Rapidly solidified and optimally constraint-aged Ni51Ti49 shape memory alloy aiming at making a purpose-designed bio-actuator

Publication date: 15 March 2017
Source:Materials & Design, Volume 118
Author(s): Y.Y. Li, X.Y. Yao, S.S. Cao, X. Ma, C.B. Ke, X.P. Zhang
Ni51Ti49 alloy prepared by rapid solidification was trained through constraint-aging to possess two-way shape memory effect (TWSME), so as to make a purpose-designed bio-actuator. Influences of aging conditions (i.e., temperature, time and constraint stress) on the transformation behavior and shape memory effect of the alloy were investigated to obtain the desired functional property. Results show that the rapidly solidified Ni51Ti49 alloy strip followed by constraint-aging at 400°C for 100h exhibits an optimized TWSME. With a high recovery ratio of 92.9% and a pre-designed transformation temperature range of 35–55°C, the so-obtained Ni51Ti49 alloy has great potential to be used as the bio-actuator. Results of metallographic analysis demonstrate that the microstructure of the above treated Ni51Ti49 alloy strip consists of fine elongated grains with<100>B2 preferred orientation along the direction of the maximum cooling rate. Such fine grains with preferred orientation restrict the size of Ni4Ti3 precipitates to nano-scale during aging treatment under an optimized condition. Meanwhile, the constraint stress during the aging process leads to preferred orientation of Ni4Ti3 precipitates according to the stress states in different regions of the alloy. Such coherent and uniquely distributed Ni4Ti3 precipitates in the NiTi alloy contribute to significant improvement of TWSME.

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