2, Xi'an Jiaotong University, Xi'an, , China
3, PMMH/ESPCI, Paris, , France
In the traditional view of dislocation-mediated plasticity, the mechanical properties of crystalline solids are predominantly controlled by the internal microstructure, which can be artificially tailored, from hardening or the introduction of solutes or precipitates, to improve the performance in service. The external size L, with a length scale much greater than microstructural scales, was usually regarded as unimportant. Meanwhile, at bulk scales, plastic deformation generally occurs in a smooth and continuous (“mild”) way without detectable fluctuations or instabilities. Over recent years, as a consequence of progressive miniaturization of systems and devices, the mechanical properties of metallic materials at micro- to nano-scales became a major concern. The compression of micro/nano-sized single crystals revealed an unexpected external size effect on strength. This “smaller is stronger” phenomenon is unfortunately accompanied by “wild” fluctuations manifesting directly on the loading curves through intermittent, catastrophic strain bursts with a broad range of sizes following a power-law distribution, even in FCC metals deforming smoothly at macro scales. Consequently, despite the high strength achieved at small scales, the plastic process is uncontrolled due to the stochastic nature of strain bursts, and the dislocation avalanches, possibly spanning the system size, may poison forming processes and the load-carrying capacity.
This calls for new metallurgical strategies to mitigate these plastic instabilities at small scales. From compression tests on micro-pillars of pure Al and Al-alloys single crystals strengthened by different types of solutes or precipitates, we showed that:
- Diminishing the external length scale L (miniaturization) intensifies fluctuations and contributes to criticality.
- Introducing quenched disorder shifts the transition from wild to mild plasticity towards smaller external length scales. This “dirtier is milder” effect opens the possibility to mitigate plastic instabilities at small length scales.
- The mild-to-wild transition is closely related with a transition from forest- to exhaustion-hardening, as dislocation-starved states characterizing small samples prevent dislocation entanglements that frustrate avalanches at bulk scales.
- The inter-relation of size and disorder effects reveals itself through a material-independent mapping between the power law exponent of avalanche size distribution and the degree of wildness quantifying the proportion of plastic strain occurring through dislocation avalanches. Translating the pinning strength of obstacles into an internal length scale l, we showed that a single parameter R=L/l controls both the transition from exhaustion- to forest-hardening, and the transition from wild to mild fluctuations. This gives clues towards a possible tailoring of the materials through the introduction the solutes or precipitates in order to tame undesired plastic avalanches at small scales.