A Comparative Study on the Mechanical Behavior of Porous Titanium and NiTi Produced by a Space Holder Technique

NiTi and Ti porous specimens with appropriate pore characteristics for biomedical applications are produced by space holder method. Porosities of the specimens linearly increase from 14 to 65 and 42 to 70% for the Ti and NiTi specimens, respectively, with the urea space holder. Mechanical properties such as stiffness, fracture strain, and strength of the porous NiTi and Ti are adjustable with pore characteristics. The apparent elasticity modulus of NiTi specimens decrease from 3.5 to 0.73 GPa as porosity increases. Since the initial linear part of the stress-strain curve consists of elastic behavior, formation of stress-induced martensite, deformation and/or detwinning of martensite variants, and plastic deformation, the unloading slope of stress-strain curves is a better approximation for the elasticity modulus of the NiTi porous specimens as it is proved by an isotropic cubic cell model. The unloading slope of the NiTi specimen with 61% porosity is 3.1 GPa, while the apparent elasticity or loading slope is 0.85 GPa. In comparison to Ti, the high, recoverable strain of NiTi improves capability of it as a good candidate for bone replacement. Moreover, in contrast to Ti specimens, hysteresis loops are clearly observed in the stress-strain curves of NiTi specimens.     

Processing of NiTi Foams by Transient Liquid Phase Sintering

Porous NiTi was produced by sintering pre-alloyed NiTi powders (with small Ni addition to form Ni-rich composition) with NaCl powders which are removed to create 40-60 vol.% macropores which are open to the surface, blocky in shape, and 100-400 ??m in size. The microporosity present between the NiTi powders is infiltrated by an in situ created NiTi-Nb eutectic liquid which, after solidification, densifies the NiTi powders into dense struts. This processing technique allows for separate control of the macroporous structure, and the densification and composition of the NiTi struts.     

热爆反应合成多孔NiTi形状记忆合金的性能

利用热爆方法来制备了多孔NiTi形状记忆合金.研究了在不同热爆温度下制备出的样品与其机械性能之间的关系.结果表明:在1 223 K下热爆反应制备的NiTi合金,具有大的孔隙度,高开孔率和基本各向同性,同时表现出较好的超弹性.对断口分析发现,断裂为脆性断裂和韧性断裂的复合.这表明改善孔洞分布和形态,可以极大地提高多孔NiTi形状记忆合金的机械性能和超弹性.     

LOW DENSITY AND HIGH STRENGTH NiTi MEMORY ALLOYS WITH CONTROLLABLE PORE CHARATERISTICS AND THEIR SUPERELASTICITY

采用球形尿素颗粒预造孔工艺结合梯级粉末烧结方法制备出孔形规则、孔径大的轻质高 强多孔NiTi形状记忆合金. 通过控制造孔剂(尿素)的含量可有效调节多孔合金的孔隙特征, 获得孔径均匀、孔形圆整和孔隙率可控的样品, 其中孔径为296-732 um, 孔隙率为31%-61.6%; 尿素形状和尺寸对多孔合金的孔隙特征有决定性影响, 具 有几何形态遗传性; 尿素对多孔NiTi合金的组成相影响很小, 相变仍具有马氏体相变特征; 合金具有优良的力学性能和稳定的线性超弹性.     

Generation of NiTi Nanoparticles by Femtosecond Laser Ablation in Liquid

NiTi was investigated as a model system for a binary alloy where the properties strongly depend on the relative proportion of the two elements and on the grain size. The NiTi nanoparticles were generated by laser ablation in water. For the analysis of the particle size distribution, we used transmission electron microscopy and dynamic light scattering. Here, we found a broad particle size distribution (10-200 nm). Furthermore, the temperature-resolved x-ray powder diffraction and differential scanning calorimetry (DSC) were used to evaluate the phase transition behavior of the generated NiTi nanoparticles. Here, we found an interesting effect. During the heating by DSC, an austenite phase transition and a weak martensite phase transition in the NiTi nanoparticles appeared. Moreover, the phase transformation temperature was about 40 K lower than that of the bulk target.     

Porous NiTi fabricated by self-propagating high-temperature synthesis of elemental powders

Porous NiTi shape memory alloy has been an active area of research for medical application due to their similar microstructure to human bone. In this study, porous product was obtained by self-propagating high-temperature synthesis (SHS) of compacts prepared from elemental nickel and titanium powders and the effects of the green porosity and preheating temperature were examined. The porosity in the synthesised products was in the range of 45–59 vol.%, with the green porosity being the primary source of final porosity. NiTi was present as the dominant phase in the porous product with other secondary intermetallic compounds and element powders. The effect of green porosity and preheating temperature was found to have an effect on the pore morphology and microstructure.     

