Structural Evolution in Mechanically Alloyed and Spark Plasma Sintered Iron–0.15 wt.% MWCNT Composite Priyanka Sharma, Akshay Kumar, and M.K. Banerjee (Submitted May 20, 2018) High-energy ball milling (HEBM) of mixtures of iron powder and multi-walled carbon nanotube (MWCNT) has been performed in an attempt to synthesize nano-grained steel. Even after exposure to a harsh HEBM conditions, the MWCNTs are seen to have retained their structural identity, and therefore, an MWCNT-reinforced steel matrix composite could be finally produced. Moreover, the study has revealed that the minor addition of copper leads to a significant reduction in grain size of ferrite in the so-produced steel matrix–MWCNT composite. It is also noticed that the fine grain structure of ferrite remains intact even after consolidation of the powder composite by spark plasma sintering, followed by hot forging. The micro-hardness values obtained for the composites (with/without copper) are observed as comparable with the submicron-grained steels, so far reported in the literature. Keywords grain growth inhibitor, mechanical alloying, metal matrix composite, micro-hardness, spark plasma sin- tering 1. Introduction The unique mechanical and physical properties of graphene, a single-layer graphitic-structured material, have enticed the material scientists to develop new generation nanocomposite by dispersing graphene in metal matrices (Ref 1, 2); it is demonstrated that the composites with graphene as reinforce- ment can significantly improve the mechanical properties. However, success in the development of polymer matrix–CNT nanocomposites has made carbon nanotubes attract immense interest within material scientists to develop novel functional materials by the use of MWCNTs as reinforcement materials in metal matrix composites (Ref 3-7). It is well documented in the literature that multi-walled carbon nanotubes (MWCNTs) have been used as reinforcement material for the development and characterization of nanocomposites based on metal matrices like aluminum, copper and magnesium (Ref 8-10); reportedly, the development in this direction has aimed at harnessing the potential of MWCNTs in creating new materials of fascinating properties owing primarily to its excellent combination of mechanical and physical properties, viz. high stiffness, high strength, high thermal conductivity and low weight (Ref 11- 14). However, the high aspect ratio and van der Waals forces between MWCNTs inhibit its uniform dispersion into the matrix metals (Ref 15, 16). Along with this, poor interfacial bonding between MWCNT and most matrix metals has become the major processing challenge to the researchers; till date, high-energy ball milling (HEBM) is found to have better edge over other processes of synthesis (Ref 17-19). Incidentally, HEBM makes MWCNT susceptible to damage; MWCNT undergoes shortening, collapse of tubular structure and even amorphization after prolonged ball milling (Ref 20-22). It is recorded that damage of MWCNTs becomes more extensive when it is milled with metals (Ref 5, 18). Iron is a transition metal with unfilled 3d orbital and is therefore amenable for hybridization with 2p orbital of carbon atoms in MWCNT; this can provide a better interfacial bonding between the reinforce- ments and the matrix metals. Moreover, iron is a strong carbide former, and hence, there is a chance of mechano-chemical degradation of MWCNT when ball-milled with iron; coupled with this, the high solubility of carbon in iron lends the underlying clue to the discovery of a novel technique to develop a new type of nano-grained steel of excellent mechanical properties. The steel so contemplated for its development may also act as the matrix of a nanocomposite with partially retained MWCNTs as the reinforcing agent. In view of the fact that ultrafine grains tend to coarsen with the slightest opportunity for grain boundary migration, the sustain- ability of nanograin structure seems to be better aided if some fine particles are made uniformly dispersed within the matrix so as to inhibit the grain boundary movement; copper is not soluble in bcc iron and is thought to be a good choice for being explored as grain growth inhibitor. In light of above, it seems plausible to develop a novel fine- structured ferrous material of superior properties. Hence, in the present research, attempts are made (1) to examine the possi- bility of using mechanical alloying route to synthesize ultrahigh hardness steel–MWCNT nanocomposite and (2) to understand the evolution of microstructure in high-energy ball-milled iron– MWCNT mixture with or without copper addition and its bearing on the strength properties of the materials thus produced. 2. Materials and Methods 2.1 Milling of Fe and Cu-Fe Powders Iron powder (Sigma-Aldrich) of ‡ 99% purity was ball- milled in a Fritsch Pulverisette-P6 high-energy planetary ball Priyanka Sharma, Akshay Kumar, and M. K. Banerjee, Department of Metallurgical and Materials Engineering, Malaviya National Institute of Technology, Jaipur 302017, India. Contact e-mail: mkbanerjee.meta@mnit.ac.in. JMEPEG ÓASM International https://doi.org/10.1007/s11665-018-3547-8 1059-9495/$19.00 Journal of Materials Engineering and Performance