Microstructure and phase evolution in spark plasma sintering of the NbCr 2 Laves phase matrix composite toughened with d
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ARTICLE Microstructure and phase evolution in spark plasma sintering of the NbCr2 Laves phase matrix composite toughened with ductile Cr phase Kewei Lia) and Fei Gao College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China
Lu Wang Department of Precision Instruments, Tsinghua University, Beijing 100084, China (Received 26 July 2018; accepted 10 September 2018)
A bulk NbCr2 Laves phase matrix composite toughened with ductile Cr phase has been fabricated by spark plasma sintering (SPS) using pre-alloyed NbCr2 and Cr powders. The sintering behaviour and phase morphological evolution of the sintered alloy were investigated. The results show that a series of microstructure evolutions along the sintering temperature occurred: elongated Cr phase with uniform dispersion of fine NbCr2 and Cr phase ! coarse Cr phase with matured fine NbCr2 and Cr ! coarse Cr and Nb phases with lamellar eutectics. The microstructural evolution and phase transformation along the sintering temperature are analyzed by considering the inhomogenous temperature distribution and the accelerated atomic diffusion due to the pulsed electric current applied during SPS. The room temperature fracture toughness of the sintered samples is expected to be markedly improved due to the absence of lamellar or the occurrence of ductile Cr and Nb phases.
I. INTRODUCTION
Particular attention has been devoted to developing Laves phase-based alloys capable of extended use at service temperatures exceeding 1473 K.1–4 The NbCr2 Laves phase has attracted extensive interest due to its high melting point (2003 K), appreciable creep resistance, excellent high temperature strength, and good oxidation resistance below 1473 K.5–7 However, like other intermetallic componuds, the NbCr2 Laves phase is very brittle at ambient temperatures, which remarkably hinders its commercial utilization. Accordingly, substantial efforts have been initiated to improve the deformability of the NbCr2 Laves phase in terms of grain refinement toughening, ductile phase toughening, and alloying element toughening.5,6,8,9 Amongst those toughening methods, introducing a ductile phase to form a composite with the NbCr2 Laves phase had been demonstrated effective in improving the brittleness while maintaining most of the high-temperature properties. The fracture toughness of NbCr2-based alloys has been increased to 18.9 MPa m1/2,10 which is 16 times higher than that of the as-cast NbCr2 Laves phase (1.2 MPa m1/2 Ref. 11). However, it still has a long way for the industrial application. To maximize the toughening effect, it is desirable to have the ductile phase presented in the form of a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2018.371 J. Mater. Res., 2018
a continuous network and the NbCr2 Laves phase distributed in the form of discrete particles as fine as possible. In this case, the ductile Cr phase could undertake plenty deformation and hinder the propagation of cracks initialed from the NbCr2 Laves phase. It
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