Research on the Hot Deformation Behavior of the Casting NiTi Alloy

The hot deformation behavior and processing maps of the casting NiTi alloy were studied at the deformation temperature of 650–1050 °C and the strain rate of 5 × 10−3–1 s−1 by Gleeble-3800 thermal simulating tester. The variation of the strain rate sensitivity exponent m and the activation energy Q under different deformation conditions (T = 650–1050 °C, ε˙ = 0.005–1 s−1) were obtained. The formability of the NiTi alloy was the best from 800 °C to 950 °C. The constitutive equation of the casting NiTi alloy was constructed by the Arrhenius model. The processing map of the casting NiTi alloy was plotted according to the dynamic material model (DMM) based on the Prasad instability criterion. The optimal processing areas were at 800–950 °C and 0.005–0.05 s−1. The microstructure of the casting NiTi alloy was analyzed by TEM, SEM and EBSD. The softening mechanisms of the casting NiTi alloy were mainly dynamic recrystallization of the Ti2Ni phase and the nucleation and growth of fine martensite.

Anodic Fabrication of Ti-Ni-Si-O Nanostructures on Ti10Ni5Si Alloy

Ti-Ni-Si-O nanostructures were synthesized on Ti10Ni5Si alloy through an electrochemical anodization in electrolyte solutions containing ammonium fluoride (NH4F). The anodic oxide structures were affected by the electrochemical anodization parameters, including the electrolyte viscosity, water content, anodization potential and anodization time. Using an anodization potential of 40 V for 90 min in an ethylene glycol/glycerol electrolyte with 3 vol.% deionized water, highly ordered self-organized nanotube arrays were obtained in the α-Ti phase region of the alloy substrate, with an average inner diameter of 70 nm and a wall thickness of about 12 nm. Self-organized nanopore structures with an average pore diameter of 25 nm grew in the Ti5Si3 phase region. Only etching pits were found in the Ti2Ni phase region. The Ti-Ni-Si-O nanostructures were characterized using scanning electron microscopy and energy dispersive spectroscopy. In addition, a formation mechanism of different nanostructures was presented.

Distinctive Features of the Phase Composition of Porous TiNi-based Alloys Obtained by Reaction and Diffusion Sintering

The present article is concerned with questions of reaction and diffusion sintering of porous shape-memory TiNi-based alloys. The comparative analysis of structural features of the porous alloys obtained by diffusion sintering of TiNi powder and reaction sintering of Ti and Ni powders was conducted. It is observed that the main feature of structure of the porous alloys is related to fraction of the TiNi phase which occupies about 90 vol.% at diffusion sintering, and 20÷50 % of the total volume of multiphase alloy for reaction sintering. The mechanisms of the structure formation on the solid phase and liquid phase sintering stages of these methods were considered. The role of Ti2Ni phase during sintering was disclosed. The Ti2Ni phase not only provides the necessary quality of sintering, activates recrystallization processes for diffusion sintering, modifies the phase composition of the sintered specimen for reaction sintering, but also participates in the formation of the TiNi phase, increasing its fraction.

The Microstructure and Properties of Ti50Ni47Fe3 and Ti50Ni46.75Fe3Cr0.25 Shape Memory Alloy

Comparing with Ti50Ni47Fe3alloys, the influences of Cr on the mechanical and shape memory properties of Ti50Ni47Fe3alloys are investigated by study on phase transformation and microstructure analysis. The results show that Ti50Ni47Fe3and Ti50Ni46.75Fe3Cr0.25shape memory alloys exhibit two-stage martensitic transformation. The transformation temperatures decrease with the addition of Cr. The microstructure of the Ti50Ni47Fe3and Ti50Ni46.75Fe3Cr0.25alloys consists of TiNi matrix, Ti2Ni phase. Fe element prefers to substitute for Ni in the matrix than black particles. Cr all substitute for Ni in the matrix and not be analyzed in the Ti2Ni phase. The mechanical property of Ti50Ni46.75Fe3Cr0.25alloy is better than Ti50Ni47Fe3alloy.

Influence of Heating Temperature on Microstructure Characteristics of Directionally Solidified Ni-45Ti-5Al Alloy

The microstructure evolution of Ni-45Ti-5Al(atomic fraction, %) alloys prepared by directional solidification with liquid-metal-cooling at different temperatures of 1450°C, 1550°C and 1650°C was investigated. The results showed that the macrostructure in directional solidification growth region was obvious columnar grains. The microstructure was cellular growth structure, and composed of primary NiTi matrix and Ti2Ni precipitates. The preferred orientation of NiTi and Ti2Ni were respectively [100] and [111] direction. It was found that aluminum was dissolved in both NiTi and Ti2Ni phase. The Al content in the matrix was higher than it in the precipitates. As heating temperature increasing, the microstructure evolution was inconspicuous and the average cellular spacing was 30~50μm. The Ti2Ni volume percent was most when the heating temperature was 1550°C.

Hydrogen Storage Properties of Mg76Ti12Ni12-xCrX(x=0,3,6,9) Alloys by Mechanical Alloying

Mg76Ti12Ni12-xCrx(x=0,3,6,9) alloys were synthesized by mechanical alloying(MA) approach and hydrogen storage properties of the alloys were investigated by X-ray diffraction, thermal analysis and pressure-composition isotherm measurement. It is found that Ti2Ni phase and Mg2Ni phase exist as the main phases in Mg76Ti12Ni12-xCrx(x=0,3,6,9) alloys. The Mg76Ti12Ni12-xCrx(x=0,3,6,9) alloys exhibit the hydrogen storage capacity of 4.61,4.30,4.21 and 4.12wt%, and the decomposition enthalpies of the alloy hydrides are 928.4, 898.3, 831.2 and 851.4J/g H2, respectively. Mg76Ti12Ni6Cr6 alloy shows small hysteresis and fast hydrogen absorption rate. Proper Cr content can improve the performance of the Mg76Ti12Ni12-xCrx(x =0,3,6,9) alloys.