Experimental nanoindentation of BCC metals

Indentation size effect has been noticed in micro and nano scales. An experimental study for nano-indentation size effects is presented in this paper for different body centered cubic (BCC) metals. The selected materials are: Iron, Molybdenum, Tungsten, and Tantalum. Nano-indentation tests are conducted using a Berkovich indenter with continuous stiffness measurement (CSM) procedure, where hardness is measured continuously with indentation depth. The results generally show three distinctive regions for the hardness measurement versus indentation depth for the material. Hardness is noticed to decrease with increasing indentation in two regions; at very low and at very high indentations. On the other hand, hardness increases with indentation in a region that lies between the two aforementioned regions. This behavior is successfully simulated through a power law model, which includes the effect of grain boundaries as well as the strain gradient mechanism.