M. Kimata, K. Hasegawa, H. Kato, K. Sasaki, Hokkaido University, Sapporo, Japan
The present authors examined conventional electrolytic and/or chemical polishing method for thinning the wire of NiTi shape memory alloy. The objective is to prepare continuous shape memory fiber not only with fine diameter but also with robust mechanical properties; the diameter would be less than the diameter of carbon fibers used for CFRP, typically a few microns, and the wire should posses the mechanical strength and shape memory performance comparable to those of bulk alloys. Starting material was an as drawn wire, which was 0.7mm in diameter, supplied by Furukawa Technomaterial Co. Ltd. During thinning the diameter of wire was monitored by the overall electrical resistance. As a result, it was found that successful thinning could be reproducibly performed as long as the diameter was not less than about 10μm, which was almost equal to the average diameter of grains, 8μm in the alloy. As reducing it below the size, however, it became hard to keep the surface smooth and the diameter uniform along the length. Chemical polishing would form many pits or grooves with a few microns size, which make the surface wavy. Smooth surface would be obtained in electrolytic polishing; however, preferential polishing at a local position sometimes occurred to generate an isolated pinch (necking). Conclusively, the present method can prepare a short(discontinuos) fiber with diameter less than 1μm, but has a difficulty to control the length; it needs some modification for keeping the diameter uniform along the length to prepare continuous submicron and nano fibers.
Summary: Electrolytic and chemical polishing were applied to prepare continues fiber of NiTi alloy with submicron diameter. Starting material was the wire of 0.7mm dia. Uniform polishing could be successfully done as long as the diameter was not less than the grain size, about 8 micro meter in this study. When the thinning continued to reduce the diameter below the size, it became difficult to polish it uniformly. Despite the difficulty, submicron fibers with more than 15mm length has been prepared. Martensitic transformation and mechanical properties of the fibers were studied.
