Nanoindentation Constant Contact Pressure Creep Experiments—A New Approach for Studying Thermally Activated Dislocation Mechanism

Thermally activated deformation mechanisms play an important role in the plastic deformation of many material systems, ranging from dislocation creep at high values of T/Tm, to the brittle to ductile transition at low values of T/Tm, to grain boundary driven deformation process in nanostructured alloys at intermediate temperature. In this talk we will show, how Nanoindentation testing protocols can be tailored to analyze these effects across different temperature and time scale, focusing on a new constant contact pressure creep approach.<br/>In the past different loading protocols have been developed to investigate the creep properties of materials using instrumented indentation testing technique. Recently, a new indentation creep method was presented, in which the contact pressure is kept constant, similar to the stress in a uniaxial creep experiment and the resulting creep strain rate is analyzed as a function of creep time. Applying this technique to nanostructured alloys, two distinctively different creep regimes have been observed, at low and high contact pressures. Here, the results of constant contact pressure creep tests are compared to uniaxial and constant load hold indentation creep experiments on different ultrafine grained Cu and Cu-X solid solutions. The limitations of the new creep method are discussed and compared to the results of uniaxial testing down to indentation strain rates of 10^-6 s^-1. Large reductions in the applied stress during uniaxial and indentation experiments result in a pronounced change in the power law exponent of the nanostructured alloys and relaxation processes seem to dominate the material response.