Thin intergranular films and solid-state activated sintering in nickel-doped tungsten Vivek K. Gupta a , Dang-Hyok Yoon a,1 , Harry M. Meyer III b , Jian Luo a,c, * a School of Materials Science and Engineering, Clemson University, Clemson, SC 29634, USA b High Temperature Materials Laboratory, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA c Center for Optical Materials Science and Engineering Technologies, Clemson University, Clemson, SC 29634, USA Received 7 December 2006; received in revised form 3 January 2007; accepted 4 January 2007 Available online 27 February 2007 Abstract Nickel-doped tungsten specimens were prepared with high purity chemicals and sintered. Although activated sintering starts more than 400 °C below the bulk eutectic temperature, the nickel-rich crystalline secondary phase does not wet the tungsten grain boundaries in the solid state. These results contrast with the classical activated sintering model whereby the secondary crystalline phase was pre- sumed to wet grain boundaries completely. High resolution transmission electron microscopy and Auger electron spectroscopy revealed the presence of nanometer-thick, nickel-enriched, disordered films at grain boundaries well below the bulk eutectic temperature. These interfacial films can be regarded as metallic counterparts to widely observed equilibrium-thickness intergranular films in ceramics. Assuming they form at a true thermodynamic equilibrium, these films can alternatively be understood as a class of combined grain boundary disordering and adsorption structures resulting from coupled premelting and prewetting transitions. It is concluded that enhanced diffusion in these thin intergranular films is responsible for solid-state activated sintering. Ó 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Sintering; Grain boundary wetting; Grain boundary diffusion; Interface segregation; Refractory metals 1. Introduction Solid-state activated sintering refers to the enhancement of densification caused by minor solid-state additives. A well-known example is the accelerated sintering of tungsten and other refractory metals (Mo, Nb and Ta) resulting from the addition of less than 0.5 wt.% of transition metals (e.g., Ni, Pd, Co, Fe and Pt), which can initiate below 60% of the corresponding bulk solidus or eutectic temperatures [1–15]. Since densification occurs at low temperatures where bulk diffusion is usually insignificant, grain bound- ary (GB) transport is generally implicated. In a phenome- nological model extended from liquid-phase sintering theory [16], solid-state activated sintering was attributed to the enhanced mass transport of the base material in an ‘‘activator’’ phase [1]. An effective solid-state activator should have high solubility and transport rate for the base material and should remain segregated at GBs. In principle, the solid-state activators can be (i) crystalline secondary bulk phases that penetrate along the GBs, (ii) Langmuir–McLean or truncated BET (Brunauer–Emmett– Teller) type interfacial segregation/adsorption region with- out precipitation of a discrete phase or (iii) discrete nanoscale interfacial phases that do not appear in bulk phase diagrams. Although the phenomenon of solid-state activated sintering has been studied over five decades [1–15,17,18], the exact nat- ure of these solid-state activators and how they result in enhanced sintering remain largely unknown. Solid-state activated sintering has also been observed in several ceramic systems [19–22]. A previous study [19] 1359-6454/$30.00 Ó 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2007.01.017 * Corresponding author. Address: School of Materials Science and Engineering, Clemson University, Clemson, SC 29634, USA. E-mail addresses: jianluo@clemson.edu, jluo@alum.mit.edu (J. Luo). 1 Present address: School of Materials Science and Engineering, Yeungnam University, Korea. www.elsevier.com/locate/actamat Acta Materialia 55 (2007) 3131–3142