Nanocrystalline strain glass TiNiPt and its superelastic behavior

TiNi-based shape-memory alloys are known to exhibit a strain glass state under certain conditions, generally in the presence of high-density defects such as excess solute atoms or alloying elements, dislocations, and nanoprecipitates. In this paper, we report a strain glass transition in a nanocrystalline ${\mathrm{Ti}}_{50}{\mathrm{Ni}}_{35}{\mathrm{Pt}}_{15}$ alloy. The nanocrystalline strain glass state is achieved by a combined effect of high-density grain boundaries and high concentration doping of Pt atoms in the B2 matrix. The nanocrystalline ${\mathrm{Ti}}_{50}{\mathrm{Ni}}_{35}{\mathrm{Pt}}_{15}$ strain glass alloy showed a large near-complete progressive superelasticity with a recovery strain of about 6% and a low apparent Young's modulus of about 30 GPa in a wide temperature range of over 200 °C. In situ synchrotron x-ray diffraction measurement showed that the strain glass B2 [B2(SG)] phase experienced B2(SG)→R→B19 transformation upon loading and B19→B2(SG) upon unloading. The findings of this study provide insight for the development of nanocrystalline strain glass shape-memory alloys.

Figure 1

Microstructure of the nanocrystalline ${\mathrm{Ti}}_{50}{\mathrm{Ni}}_{35}{\mathrm{Pt}}_{15}$ alloy wire after annealing at 480 °C for 10 min. (a) A hollow-cone dark-field image, revealing the nanocrystallinity of the sample. (b) Grain-size distribution as determined from the HCDF image.

Figure 2

The transformation behavior and structure evolution of the nanocrystalline ${\mathrm{Ti}}_{50}{\mathrm{Ni}}_{35}{\mathrm{Pt}}_{15}$ sample (a) DSC curves of the nanocrystalline ${\mathrm{Ti}}_{50}{\mathrm{Ni}}_{35}{\mathrm{Pt}}_{15}$ sample and a coarse-grained ${\mathrm{Ti}}_{50}{\mathrm{Ni}}_{35}{\mathrm{Pt}}_{15}$ sample annealed at 700 °C for 10 min. (b) Normalized (at 300 K) electrical resistivity-temperature curve of the nanocrystalline ${\mathrm{Ti}}_{50}{\mathrm{Ni}}_{35}{\mathrm{Pt}}_{15}$ alloy. (c) XRD patterns of the nanocrystalline ${\mathrm{Ti}}_{50}{\mathrm{Ni}}_{35}{\mathrm{Pt}}_{15}$ alloy at different temperatures.

Figure 3

Dynamic mechanical properties of the nanocrystalline ${\mathrm{Ti}}_{50}{\mathrm{Ni}}_{35}{\mathrm{Pt}}_{15}$. (a) Effect of temperature on storage modulus of the nanocrystalline ${\mathrm{Ti}}_{50}{\mathrm{Ni}}_{35}{\mathrm{Pt}}_{15}$ at different frequencies. (b) ZFC/FC curves of the nanocrystalline ${\mathrm{Ti}}_{50}{\mathrm{Ni}}_{35}{\mathrm{Pt}}_{15}$. The sample was cooled to −150 °C under zero applied force (denoted zero-field cooling, or ZFC). Then a tensile stress of 50 MPa was applied and the sample was heated to 300 °C under this stress, presented as the ZFC curve in the figure. Then the sample was cooled to −150 °C under the 50-MPa tensile stress, denoted the field cooling and then heated to 300 °C again under the same stress, presented as the FC curve in the figure. For each curve, the effect of thermal dilation of the sample has been removed by subtracting the strain-temperature curve measured upon the first cooling under zero stress.

Figure 4

Mechanical behavior of the nanocrystalline ${\mathrm{Ti}}_{50}{\mathrm{Ni}}_{35}{\mathrm{Pt}}_{15}$ alloy. (a) Tensile stress-strain curves at different temperatures. (b) Effect of testing temperature on the apparent Young modulus and elongation. (c) Temperature dependence of the critical stress for stress-induced transformation, (d) Progressive deformation curves at the room temperature and −60 °C.

Figure 5

In situ XRD analysis of the transformation behavior of the nanocrystalline ${\mathrm{Ti}}_{50}{\mathrm{Ni}}_{35}{\mathrm{Pt}}_{15}$ alloy during tensile deformation. (a) 1D XRD spectra integrated over ±5 ° azimuthal angle of the loading direction from the 2D XRD patterns measured by synchrotron high-energy XRD. (b), (c) Enlarged views of sections of the diffraction patterns identified by the boxes in (a).

Figure 6

The transformation behavior of the nanocrystalline and coarse-grained ${\mathrm{Ni}}_{51.5}{\mathrm{Ti}}_{48.5}$ alloys. (a) Normalized electrical resistivity curves of a nanocrystalline sample and a solid solution treated coarse-grained sample of ${\mathrm{Ni}}_{51.5}{\mathrm{Ti}}_{48.5}$. Dynamic mechanical properties of the nanocrystalline ${\mathrm{Ni}}_{51.5}{\mathrm{Ti}}_{48.5}$ (b) and the solid solution treated coarse-grained ${\mathrm{Ni}}_{51.5}{\mathrm{Ti}}_{48.5}$ (c).

Figure 7

Schematic illustrations of the unit-cell configurations in (a) coarse-grained Ni-rich TiNi alloy, (b) nanocrystalline Ni-rich TiNi alloy, (c) coarse-grained ${\mathrm{Ti}}_{50}{\mathrm{Ni}}_{35}{\mathrm{Pt}}_{15}$ alloy, and (d) nanocrystalline ${\mathrm{Ti}}_{50}{\mathrm{Ni}}_{35}{\mathrm{Pt}}_{15}$ alloy.