液氮球磨制备纳米SiCp/Al基复合材料的研究

利用液氮球磨技术制备了纳米SiC颗粒增强铝基复合材料粉末,对该纳米粉末进行真空热压和热挤压,获得纳米铝基复合材料块体.采用扫描电子显微镜(SEM)、X射线衍射仪(XRD)研究了纳米SiC粉和Al-Zn-Mg-Cu粉在液氮球磨过程中形貌、组织和相组成等.结果表明,液氮球磨可以使复合材料粉末达到纳米级,且纳米SiC可均匀地分布于铝合金中.     

Enhanced corrosion resistance of porous NiTi with plasma sprayed alumina coating

In this study, corrosion behaviour of porous NiTi modified by plasma sprayed alumina coating has been investigated. Scanning electron microscopy and X-ray diffraction techniques were applied for the morphology and microstructure characterisation, while linear sweep voltammetry and electrochemical impedance spectroscopy were used for investigation of corrosion behaviour of coated and uncoated NiTi specimens. Induced couple plasma was conducted to measure ion release of the specimens in simulated body fluid at 37°C. The plasma sprayed Al₂O₃ coating on the porous NiTi improved the surface characteristics for biomedical applications. The alumina coating significantly hampered Ni ion release from the surface. In spite of slight decrease in corrosion resistance of the coated specimens, the corrosion mechanism changed from pitting to general corrosion. The breakdown phenomenon was not detected in the coated specimens, as well. Overall, it can be concluded the longevity of the coated specimen in the simulated biological system was enhanced, comparing to bare NiTi specimens.     

Highly porous open cellular TiAl-based intermetallics fabricated by thermal explosion with space holder process

High porosity TiAl-based intermetallics were prepared through thermal explosion (TE) from Ti–50Al at.% powders with NaCl as soluble template. The results showed that the space holder particles of NaCl were removed completely in green compacts, and porous Ti–Al materials were synthesized via a low-energy consumption method of TE at a temperature of 600 °C. TiAl was evolved as dominant phase in sintered materials at 1100 °C. With adding 80 vol.% NaCl to Ti–50Al at.% powders, the open porosity was significantly elevated up to 84%. Moreover, the porous materials exhibited a bimodal pore size distribution: large pores (200–500 μm) replicating NaCl particles and small pores (<50 μm) embedded in pore walls. The interconnected small and large pores make open cellular porous TiAl materials, which prescribe them promising for a wide range of applications in separation, heat insulation and catalysis.     

Influence of nanocrystalline Ni–Ti reaction agent on self-propagating high-temperature synthesized porous NiTi

Nanocrystalline Ni–Ti was used in self-propagating high-temperature synthesis (SHS) to fabricate porous NiTi. The SHS of porous NiTi using elemental powders was also prepared for comparison. Results showed that the main phase was NiTi with unreacted Ni when using elemental powders, which is detrimental to medical use. A large amount of Ti₂Ni secondary phase was also detected. By employing mechanically alloyed nanocrystalline Ni–Ti as a reaction agent, the secondary intermetallic phase (i.e. Ti₂Ni) was significantly reduced and the unreacted Ni was eliminated. The addition of 25 wt% nanocrystalline Ni–Ti reaction agent produced porous NiTi with an average porosity of 52–55 vol% and a general pore size of 100–600 μm under preheating temperatures of 200 and 300 °C. This general pore size in the range of 100–600 μm is beneficial to biomedical application for osseointegration. By further increase of the reaction agent to 50 wt% in the reactant, a porous NiTi part was produced at ambient temperature (i.e. no preheating was necessary) and a dense part was formed at preheated temperature of 200 °C due to the large amount of energies in the nanocrystalline reaction agent. This revealed that the use of nanocrystalline reaction agent effectively lowered the activation barriers for combustion synthesis reaction.     

