Effects of Al and Cu Addition on Pressureless Sintering Behavior and Phase Transformation of β-FeSi2

• Published in: Materials Transactions, JIM Vol. 41; no. 7; pp. 857 - 864
• Main Authors: Ito, Mikio, Nagai, Hiroshi, Tanaka, Takashi, Katsuyama, Shigeru, Majima, Kazuhiko
• Format: Journal Article
• Language: English
• Published: Sendai The Japan Institute of Metals 2000; Japan Institute of Metals
• Subjects: Applied sciences; Cross-disciplinary physics: materials science; rheology; densification; Exact sciences and technology; liquid phase sintering; low melting point metal; Materials science; Materials synthesis; materials processing; Metals. Metallurgy; Physics; Powder processing: powder metallurgy, compaction, sintering, mechanical alloying, and granulation; pressureless sintering; swelling; β phase transformation; β-FeSi2; Densification; Swelling; Phase transformations; Vacuum sintering; XRD; Liquid phase sintering; Experimental study; Powder metallurgy; Melting points; DTA; Dilatometry; Sintering; Iron silicides
• ISSN: 0916-1821; 2432-471X
• DOI: 10.2320/matertrans1989.41.857

The preparation of β-FeSi2 thermoelectric material by liquid phase sintering with addition of a low melting point metal has been tried. Aluminum and copper were selected as an additive. The effects of Al or Cu on the sintering behavior and the phase transformation of β-FeSi2 have been investigated. In the case of sample with Al addition before mechanical grinding (MG), the liquid phase was not formed because pure Al disappeared by diffusion of Al atoms into the matrix before the sintering temperature reached the melting point of Al. On the other hand, in the case of the sample with Al addition after MG, Al–Si liquid phase was formed around 850 K, the eutectic point of Al88.7Si11.3. The liquid aluminum preferentially diffused into the solid phase and the pores were formed at previously occupied Al particle sites. This pore formation resulted in swelling of the compacts. Additionally, Al addition accelerated decomposition of the β phase to the α and ε phases. In the case of the sample with Cu addition, Cu–Si liquid phase was formed around 1075 K . This liquid phase was highly effective for densification of the compacts. Cu addition caused the transformation from the mixture of α and ε phase to the β phase around 1130 K during cooling after the sintering, which suggests that the subsequent annealing for the formation of the β phase in the conventional method can be omitted. The β phase transformation during cooling is considered to consist of the following two mechanisms. (1) Cu addition accelerates the decomposition reaction α→β+(Cu–Si)liquid instead of the ordinary α→β+Si. (2) The Cu–Si liquid phase spreads over the particles and facilitates the reaction ε+Si→β by supplying Si atoms to the ε phase through the liquid.