Lijian Rong

Synthesis of porous Ni–Ti shape-memory alloys by self-propagating high-temperature synthesis: reaction mechanism and anisotropy in pore structure

Porous Ni-Ti shape-memory alloys (SMAs) have attracted a great deal of attention recently because they have a similar microstructure to human bone and have significant prospects in medical applications. In the present study, equiatomic porous Ni-Ti SMAs, especially those with an unusual kind of linear-aligned elongated pore structure, have been successfully prepared by self-propagating high-temperature synthesis (SHS) using elemental nickel and titanium powders. The porous Ni-Ti SMAs thus obtained have an open porous structure with about 60 vol.% porosity, and the channel size is about 400 mu m. The corresponding microstructural characteristics and the effect of preheating temperature on the microstructure have been investigated. It is found that the combustion temperature increases with increasing preheating temperature and results in melting of the NiTi compound above 450 degrees C. Moreover, the preheating temperature has been shown to have a significant effect on the microstructure of the SHS-synthesized porous Ni-Ti SMAs, and the mechanism of anisotropy in pole structure is attributed to the convective flows of Liquid and argon during combustion

A recent development in producing porous Ni-Ti shape memory alloys

Abstract Porous Ni–Ti shape memory alloys (SMAs) are promising candidates for biomedical materials. In the present study, a bulk porous Ni–Ti SMA with a banded structure of channels and 54 vol.% porosity has been successfully prepared by self-propagating high-temperature synthesis (SHS). The Ni–Ti SMA synthesized has an open porous structure, also its pores and channels with various size and shape are three-dimensionally interconnected. Compared to the conventional powder sintering method, the present method extends the porosity range of porous Ni–Ti SMAs. A schematic representation of SHS synthesized Ni–Ti SMAs has been suggested.

Stress-strain behavior of porous Ni-Ti shape memory intermetallics synthesized from powder sintering

The microstructure and stress-strain behavior of porous Ni-Ti intermetallics fabricated by powder sintering of Ni, Ti and/or TiH2 are investigated. The pores are small and well distributed in the present porous Ni-Ti alloys and the phase constituent and stress-strain behavior of porous Ni-Ti alloys are significantly influenced by the sintering conditions (sintering temperature and sintering time) and TiH2 additions. With increasing sintering temperature, the pseudoelasticity (PE) increases while the hysteretic width decreases. A second compression also leads to an increase of PE, elastic modulus and related deformation resistance. Further, the stress-strain behavior of porous Ni-Ti alloys is different in some way from that of bulk Ni-Ti alloys and other porous materials

Thermomechanical fatigue of shape memory alloys

As shape memory alloys (SMAs) gain popularity as high energy density actuators, one characteristic that becomes particularly important is the thermomechanical transformation fatigue life, in addition to maximum transformation strain and stability of actuation cycles. In this paper, a novel test frame design and testing protocol are discussed, for investigating the thermally activated transformation fatigue characteristics of SMAs under various applied loads for both complete and partial phase transformation. A Ni50Ti40Cu10 (at.%) SMA was chosen for this investigation and the effects of various heat treatments on the transformation temperatures and the transformation fatigue lives of actuators were studied. For selected heat treatments, the evolution of recoverable and irrecoverable strains up to failure under different applied stress levels was studied in detail. The influence of complete and partial transformation on the fatigue life is also presented. The irrecoverable strain accumulation as a function of the number of cycles to failure for different stress levels is presented by a relationship similar to the Manson–Coffin law for both partial and complete transformations.

The influence of porosity on corrosion characteristics of porous NiTi alloy in simulated body fluid

Porous NiTi shape memory alloy (SMA) is a novel biomedical candidate used for surgical implant application. Corrosion characteristics of porous NiTi SMAs with different porosity prepared from combustion synthesis were investigated in this paper by potentiodynamic polarization measurements in physiological and Hanks solutions at 37 °C, respectively, while solid NiTi SMA was chosen as a reference. The result indicated that the corrosion resistance of porous NiTi SMA decreased with the increasing porosity. Porous NiTi SMA was less corrosion resistant than the solid one.

Fabrication of cellular NiTi intermetallic compounds

Self-propagating high-temperature synthesis (SHS) has been successfully developed for the fabrication of cellular NiTi intermetallic compounds, which have an open cellular structure with about 60 vol% porosity and more than 95% open-porosity ratio. The SHS reactions lead to the formation of TiNi, Ti2Ni, Ni3Ti, and Ni4Ti3 intermetallics. The SHS process can be controlled by regulating the preheating temperature, which has effects on the phase amount and the shape as well as macrodistribution of pores in the products.

Effect of pores on corrosion characteristics of porous NiTi alloy in simulated body fluid

Porous NiTi shape memory alloy (SMA) is a novel biomaterial used for human surgical implant. The corrosion characteristics of porous NiTi SMAs prepared from combustion synthesis have been investigated by potentiodynamic polarization tests in physiological and Hanks solutions at 310 K. Meanwhile, solid NiTi SMA has been chosen as a control. The preliminary data indicate that pores have strong effects on corrosion characteristics of porous NiTi SMA. Porous NiTi SMA is less corrosion resistant than solid one

ELECTRIC RESISTANCE PHENOMENA IN POROUS Ni-Ti SHAPE-MEMORY ALLOYS PRODUCED BY SHS

chinese acad sci, inst met res, shenyang 110015, peoples r china. inst med mat & shape memory implants, tomsk 634034, russia.;li, by (reprint author), chinese acad sci, inst met res, shenyang 110015, peoples r china

The influence of addition of TiH2 in elemental powder sintering porous Ni–Ti alloys

Abstract TiH 2 powder is added as a pore-forming and active agent in synthesis of porous Ni–Ti alloys by using a conventional powder sintering technique. The influence of addition of TiH 2 on microstructure, memory characteristics and pseudoelasticity (PE) of porous Ni–Ti alloys is investigated. It is found that porous Ni–Ti alloys produced with or without addition of TiH 2 exhibit good memory effect and PE, and the addition of TiH 2 not only has a pronounced effect on the microstructure but also increases the shape memory effect (SME) and volume memory effect of porous Ni–Ti alloys. Further, the addition of TiH 2 has a significant influence on the elastic behavior and Young’s modulus of porous Ni–Ti alloys.

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 1 223 K-9 h. Compared with the biomedical criteria for choice of implanting materials, porous NiTi alloy is satisfying to a great degree.