Deformation Behavior, Fatigue and Fracture Surface Microstructure of Porous Titanium Nickelid

Background: The porous SHS–TiNi alloy is a widely used material for repairing defects in bone tissues. Objective: The objective of the study is to comprehensively investigate porous SHS–TiNi alloy samples for fatigue strength under cyclic bending, to study deformation characteristics under quasistatic tension and bending, and to carry out the fractographic analysis of fracture features. Method: The study employed the electrospark method for cutting plates from a porous isotropic SHS–TiNi rod 30 mm in diameter and 300 mm in length. Results: Deformation behaviour under tension and three-point bending of porous plates showed that porous samples undergo viscoelastic deformation due to the austenite–martensite (A→M) phase transformation. The fracture surfaces of elastic porous samples were studied by SEM. Microscopic studies of fracture surfaces revealed zones of quasi-brittle fracture of martensite and viscous fracture of austenite. The porous framework of intermetallic alloy exhibits a continuous brittle layer and numerous brittle non-metallic inclusions. However, successful fatigue tests showed that brittle phases and inclusions do not significantly affect deformation and fatigue characteristics of porous titanium nickelide. It was found that 70% of porous samples sustain 106 cycles of deformation without fracture due to reversible A→M→A phase transformations in the TiNi phase, which is one of the components of multiphase porous alloy. Conclusion: Viscoelastic behavior of the porous sample and its high fatigue strength under cyclic loading is due to reversible deformation of the TiNi phase. The corrosion-resistant layer of the porous framework allows an effective use of SHS–TiNi.

Effect of Heat Treatment Temperature on Martensitic Transformation and Superelasticity of the Ti49Ni51 Shape Memory Alloy

The martensitic transformation and superelasticity of Ti49Ni51 shape memory alloy heat-treatment at different temperatures were investigated. The experimental results show that the microstructures of as-cast and heat-treated (723 K) Ni-rich Ti49Ni51 samples prepared by rapidly-solidified technology are composed of B2 TiNi phase, and Ti3Ni4 and Ti2Ni phases; the microstructures of heat-treated Ti49Ni51 samples at 773 and 823 K are composed of B2 TiNi phase, and of B2 TiNi and Ti2Ni phases, respectively. The martensitic transformation of as-cast Ti49Ni51 alloy is three-stage, A→R→M1 and R→M2 transformation during cooling, and two-stage, M→R→A transformation during heating. The transformations of the heat-treated Ti49Ni51 samples at 723 and 823 K are the A↔R↔M/A↔M transformation during cooling/heating, respectively. For the heat-treated alloy at 773 K, the transformations are the A→R/M→R→A during cooling/heating, respectively. For the heat-treated alloy at 773 K, only a small thermal hysteresis is suitable for sensor devices. The stable σmax values of 723 and 773 K heat-treated samples with a large Wd value exhibit high safety in application. The 773 and 823 K heat-treated samples have large stable strain–energy densities, and are a good superelastic alloy. The experimental data obtained provide a valuable reference for the industrial application of rapidly-solidified casting and heat-treated Ti49Ni51 alloy.

Влияние легирующих добавок кобальта и молибдена на структуру и параметры памяти формы пористого реакционно-спеченного никелида титана

We have studied the structure and properties of porous nickel titanium (TiNi) alloys obtained upon reaction sintering of Ti and Ni powders with Co and Mo additives. It is established that Co and Mo doping additives retain the compaction of Ni powder achieved at the initial stage of sintering. The maximum deformation of porous samples loaded in the austenite state was observed upon adding Co, while the addition of Mo resulted in minimum deformation. The addition of Co leads to single-stage martensitic transformation in TiNi phase, while the addition of Mo leads to the two-stage transformation that is more homogeneous over the volume. Both Co and Mo additives lead to increase in the maximum accumulated strain due to the formation of favorably oriented stress-induced martensite and reoriented quench-induced martensite.

Effect of sintering methods and cobalt addition on the shape memory properties of porous TiNi-based alloy

The changes of shape memory characteristics and properties of the porous sintered TiNi-based alloy are possible by a choice of the sintering methods or use of cobalt doping additive, as the present investigation has showed. The comparative analysis of the temperature dependences of electric resistance and macrodeformation both alloys, obtained by reaction and diffusion sintering was conducted. Diffusion-sintered alloy have showed high shape memory parameters and a more uniform passing of martensitic transformations. This is connected with a larger fraction of TiNi phase (about 90 vol.%) after diffusion sintering. It was found that the martensitic transformation characteristics and reversibility of martensitic strain in the porous nickelid titanium depend on level of intrinsic stresses in the TiNi phase and stresses caused by Co impurity. The addition to 1.0 at.% Co decreases the internal stresses in the TiNi phase, and more than 1.0 at.% Co increases their due to the effect of precipitation hardening of the alloy.

