Powder Metallurgical Processes for NiTi Shape Memory Alloys

2010 ◽  
Vol 636-637 ◽  
pp. 928-933
Author(s):  
Filipe Neves ◽  
Francisco Manuel Braz Fernandes ◽  
Isabel M. Martins ◽  
Jose Brito Correia ◽  
Manuela Oliveira ◽  
...  
Keyword(s):  
Phase Transformation ◽  
Shape Memory Alloys ◽  
Shape Memory ◽  
Martensitic Transformations ◽  
Reactive Extrusion ◽  
Powder Mixtures ◽  
Heat Treated ◽  
Niti Shape Memory Alloys ◽  
Metallurgical Processes ◽  
Niti Matrix

Two promising powder metallurgy (PM) processes were used for the fabrication of NiTi shape memory alloys (SMA): Mechanically Activated Reactive FOrging Synthesis (MARFOS) and Mechanically Activated Reactive Extrusion Synthesis (MARES). In these two processes, equimolar powder mixtures of elemental Ni and Ti are first mechanically activated and then forged/extruded at relatively low temperature. Afterwards, heat treatments are used to promote homogenization and to adjust the composition of the NiTi matrix. When MARFOS and MARES processes are compared some differences have been observed but only in relation to the extent of phase transformation and to the degree of densification. The crystallite size was less than 100 nm for all the phases which indicates nanostructured materials and multi-step martensitic transformations could be observed in heat treated materials.

Orientation relationship between TiC carbides and B2 phase in as-cast and heat-treated NiTi shape memory alloys

2006 ◽  
Vol 438-440 ◽  
pp. 879-882 ◽  
Author(s):  
Zhonghua Zhang ◽  
Jan Frenzel ◽  
Christoph Somsen ◽  
Josef Pesicka ◽  
Klaus Neuking ◽  
...  
Keyword(s):  
Shape Memory Alloys ◽  
Shape Memory ◽  
Orientation Relationship ◽  
Heat Treated ◽  
B2 Phase ◽  
Niti Shape Memory Alloys

On the effect of aging on martensitic transformations in Ni-rich NiTi shape memory alloys

2005 ◽  
Vol 14 (5) ◽  
pp. S186-S191 ◽  
Author(s):  
Gunther Eggeler ◽  
Jafar Khalil-Allafi ◽  
Susanne Gollerthan ◽  
Christoph Somsen ◽  
Wolfgang Schmahl ◽  
...  
Keyword(s):  
Shape Memory Alloys ◽  
Shape Memory ◽  
Martensitic Transformations ◽  
Niti Shape Memory Alloys

Martensitic Transformations and Functional Stability in Ultra-Fine Grained NiTi Shape Memory Alloys

2008 ◽  
Vol 584-586 ◽  
pp. 852-857 ◽  
Author(s):  
Juri Burow ◽  
Egor Prokofiev ◽  
Christoph Somsen ◽  
Jan Frenzel ◽  
Ruslan Valiev ◽  
...  
Keyword(s):  
Grain Size ◽  
Shape Memory Alloys ◽  
Shape Memory ◽  
Thermomechanical Processing ◽  
High Pressure Torsion ◽  
Martensitic Transformations ◽  
Coarse Grained ◽  
Functional Stability ◽  
Fine Grained ◽  
Niti Shape Memory Alloys

Martensitic transformations in NiTi shape memory alloys (SMAs) strongly depend on the microstructure. In the present work, we investigate how martensitic transformations are affected by various types of ultra-fine grained (UFG) microstructures resulting from various processing routes. NiTi SMAs with UFG microstructures were obtained by equal channel angular pressing, high pressure torsion, wire drawing and subsequent annealing treatments. The resulting material states were characterized by transmission electron microscopy and differential scanning calorimetry (DSC). The three thermomechanical processing routes yield microstructures which significantly differ in terms of grain size and related DSC chart features. While the initial coarse grained material shows a well defined one-step martensitic transformation on cooling, two-step transformations were found for all UFG material states. The functional stability of the various UFG microstructures was evaluated by thermal cycling. It was found that UFG NiTi alloys show a significantly higher stability. In the present work, we also provide preliminary results on the effect of grain size on the undercooling required to transform the material into B19’ and on the related heat of transformation.

Experimental and FEM Analysis of the Fracture Behavior in NiTi Shape Memory Alloys

2006 ◽  
Vol 324-325 ◽  
pp. 919-922 ◽  
Author(s):  
Xin Mei Wang ◽  
Zhu Feng Yue
Keyword(s):  
Fracture Toughness ◽  
Phase Transformation ◽  
Shape Memory Alloys ◽  
Shape Memory ◽  
Fracture Behavior ◽  
Micromechanical Model ◽  
Crack Tip ◽  
Fem Analysis ◽  
Transformation Behavior ◽  
Niti Shape Memory Alloys

In the present work, the fracture toughness of a NiTi pseudoelastic alloy has been obtained by experiments on CT specimens, which is KIC =39.38MPa·m1/2. Then the stress induced phase transformation behavior in front of the crack tip of the CT specimen is simulated by a micromechanical model considering the different elastic properties between martensite and austenite. The results show that the pre-crack promotes phase transformation at the crack tip. And the phase transformation is localised near the crack tip. It is also shown that phase transformation reduces the Mises stress around the crack tip.

The Effect of Chemical Composition on Microstructure and Transformation Behavior of NiTi Shape Memory Alloy Prepared by Vacuum Arc Melting

2008 ◽  
Vol 1 ◽  
pp. 91-97
Author(s):  
Mandana Bornapour ◽  
Y. Motemanni ◽  
Mahmoud Nili-Ahmadabadi ◽  
S. Raygan
Keyword(s):  
Phase Transformation ◽  
Shape Memory Alloys ◽  
Shape Memory ◽  
Niti Alloy ◽  
Martensite Phase ◽  
Vacuum Arc ◽  
Niti Shape Memory Alloy ◽  
Arc Melting ◽  
Vacuum Arc Melting ◽  
Niti Shape Memory Alloys

NiTi shape memory alloys are a group of materials which have a lot of applications especially in aerospace industries and medical equipments because of their excellent properties. Shape memory effect (SME), pseudo-elasticity (PE), high corrosion resistance and biocompatibility is special properties of these alloys which lead to their extensive applications. The superior behavior of NiTi alloy is due to thermoelastic martensitic phase transformation. In the present paper, two NiTi shape memory alloys were prepared by non-consumable vacuum arc melting technique in copper water cooled crucible. One of them had commercial elements and the other had high purity elements. Metallographic investigation, chemical analysis, XRD and DSC were carried out on two alloys. Metallographic observation and XRD shows that structure at ambient temperature consists of austenite phase besides Ti2Ni, Ni3Ti intermetallic compounds and martensite phase. Transformation investigation determines that the impurity such as iron in commercial alloy causes two stage phase transformation B2→R→B19′.

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