KEYWORDS: Shape memory alloys, Temperature metrology, Photomicroscopy, X-ray diffraction, Titanium, Materials science, Life sciences, Medical devices, Smart structures, System integration
Ni-free shape memory alloys are promising functional materials for medical applications. A newly developed
Ti-Mo based shape memory alloy shows superelasticity after thermomechanical treatment. However, the microstructure
evolution and precipitation during thermomechanical processes are still not well understood. In the present paper,
compressive deformation behavior at a series of temperatures of 298K - 973K and tensile deformation behavior of the
alloy after aged at 573K - 973K have been investigated systematically. It is found that the compressive yield stress and
ultimate compressive strength change with the deformation temperature. The ultimate tensile strength and yield stress of
aged specimens also change with the aging temperature following a non-linear relationship. Microstructures of aged
specimens as well as effects of lattice softening and aging-induced precipitates on the deformation behavior have been
investigated and discussed.
Polymer composites inserted with high volume fraction (up to 70 Vol%) of NiMnGa powders were fabricated and their
damping behavior was investigated by dynamic mechanical analysis. It is found that the polymer matrix has little
influence on the transformation temperatures of NiMnGa powders. A damping peak appears for NiMnGa/epoxy resin
(EP) composites accompanying with the martensitic transformation or reverse martensitic transformation of NiMnGa
powders during cooling or heating. The damping capacity for NiMnGa/EP composites increases linearly with the
increase of volume fraction of NiMnGa powders and, decreases dramatically as the test frequency increases. The fracture
strain of NiMnGa/EP composites decrease with the increase of NiMnGa powders.
The effects of aging and thermo-mechanical training on the two-way shape memory effect (TWSME) of a NiAl-Fe alloy have been investigated. It was found that the two-way shape memory property of NiAl-Fe alloy was increased obviously after training. The two-way shape recovery of NiAl-Fe specimens aged at 400 degree(s)C for 1 hour was higher than that of specimens without aging, and kept unchanged up to 1x103 heating-cooling cycles. The fatigue life of NiAl- Fe specimens aged at 400 degree(s)C for 1 hour showing TWSME was more than 1x103 cycles. The mechanisms for the effects of thermo-mechanical training, as well as aging precipitates on the TWSME have also been discussed.
In off stoichiometric Ti50+xPd30Ni20+x alloys, it has been confirmed that B2-B19 phase transformations take place as temperature changes, and found that the phase transformation temperatures decrease with the increase of Ti content deviation from 50 at. %. Shape memory effect (SME) was determined at room temperature and at temperature higher than the austenite finish temperature, respectively. The room temperature SME is evaluated as a total strain of 7.2% with a recovery rate of 100%, and weakens with the increase of tensile strain. SME at high temperature indicates that stress-induced martensite remains in the alloys after unloading. Complete linear superelasticity has been observed by training the specimen under loading-unloading cycling. The shapes of superelastic cycles for specimens of different phases are different, and superelastic strain can be changed according to the load level of training. The yield stress for specimen in austenitic state is much higher than that in martensitic state, and the two kinds of specimens show different strain-hardening ability. The fracture surface shows overall characteristics of brittle failure for specimens both in austenitic and martensitic state. The microstructures of the specimens were also investigated. It is revealed that thermomechanical treatment and training process are necessary steps for preparation the superelastic alloy.
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