The domain of mesoscale science, where the discrete granularity of atoms and quantization of energy give way to apparently continuous and infinitely divisible matter and energy, presents a new frontier of scientific opportunity and yields new complex architectures, phenomena, and functionalities. In this article, we describe some hallmarks of mesoscale science and highlight research directions that are described in greater detail in subsequent articles in this issue of MRS Bulletin. The exciting progress of the past several years and the rich unexplored opportunities at the mesoscale offer extraordinary prospects for future advances.

Taking the mesoscale to mean length and time scales at which a material’s behavior is too complex to be understood by construction from the atomistic scale, we focus on three-dimensional characterization and modeling of mesoscale responses of polycrystals to thermal and mechanical loading. Both elastic and plastic internal structural responses are now accessible via high-energy x-ray probes. The combination of diffraction experiments and computed tomography, for example, is yielding new insights into how void formation correlates with microstructural features such as grain boundaries and higher-order junctions. The resulting large, combined data sets allow for validation of micromechanical and thermal simulations. As detectors improve in resolution, quantum efficiency, and speed of readout, data rates and data volumes present computational challenges. Spatial resolutions approach one micrometer, while data sets span a cubic millimeter. Examples are given of applications to tensile deformation of copper, grain growth in nickel and titanium, and fatigue cracks in superalloys.

Precise control of structural parameters through nanoscale engineering to continuously tailor optical and electronic properties of functional nanomaterials remains an outstanding challenge. Previous work focused largely on chemical or physical interactions that occur under ambient pressures. In this article, we introduce a new pressure-directed assembly and fabrication method that uses a mechanical compressive force applied to nanoparticles (NPs) to induce structural phase transitions and consolidate new nanomaterials with precisely controlled structures and tunable properties. By manipulating NP coupling through external pressure instead of through chemistry, a reversible change in assembly structure and properties can be demonstrated. In addition, over a certain threshold, the external pressure forces these NPs into contact, allowing the formation and consolidation of one- to three-dimensional nanostructures. Through stress-induced NP assembly, unusual materials engineering and synthesis, in which morphology and architecture can be readily tuned to produce desired optical and electrical properties, appear feasible.

Indentation as a means to extract creep properties has the advantage that it can be applied directly to micro/nano-structures. Many studies on indentation creep reported at least partially poor agreement with creep parameters derived from uniaxial test. One important reason for the incompatibility is the neglect of transient creep. Another one is the choice of equivalent stress and strain measures to relate the different material responses. Applying a material model that accounts for transient creep effects we propose an efficient method for deriving creep properties from short-time spherical indentation tests. We first determine a subsurface point where the material response is very close to that observed in uniaxial tests. We then map the load–displacement data to the material response, expressed in terms of two dimensionless variables, at this point. Converting the dimensionless variables data to stress, strain, and strain rate data, we finally determine the material's creep coefficient and exponent.

The work hardening behavior of electrodeposited nanocrystalline nickel (29 and 19 nm) was investigated under multiaxial loading and compared with coarse-grained nickel. Plastic strain gradients were introduced into the materials using large Rockwell D hardness indentations, and measured through cross-sectional hardness profiles. The results showed that the coarse-grained material exhibited substantial hardening up to twice the hardness of the deformation-free area due to dislocation mediated deformation, while the nanocrystalline materials displayed small hardness variations along the strain gradient, indicative of considerably reduced dislocation interactions. Moreover, the grain structure analysis (cumulative volume fraction and size distribution) for the nanocrystalline materials suggested the operation of both dislocation mediated and grain boundary controlled deformation mechanisms, the latter becoming more significant with increasing cumulative sample volume of very small grains. The plastic deformation zone sizes under Rockwell indentation of the 29 nm Ni are similar to those conventional materials with reduced strain hardening. Microhardness-indentation size effects were negligible in both the nanocrystalline and coarse-grained materials.