Nanocomposites and nanomaterials

Field assisted sintering of Ti-Ni-Hf powders prepared by spark-erosion method 
G.E. Monastyrsky1, A.V. Gilchuk1, M. Rosiński2
1 NTUU “Igor Sikorsky Kyiv Polytechnic Institute”, 37, Peremogy Av., UA-03506 Kyiv, Ukraine 
E-mail: monastyrsky@yahoo.com
2 Faculty of Materials Science and EngineeringWarsaw University of TechnologyWarsawPoland 

Two modifications of field assisted sintering, namely, spark-plasma sintering (SPS) and pulse-plasma sintering (PPS) were used for the consolidation of Ni50Ti40Hf10 powders produced by spark-erosion technique in liquid argon. In both cases specimens sintered under appropriate conditions had dense composite structure. More faster and intensive PPS technique together with much better vacuum conditions provided preservation of the initial substructure of powder particles as well as their chemical composition. In SPS specimens the micron sized spherical particles were embedded in inter-particles phase formed from sintered nano-sized particles of powder. Chemical composition of micron sized grains was different from the inter-particles phase. Moreover the intensive decomposition of the B2 phase into Ti4Ni3 and Ti3Ni2 phases were observed inside micron-sized grains. The composition stratification, layered substructure and internal oxidation with creation mainly m-HfO2 as well as Ti4Ni2Оx and Ti3Ni3O oxides were the peculiarities of SPS samples too.            
The Bernard-Granger creep model [1], has been used to describe the densification kinetics at different sintering stages of SPS samples. It presume the power law creep mechanism of the densification. The activation energy estimated from densification curves was lower than that of expected from hot deformation behavior of Ni-Ti-Hf shape memory alloy under hot compression tests. This, as well as high reactivity of powders in SPS experiments is a result of heavy current, that assists the sintering in FAST techniques. 

1.	Bernard-Granger G., Guizard C. Spark plasma sintering of a commercially available granulated zirconia powder: I. Sintering path and hypotheses about the mechanism(s) controlling densification // Acta Mater.-2007.-55(10).-P. 3493–3504.