Deformation of nanostructured materials: insights from experimental observations

Increased interest in nanoscale deformation has been spurred by the rapid miniaturization of features and components in microelectronic devices, magnetic storage media, and MEMS/NEMS, i.e., micro- and nanoelectromechanical systems, respectively. Materials for these applications are often chosen because of their electronic, optical, or magnetic properties. Frequently, however, the mechanical properties are the performance-limiting factor. Until recently, the techniques used to produce samples resulted in a high number of induced artefacts and, hence, irregular mechanical properties. Thanks to progress in processing methods, we can now make nanograin samples of high purity and high density that, under experimental conditions, show reproducible characteristics. Such fully dense materials are excellent candidates for fundamental mechanical property studies as well as for high-performance applications.

Compared with their microcrystalline counterparts, nanostructured metals with grain size smaller than 100nm possess attractive properties such as high yield and fracture strength¹ and improved wear resistance.² But evidence also suggests that miniaturization may influence their microstructural^3,4 and mechanical stability,⁵ resulting in undesirable shape change, mechanical response, thermal behavior, and electrical performance. These instability phenomena are the main focus of our research efforts. Small sample size generally rules out standardized tests, such as tensile and compression tests. But advances in methods such as nanoindentation accommodate very small amounts of material.

Traditional nanoindentation testing measures the elastic modulus and hardness of engineering materials by pushing a probe into the material while monitoring load and penetration depth. Now, instrumentation offers new possibilities. For example, we perform cyclic nanoindentation tests with up to a million load cycles to investigate microstructural changes due to repeated contact.^4,6 Monitoring the mechanical response enables us to relate the onset of microstructural changes and damage evolution to the number of cycles. Another recent test procedure⁷ consists of machining cylindrical microcompression samples into the surface of bulk materials using a focused ion beam and then compressing these columns with a flat probe in a nanoindenter. By varying the dimensions of the microcompression samples, we study the interplay of external size and internal structure on deformation behavior.

Prominent microstructural changes as well as changes in the mechanical response were observed after cyclic indentation of nanocrystalline nickel (Figure 1). Significant grain growth was observed at locations where the highest stresses occurred. The changes were most apparent in materials with a very fine initial grain size. At sufficiently small load amplitudes, the microstructure remained stable and the mechanical properties did not change. The underlying mechanisms are not yet clear, but our observations point toward stress-assisted grain boundary migration and grain rotation. Microcompression tests⁸ show no influence of specimen size on yield stress for columns with diameters ranging from 0.3 to 7 microns. Plastic buckling, which occurred frequently with smaller grain sizes, can be explained by the absence of significant strain hardening and strain localization. See Figure 2(a) and (b).

The microstructural and mechanical instabilities we observed are of considerable practical importance. An unstable grain structure that develops during mechanical loading will alter the properties of a material and possibly result in catastrophic failure. This effect limits the application of nanostructured materials. Our current efforts focus on identifying the relevant mechanisms and conditions under which microstructural and mechanical instabilities occur in pure metals and alloys. The insights gained will contribute to better tailoring of small-scale materials for high-performance applications.