Martensitic Transformation And Structural Stability In Pulse Electrodeposited Nano-grained Co, Feni And Coni

Materials with nano-sized grains always show certain unique properties, such as electronic, optical and catalytic characteristics. However, the materials must keep their phase stability upon heating or cooling to keep those characteristics. The phase transformations and grain growth always take place upon heating. So it's important to investigate grains growth and phase stability in nano-grained materials.Previous investigations focused on the micro-sized materials, and recently some researchers manufactured nano-grained Co and FeNi by hydrogen plasma-metal reaction and surface mechanical attrition treatment (SMAT) to investigate the phase stability in nano-grained materials. However, these method always introduce a lot of dislocations and contaminations, and the shape of the materials are always particles. This factors could complicate the analysis. So in order to study the intrinsic mechanism of phase stability in nano-grained materials, the samples with fewer defects and impurities are required. Fortunately, pulse electrodeposition just provides a relatively simple way to synthesize such 3-dimendional nano-grained samples. Less defects, bulk, nano-grained Co, FeNi and CoNi films, with the grain size of 15nm, 25nm and 12~15nm respectively, were prepared by pulse electrodeposition. Through controlling the parameters of electrodeposition, three different nickel contents in CoNi alloy were manufactured. The reason for choosing these materials is that they undergo classical martensitic transformations upon heating and cooling. The grain growth and phase transformations were investigated by DSC, XRD and MPMS.Upon heating in DSC, there is an obvious exothermic peaks in nano-grained Co and CoNi, which represent the grain growth. According to the Kissinger method, the calculated activation energy values for grain growth in nano-grained Co is close to grain boundary self-diffusion, which indicated that boundary diffusion may play a key role during the grain growth. The phase transformations were both observed in nano-grained Co, FeNi and CoNi upon heating. The end temperatures of transformation (Af) in nano-grained CoNi decreased with increasing the nickel content, which means the phase transformations upon heating were influenced by the compositions. And the great decreasing of onset temperatures of hcp-to-fcc and bcc-to-fcc in nano-grained Co and FeNi implies that the grain growth promotes the phase transformations upon heating and significantly reduces the low temperature phaseε-Co andα-FeNi stability. The forward martensitic transformation upon cooling had took place in annealed nano-grained FeNi which was confirmed by in-situ XRD and MPMS. However, the forward martensitic transformations were greatly suppressed in annealed Co and CoNi. The abnormal stability of the high temperature phaseγ-Co with the grain size 500nm cooled to LN may be attributed to the inhibition of the nanoscale twin boundaries (TBs), which are easily formed in nano-grained Co upon heating due to the low stacking fault free energy ofγ-Co. The relative higher stacking fault free energy ofγ-FeNi may be the reason for the appearance of the forward martensitic transformation in annealed FeNi.Before the martensitic transformation, there possibly exist order-to-disorder transformations upon relative higher cooling (>100K). It could be attributed to the high mobility of atoms in nano-grained FeNi upon relative low temperature heating (<773K), which may induce the formation of Fe3Ni structure. The similar order-to-disorder transformations were also observed in nano-grained CoNi that need further experiments to confirm.