Interaction between refractory crucible materials and the melted NiTi shape-memory alloy
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I. INTRODUCTION
EXCELLENT shape memory, superelasticity, and mechanical properties have introduced NiTi as an extremely useful family of newly developed alloys. These properties depend greatly on the exact chemical composition, processing history, and smallness of undesirably dissolved elements.[1] Contaminants such as oxygen and carbon can dramatically affect the properties of the NiTi shape memory alloy. Their penetration occurs basically during production and processing of the alloy. Commercial production processes usually involve induction melting of the alloy under heavy vacuum. A major source of contaminants is the melting crucible, which needs to be carefully chosen. Numerous investigators[2,3,4] have tried to solve the problem, but they have generally not been able to obtain a satisfying result, because the contaminating behavior of the ordinary materials (oxides, borides, silisides, sulfides, nitrides, fluorides, Mo3Al, and W) could never be totally stopped. In an early study, zirconia stabilized with titanium was found to be the least reactive one.[2] Later investigators[4,5] indicated that Y2O3 performs even better than ZrO2. Other investigators[5] indicated that Y2O3 stabilized with 8 to 15 wt pct of titanium has the best performance. Yttria is, however, fairly expensive, which is a drawback. Induction and arc skull melting processes both were used to prevent the molten metal contamination. These processes possessed, however, very low energy efficiency and great difficulty in obtaining sufficient superheats generally needed for a better molten metal homogenization.[6] In order to improve the quality of NiTi casting objects and reduce the production costs, it is, therefore, necessary to introduce a refractory material capable of melting highly reactive
NiTi alloys. The present study deals with quantification of instability and contamination formation behavior of various refractory crucibles in order to help the development of low cost materials such as zirconia, alumina, and silicon carbide for melting of the NiTi shape memory materials. II. EXPERIMENTS A. Melting Procedure High content zirconia, zircon type A (ASTM C574-70), recrystallized alumina, slurry cast alumina, commercially pure SiC, and SiC-5Al2O3-5SiO2 commercial crucibles with chemical compositions given in Table I were used in this investigation. For evaporation of the absorbed humidity, all crucibles were heated to 200 °C 10 °C in a resistance furnace and wrapped with a piece of aluminum foil in order to prevent the possible re-absorption of humidity. The interaction between the crucibles and the melt was studied in a vacuum resistance furnace with heating elements made of tungsten. The chemical composition of the charge material was Ni-45 wt pct Ti. After placing the crucible inside the chamber and evacuating the chamber for up to 106 mbar, electric heating was started. The temperature of the crucible was gradually increased up to 1450 °C, i.e., 140 °C above the melting point of NiTi (1310 °C). Although soaking for 30 minutes at this t
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