Small scale mechanical testing affords the possibility of measuring constitutive materials properties and studying intrinsic size effects on mechanical behavior. Accordingly, Uchic and colleagues developed a microcompression testing technique about five years ago. Published studies employing this technique have clearly demonstrated that size-scale effects exist independently of other previously known size effects such as nucleation-controlled deformation (whiskers) or the presence of imposed strain gradients (nanoindentation). The overwhelming majority of these studies have focused on metals with the FCC crystal structure. In this study, the microcompression test methodology was employed to evaluate Rene N5, an engineering alloy with a heterogeneous microstructure containing features which vary over a range of length scales. Compression samples were tested using sample diameters in a range from 2.5 to 80 um, while selectively testing samples from both dendrite core and interdendritic regions. A size-dependent flow response, consistent with exhaustion hardening, was observed. Region-specific properties were also observed, where samples tested from interdendritic regions had varied and on average decreased flow stress values compared to those tested from dendrite core regions. A representative sample diameter, where bulk properties would be matched, was extrapolated to be 400 um.
Additionally, a custom in-situ SEM testing device for performing uniaxial mechanical tests on micrometer-scale samples was employed. This device allows one to access both tensile and compressive test modes, and also directly observe the spatial and temporal distribution of deformation events through continuous recording of SEM images. Microcompression experiments with this device demonstrated that the degree of lateral constraint imposed by the compression platen affects many aspects of the observed response, such as the strain hardening rate, crystal rotations, elastic modulus, dislocation intermittency, and the spatial distribution of slip bands. A microtension testing methodology was also developed, and lateral constraints were shown to not strongly affect the observed response.