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Temperature-activated dislocation avalanches signaling brittle-to-ductile transition in BCC micropillars

Yang Li, Inam Lalani, Matthew Maron, William Hixson, Biao Wang, Nasr Ghoniem, Giacomo Po

Abstract

We carry out strain-controlled in-situ compression experiments of micron-sized tungsten (W) micropillars in the temperature range 300-900 K, together with simulations of three-dimensional discrete dislocation dynamics (DDD) at the same scale. Two distinct regimes are observed. At low temperatures, plastic deformation appears smooth, both temporally and spatially. Stress fluctuations are consistent with a Wiener stochastic process resulting from uncorrelated dislocation activity within the pillars. However, high-temperature stress fluctuations are highly correlated and exhibit features of self-organized criticality (SOC), with deformation located within well-defined slip bands. The high-temperature stress relaxation statistics are consistent with a thermally activated nucleation process from the surface. The nature of the transition between the two regimes is a manifestation of the brittle to ductile transition in BCC metals.

Temperature-activated dislocation avalanches signaling brittle-to-ductile transition in BCC micropillars

Abstract

We carry out strain-controlled in-situ compression experiments of micron-sized tungsten (W) micropillars in the temperature range 300-900 K, together with simulations of three-dimensional discrete dislocation dynamics (DDD) at the same scale. Two distinct regimes are observed. At low temperatures, plastic deformation appears smooth, both temporally and spatially. Stress fluctuations are consistent with a Wiener stochastic process resulting from uncorrelated dislocation activity within the pillars. However, high-temperature stress fluctuations are highly correlated and exhibit features of self-organized criticality (SOC), with deformation located within well-defined slip bands. The high-temperature stress relaxation statistics are consistent with a thermally activated nucleation process from the surface. The nature of the transition between the two regimes is a manifestation of the brittle to ductile transition in BCC metals.
Paper Structure (1 section, 4 figures)

This paper contains 1 section, 4 figures.

Table of Contents

  1. Acknowledgments

Figures (4)

  • Figure 1: (a) Micropillar compression stress-strain curves from 300 K to 900 K. An incremental $1\%$ strain shift is applied to each temperature for clarity. The SEM images in the insert indicate the occurrence of a large strain burst. (b) Size of representative pre-compressed pillar and post-compression SEM images of micropillars at 300 K, 600 K and 800 K.
  • Figure 2: (a) Complementary cumulative distribution function (CCDF) of avalanche size at different temperatures. The inset shows that the truncated power law distribution of the "size" $\mathcal{S}$ is consistent with a stochastic Wiener process at low temperature. (b) PSD of stress fluctuations at different temperatures. For clarity, a shift along the $y$ -axis is applied.
  • Figure 3: (a) Stress-strain curves of DDD micropillar compression simulations at 300 K and 900 K with and without surface nucleation. Each case is simulated multiple times with different initial configurations. (b) Corresponding power spectrum of the flow stress from the simulations.
  • Figure 4: Cumulative probability function of the nucleation stress. The dashed lines are model predictions from the supplementary equation 11.