Microstructure Evolution of Fine-Grained Cobalt T400 Tribaloy Processed by Spark Plasma Sintering or Hot Isostatic Press
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ODUCTION
IN order to increase the surface hardness of parts for wear applications, carburizing, nitriding or surface coating can be used.[1] Coating is applied by spraying high hardness alloy powders.[2–4] Cobalt base alloy powders of the Stellite or Tribaloy type are marketed for this type of application.[5,6] Instead, this project aims to develop a bulk material of this alloy by a powder metallurgy processing route. The alloy chosen is T400, from the Tribaloy family. These alloys exhibit very good wear resistance at high temperatures combined with good corrosion resistance.[1] Tribaloys are cobalt alloys enriched in molybdenum and sometimes chromium, supplemented by a few percent Silicon (Table I). Their microstructure after casting consists of an intermetallic hard phase (Laves phase) dispersed in a softer matrix of eutectic solution (aggregate of a solid cobalt solution and Laves phases).[7] This principle of mechanical reinforcement by a hard phase in a ductile matrix is similar to Stellite alloys, which are reinforced by the presence of
X. BOULNAT and C. LAFONT are with the Universite´ de Lyon, INSA-Lyon, MATEIS UMR CNRS 5510, 69621 Villeurbanne, France. Contact e-mail: [email protected] J.-B. COUDERT and C. DAYOT are with SKF Aerospace France, 22 Rue Brillat Savarin, 26300 Chaˆteauneuf-sur-Ise`re, France. Manuscript submitted November 26, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
chromium carbides (M23C6, M7C3).[8] These Stellites are therefore richer in chromium and carbon than Tribaloys—for example Stellite 6 has 28 wt pct Cr and 1 pct wt carbon—and also exist in powder form and thus parts can be produced by HIP.[9,10] The mechanical properties of Tribaloys are highly dependent on the fraction of Laves phases.[11] The percentage of Laves phases, generally of composition Co3Mo2Si or CoMoSi, directly depends on the composition of the alloying elements Co, Mo, Si.[12,13] There are several types of Tribaloys known as T100-T400-T800 whose fraction of Laves phases varies from 40 to 65 pct in volume. Chromium is placed in solid solution in the cobalt matrix but can sometimes substitute itself for silicon in the composition of the Laves phase.[12] Silicon plays a decisive role in the volume fraction of Laves phases. T401 has less silicon but more chromium than T400, which reduces the fraction of Laves phases and thus increases the ductility of the material, but reduces hardness and wear resistance.[13] This study focuses on the T400 alloy, whose microstructure and wear behavior are well documented for cast materials, making it possible to compare it with materials obtained by powder metallurgy. Powder metallurgy could be a way to avoid large heterogeneities found in cast alloys, such as segregation bands, and thus to improve bulk toughness when keeping a high level of hardness. Two powder consolidation techniques will be used:
(i)
(ii)
The flash sintering technique, or Spark Plasma Sintering (SPS), which makes it possible to obtain dense parts by unidirectional pressure and Joule heating of
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