纳米金属Ni粉的电爆炸法制备与表征

用电爆炸法制备纳米金属Ni、Al、Cu和Fe粉。以纳米金属Ni粉为研究对象,用X射线衍射分析仪(XRD)和能谱分析仪(EDS)分析其相组成和成分,用扫描电镜(SEM)和透射电镜(TEM)表征其形貌特征、晶体结构和粉体粒径,用差示扫描量热仪(DSC)表征其热稳定性和热焓值。结果表明:电爆炸法可以制备多种纳米金属粉体材料及熔点相差较大的合金材料;电爆炸法生产的金属粉体均为单质晶体,产品纯度高,且具有良好的热学性能;生产条件对样品的粒径和形貌影响较大,可以通过设定相应的制备参数来获得不同粒径和不同形貌的纳米金属粉体,如粒径分布在30～140nm范围内(平均粒径为80nm左右)的球状金属粉体,或粒径分布在30～50nm范围内(平均粒径为40nm左右)的不规则状金属粉体;电爆炸法生产能力大,产品质量高,可以实现工业化生产。     

颗粒尺寸对微波烧结多孔NiTi合金的显微结构及力学性能的影响

以不同颗粒尺寸的Ni/Ti粉末为原料,采用微波烧结技术制备了多孔NiTi合金,并系统考察了颗粒尺寸对多孔NiTi合金的显微结构和力学性能的影响。结果表明,随着颗粒尺寸的减小,多孔NiTi合金中的Ti2Ni和Ni3Ti第二相减少而单质Ni相消失。同时,多孔NiTi合金的孔隙形貌由带尖角的不规则形状向近球形转变。此外,多孔NiTi合金的孔隙率和孔径随着颗粒尺寸的增大而增大,而洛氏硬度、抗压强度和抗弯强度均下降。因此,减小颗粒尺寸有利于获得理想的显微结构(纯净的物相和均匀的孔隙结构)和提高微波烧结多孔NiTi合金的力学性能。     

Mechanical Properties of Highly Porous NiTi Alloys

Highly porous NiTi alloys with pseudoelastic properties are attractive candidates for biomedical implants, energy absorbers, or damping elements. Recently, a new method was developed for net-shape manufacturing of such alloys combining metal injection molding with the application of suitable space-holder materials. A comprehensive study of mechanical properties was conducted on samples with a porosity of 51% and a pore size in the range of 300-500 ??m. At low deformations <6%, fully pronounced pseudoelasticity was found. Even at higher strains, a shape recovery of maximum 6% took place, on which the onset of irreversible plastic deformation was superposed. Results of static compression tests were also used to calculate the energy-absorbing capacity. Fatigue of porous NiTi was investigated by cyclic loading up to 230,000 stress reversals. The failure mechanisms responsible for a reduction of shape recovery after an increased number of load cycles are discussed.     

Porous titanium implants fabricated by metal injection molding

Sodium chloride (NaCl) was added as a space holder in synthesis of porous titanium by using metal injection molding(MIM) method. The microstructure and mechanical properties of porous titanium were analyzed by mercury porosimeter, scanning electron microscope(SEM) and compression tester. The results show that the content of NaCl influences the porosity of porous titanium significantly. Porous titanium powders with porosity in the range of 42.4%–71.6% and pore size up to 300 μm were fabricated. The mechanical test shows that with increasing NaCl content, the compressive strength decreases from 316.6 to 17.5 MPa and the elastic modulus decreases from 3.03 to 0.28 GPa.     

Nonequiatomic NiTi Alloy Produced by Self Propagating High Temperature Synthesis

Shape memory alloy NiTi in porous form is of high interest as implantable material, as low apparent elastic modulus, comparable to that of bone, can be achieved. This condition, combined with proper pore size, allows good osteointegration. Porous NiTi can be produced by self propagating high temperature synthesis (SHS), starting from mixed powders of pure Ni and Ti. Process parameters, among which powder compaction degree and preheating temperature, strongly influence the reaction temperature and the resulting product: at low reaction temperatures, high quantity of secondary phases are formed, which are generally considered detrimental for biocompatibility. On the contrary, at higher reaction temperatures, the powders melt and crystallize in ingots. The porous structure is lost and huge pores are formed. Mechanical activation of powders through ball milling and addition of TiH _x are investigated as means to reduce reaction temperature and overheating, in order to preserve high porosity and limit secondary phases content. Both processes affect SHS reaction, and require adjustment of parameters such as heating rate. Changes in porous shape and size were observed especially for TiH _x additions: the latter could be a promising route to obtain shaped porous products of improved quality.     

