UCLA

The B1-structured group 5 transition metal carbides (VC, NbC, and TaC) are refractory compounds that exhibit a remarkable mixture of ionic, covalent, and metallic bonding. They are attractive materials for numerous and diverse applications and are of great interest to the scientific community. In this dissertation I investigated their mechanical behavior, observed plasticity at room-temperature, measured anisotropic yield strengths as a function of crystal (pillar) size, and determined their deformation mechanism. With in situ scanning electron microscopy based uniaxial microcompression testing, I show that single-crystalline sub-micrometer-size transition-metal carbides exhibit orientation- and size-dependent room-temperature plasticity. I find that for all the group 5 carbides, the yield strength increases with decreasing pillar size. For NbC(001) pillars, I observed that the extent of plastic deformation increases with increasing diameter. Surprisingly, in the smallest pillars the {110}1"1" ̅0 slip system is activated and for relatively larger pillars, the {111}1"1" ̅0 slip system is activated, indicating a transition in slip system based on size. I show that the largest pillars sustain extended plastic deformation. For VC, I present the microcompression test results of 001, 110, and 111 crystal orientations, where I have identified the operation of up to three slip systems dependent on the crystal orientation. I find that the mechanical behavior for VC(001) is similar to that of NbC(001) with a size-dependent transition in the operating slip systems. In VC(110) pillars, for all sizes, I observed minimal local plastic deformation followed by local fracture resulting in several large slip bands and is therefore described as brittle. In VC(111), the pillars exhibit size-dependent plasticity that increases with increasing diameter, however to a lesser extent than VC(001). For TaC, I investigated the mechanical responses of 110- and 111- oriented pillars. Similar to VC(110), the TaC(110) pillars are brittle. The TaC(111) pillars exhibit plasticity, however, to a lesser extent than in VC(111) pillars. Surprisingly, the largest diameter TaC(111) pillars are brittle. I have identified the operation of two slip systems dependent on the crystal orientation and pillar diameter, and also the observation of a transition in slip system for both orientations. My results also point to the exciting possibility of designing refractory TMCs with superior plasticity by optimizing the grain sizes, orientations, and compositions that promote the activation of desired slip systems.