Investigation on the Strengthening Mechanisms of Nickel Matrix Nanocomposites

Abstract

The strengthening effect of carbon nanotubes (CNTs) in metal matrix nanocomposites occurs due to several mechanisms that act simultaneously. The possible strengthening mechanisms for metal matrix nanocomposites reinforced with CNTs consist of: (1) load transfer, (2) grain refinement and texture strengthening, (3) second phase strengthening, and (4) strain hardening. The main focus of this work is to identify the strengthening mechanisms that play a role in the case of the Ni-CNT nanocomposite produced by powder metallurgy. For the dispersion and mixing of the metallic powders with CNTs, two different routes were performed by ultrasonication and ball milling. The results indicated that four different strengthening mechanisms are present in the nanocomposites and had a different contribution to the final mechanical properties. The load transfer and the increase in dislocation density seem to strongly affect the properties and microstructure of the nanocomposite. The grain refinement and the presence of second phase particles have a small contribution in the strengthening of this nanocomposite, since the introduction of CNTs in the Ni matrix slightly affects the size and orientation of the grains in the matrix and a few nanometric particles of Ni3C were identified.

Heat-Treated Ni-CNT Nanocomposites Produced by Powder Metallurgy Route

“…Only an increase of the time at 1100 °C promotes the formation of recrystallized grains. For Ni samples, some recrystallization is observed during the sintering, as already reported [ 24 , 25 , 28 ]. The heat treatment promotes an increase in Ni recrystallization of the grains.…”

“…The XRD analysis did not detect the Ni 3 C phase. In fact, for the sintered Ni-CNT samples, in previous research [ 24 ], the XRD patterns did not detect Ni 3 C, which is formed in nanometric particles. This could mean that the heat treatments did not promote an extensive reaction of the CNT into the carbide phase and, therefore, it is not an essential factor for the thermal stability of the nanocomposites.…”

“…For the nanocomposite, LAGB density is higher. Previous research [ 24 , 25 , 28 ] has already shown that after sintering, nanocomposites reveal more LAGB than the metal without reinforcement due to the CNTs’ influence in increasing the dislocation density. It was expected that with the heat treatments, LAGB density would decrease.…”

“…The nanocomposites used in this research were produced by powder metallurgy. Nickel powder was provided by Goodfellow Cambridge Ltd. (Huntingdon, UK) and multi-walled carbon nanotubes (MWCNTs) were obtained from Fibermax Nanocomposites Ltd. (London, UK), which were already characterized in previous research [ 14 , 24 , 25 , 26 , 27 ].…”

“…For this characterization, a high-resolution Thermo Fisher Scientific QUANTA 400 FEG SEM (Thermo Fisher Scientific, Hillsboro, OR, USA) coupled with an EBSD detector TSL-EDAX EBSD Unit (EDAX Inc. (Ametek), Mahwah, NJ, USA) was used. EBSD data was analyzed using TSL OIM Analysis 5.2 2007 (EDAX Inc. (Ametek), Mahwah, NJ, USA) and different maps were performed similar to that of previous research [ 24 , 25 ], to obtain detailed information on the microstructures of the samples. Before all the maps were elaborated, all the raw data was subjected to a clean-up process, defining a grain tolerance angle of 15° and a minimum grain size of 2 points.…”