Thermal arrest memory effect

Dilatation characteristics were measured to investigate the thermal arrest memory effect inNi53.6Mn27.1Ga19.3andNi54.2Mn29.4Ga16.4alloys. Interruption of the martensite-austenite phase transformation is connected with the reduction of the sample length after thermal cycle. If a total phase transformation took place in the complete thermal cycle following the interruption, then the sample length would return to its original length. Analysis of these results has shown that the thermal arrest memory effect is a consequence of a stress-focusing effect and shape memory effect. The stress-focusing effect occurs when the phase transformation propagates radially in a cylindrical sample from the surface, inward to the center. Evolution and release of the thermoelastic deformations in both alloys during heating and cooling are analyzed.

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Two-Way Memory Effect in NiTi Shape Memory Alloys

Advances in Science and Technology ◽

10.4028/www.scientific.net/ast.59.77 ◽

2008 ◽

Vol 59 ◽

pp. 77-85 ◽

Cited By ~ 1

Author(s):

Kiyohide Wada ◽

Yong Liu

Keyword(s):

Shape Memory ◽

Internal Stress ◽

Memory Effect ◽

Back Stress ◽

Nucleation And Growth ◽

Reverse Transformation ◽

Thermal Arrest ◽

Accommodation Process ◽

Stress Formation ◽

Niti Shape Memory Alloys

In general, the development mechanisms of TWME have long been understood as the nucleation and growth of preferentially oriented martensite guided by the internal stress. This work extends the study by investigating the effects of martensite deformation, constrained stress and retained martensite via partial reverse transformation through thermal arrest during heating on the stress-assisted two-way memory effect (SATWME) and TWME. It was observed that the generation of maximum SATWME was caused by the development of optimum internal stress. The increase of internal stress was accompanied by the increase of martensitic strain resulting from constrained cooling. When the martensitic strain exceeded the initial pre-strain, it directly influenced on the magnitudes of SATWME and TWME. The accommodation process of stress-assisted and detwinned martensite variants as a result of partial reverse transformation caused the formation of internal forward and back stresses. TWME was promoted by the dominant internal forward stress formation, while the dominance of internal back stress decreased the TWME by decreasing the martensitic strain.

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New observations on the thermal arrest memory effect in Ni–Ti alloys

Scripta Materialia ◽

10.1016/j.scriptamat.2005.05.031 ◽

2005 ◽

Vol 53 (7) ◽

pp. 875-879 ◽

Cited By ~ 24

Author(s):

Madangopal Krishnan

Keyword(s):

Memory Effect ◽

Thermal Arrest ◽

Ti Alloys

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Use of 2½ D imaging in analysis of NiTi martensite

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100110192 ◽

1978 ◽

Vol 36 (1) ◽

pp. 610-611

Author(s):

G. M. Michal

Keyword(s):

Memory Effect ◽

Monoclinic Phase ◽

Reaction Path ◽

Structural Characteristics ◽

Salient Feature ◽

Martensite Phase ◽

Orientation Relationships ◽

Low Symmetry ◽

Hydride Phases ◽

Unusual Shape

Several TEM investigations have attempted to correlate the structural characteristics to the unusual shape memory effect in NiTi, the consensus being the essence of the memory effect is ostensible manifest in the structure of NiTi transforming martensitic- ally from a B2 ordered lattice to a low temperature monoclinic phase. Commensurate with the low symmetry of the martensite phase, many variants may form from the B2 lattice explaining the very complex transformed microstructure. The microstructure may also be complicated by the enhanced formation of oxide or hydride phases and precipitation of intermetallic compounds by electron beam exposure. Variants are typically found in selfaccommodation groups with members of a group internally twinned and the twins themselves are often observed to be internally twinned. Often the most salient feature of a group of variants is their close clustering around a given orientation. Analysis of such orientation relationships may be a key to determining the nature of the reaction path that gives the transformation its apparently perfect reversibility.

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Tensile Failure of 55-Nitinol Wire

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100113858 ◽

1975 ◽

Vol 33 ◽

pp. 46-47

Author(s):

F. I. Grace

Keyword(s):

Shape Memory ◽

Shape Memory Effect ◽

Memory Effect ◽

Stoichiometric Composition ◽

Tensile Failure ◽

Initial State ◽

Initial Shape ◽

Nitinol Wire ◽

Cscl Type ◽

Shear Transformation

An interest in NiTi alloys with near stoichiometric composition (55 NiTi) has intensified since they were found to exhibit a unique mechanical shape memory effect at the Naval Ordnance Laboratory some twelve years ago (thus refered to as NITINOL alloys). Since then, the microstructural mechanisms associated with the shape memory effect have been investigated and several interesting engineering applications have appeared.The shape memory effect implies that the alloy deformed from an initial shape will spontaneously return to that initial state upon heating. This behavior is reported to be related to a diffusionless shear transformation which takes place between similar but slightly different CsCl type structures.

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