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.

Influence of Processing State on Recovery Stress in NiTiNb Shape Memory Alloy

Self-made tension machine was used to measure the evolution of recovery stress under different processing state for NiTiNb alloy. Then, SEM was used to investigate the microstructure evolution. The results show that the range of the highest recovery stress for forged NiTiNb alloy is between 210-215MPa. Otherwise, the recovery stress level of the samples enduring cold drawing and hot rolling is basically same, which all belong to the rage of 210-220MPa. After forging, the firstly precipitated TiNi phase particles become fine, only 5-8μm. The Nb tablets in eutectic microstructure, which originally contributed between TiNi phase particles, appear spheroidization.

Hot Dynamic Densification of Materials by Utilizing Underwater Shock Wave

Hot dynamic densification method was developed by combining self-propagating high temperature synthesis (SHS) with explosively shock powder compaction technique. This method is extremely short time processing. The main purpose in this study is to perform from synthesis to densification of TiB2-TiN system high temperature ceramic composites and TiB2-TiNi-Cu system functionally graded materials (FGMs) in one step. In TiN-TiB2 ceramic composites, they showed up to 95% of relative density. It was appeared by TEM observations that both the two phases joined tightly each other. The FGMs also were produced by the same technique. They indicated no interlayer exfoliation and no macro cracks after thermal shock tests from 973 K to room temperature. It was shown that thermoelastic property of intermetallic TiNi phase as intermediate layer between ceramics and metal layers operated effectively.

Study on Preparation of Gradient Porous NiTi Alloys by Powder Metallurgy Method

Gradient porous NiTi alloys were fabricated by powder metallurgy method using NH4HCO3as space-holder. The effect of content and distribution of NH4HCO3on pore characteristic, phase composition and compressive properties was studied. The results showed the content of TiNi phase increased with the decrease of the content of NH4HCO3. When the distribution of NH4HCO3varying form 12wt%-12wt%-12wt% to 12wt%-6wt%-12wt% and 12wt%-0wt%-12wt%, the stress and elastic modulus of porous NiTi alloys increased from 228MPa to 321MP and 446MPa, from 4.8GPa to 5.6GPa and 6.8GPa, respectively. Compared with uniform porous materials, gradient porous NiTi alloy exhibited better superelasticity.

Dynamic Responses of TiNi Phase Transformation Cylindrical Shell under Radial Impact Load

In this paper, radial impact responses of TiNi phase transformation cylindrical shell were experimentally studied on two-directional constraint. The nominal load-displacement curve was gained by advanced SHPB; meanwhile, the deformation process of the cylindrical shell was recorded by the high speed camera (30000 fps). The effect of stress wave on the whole structure during load process was as follows: the load-displacement curve fluctuated fiercely and rose gradually in the beginning period, whose frequency was basically the same as that of the stress wave propagation in the Striker 2; After that, the load-displacement curve tended to flat, the shell structure became to be ellipse as the compression increases, and the whole unload process slowed down. Finally, the surface strain was obtained by processing the shell deformation image, and the dynamic effect of the material phase transition and phase transition hinge on the cylindrical shell was examined.

Study on Characteristic and Compressive Property of Porous NiTi Alloys Fabricated by Thermal Explosion Method

Porous NiTi shape memory alloys were fabricated by thermal explosion method using different Ti and Ni powder as initial materials. The effect of process parameters including heating rate, and particle size of Ti on pore characteristic and phase composition was analyzed. Microstructure, phase composition, and mechanical properties were studied by SEM, XRD, and compression test, respectively. The mechanism of thermal explosion reaction was studied. The results show higher heating rate and smaller Ti particle size result in higher porosity and bigger pores. The thermal explosion reaction starts with the melting of a eutectic between β-Ti(Ni) and Ti2Ni and the main phases of as-reacted products are TiNi phase which are the desired phases. NiTi2 and TiNi3 phases are also present in small amounts. The content of TiNi phase increases with increasing heating rate or decreasing Ti particle size. The compressive strength and Young’s modulus of compacts decrease with the increase of the porosity.