In this paper, we will first present a new preparation method to obtain ultrafine Co and/or Ni particles using a gamma-irradiation techniques, with which the particle size could be controlled from 20 nm to a few microns. By adding proper surfactants, the production and protection of metallic particles could occur simultaneously. We then prepared magneto-rheological suspensions containing iron-based complex phase ultrafiines such that the micron-sized iron particles were decorated by nano-sized Co particles on their surface. We then measured the yield stress of suspensions containing these complex phase particles with a channel flow mode testing unit. Experimental results showed that a small amount of Co ultrafine particles as the modifier of the iron particles could lead to a significant enhancement of the yield strengths compared with the suspension without Co particles.
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