Two-Way Memory Effect in NiTi Shape Memory Alloys

2008 ◽  
Vol 59 ◽  
pp. 77-85 ◽  
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.

Grain Nucleation and Growth in Deformed NiTi Shape Memory Alloys: An In Situ TEM Study

2017 ◽  
Vol 3 (4) ◽  
pp. 347-360 ◽  
Author(s):  
J. Burow ◽  
J. Frenzel ◽  
C. Somsen ◽  
E. Prokofiev ◽  
R. Valiev ◽  
...  
Keyword(s):  
Shape Memory Alloys ◽  
Shape Memory ◽  
Nucleation And Growth ◽  
In Situ Tem ◽  
Niti Shape Memory Alloys

Sintering and Morphology of Porous Structure in NiTi Shape Memory Alloys for Biomedical Applications

2012 ◽  
Vol 570 ◽  
pp. 87-95 ◽  
Author(s):  
Irfan Haider Abidi ◽  
Fazal Ahmad Khalid
Keyword(s):  
Elastic Modulus ◽  
Shape Memory Alloys ◽  
Porous Structure ◽  
Shape Memory ◽  
Shape Memory Effect ◽  
Memory Effect ◽  
Biomedical Implants ◽  
Recoverable Strain ◽  
Porous Niti ◽  
Niti Shape Memory Alloys

The combination of attractive properties of porous NiTi shape memory alloys like high recoverable strain due to superelasticity and shape memory effect, good corrosion resistance, improved biocompatibilty, low density and stiffness along with its porous structure similar to that of bone make them best materials for biomedical implants. In current study porous NiTi SMAs have been fabricated successfully by space holder technique via pressureless sintering using NaCl powder as a spacer. Various volume fractions of NaCl powders have been involved to study their effect on the pore characteristics as well as on mechanical properties of foam. Porous NiTi with average porosity in the range of 44.3%-63.5% have been fabricated having average pore size 419µm which were very appropriate for various biomedical implants. Porous NiTi SMAs exhibited superelasticity at room temperature and shape memory effect was also determined. Maximum recoverable strain of 6.79% was demonstrated by the porous NiTi alloy with 44.3% porosity and it was diminishing with increasing porosity. Compression strength and elastic modulus have shown a decreasing trend with increasing porosity content. Elastic modulus of porous NiTi extends from 1.38 to 5.42GPa depending upon the pore volume which was very much comparable to that of various kinds of bones.

Understanding indentation-induced two-way shape memory effect

2007 ◽  
Vol 22 (10) ◽  
pp. 2851-2855 ◽  
Author(s):  
Yijun Zhang ◽  
Yang-Tse Cheng ◽  
David S. Grummon
Keyword(s):  
Experimental Data ◽  
Shape Memory ◽  
Shape Memory Effect ◽  
Memory Effect ◽  
Contact Radius ◽  
Element Analysis ◽  
Austenitic Phase ◽  
Heating And Cooling ◽  
Surface Protrusion ◽  
Niti Shape Memory Alloys

Spherical indents in NiTi shape memory alloys can have reversible depth change: deeper depth in the martensitic phase at low temperature and shallower depth in the austenitic phase at high temperature. This is the indentation-induced two-way shape memory effect. After polishing the indents, two-way reversible surface protrusions can occur on the shape memory alloy surfaces upon heating and cooling. The height of the surface protrusion is about the same as the depth of the reversible indent. Further polishing reduces the height of the surface protrusion, which disappears completely when the polished depth is about the length of the contact radius. By comparing finite element analysis and experimental data, we show that the depth at which a protrusion disappears is close to the 10% strain boundary. This suggests that slip-plasticity is responsible for the observed indentation-induced two-way shape memory effect.

Indentation-induced two-way shape memory surfaces

2009 ◽  
Vol 24 (3) ◽  
pp. 823-830 ◽  
Author(s):  
Xueling Fei ◽  
Yijun Zhang ◽  
David S. Grummon ◽  
Yang-Tse Cheng
Keyword(s):  
Shape Memory Alloys ◽  
Shape Memory ◽  
Shape Memory Effect ◽  
Memory Effect ◽  
Deformation Zone ◽  
Plastic Strains ◽  
Spherical Case ◽  
Indent Depth ◽  
Niti Shape Memory Alloys ◽  
Made In

A method is described for the creation of surfaces with cyclically reversible topographical form. Using spherical and cylindrical indenters applied to NiTi shape-memory alloys, an indentation-planarization technique is shown to result in a two-way shape memory effect that can drive flat-to-wavy surface transitions on changing temperature. First, it is shown that deep spherical indents, made in martensitic NiTi, exhibit pronounced two-way cyclic depth changes. After planarization, these two-way cyclic depth changes are converted to reversible surface protrusions, or “exdents.” Both indent depth changes and cyclic exdent amplitudes can be related to the existence of a subsurface deformation zone in which indentation has resulted in plastic strains beyond that which can be accomplished by martensite detwinning reactions. Cylindrical indentation leads to two-way displacements that are about twice as large as that for the spherical case. This is shown to be due to the larger deformation zone under cylindrical indents, as measured by incremental grinding experiments.

Sign in / Sign up

Export Citation Format

Share Document