Anomalous hardening in magnesium driven by a size-dependent transition in deformation modes

Abstract

Magnesium (Mg) and its alloys hold great potential as an energy-saving structural material for automative, aerospace applications. However, the use of Mg alloys has been limited due to poor ductility and formability. Poor mechanical properties of Mg alloys origin from the insufficient number of slip systems, and deformation twinning plays an important role to accommodate plastic deformation. Here, we report a comprehensive experimental and modeling study to understand crystal size effect on the transformation in deformation modes in twin oriented Mg single crystals. The experiments reveal two regimes of size effects: (1) single twin propagation, where a typical "smaller the stronger" behavior was dominant in pillars ≤ 18 µm in diameter, and (2) twin-twin interaction, which results in anomalous strain hardening in pillars > 18 µm. Molecular dynamics simulations further indicate a transition from twinning to dislocation mediated plasticity for crystal sizes below a few hundred nanometers. Our results provide new understanding of the fundamental deformation modes of twin oriented Mg from nano-scale to bulk, and insights to design Mg alloys with superior mechanical properties through dimensional refinement. This subsequently can materialize into more utilization of Mg alloys as a structural material in technologically relevant applications.

Effect of Al content on the critical resolved shear stress for twin nucleation and growth in Mg alloys

“…The stress-strain curves of the micropillars oriented for twinning showed a region with strong strain hardening when the applied strain was higher than 5%. Similar behaviour has been reported during micropillar compression [28,31] as well as uniaxial [54] and plane strain compression [55] Mg and Mg alloys single crystals oriented for twinning. The origin of this strain hardening has been attributed to the activation of <c+a> pyramidal slip to accommodate the deformation in the twinned crystal, which is not suitable oriented for basal slip.…”

“…(c-d) Effect of micropillar dimension on the CRSS for twin nucleation and growth for pure Mg. Results from this investigation and from the literature[17,26,28,30,31,35].A size effect of the type 'smaller is stronger' was found in the CRSS for twin nucleation in the micropillars of 3x3 µm 2 of Mg and Mg-9 at.%Al. Nevertheless, the values of + were similar for micropillars of 5x5 µm 2 and 7x7 µm 2 , regardless of the Al content.…”

Effect of solute content and temperature on the deformation mechanisms and critical resolved shear stress in Mg-Al and Mg-Zn alloys

“…Micromechanical testing techniques, such as nanoindentation and micropillar compression, offer an attractive approach to obtain reliable values of the CRSS for basal slip as a function of the solute content. The main problem of these techniques is associated with the overestimation of the strength when the volume of the tested material is of the order of tenths of µm 3 or smaller [23][24][25][26][27], due to the limited number of mobile dislocations and the scarcity of dislocation sources in the deformed volume. For instance, the CRSS for basal slip measured by K. Eswar Prasad et al [25] in pure Mg micropillars of 3 µm in diameter was six times higher than the one measured in square specimens of 3 x 3 mm 2 cross-section.…”

Review of Size Effects during Micropillar Compression Test: Experiments and Atomistic Simulations

“…Variation of the critical resolved shear stress normalized by the shear modulus versus the sample diameter normalized by the Burgers vector in HCP metals. The original experimental data are reported by Byer et al [47], Yu et al [49], Lilleodden [46], Ye et al [48], Sun et al [45], Kim [50], and Sim et al [51].…”