Shedding new light on the dislocation-mediated plasticity in wurtzite ZnO single crystals by photoindentation

Dislocation-mediated plasticity in inorganic semiconductors and oxides has attracted increasing research interest because of the promising mechanical and functional properties tuned by dislocations.In this study,we investigated the effects of light illumination on the dislocation-mediated plasticity in hexagonal wurtzite ZnO,a representative third-generation semiconductor material.A(0001)45o off sample was specially designed to preferentially activate the basal slip on(0001)plane.Three types of nanoindentation tests were performed under four different light conditions(550 nm,334 nm,405 nm,and darkness),including low-load(60μN)pop-in tests,high-load(500μN)nanoindentation tests,and nanoindentation creep tests.The maximum shear stresses at pop-in were found to approximate the theoretical shear strength regardless of the light conditions.The activation volume at pop-ins was calculated to be larger in light than in darkness.Cross-sectional transmission electron microscope images taken from beneath the indentation imprints showed that all indentation-induced dislocations were located beneath the indentation imprint in a thin-plate shape along one basal slip plane.These indentation-induced dislocations could spread much deeper in darkness than in light,revealing the suppressive effect of light on dislocation behavior.An analytical model was adopted to estimate the elastoplastic stress field beneath the indenter.It was found that dislocation glide ceased at a higher stress level in light,indicating the increase in the Peierls barrier under light illumination.Furthermore,nanoindentation creep tests showed the suppression of both indentation depth and creep rate by light.Nanoindentation creep also yielded a larger activation volume in light than in darkness.

Prediction of pressure-promoted thermal rejuvenation in metallic glasses

Rejuvenation is the structural excitation of glassy materials,and is a promising approach for improving the macroscopic deformability of metallic glasses.This atomistic study proposes the application of compressive hydrostatic pressure during the glass-forming quenching process and demonstrates highly rejuvenated glass states that have not been attainable without the application of pressure.Surprisingly,the pressure-promoted rejuvenation process increases the characteristic short-and mediumrange order,even though it leads to a higher-energy glassy state.This‘local order’–‘energy’relation is completely opposite to conventional thinking regarding the relation,suggesting the presence of a well-ordered high-pressure glass/high-energy glass phase.We also demonstrate that the rejuvenated glass made by the pressure-promoted rejuvenation exhibits greater plastic performance than as-quenched glass,and greater strength and stiffness than glass made without the application of pressure.It is thus possible to tune the mechanical properties of glass using the pressure-promoted rejuvenation technique.

Mechanical properties of Fe-rich Si alloy from Hamiltonian

The physical origins of the mechanical properties of Fe-rich Si alloys are investigated by combining electronic structure calculations with statistical mechanics means such as the cluster variation method,molecular dynamics simulation,etc,applied to homogeneous and heterogeneous systems.Firstly,we examined the elastic properties based on electronic structure calculations in a homogeneous system and attributed the physical origin of the loss of ductility with increasing Si content to the combined effects of magneto-volume and D03 ordering.As a typical example of a heterogeneity forming a microstructure,we focus on grain boundaries,and segregation behavior of Si atoms is studied through high-precision electronic structure calculations.Two kinds of segregation sites are identified:looser and tighter sites.Depending on the site,different segregation mechanisms are revealed.Finally,the dislocation behavior in the Fe-Si alloy is investigated mainly by molecular dynamics simulations combined with electronic structure calculations.The solid-solution hardening and softening are interpreted in terms of two kinds of energy barriers for kink nucleation and migration on a screw dislocation line.Furthermore,the clue to the peculiar work hardening behavior is discussed based on kinetic Monte Carlo simulations by focusing on the preferential selection of slip planes triggered by kink nucleation.