With the advent of nanocrystalline (NC) and ultra-fine grained (UFG) metals, severe plastic deformation processing is a promising technique for producing bulk nanostructured samples with enhanced mechanical properties. However, the size of specimens is commonly limited to a maximum of approximately 20 × 20 × 40 mm, so that sample characterisation by mechanical testing using tensile or compression samples of conventional size is an issue. Due to this, we applied small punch testing (SPT) to specimens previously subjected to different amounts of deformation by equal channel angular extrusion (ECAE), and compared them with tensile and compression results obtained from samples with reduced dimensions. Despite the difference in strain paths between uniaxial tension/compression and shear punch tests, (the latter characterized by a compression and shear component), a good correlation has been found between the two techniques. A comparison of the tensile and punch curves of the annealed specimen compared to the tensile and punch curves of the ECAE Cu specimens shows a significantly lower ductility in tension than in shear for the ECAE specimens. Therefore, ultrafine grained metals are more ductile under a compression/shear stress than under tensile conditions if compared to their coarse-grained counterparts. A comparison of the tensile strain, in which there is typically a significantly larger amount of unstable (necking) elongation for ECAE metals, and the shear strain measured by punch tests, reveals that plastic instability, and consequent failure occur significantly 'earlier' in tensile deformation. Strain rate sensitivity values, m, measured by the punch tests, coincide with values of m measured in cyclic strain rate jump tests under compression. The microstructural evolution with accumulated strain was also followed using transmission electron microscopy.
|Number of pages||5|
|Publication status||Published - 5 Sep 2005|