Though strength and fracture properties are con­trolled by sample processing, the elastic and plastic deformation behavior inherent to ZrC may be under­stood in terms of its chemical bonding. Transition metal carbides are known to be brittle at ambient temperatures but ductile at high temperatures.

The metal-like conductivity and the fcc structure of ZrC suggest that deformation along the fcc metal slip systems is possible. In fcc, the {111} (110) system corresponds to the slip of close-packed planes along close-packed directions and requires the lowest stress to form and move a dislocation. The same is assumed for the rocksalt structure.

However, many crystals with the NaCl structure are ionic, and slip along the above system is inhibited due to the energy required to overcome strong Cou — lombic repulsion when in the half-glided position.3 Instead, ionic rocksalt compounds prefer to slip along {110} planes, maintaining attractive Coulombic forces. If ZrC is known to slip along {111} planes, the degree of ionic bonding must not be large.

Even if there is no ionic prohibition to slip in ZrC, the directed nature of its covalent bonding is still an impediment to slip. Strong metal-carbon bonds in an octahedral coordination inhibit slip on close-packed {111} planes, leading to high shear stresses required for dislocation mobility at low temperatures. The preferred mechanism of deformation is then brittle fracture, which occurs by cleavage on {100} planes.12,114 Brittle fracture persists at least to 1000 K.3

At elevated temperatures, however, a ductile-brittle transition has been observed. Based on compressive loading of single-crystal ZrC09,115 ZrC0 945116, and arc- melted ZrC094,117 plastic yield was reported at 1172, 1353, and 1473 K. Microplasticity was reported at 1273 K, with gross plastic yield above 1773 K, as observed by fractography of tensile specimens.118 Trans­mission electron microscopy (TEM) of dislocations in ZrC0.98 after elevated temperature compression re­vealed microplasticity above 1420 K.119

The crystallography of the slip system has been investigated. Lee and Haggerty116 measured the crit­ical resolved shear stress (CRSS) in the compression of single crystal ZrC0945 samples grown along (100) and (111) directions, and one sample whose axis corresponded to the ‘0.5’ orientation for {111} (110) slip, indicating that one of these 12 equivalent slip systems was oriented at 45° to the crystal axis. When loaded uniaxially along the crystal axis, a maximum resolved shear stress (t) of half of the applied stress (or 0.5s) would be achieved, according to Schmid’s law,

t = a cos f cos l [11]

where f and l are, respectively, the angles separating the slip plane normal and the slip direction from the load axis. CRSS as a function of temperature for the various samples are plotted in Figure 27. The results of Williams115 for compression of (100) oriented single crystal ZrC0.875 are consistent with those of Lee and Haggerty, plotted in the same figure.

The slip planes were identified by slip traces on the samples, while the Burgers vector was confirmed to be 1/2(110) by diffraction contrast of dislocations, a result consistent with the TEM analysis of Britun
eta/.119 Slip was induced on either {100}, {110}, or {111} planes, depending on the crystal axis and its slip system of maximum resolved shear stress. For loading along the (100) direction, the maximum resolved shear stress was for the {110} plane; for (111) loading, slip was along {100} planes; and for the ‘0.5’ oriented sample, slip was along {111} planes. CRSS for slip along {100} was highest, and along {110} and {111} was approximately equal over the temperature range where they overlapped.

Hannink et a/’22 characterized the anisotropy of room-temperature Knoop hardness on the {100} surface of single crystal ZrC0.94, rotating the long axis of the Knoop indenter azimuthally; hardness as a function of rotation angle is plotted in Figure 28. Hardness varied sinusoidally with rotation, with a minimum occurring when the indenter was aligned along (100) directions, and maximum for indenter alignment with (110) directions. This indicated that the slip system was {110} (110), normally asso­ciated with ionic crystals. The authors also measured Vickers hardness, which exhibited anisotropy in the same sense as the Knoop measurements, but with lower amplitudes, in agreement with the results of Kumashiro eta/}2

Hannink et at}22 suggested that the active slip system is dependent on temperature, as they observed for TiC096 and VC0 83. At ‘low’ tempera­tures (room temperature for TiC and ZrC, 87 K for VC), the {110} (110) slip system was active, as seen by the hardness anisotropy described earlier. At higher temperatures (883 K for TiC, 623 K for VC), maxi­mum and minimum hardness occurred respectively for indenter alignment with (100) and (110), which is the opposite to that observed for {110} (110) slip. ‘High’ temperature slip in TiC and VC was on the {111} (110) or {100} (110) system. Kohlstedt101 pro­posed that covalent, directional bonding dominates at low temperatures, prohibiting slip on {111} planes and resulting in high hardness, while at high tem­peratures, the degree of covalent bonding is decreased as the {111} (110) slip characteristic of fcc metals is favored, resulting in the observed hardness drop with temperature.

Capacity for plastic deformation has also been observed to vary with the C/Zr ratio. A monotonic decrease in hardness with decreasing C/Zr ratio is seen in Figure 23. Although not determined for ZrC, CRSS of TiCx was observed to decrease with decreasing C/Ti ratio.115 The author explains this intuitively in terms of fewer C—Ti bonds that must be broken during dislocation motion. Hollox12 attrib­uted these results to a decrease in the contribution made by carbon atoms to cohesion in TiC as the C/Ti ratio is reduced, further citing the band structure calculations of Lye and Logothetis11 which
indicated that carbon donates electrons to and strengthens metal-metal bonds. Hollox also inferred that the DBTT might decrease as the carbon-to — metal ratio is decreased, but this has not been demon­strated conclusively.