Rate sensitivity and tension-compression asymmetry in AZ31B magnesium alloy sheet.

The constitutive response of a commercial magnesium alloy rolled sheet (AZ31B-O) is studied based on room temperature tensile and compressive tests at strain rates ranging from 10(-3) to 10(3) s(-1). Because of its strong basal texture, this alloy exhibits a significant tension-compression asymmetry (strength differential) that is manifest further in terms of rather different strain rate sensitivity under tensile versus compressive loading. Under tensile loading, this alloy exhibits conventional positive strain rate sensitivity.

Reducing deformation anisotropy to achieve ultrahigh strength and ductility in Mg at the nanoscale.

In mechanical deformation of crystalline materials, the critical resolved shear stress (CRSS; τCRSS) is the stress required to initiate movement of dislocations on a specific plane. In plastically anisotropic materials, such as Mg, τCRSS for different slip systems differs greatly, leading to relatively poor ductility and formability. However, τCRSS for all slip systems increases as the physical dimension of the sample decreases to approach eventually the ideal shear stresses of a material, which are much less anisotropic.

The nanostructured origin of deformation twinning.

We have revealed the fundamental embryonic structure of deformation twins using in situ mechanical testing of magnesium single crystals in a transmission electron microscope. This structure consists of an array of twin-related laths on the scale of several nanometers. A computational model demonstrates that this structure should be a generic feature at the incipient stage of deformation twinning when there are correlated nucleation events.

Mechanical annealing and source-limited deformation in submicrometre-diameter Ni crystals.

The fundamental processes that govern plasticity and determine strength in crystalline materials at small length scales have been studied for over fifty years. Recent studies of single-crystal metallic pillars with diameters of a few tens of micrometres or less have clearly demonstrated that the strengths of these pillars increase as their diameters decrease, leading to attempts to augment existing ideas about pronounced size effects with new models and simulations. Through in situ nanocompression experiments inside a transmission electron microscope we can directly observe the deformation of these pillar structures and correlate the measured stress values with discrete plastic events.