Role of Ag addition on microstructure,mechanical properties,corrosion behavior and biocompatibility of porous Ti-30 at%Ta shape memory alloys

In the present study,the thermal,mechanical,and biological properties of xAg/Ti-30Ta(x=0,0.41,0.82 and 2.48 at%)shape memory alloys(SMAs)were investigated.The study was conducted using optical and scanning electron microscopy(SEM),X-ray diffractometry(XRD),compression test,and shape memory testing.The xAg/Ti-Ta was made using a powder metallurgy technique and microwave-sintering process.The results revealed that the addition of Ag has a significant effect on the pore size and shape,whereas the smallest pore size of 11μm was found with the addition of 0.41 at%along with a relative density of 72%.The fracture stress and strain increased with the addition of Ag,reaching the minimum values around 0.41 at%Ag.Therefore,this composition showed the maximum stress and strain at fracture region.Moreover,0.82 Ag/Ti-Ta shows more excellent corrosion resistance and biocompatibility than other percentages,obtaining almost the same behaviour of the pure Ti and Ti-6Al-4V alloys,which can be recommended for their promising and potential response for biomaterial applications.

REVERSIBLE TRANSFORMATION BETWEEN MATRIX AND MARTENSITE IN Ni-Ti SHAPE MEMORY ALLOYS

The reversible transformation between matrix and martensite in Ni-Ti shape memory alloys has been dynamically observed under TEM.The orientation relation between martensite and austenite as well as the structural change near the transition temperature has been also studied with the help of HREM SADP.The results show that the orientation relation between martensite and austenite is[11]_A//[10]_M,[110]_A//[001]_M,(110)_A//(001)_M and the angle between(110)_A and(010)_M is about 6.5°.The crystal defects of martensite are found to be twin and stacking fault,and the twin plane as(100).

Constitutive behavior of Ni-Ti shape memory alloy under hot compression

Constitutive behavior of nickel-titanium shape memory alloy （Ni-Ti SMA） under hot deformation was investigated by means of the compression tests and the linear fitting method. Based on the true stres-strain curves of Ni-Ti SMA under compression at the strain rates of 0.001-1 s land at the temperatures ranging from 600 to 1 000 ℃, the constitutive equation of Ni-Ti SMA with respect to the Zener-Hollomon parameter was established according to the high stress level and the low stress level at various temperatures so as to more accurately describe the deformation behavior of Ni-Ti SMA during hot working. Dynamic recovery and dynamic recrystallization of Ni-Ti SMA occur under hot compression, which lays the theoretical foundation for understanding the constitutive behavior of Ni-Ti SMA.

Modeling of Superelastic–plastic Behavior of Porous Shape Memory Alloys Incorporating Void Shape Effects

A new constitutive model for describing the superelastic–plastic behavior of porous shape memory alloys(SMAs)is proposed.The model incorporates the influences of void shape and hydrostatic pressure as well as the elastic modulus mismatch between austenite and martensite.In addition,the interactions between plastic strain and transformation strain are considered via the plastic back stress.The porous SMAs are considered as two-phase composites with the dense SMA matrix and the second phase representing ellipsoidal voids.Based on Gurson’s formulation,the transformation and plastic flow potentials accounting for the transformation–plasticity coupling are developed.The numerical results present good agreement with available experimental data for various levels of porosity,which proves that the model is capable of capturing stress-induced phase transformation and plastic deformation of porous SMAs.Using the proposed model,the influence of plastic strain on reverse transformation and the effects of porosity and void shape on the pseudoelastic and plastic behavior of porous SMAs are investigated.

Influence of Ti Powder Characteristics on Combustion Synthesis of Porous NiTi Alloy

Porous NiTi shape memory alloy (SMA) is a novel biomedical material used for human hard tissue implant. The influence of elemental titanium powder characteristics such as powder morphology, particle size and specific surface area (SSA) on the minimal ignition temperature, combustion temperature and final product of porous NiTi SMA fabricated by combustion synthesis method was investigated in this paper by scanning electron microscopy (SEM) and laser diffraction. The preliminary data indicated that the titanium powder characteristics had a strong effect on combustion synthesis of porous NiTi SMA.

