Ti-Ni alloys have been known to be very attractive for actuators because of their dual functional ability, i.e. sensing and operating in a single body. Especially, for applications to robotic devices, a linear relationship between the transformation strain and a physical quantity(usually temperature) together with a very small hysteresis is required. However, the transformation strain(ε) occurs steeply in a narrow temperature(T) range(usually within 3-5 K) in most of Ti-Ni based alloys, which makes difficult to control the position of actuators working according to changes in temperature. For an accurate position control responding to temperature changes, Ti-Ni alloys developing a gradual transformation strain(ε) over a wide temperature(T) range(i.e., low dε/dT) are desirable.
One method to obtain the gradual transformation strain over a wide temperature range is thought to let Ti-Ni alloy actuators have transformation temperatures varying continuously along a working direction. This can be accomplished by temperature gradient annealing method - annealing Ti-Ni alloys cold worked under a temperature gradient- because transformation temperatures of cold worked Ti-Ni alloys were known to depend largely on annealing temperatures.
An equiatomic Ti-Ni alloy was cold rolled by 25 %, and then annealed under a temperature gradient from 823 K to 466 K. Changes in TR (B2-R transformation start temperature) and Ms(R-B19’ transformation start temperature) by the temperature gradient annealing were found to be 17 K and 24 K. Stress-strain curves of specimens annealed under the temperature gradient did not show a stress-plateu suggesting a gradual elongation of specimens due to a gradient in transformation temperatures. Elongation vs. temperature curves showed very small dε/dT(less than 0.01 %/K) comparing to those obtained from specimens annealed at a specific temperature.

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