Non-Equilibrium Processing of Ni-Si Alloys at High Undercooling and High Cooling Rates

Abstract:

Melt encasement (fluxing) and drop-tube techniques have been used to solidify a Ni-25 at.% Si alloy under conditions of high undercooling and high cooling rates respectively. During undercooling experiments a eutectic structure was observed, comprising alternating lamellae of single phase γ (Ni₃₁Si₁₂) and Ni-rich lamellae containing of a fine (200-400 nm) dispersion of β₁-Ni₃Si and α-Ni. This is contrary to the equilibrium phase diagram from which direct solidification to β-Ni₃Si would be expected for undercoolings in excess of 53 K. Conversely, during drop-tube experiments a fine (50 nm) lamellar structure comprising alternating lamellae of the metastable phase Ni₂₅Si₉ and β₁-Ni₃Si is observed. This is also thought to be the result of primary eutectic solidification. Both observations would be consistent with the formation of the high temperature form of the β-phase (β₂/β₃) being suppressed from the melt.

• Periodical: Materials Science Forum (Volumes 790-791)
• Pages: 22-27
• DOI: https://doi.org/10.4028/www.scientific.net/MSF.790-791.22
• Online since: May 2014
• Authors: Andrew M. Mullis*, Lei Gang Cao, Robert F. Cochrane
• Keywords: Intermetallic, Rapid Solidification, Undercooling

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