High temperature hardness and slip system of NbB2 and TaB2 single crystals

The high temperature Knoop hardness of NbB2 and TaB2 single crystals was measured in the temperature range 20-11000C on the (0001) plane in the <10-10> and <11-20> indentation directions and on the [10-10] plane in the <1-210> and [0001] directions. In every case the hardness value H decreased monotonically with increasing temperature, but the H-T curves all showed a step at around 0.3T/Tsub(m) where Tsub(m) is the melting temperature, suggesting a change in the deformation mechanism at this temperature. This hardness anisotropy indicates that slip was mainly on the prismatic system and this was confirmed by slip line observations. (Auth.)