Thermomechanical Properties of Porous NiTi Alloy Produced by SHS

Samples obtained from relatively large powders (<150 µm), with total porosity in the range 30-68%, were characterized mainly from a morphological point of view. Total porosity, as well as pore size, shape and distribution, was analyzed. Sample microstructure was also investigated, indicating that the main phase produced during the self-propagating high-temperature synthesis (SHS) reaction is Ti reach NiTi phase, as confirmed by calorimetric analyses. Moreover, the presence of secondary phases, suggested by the low transformation enthalpy, was confirmed by SEM observations. In fact, EDS microanalyses and EBSD mapping helped in the identification of such secondary phases, such as Ni₃Ti, Ti₂Ni and Ti₄Ni₂O _x . Other samples were successively produced starting from the same powders but introducing a different powder compression methodology and operating conditions. In this way, the obtained samples showed higher porosity featured by more uniform size, shape and distribution while, from a micro-structural point of view, no significant differences were observed. Mechanical compression tests were carried out at room temperature and, on selected samples, also above A _f in order to highlight the influence of pore shape and distribution. Results obtained at room temperature show that the mechanical properties decrease with the porosity augmentation. For higher temperatures, the samples presented a pseudoelastic behavior. Dilatometric tests were also performed and the results well indicated the martensite to austenite transformation at the same temperature showed by the DSC analyses. Thermal analysis was completed by evaluating the thermal diffusivity temperature and porosity dependence using an experimental-numerical approach especially developed.     

孔隙对NiTi形状记忆合金中B2-R相变影响的相场模拟

建立了适用于含孔隙NiTi合金中B2-R相变的相场模型,并用该相场模型研究了多孔NiTi合金中B2-R转变的微观组织演化过程以及孔隙率和孔尺寸对R相变体生长动力学行为的影响.多孔NiTi合金中R相变体以相互协调的方式形成"带状"的三维结构和"鱼骨"状的二维组织,变体之间形成的孪晶面包括{101}B2和{001}B2 2...     

Characterization of porous Ti-bioglass composite produced by mechanical milling and space holder sintering

In this study, porous titanium-10 wt% bioglass(BG) composites were fabricated by the process of combining mechanical alloying with space holder sintering. The pore morphology and phase constituents of the milled powders and porous compacts were characterized by scanning electron microscopy(SEM), X-ray diffractometry(XRD), and Fourier transform infrared spectroscopy(FT-IR). The mechanical properties were determined by running compression test. The porosity of the sintered samples shows a downward trend with the increase of milling time. As the porosity increases, both the compressive strength and elastic modulus decrease. The results illustrate that the fabricated porous compacts with high porosity and suitable mechanical properties have the potential application in bone tissue engineering.     

The mechanism of multistage martensitic transformations in aged Ni-rich NiTi shape memory alloys

Usually aged Ni-rich NiTi alloys undergo martensitic transformations on cooling from high temperatures in two steps: B2 to R and then R to B19′ (normal behaviour). But under certain ageing conditions, the transformation can also occur in three or more steps (unusual multiple step behaviour). In the present study we use differential scanning calorimetry (DSC) for a systematic investigation of the evolution of transformation behaviour with ageing temperature and time. We demonstrate that during ageing of Ni-rich NiTi alloys, DSC curves exhibit two transformation peaks on cooling after short ageing times, three after intermediate ageing times and finally again two peaks after long ageing times (2–3–2 transformation behaviour). In the present study we propose a new explanation for the 2–3–2 transformation behaviour which consists of two basic elements: (1) The composition inhomogeneity which evolves during ageing as Ni₄Ti₃ precipitates grow. (2) The difference between nucleation barriers for R-phase (small) and B19′ (large). These two elements explain all features of the evolution of DSC charts during ageing including the number of distinct DSC peaks and their positions.     

Formation of the Nanocrystalline Structure in an Equiatomic NiTi Shape-Memory Alloy by Thermomechanical Processing

The microstructural evolution during cold rolling followed by annealing of an equiatomic NiTi shape-memory alloy was investigated. The high purity Ni₅₀Ti₅₀ alloy was cast by a copper boat vacuum induction-melting technique. The as-cast ingots were then homogenized, hot rolled, and annealed to prepare the suitable initial microstructure. Thereafter, annealed specimens were cold rolled up to 70 % thickness reduction at room temperature. Post-deformation annealing was conducted at 400 °C for 1 h. The microstructure was characterized using scanning electron microscopy, transmission electron microscopy, x-ray diffraction, and differential scanning calorimetry techniques. The initial microstructure was free from segregation and Ti- or Ni-rich precipitates and was composed of coarse grains with an average size of 50 μm. The cold rolling of NiTi alloy resulted in a partial amorphization and the deformation-induced grain refinement. A nanocrystalline structure with the grain size of about 20-70 nm was formed during the post-deformation annealing.