructure and mechanical properties of Laves phase NbCr 2 -based composites toughened with Cr phase fabricated by s
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Laves phase NbCr2-based composites toughened with different volume fractions of Cr phase were fabricated by spark plasma sintering (SPS). The microstructural evolution and mechanical properties of these spark plasma sintered NbCr2/Cr composites were investigated. The mechanical properties evaluations indicate that the introducing of Cr phase consolidated by SPS has a beneficial effect on the mechanical properties of the NbCr2 Laves phase. When the content of Cr phase in the NbCr2 Laves phase increases to 30 wt%, the hardness measured by a Berkovich nanoindenter operated with the continuous stiffness measurements mode attains the maximum value of 13.44 GPa, which is increased by about 56% over the as-cast NbCr2 Laves phase. More importantly, the room-temperature fracture toughness of the NbCr2-30wt%Cr alloy is increased to 18.9 MPam1/2, which is 16 times higher than that of the as-cast NbCr2 Laves phase (1.2 MPam1/2). The microstructural analysis indicates that the residual of Cr phase and formation of Nb solid solution can provide remarkable toughening of the NbCr2 Laves phase by fine grain toughening, dual ductile phase toughening, and interface toughening mechanisms. A possible formation mechanism of Nb solid solution during SPS has been proposed by considering the composition distribution.
I. INTRODUCTION
The high melting temperature (2003 K), appreciable creep resistance, and excellent high strength of NbCr2 Laves phase make it an attractive candidate for hightemperature structural applications.1–3 The major limitation of NbCr2 Laves phase has been the lack of ductility and toughness in the monolithic intermetallic at room temperatures. However, dual phase alloys of a Laves phase and bcc phase have shown remarkable enhancement in fracture toughness, which may combine the excellent high-temperature properties of NbCr2 Laves phase and toughen from the second phases by means of mechanisms such as ductile phase toughening, fiber toughening, and grain refinement toughening. For this reason, a considerable effort was initiated to study the effects of adding various metal and ceramic phases to NbCr2 Laves phase.4–10 Takeyama6 found that the incorporation of a ductile Cr-solid solution showed promising effects in improving the fracture toughness of the NbCr2 Laves phase. Also, Aoyama10 reported that the NbCr2/Cr in situ composites exhibited extremely high yield stresses at elevated temperatures. Shayesteh5 has explored the potentially beneficial effects of adding Contributing Editor: Jürgen Eckert a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.231
Al2O3 to NbCr2 Laves phase. However, the maximum toughening increment is achieved only when the second phase can exhibit significant plastic deformation prior to fracture. This is easier to achieve when the second phase is in the form of a continuous network but in terms of maintaining some elevated temperature capability it may be desirable to have the metallic phase in the form of discrete particles. Unfortunately, this special kin
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