High temperature nano-indentation on the mechanical properties of Zr and Zr–Fe alloys: Experimental and theoretical analysis

Abstract In this work, the microstructure and subsurface hardness of pure Zr and Zr-xFe (x = 0.2, 0.4, 0.6) model alloys were investigated at elevated temperatures (298–573 K) by means of scanning electron microscopy-electron backscatter diffraction (SEM-EBSD) analysis, transmission electron microscope (TEM) and high temperature nano-indentation test (HTNIT). Corresponding experimental results indicated that the addition of Fe element could lead to grain refinement for Zr–Fe alloys, and facilitate the formation of second phase particles (SPPs) like Zr3Fe precipitates and spherical particles in the matrix, which were in a close relationship with materials hardening. In order to address the hardness-indentation depth relation of Zr–Fe alloys with temperature effect, a mechanistic model was developed that covered the hardening mechanisms of lattice friction, precipitation hardening and dislocation interaction. Related theoretical analysis showed that all the three hardening mechanisms got weakened at high temperatures, which led to the decrease of materials hardness with increasing temperature. Moreover, the noticeable indentation size effect (ISE) was dominated by the formation of geometrically necessary dislocations (GNDs), and increasing temperature could result in the density decrease of GNDs due to the expansion of the plastic zone at elevated temperatures. With the increase of the indentation depth, the dominant hardening mechanism changed from the contribution of GNDs to that of statistically stored dislocations (SSDs). The results obtained in this work can help comprehend the effect of Fe element on both the microstructure and mechanical properties of Zr–Fe alloys at high temperatures.