Shape Memory Behaviour in Rapidly Solidified TiNi Alloys
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SHAPE MEMORY BEHAVIOUR IN RAPIDLY SOLIDIFIED TiNi ALLOYS
m. IrGEA AND J.V. WOOD The Open University, Dept. 6AA, UK.
of Materials, Walton Hall,
Milton Keynes,
MK7
ABSTRACT Equiatomic TiNi alloy has been rapidly solidified by melt spinning. The M. temperature of ribbon is depressed with respect to samples made from elemental powders. This phenomenon is related to grain refinement. Some of the ribbons have been crushed and the consolidation characteristics of the resulting pre-alloyed powder is compared with that of elemental blends. Sintering activity is higher in elemental compacts as a result of the dominant effect of the a]loy formation energy in elemental blends. Introduction Although the process of rapid solidification of shape memory alloys has been extensively investigated [e.g 1-3] most of this work has concentrated on copper-base alloys and little has been reported on other alloy systems eg TiNi. The compound TiNi undergoes a thermoelastic martensitic transformation near room temperature. This reaction is a prerequisite of the shape memory effect (SME). Alloys based on TiNi are widely used for a number of applications although there are considerable problems involved in processing. In a previous attempt to make TiNi by sintering elemental powders, it was observed that as a result of the unequal interdiffusion coefficients of nickel and titanium, extensive porosity remained in the compact [4]. Experimental Details TiNi mixtures were produced by compaction and sintering elemental powders. About 10 gram of the pre-sintered compacts were rapidly solidified in an enclosed melt spinning unit under an argon atmosphere. Quartz crucibles were employed with an 0.8 mm orifice diameter. Melt was quenched on to a 230 mm diameter water cooled brass wheel with a peripheral 1
speed of 27 msusing an ejection pressure of 100 KPa. Under these conditions, ribbons of about 3 mm width and up to 50 Aim in thickness were produced. Subsequently some of the ribbons were comminuted to powder followed by die compaction and vacuum sintering. Samples for microstructural examination were etched in a mixture of 6 parts of HF, 3 parts of HNO 3 in water. The transformation temperatures of RS and sintered specimens were determined using a DuPont 1090 differential scanning calorimeter (DSC).
iesults and Discussion As Solidified Structures: Scanning electron micrographs of rapidly solidified ribbon in both the longitudinal axis and plane cross-section near the bottom surface are shown in Fig. 1. The structure consists predominantly of columnar grains extending from the wheel contact surface through the entire thickness for thin ribbons (]a), although equiaxed grains are found in thicker material near the top surface (3b). Fig Ic demonstrates the degree of grain refinement resulting from melt spinning compared with material made from elemental powders. The grain size ranges from 0.5 to 10 gm with a mean value of 4 A•m which is at least an order of magnitude smaller.
Mat. Res. Soc. Symp. Proc. Vol. 58. 1986 Materials Research Society
38
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