Temperature dependent structural evolution in nickel/carbon nanotube composites processed by high-pressure torsion

Abstract

Nickel/Carbon nanotube (CNT) composites with varying amounts of CNTs were processed at different temperatures by high-pressure torsion (HPT) with the aim to optimize the process parameters to obtain a homogenous dispersion of CNTs in the metallic matrix. As the CNT distribution has an enormous influence on the composite properties, the structural evolution with increasing strain and the final microstucture of the composites are investigated by scanning and transmission electron microscopy. Microhardness measurements were additionally performed. Microhardness increases up to 800 Vickers (HV) and the mean grain size decreases to an equivalent radius smaller than 40 nm for HPT at room temperature (RT), while the CNTs form rather large agglomerates. HPT deformation at 200 °C shows no significant change in hardness, grain size and CNT agglomerate size. For HPT deformation at 300°C and 400°C grain sizes increase to 60 nm respectively 90 nm, microhardness decreases to 500 HV respectively 400 HV and the size of the CNT agglomerates decreases from more than 5 times the grain size at RT to smaller than the grain size. It could be shown that the optimal HPT processing route to improve the CNT distribution is a combination of deformation at 400°C with subsequent deformation at RT.