NiTi Shape Memory Alloy Active Element Behavior in Long Time Solicitation Conditions

Equi-qtomic NiTi (nitinol) shape memory alloy (SMA) is a good potential candidate material for use as thermo-mechanical actuator in a large variety of engineering like automotive and aerospace applications. Shape memory alloy in action are required to perform a large number of actuation cycles under cyclic thermo-mechanical loads and therefor are subject of fatigue. A shape memory alloy, supplied from Nimesis Technology, with martensite to austenite temperature transformation domain 76-80 °C. The material characteristics were investigated through differential calorimetry (DSC) before and after the thermo-mechanical solicitations. Under Joule effect and a timer, the active element goes up to 3000 cycles with a 500g weight on. The properties of thermo-elastic martensite transformation are the elastic accommodation of volume and shape change that takes place due to change in crystal structure upon phase transformation. A modification of the first coil of the intelligent arch-wire suffer a modification of the temperature transformation domain increasing the As and Af temperature values.

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Experimental Analysis of NiTi Alloy during Strain-Controlled Low-Cycle Fatigue

Materials ◽

10.3390/ma14164455 ◽

2021 ◽

Vol 14 (16) ◽

pp. 4455

Author(s):

Pedro Cunha Lima ◽

Patrícia Freitas Rodrigues ◽

Ana Sofia Ramos ◽

José D. M. da Costa ◽

Francisco Manuel Braz Fernandes ◽

...

Keyword(s):

Shape Memory Alloy ◽

Shape Memory ◽

Low Cycle Fatigue ◽

Niti Alloy ◽

Niti Shape Memory Alloy ◽

Cycle Fatigue ◽

Maximum Deformation ◽

Transmission Electron ◽

Before And After

The interaction between the stress-induced martensitic transformation and resistivity behavior of superelastic NiTi shape memory alloy (SMA) was studied. Strain-controlled low-cycle fatigue up to 6% was monitored by in situ electrical resistivity measurements. The experimental results show that a great motion of martensite fronts results in a significant accumulation of defects, as evidenced by transmission electron microscopy (TEM), before and after the tensile cycles. This gives rise to an overall increase of the resistivity values up to the maximum deformation. Therefore, the research suggests that shape memory alloy wire has great potential as a stress sensor inside bulk materials.

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Effect of Hatch Angle Rotation on the Thermal and Mechanical Properties of Micro Selective Laser Melted NiTi Shape Memory Alloy

Volume 1: Additive Manufacturing; Advanced Materials Manufacturing; Biomanufacturing; Life Cycle Engineering; Manufacturing Equipment and Automation ◽

10.1115/msec2020-8235 ◽

2020 ◽

Author(s):

Jin Fu ◽

Zhiheng Hu ◽

Xu Song ◽

Mingwang Fu

Keyword(s):

Shape Memory Alloy ◽

Shape Memory ◽

Niti Shape Memory Alloy ◽

Secondary Phases ◽

Aerospace Applications ◽

Columnar Grains ◽

Angle Rotation ◽

Increasing Demand ◽

Equiaxed Grains ◽

Similar Phase

Abstract NiTi shape memory alloy (SMA) has been widely used for bio-medical and aerospace applications due to its unique properties, i.e. shape memory effect and pseudoelasticity. However, the high ductility and work-hardening effect of NiTi lead to poor machinability. Additive manufacturing (AM), with excellent capability of fabricating complicated structures, has been used to fabricate NiTi components. To meet the increasing demand of product miniaturization, micro selective laser melting (μSLM) system equipped with finer laser beam has been developed to improve manufacturing resolution. This work studies the fabrication of NiTi SMA parts by μSLM for the first time. The effect of hatch angle rotation on the thermal and mechanical behaviors of μSLMed NiTi is analyzed. Columnar grains accompanied with equiaxed grains are observed in μSLMed NiTi. Laser rotation angles of 45/60/90° lead to weak crystallographic texture. Ti-rich secondary phases including Ti2Ni/Ti4Ni2Ox and TiC1-xNx are detected in the raw NiTi powder and the as-printed NiTi parts, respectively. The as-printed parts under different hatch angles show similar phase constitution. The thermal-induced transformation behavior was depressed with absence of transformation peak. The variation of hatch angle cannot activate the transformation peak. Varying hatch angle from 45° to 90°, the compressive strength and ductility reduce, and the hardness increases. The depressed thermal-/stress-induced phase transformation of the μSLM NiTi can be attributed to the Ti-rich secondary phases, which cause variation of matrix Ni/Ti ratio and inhomogeneous microstructure.

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