A MODEL CONSIDERING HYDROSTATIC STRESS OF POROUS NITI SHAPE MEMORY ALLOY

Based on the micromechanical method and thermodynamic theory,a constitutive model for the macroscopic mechanical behavior of porous NiTi shape memory alloy is presented.The hydrostatic stress is considered for porous NiTi according to the transformation function of dense NiTi.The present model takes account of the tensile-compressive asymmetry of NiTi,and can degenerate to model dense material.Numerical calculations,which only need material parameters of dense NiTi,are conducted to investigate the nonlinear and hysteretic strain of porous NiTi,and the predicted results are in good agreement with the corresponding experiments.

Traces of Defects in the Electronic Structure of Porous Ni—Ti Alloys

The electronic structures of Ni-Ti shape-memory alloy samples were investigated by X-ray absorption fine structure （XAFS） spectroscopy both experimentally and theoretically. In the experimental section, the samples were measured at low temperature to determine the persistent traces of both preheating process and atomic concentration effects on the crystal and electronic structure by X-ray absorption near-edge structure （XANES） spectroscopy. As a second step, the extended-X-ray absorption fine structure （EXAFS） calculations, which are based on different choices of one electron potentials according to Ti coordinations by using the real space multiple scattering method FEFF 8.2 code, were performed. The crystallographic and electronic structures of the porous Ni-Ti alloys were tested at various temperatures ranging from 5 to 1323 K.

Martensitic Characterization of the Ti_45.3Ni_54.7Melt Spun Alloy

The ribbons of the Ti45.3Ni54.7 shape memory alloy were prepared through the melt spinning technique. The study was focused on investigating the effect of the rapid solidification and grain size at characteristic start martensitic (Ms), final martensitic (Mf), start austenite (As) and final austenite (Af) transformation temperatures. Changes on martensitic transformation temperatures in Ti45Ni55 melt spun ribbons were observed as grain size is reduced. Results of optical microscopy and differential scanning calorimetry (DSC) were used to associate grain size with transformation temperatures.

Compression Behavior and Failure Mechanisms of Bionic Porous NiTi Structures Built via Selective Laser Melting

Inspired by the crystal microstructure of metals and the bamboo,the bionic porous NiTi structures with the porosities in the range of 75.8%–84.9%were built via selective laser melting(SLM).The compression behavior and the failure mechanisms of the porous NiTi structures were evaluated.It demonstrated an increase in the elastic modulus and ultimate strength when the porosity was decreased,from 3.06 to 7.66 GPa and from 34.1 to 147.6 MPa,respectively.The relationship between the elastic modulus and the porosity obtained by the finite element analysis exhibited similar tendency with the experiment,and agreed well with the Gibson-Ashby model’s prediction.Based on the theoretical model above and the observation of the deformation processing,the plastic deformation behavior and failure mechanisms of the SLMed porous NiTi structures were analyzed.

Microstructure and superelasticity of porous NiTi alloy

The microstructure, porosity, phase composition and superelasticity (SE) in porous NiTi alloys produced by elemental powder sintering are examined by SEM, image analysis and XRD. It is found that it is feasible to produce porous NiTi alloy by elemental powder sintering, and the porosity of sintered porous NiTi alloy is in the range of 36.0 %-41.5 %. The pores are interconnected and the microstructure is sponge-like. Meanwhile, porous NiTi alloy has good SE. XRD patterns show that there is no pure Ni in alloy sintered at 1223 K-9 h. Compared with the biomedical criteria for choice of implanting materials, porous NiTi alloy is satisfying to a great degree.

An Evaluation of the Machinability of Nitinol SMA Using Combined Process of EP and MR Polishing

Nitinol,a shape memory alloy(SMA),is manufactured from titanium and nickel.It is employed in various fields for use in devices such as micro sensors,ultra-precision devices and satellite wings.It is highly recommended as a material in medical stents for insertion into the body because it has excellent organic compatibility.However,because they are intended to be inserted into the human body,products such as medical stents require a high-quality surface.Because nitinol has more of the characteristics of titanium than of nickel,one of its drawbacks is that heat generated in nitinol during machining is not discharged smoothly,leading to areas of inner stress which occur when traditional machining methods are used.To overcome this difficulty,various non-traditional machining methods,including non-contact machining,have been investigated for use with nitinol.This study is focused on the application of an effective fabrication method that combines electrolytic polishing(EP)and MR polishing to improve the surface integrity of nitinol through a series of experiments.Surface roughness variations of the polished nitinol were investigated by changing the imposed polishing conditions of EP and MR polishing,respectively.Finally,the variations in the surface roughness were observed with polished previously performed via the EP process.