Correlations between milling conditions and iron contamination, microstructure and hardness of mechanically alloyed cubic BN particle reinforced NiCrAl matrix composite powders

Contamination from milling vials and grinding balls is one of the major problems for mechanically alloyed materials especially when brittle hard components are included. In this study, 40 vol.% cubic BN/NiCrAl alloy composite, a typical example in ductile-brittle materials system, was prepared by mechanical alloying (MA) process. To minimize iron contamination arising from the stainless steel milling medium, two types of precautions, including coating NiCrAl alloy on milling medium surface by mechanical milling and a novel 'step-fashion' MA method, were employed. Without any approaches, a high iron content of 10.9 wt.% was yielded. After coating a ∼20 μm thick NiCrAl layer on milling medium surface, the iron contamination was decreased to 7.3 wt.% because the coated NiCrAl layer played an important role in preventing the milled powders from directly tearing and wearing the milling medium. By the 'step-fashion' MA method, the iron contamination was reduced to 4.4 wt.%. Combining these two pretreatments, a low iron content of 2.2 wt.% was achieved. After 40 h of milling, crystals of NiCrAl matrix for powders prepared under all of the conditions were refined to be sizes of 30-40 nm. Furthermore, by the 'step-fashion' milling method, NiCrAl powders exhibited an enhanced ability to incorporate more cBN ceramic dispersoids. As many as 40 vol.% of cBN was embedded into NiCrAl matrix for composite powders prepared by the 'step-fashion' method and thus resulted in a high hardness of ∼1150 HV. However, powders prepared by conventional method incorporated only 20 vol.% of cBN and exhibited a relative low hardness of 800 HV. To make clear the mechanisms that how milling conditions actually influence the iron contamination level, the interactions between milled powders and milling medium during MA were also investigated.