Nanoparticle consolidation using equal channel angular extrusion at room temperature

In the present work, we demonstrate the use of equal channel angular extrusion (ECAE) for the consolidation of metallic nanoparticles at room temperature as a bottom up approach to fabricating nanocrystalline (NC) metals. Three different initial average particle sizes of pure copper were used: -325 mesh micron size particles, 130 nm and 100 nm nanoparticles. The processing work was divided into three major stages (Stages I-III), depending on the powder filling procedure used prior to ECAE, to investigate the effect of processing parameters such as extrusion rate and ECAE route, powder filling environment, and hydrostatic pressure on the final performance of the consolidates. Microstructure of the consolidates and monotonic mechanical behavior were determined at room temperature. The Stage I experiments revealed what can materials, ECAE routes and range of extrusion rates to use for achieving near full density consolidates. In Stages II and III, the effect of initial compact density on the resulting mechanical behavior was investigated. It was found that the prior compaction is helpful in breaking down the initial nanoparticle agglomerates and achieving high tensile strength and ductility levels in the ECAE consolidates. Tensile strength as high as 800 MPa and tensile ductility as high as 7% were achieved in 100 nm Cu particle consolidates, which were more than 1.5 cm in diameter and 10 cm in length, with a bimodal grain size distribution in the range of 50-100 nm and 300 nm-600 nm. ECAE was also used to consolidate 316 L stainless steel nanoparticles resulting in bulk samples with tensile strength of 1180 MPa and 4% ductility. The present study shows that ECAE can be a feasible method for fabricating bulk NC materials with all dimensions in the centimeter range. Future work is needed to further optimize the processing parameters for improving the ductility level further and controlling the grain size distribution.