Magnetic Field Tunable Small-scale Mechanical Properties of Nickel Single Crystals Measured by Nanoindentation Technique

Nano- and micromagnetic materials have been extensively employed in micro-functional devices. However, measuring small-scale mechanical and magnetomechanical properties is challenging, which restricts the design of new products and the performance of smart devices. A new magnetomechanical nanoindentation technique is developed and tested on a nickel single crystal in the absence and presence of a saturated magnetic field. Small-scale parameters such as Young's modulus, indentation hardness, and plastic index are dependent on the applied magnetic field, which differ greatly from their macroscale counterparts. Possible mechanisms that induced 31% increase in modulus and 7% reduction in hardness (i.e., the flexomagnetic effect and the interaction between dislocations andmagnetic field, respectively) are analyzed and discussed. Results could be useful in the microminiaturization of applications, such as tunable mechanical resonators and magnetic field sensors.Comment: 6 pages, 4 figure

External uniform electric field removing flexoelectric effect in epitaxial ferroelectric thin films

Using the modified Landau-Ginsburg-Devonshire thermodynamic theory, it is found that the coupling between stress gradient and polarization, or flexoelectricity, has significant effect on ferroelectric properties of epitaxial thin films, such as polarization, free energy profile and hysteresis loop. However, this effect can be completely eliminated by applying an optimized external, uniform electric field. The role of such uniform electric field is shown to be the same as that of an ideal gradient electric field which can suppress the flexoelectricty effect completely based on the present theory. Since the uniform electric field is more convenient to apply and control than gradient electric field, it can be potentially used to remove the flexoelectric effect induced by stress gradient in epitaxial thin films and enhance the ferroelectric properties.Comment: 5 pages, 3 figure

Preliminary study on ductile fracture of imperfect lattice materials

AbstractThe ductile fracture behavior of two-dimensional imperfect lattice material under dynamic stretching is studied by finite element method using ABAQUS/Explicit code. The simulations are performed with three isotopic lattice materials: the regular hexagonal honeycomb, the Kagome lattice and the regular triangular lattice. All the three lattices are made of an elastic/visco-plastic metal material. Two typical imperfections: vacancy defect and rigid inclusion are introduced separately. The numerical results reveal novel deformation modes and crack growth patterns in the ductile fracture of lattice material. Various crack growth patterns as defined according to their profiles, “X”-type, “Butterfly”-type, “Petal”-type, are observed in different combinations of imperfection type and lattice topology. Crack propagation could induce severe material softening and deduce the plastic dissipation of the lattices. Subsequently, the effects of the strain rate, relative density, microstructure topology, and defect type on the crack growth pattern, the associated macroscopic material softening and the knock-down of total plastic dissipation are investigated

Numerical Simulation for Thermal Shock Resistance of Ultra-High Temperature Ceramics Considering the Effects of Initial Stress Field

Taking the hafnium diboride ceramic as an example, the effects of heating rate, cooling rate, thermal shock initial temperature, and external constraint on the thermal shock resistance (TSR) of ultra-high temperature ceramics (UHTCs) were studied through numerical simulation in this paper. The results show that the external constraint has an approximately linear influence on the critical rupture temperature difference of UHTCs. The external constraint prepares a compressive stress field in the structure because of the predefined temperature field, and this compressive stress field relieves the tension stress in the structure when it is cooled down and then it improves the TSR of UHTCs. As the thermal shock initial temperature, a danger heating rate (or cooling rate) exists where the critical temperature difference is the lowest

Electric-field-tunable mechanical properties of relaxor ferroelectric single crystal measured by nanoindentation

Electric field dependent mechanical properties of relaxor ferroelectric material Pb(Mn1/3Nb2/3)O3-PbTiO3 are investigated with the nanoindentation technique. Giant electric-field-tunable apparent elastic modulus (up to -39%), hardness (-9% to 20%) and energy dissipation (up to -13%) are reported. Based on experimental data, a characterization method of electromechanical coupled nanoindentation is proposed. In this method, an electric field tunable scaling relationship among elastic modulus, hardness and indentation work for ferroelectric materials can be determined. In addition, this method can be used to obtain the electric-field-dependent elastic modulus and hardness, and avoid the estimate of contact area in the Oliver-Pharr method. Finally, the different effects on elastic modulus between positive and negative electric fields can be explained by the flexoelectric effect.Comment: 14 pages, 4 figure