Twinning, particle strengthening, disloctions and continuum plasticity

October 14, 2011

[1] Significance of mechanical twinning in hexagonal metals at high pressure

W Kanitpanyacharoen et al

Diamond anvil cells (DAC) in radial synchrotron X-ray diffraction geometry were used to investigate texture development and identify deformation mechanisms in zinc and osmium at the Advanced Photon Source (APS) and the Advanced Light Source (ALS), respectively. Further experiments on cadmium and hafnium wires were carried out in the Deformation-DIA (D-DIA) multi-anvil press at APS to study the simultaneous effects of pressure, temperature and strain. At room temperature and increasing pressure the c-axis aligns near the compression direction in all hexagonal metals, but with considerable differences. Texture in zinc evolves gradually between 10 and 15 GPa and strengthens as pressure is increased to 25 GPa. In osmium, texture development starts very early (4 GPa). At ambient temperature cadmium and hafnium develop a similar textures as zinc and osmium, respectively. Texture in cadmium evolves gradually with axial shortening to 34%, whereas in hafnium texturing develops immediately after small strains. When hafnium is simultaneously heated to 700 K and deformed in compression, a texture develops with compression axes near View the MathML source. Simulations from a visco-plastic self-consistent (VPSC) polycrystal plasticity model suggest that the gradual texture evolution observed in zinc and cadmium is controlled primarily by View the MathML source basal slip and later accompanied by View the MathML source tensile twinning when the c/a ratio is below View the MathML source. Conversely, early texture development in osmium and hafnium at room temperature is contributed mainly by View the MathML source tensile twinning. However, the View the MathML source texture in hafnium at high temperature is attributed to basal and prismatic slip.

[2] Particle strengthening in fcc crystals with prolate- and oblate-shaped precipitates

B Sonderegger and E Kozeschnik

The prediction of precipitation hardening is a key factor for optimizing strength or creep performance of advanced structural materials. An essential part of this task is a reliable assessment of precipitate distances based on arbitrary discrete size distributions. Up to now, models are available for spherical precipitate shape. The present work advances these approaches to all kinds of spheroids. The results are expressed in compact form as a correction factor to the spherical case, only depending on the precipitate shape.

[3] Continuum modeling of dislocation starvation and subsequent nucleation in nano-pillar compression

A Jerusalem et al

The mechanical behavior of single crystalline aluminum nano-pillars under uniaxial compression differs from coarse-grained Al in that the former is characterized by a smoother transition from elasticity to plasticity. We propose an extension of the phenomenological model of dislocation starvation originally proposed in [Greer and Nix, Phys Rev B, 73:245410 (2006)] additionally accounting for dislocation nucleation. The calibrated and validated continuum model successfully captures the intrinsic mechanisms leading to the transition from dislocation starvation to dislocation nucleation in fcc nano-pillars.

[4] A Crystal-Plasticity Finite Element Method Study on Effect of Abnormally Large Grain on Mesoscopic Plasticity of Polycrystal

Y S Choi and T A Parthasarathy

In order to investigate the effect of an abnormally-large grain on plastic responses of polycrystal, elasto-viscoplastic crystal plasticity finite element simulations were performed using synthetic three dimensional microstructures with and without an abnormally-large grain. Results indicated that abnormally-large grain can be an important feature to cause the instability of mesoscopic plasticity by drawing hot spots of plastic shear. In particular, the maximum slip system shear hot spots are pronounced in the presence of the abnormally-large grain oriented in single slip.

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