DIRECTED CRYSTALLIZATION OF PERITECTIC TRANSFORMED ALLOYS

Transcription

1 DIRECTED CRYSTALLIZATION OF PERITECTIC TRANSFORMED ALLOYS I.V. Belyaev, OAO NPO Magneton, Vladimir, Russia; M. Kursa and Ya. Drapala, The Mining and Metallurgical Institute, Technical University, Ostrava, Czech Republic. In this work we studied the microstructure of Al-Ni and Cu-Sn-based alloy ingots obtained by the controlled and noncontrolled directed solidification techniques. The alloy compositions are given in Table #1. Table #1. The Chemical Compositions of the Studied Alloys by Mixture (nominator) and by Chemical Analysis (Denominator) Alloy Elemental Content of Alloy, Mass% No. Sn Cu Ni Al 1 5.0/5.1 Bal ,0/10,3 Bal ,0/15,2 Bal ,0/20,1 Bal ,024,5 Bal ,0/31,8 Bal ,0/68.8 Bal Alloys 1 and 2 were crystallized as solid solutions. Alloys 3-6 were peritectically transformed during crystallization. Alloy7 was crystallized as congruently melted intermetallic. For alloy melting the pure metals were used. The melting of Al-Ni alloys was carried out under argon atmosphere in a vacuum induction furnace. The furnace lining is made of synthetic corundum. The Cu-Sn alloys were melted in a graphite- clay crucible under charcoal in a resistance furnace. The melt temperature was monitoring by Raytek (USA) laser pyrometer and platinum-platinum rhodium thermocouple PR30/6 in combination with potentiometer KSP-4. Cu-Sn-based alloys were heated up to 100 C over their liquidus temperature and cast into bottomless ceramic moulds preheated to the solidus temperature and placed on a watercooled copper chill. To create the conditions for directed solidification the moulds had massive casting heads. The columnar grain lengths and microstructures of these alloys were investigated on the ingots obtained. The Al-Ni-based alloys were directedly solidified under argon in the vacuum crystallizer using Bridgman method. The tapered bottom alumina crucible, mounted on the end of water-cooled copper chill, was utilized. The grown ingots structures were metallographically examined. The existence of a single crystal structure was determined by etched metallographic specimen. Chemical compositions and constituents of the alloys were studied by wet chemical analysis and X-ray diffraction analysis, respectively. By experimental studies it has been found that the columnar grains in the peritectic based alloy ingots have significantly smaller length compared to that of the solid solution ones. In the alloys Cu +20.1%Sn and Cu+24.5%Sn, in which the peritectic transformation 1

2 starts at the upper part of the crystallization temperature range, the columnar structure is completely absent. The dependence of columnar length from the alloy position in the phase state diagram is shown in Fig.1. Ingot metallographic studies showed a existence of α- and β solid solution grains in the microstructure of the 3-4 alloys presented in Table 1. In Alloy5 (Cu+24.5%Sn) the only grains of β- solid solution were presented. No α- solid solution grains have been found in this alloy microstructure. The examination of longitudinal and traverse metallographic specimen, obtained from the Al-Ni alloy ingots grown in the crystallizer, showed that as grown Al+31.8%Ni ingots had no single crystal structure. This alloy experienced peritectic transformation on crystallization (Fig.2). X-ray diffraction studies quantified the amounts of Al 3 Ni phase and Al- phase in the given alloy microstructure to be approximately ~ 67% and ~ 33%, respectively. The Al+68.8% Ni ingot structure was close to single crystal. This alloy crystallization proceeded forming congruently melted NiAl inter-metallic compound (Fig.2). This compound was the only phase in the alloy. The obtained result confirmed the earlier findings that peritectic transformation hindered the formation of a columnar structure under non-controlled directed solidification as well as a single crystal structure under controlled directed solidification [1, 2]. It is related to the low rate of peritectic transformation due to low diffusion rate in a solid. Even a small increment in alloy cooling rate within a crystallization range leads to strongly inhibited and even suppressed diffusion in a solid phase, and as a result to suppression of the peritectic reaction. Under such conditions the growth of columnar grains and single crystals becomes impossible. It becomes more favorable thermo-dynamically to form a new phase with its own interface. The new phase nucleation at the crystallization front suppresses the columnar grain growth under non-controlled directed solidification and impedes the development of single crystals under controlled directed solidification. The appearance of a new phase (e.g. β-phase in Cu+20.1%Sn), which generally forms through peritectic reaction directly from the melt as original crystals, is possible, when the crystallization front temperature descends below the temperature of the peritectic transformation onset. However, after the formation of post-peritectic grains has been completed and crystallization heat has been evolved the melt temperature at the crystallization front can rise again and exceed the temperature of the peritectic transformation onset. In this case again the crystals of hypo-peritectic composition begin to precipitate from the melt. They will envelop the precipitated post- peritectic grains and hence suppress their development. This process can be multiply repeated. Consequently, more and more new crystals with interface of their own that hinder developing columnar grains would appear at the crystallization front. It makes the growth of columnar grains impossible. The ingot structure becomes equiaxial and fine-grained. These ingot columnar and even single crystal structures can be formed during directed solidification of the peritectic alloys only in the case, if the melt overcooling ( t) prior to the crystallization onset is higher than the difference between the melt liquidus temperature (t l ) and the temperature of peritectic transformation onset (t per ), i.e. t>t l t per, proving such a situation would be kept up to crystallization completion. Thus much will depend on post-peritectic alloy susceptibility (potential ability) to columnar crystallization. It will be that above, than higher value M melt of this alloy [2, 3]. In the best way both a columnar and single crystal structure will be formed from the chemical compounds of the fixed composition. 2

3 REFERENCES 1. Belyaev I.V., Pikunov M.V., The Crystallization Parameter Design for Peritetic Transformed Alloys// Izv. AN USSR. Metals- No3, 1991,p M.V.Pikunov, I.V.Belyaev, E.V.Sidorov: Alloy Crystallization and Ingot Directed Solidification,-Vladimir. 2002, p Pikunov M.V, Belyaev I.V.: The Regularities in Crystallization of Multi-component Solid Solution Alloys for Cast Permanent Magnets.//The Proceedings og the International Scientific and Technical Conference Scientific Concepts of S.T.Kishkin and Advanced Materials Science, April 25-26, M.: VIAM. 2006, p

4 Fig.1. The dependence of columnar crystal length in ingot from alloy position in the Cu-Sn phase-state diagram 4

5 Fig.2. System Al-Ni equilibrium phase-state diagram 5