Effect of orientation and loading rate on the incipient behavior of Ti60(AlCrVNb)40 medium entropy alloy

The influence of orientation and loading rate on the pop-in behavior of Ti-60(AlCrVNb)(40) medium entropy alloy (MEA) under nanoindentation was investigated. The crystal structure of Ti-60(AlCrVNb)(40) is a single body centered cubic phase, confirmed by both X-ray diffractometer and transmission electron microscopy. Nanoindentation experiments were carried out on the (100), (110) and (111) orientations and over loading rates of 10-500 mu N/s. The incipient plasticity showed a clear orientation and loading rate dependences. Specifically, the (110) data exhibit the lowest pop-in load and pop-in displacement, and the associated loads of all orientations increase with increasing loading rate. In addition, the spread of the load at pop-in for (110) is narrower compared to those for (100) and (111), which is caused by the lower influence from defects on (110). The activation volume was estimated to be similar to 0.7 b(3), which falls between those of conventional metals (similar to 0.3 b(3)) and high entropy alloys (HEM) (similar to 1.7 b(3)), suggesting that dislocation activities in MEAs is more difficult than that in traditional metals but easier than that in HEAs. Such a medium activation volume implies that the onset of yielding is associated with a mixture of two mechanisms: heterogeneous nucleation of dislocation through atomic-sized precursors and/or expansion of dislocation loop by pre-existing dislocation multiplication. The crystalline orientation effects can be rationalized by the different densities and dislocation configurations resulted from the different numbers of activated slip systems and Schmid factors for different loading planes.