Surface Ni sulfides layers were formed on the surface of a Ti-50.0Ni alloy by reacting sulfur and Ni film deposited on
the alloy, and then microstructures, transformation behavior, shape memory characteristics, superelasticity and
electrochemical properties of a Ti-50.0Ni(at%) alloy with the sulfides were investigated. When Ni film deposited on a
Ti-50.0Ni alloy was annealed under the sulfur pressure of 100 kPa at 623 K, sulfides layers consisted of NiS and NiS1.97
were formed. When annealing was made at 648 K annealing with annealing time less than 0.9 ks, sulfides layers
consisted of NiS and NiS1.97 were formed also, while only NiS1.97 was formed when it was made for 1.8 ks. When
annealing was made at 673 K annealing with annealing time longer than 0.9 ks, only NiS1.97 was formed. A Ti-
50.0Ni(at%) alloy with surface NiS1.97 layer showed the two-stage B2-R-B19' transformation behavior, the perfect shape
memory effect and a partial superelasticity with a superelastic recovery ratio of 78 %. NiS1.97 cathode showed a clear
discharge behavior with multi voltage plateaus. Discharge capacity of NiS1.97 cathode decreased abruptly with
increasing number of cycles up to 3, above which it decreased gradually.
Transformation behavior, shape memory characteristics and superelasticity of Ti-(40-x)Ni-10Cu(at%)(x=0~2) alloy
ribbons were investigated. All ribbons showed the two-stage B2-B19-B19' transformation behavior. The temperature
gap between Ms'(the B2-B19 transformation start temperature) and Ms(the B19-B19' transformation start temperature)
was large(~ 40 K), which was ascribed to internal stress fields around dislocations introduced by melt spinning. Ti2Ni
particles were observed in Ti-39Ni-10Cu and Ti-38Ni-10Cu alloy ribbons, while they were not found in a Ti-40Ni-10Cu
alloy ribbon. Volume fraction of Ti2Ni particles increased from 3 % to 12 % with increasing Ti-content from 51 at% to
52 at%. Both Ms and Ms' decreased with increasing Ti-content, which was ascribed to formations of Ti2Ni particles
resulting in a decrease in Ti-content of matrix. Ti-(40-x)Ni-10Cu alloy ribbons showed perfect shape memory effect
and superelasticity. The transformation hysteresis associated with the B2-B19 transformation was 6-10 K depending on
Ti-content.
Ti-Ni-Cu alloys have been known to be attractive shape memory alloys for applications as actuators because of large
transformation elongation and small transformation hysteresis. However, Cu addition exceeding 10at% makes the alloys
brittle and spoils the formability. Arc melt overflow enables Ti-Ni-Cu alloys with high Cu-content to be fabricated into
thin strips. Then, the shape memory alloy strips of Ti50Ni30Cu20 and Ti50Ni25Cu25 have been fabricated by arc melt
overflow technique. Their microstructures and shape memory characteristics were investigated by means of XRD,
optical microscopy and DSC. The microstructure of as-cast strips exhibited columnar grains normal to the strip surface.
X-ray diffraction analysis showed that the martensitic transformation of B2-B19 occurred in the alloy strips. During
thermal cyclic deformation with the applied stress of 60 MPa, transformation hysteresis and elongation associated with
the B2-B19 transformation were observed to be 4.9°C and 1.8% in Ti50Ni30Cu20 alloy strip and 3.5°C and 1.7% in
Ti50Ni25Cu25 alloy strip. The as-cast strip of Ti50Ni25Cu25 alloy also showed a perfect superelasticity and its stress
hysteresis was as small as 14 MPa. These mechanical properties and shape memory characteristics of the alloy strips
were ascribed to B2-B19 transformation and the controlled microstructures produced by rapid solidification of the arc
melt overflow process.
Transformation behavior, shape memory characteristrics and superelasticity of Ti-(45-x)Ni-5Cu-xV(at%)(x=0.5-2.0)
alloys were investigated. The B2-B19-B19' transformation occurs when V content was more than 1.0 at%, while the
B2-B19' transformation occurred when V content was 0.5 at% in solution treated specimens. However, the B2-B19 and
B19-B19' transformations were not separated clearly in the thermo-mechanically treated specimens. A temperature
gap between Ms'(the B2-B19 transformation start temperature) and Ms(the B19-B19' transformation start temperature)
increased from 8 K to 22 K with increasing V content from 1.0at % to 2.0 at% in solution treated specimens. One-stage
elongation associated with the B2-B19' transformation occurred in a Ti-44.5Ni-5.0Cu-0.5V alloy, while two-stage
elongation associated with the B2-B19-B19' transformation occurred in a Ti-43.0Ni-5.0Cu-2.0V alloy. The
superelastic recovery ratio increased from 65.0 % to 86.1 % with increasing V content from 0.5 % to 2.0 % in solution
treated specimens. The thermo-mechanically treated specimens showed the superelastic recovery ratio more than 95 %.
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