Investigation of time-dependent characteristics of materials and micromechanisms of plastic deformation on a submicron scale by a new pulse indentation technique

Abstract

A new pulse technique is proposed to investigate dynamics and micromechanisms of plastic deformations of materials underneath the indenter during microindentation. It is established that the process of indenter penetration under pulse loading conditions can be described by several distinct stages (as many as four in some cases) differing in kinetics and activation parameters. In each investigated material, the first stage was characterized by a high strain rate (≥103 s−1) and the high contact stresses (dynamic hardness), which exceeded static hardness by factor of 5–10. Typical values of activation volumes during the first stage were of the order of 10−30 m3, i.e. close to the volume occupied by an atom (ion) in a lattice. During the second and the subsequent stages, the activation volume in ionic crystals increased up to about 10−28 m3,( i.e. 10b 3, where b is Burger's vector of glide dislocations). Dynamics of initial stages of the indenter penetration was, therefore, determined by elastic and subsequently by plastic deformation, which is carried out by non-equilibrium point defects (most likely by interstitials or crowdions). A relative role of point defects in the process of mass transfer underneath the indenter and its contribution to microhardness is estimated. In soft materials (NaCl, LiF, Pb) during long indentation times (≥1 s) contribution of point defects can be estimated as more than 10%, and in the case of hard materials (Si, amorphous alloys) became closer to 100%. Dynamics of the final stages of the indenter penetration in soft crystals were governed by the dislocation creep. In all investigated materials for short indentation times (≪1 s) plastic deformation underneath the indenter occurred predominantly via generation and motion of interstitials and their clusters containing a few atoms.