Study of the Ni 41.3 Ti 38.7 Nb 20 wide transformation hysteresis shape-memory alloy
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6/30/04
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Study of the Ni41.3Ti38.7Nb20 Wide Transformation Hysteresis Shape-Memory Alloy XIANG-MING HE, LI-JIAN RONG, DE-SHENG YAN, ZHI-MIN JIANG, and YI-YI LI The shape-memory characteristics in the Ni41.3Ti38.7Nb20 alloy have been investigated by means of cryogenic tensile tests and differential scanning calorimetry measurement. The martensite start temperature Ms could be adjusted to around the liquid nitrogen temperature by controlling the cooling condition. The reverse transformation start temperature As rose to about 70 °C after the specimens were deformed to 16 pct at different temperatures, where the initial states of the specimens were pure austenite phase, martensite phase, or duplex phase. The shape-memory effect and the reverse transformation temperatures were studied on the specimens deformed at (Ms 30 °C). It was found that once the specimens deformed to 16 pct, a transformation hysteresis width around 200 °C could be attained and the shape recovery ratio could remain at about 50 pct. The Ni41.3Ti38.7Nb20 alloy is a promising candidate for the cryogenic engineering applications around the liquid nitrogen temperature. The experimental results also indicated that the transformation temperature interval of the stress-induced martensite is smaller by about one order of magnitude than that of the thermal-induced martensite.
I. INTRODUCTION
II. EXPERIMENTAL PROCEDURE
THE NiTi-based alloys are known to be the most important shape-memory alloys (SMAs) because of their various applications on a commercial basis, based on their good mechanical properties and remarkable shape-memory effect.[1–4] It is important to control the transformation temperatures to meet the needs of any kinds of applications of SMAs. For Ni-Ti binary alloys, the martensitic transformation temperatures are strongly dependent of the composition (especially the addition of a third element)[5–8] and heat treatment.[9,10,11] As a ternary element, Nb has recently been found to be quite attractive. The wide hysteresis Ni-Ti-Nb SMAs are developed to avoid storing and installing of parts at a cryogenic temperature, and they have been widely used as couplings in aircraft. However, most of the work was done only for the typical alloy with Ni47Ti44Nb9 (in at. pct) composition.[12,13] The martensitic starting temperature Ms of the Ni47Ti44Nb9 alloy could be adjusted in the range between 110 °C and 50 °C by variation of heat treatment.[14] So, this alloy is not suitable for cryogenic engineering applications around the liquid nitrogen temperature. It has found that an increase in the Nb content will result in an obvious decrease of the Ms temperature.[15] Therefore, it is expected to develop a Ni-Ti-Nb SMA with much lower transformation temperatures by further addition of Nb. The purpose of the present study is to investigate the shape-memory characteristics of the high-Nb-content Ni41.3Ti38.7Nb20 alloy, which is the candidate with the potentiality of low-temperature SMA, such as transformation temperatures, transformation
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