Nanomechanical Behavior of Advanced Structural Materials

Abstract

The solids deviating from the typical crystalline structure may exhibit attractive features, like enhanced yield/fracture strength, superior wear resistance, possibly good formability, and other interesting physical properties. Therefore, during the last few decades several advanced metals, such as bulk metallic glasses (BMGs), nanocrystalline/nanotwinned (nc/nt) metals, and ultrafine grained (ufg) metals, have been developed based on this design concept. Despite significant advances in the processing, characterizing, and structural applications of these materials, the fundamental of mechanical behavior and its relation to the microstructural feature has not been fully understood yet. Although a number of the experimental findings on mechanical behavior of advanced structural metals are available, there are some issues remaining unsolved. In this thesis, I attempt to shed a light on the issues and thus to extend our current knowledge on the mechanical behavior of these advanced materials. In the BMG studies, the size of the shear transformation zone (STZ) that initiates the elastic-to-plastic transition in a Zr-based BMG was estimated by conducting a statistical analysis of the first pop-in events during spherical nanoindentation tests. A series of experiments led us to a successful description of the distribution of critical shear strength for the transition and its dependence on the loading rate. From the activation volume determined by statistical analysis of the STZ size was estimated based on a cooperative shearing model. Additionally, the existence of an indentation size effect (ISE) in the onset of plasticity in a Zr-based BMG is investigated by employing spherical-tip nanoindentation experiments. Statistically significant data on the load at which the first pop-in in the displacement occurs were obtained for three different tip radii and in two different structural states (as-cast and structurally relaxed) of the BMG. Hertzian contact mechanics were employed to convert the pop-in loads to the maximum shear stress underneath the indenter. Results establish the existence of an ISE in the BMG of both structural states, with shear yield stress increasing with decreasing tip radius. Structural relaxation was found to increase the yield stress and decrease the variability in the data, indicating “structural homogenization” with annealing. Statistical analysis of the data was employed to estimate the STZ size. Results of this analysis indicate an STZ size of ~25 atoms, which increases to ~34 atoms upon annealing. These observations are discussed in terms of internal structure changes that occur during structural relaxation and their interaction with the stressed volumes in spherical indentation of a metallic glass. In the nc-metal studies, time-dependent plastic deformation (creep) behavior of nc-Ni at room temperature (RT) was systematically explored by performing two novel testing methods. First, to overcome the difficulties in estimating the creep exponent through the popular constant-load, sharp-tip indentation creep method, I propose here a modified way that involves using a spherical tip. Both sharp and spherical indentation creep experiments were performed on nc-Ni (d~30 nm), which is known to show creep-like behavior at room temperature. The results suggest that nc-Ni exhibits a strong strain-rate-dependent deformation mechanism, and that spherical indentation creep may produce more reliable data than sharp indentation creep. Then, I turned my attention to the second method: Nanoscale creep behavior of nc-Ni, at low stresses and at RT, was systematically explored through uniaxial creep experiments performed on nano-/micro-pillars (with diameters of 600, 1000, and 2000 nm). It was revealed that the creep indeed occurs at RT, and exhibits a creep strain of ~2 × 10^-4 - 9 × 10^-3 (for 200 s load-holding) at stresses below the nominal yield strengths of the pillars. At a given stress, much higher total creep strains and strain rates accrue in the smaller pillars, which is likely due to the increased contributions of free surfaces. Estimation of the stress exponent and the activation volume suggests that the nanoscale creep event under low stresses may be dominated by diffusion-controlled mechanisms such as free surface assisted grain boundary (GB) diffusion and GB sliding. In the nt-metal study, the influence of strain on the mechanical properties and deformation kinetic parameters of nt-Cu is investigated by a series of nanoindentation experiments, which were performed by employing sharp indenters with five varying centerline-to-face angles (ψ). Comparison experiments were also conducted on (110) single crystalline Cu. Experimental results indicate that unlike coarse-grained materials, nt-Cu is prone to plastic flow softening with large material pile-up around the indentation impression at high levels of strains. Localized detwinning becomes more significant with the decreasing ψ, concomitant with reduced strain-rate sensitivity (SRS) and enhanced activation volume. The SRS of nt-Cu is found to depend sensitively on the ψ with a variation of more than a factor of 3, whereas the activation volume exhibits a much less sensitive trend. I discuss the validation of the experimental techniques and implications of various deformation kinetic parameters on the underlying deformation mechanisms of nt-Cu. In the ufg metal studies, the evolution of plasticity, SRS and the underlying deformation mechanism of a Zn-22%Al eutectoid alloy during high-pressure torsion (HPT) process. The experiments reveal an optimal torsional straining condition for achieving the largest plasticity and beyond this condition the SRS decreases and activation volume increases. The results are discussed in terms of changes in microstructure and the underlying deformation mechanism. In addition, its high-cycle fatigue behavior was also explored using novel small-scale bending fatigue experiments. Testing of the finest grain region in each HPT disk showed that the fatigue life decreases continuously with increasing numbers of torsional revolutions. The results are discussed in terms of the HPT-induced hardness change and the underlying fatigue failure mechanism. On the other hand, a Mg-Zn-Zr alloy was processed by HPT for up to 2 turns to produce significant grain refinement together with enhanced plasticity and strength. Measurements were performed to determine the SRS, shear yield strength and activation volume as a function of the processing conditions. The results suggest there is a significant contribution from GB sliding to the flow process and the onset of plasticity is associated with heterogeneous dislocation nucleation.