Porous Metal Sheets of NiTi made by Wet Powder Spraying

As a promising method to produce thin porous NiTi sheets, Wet Powder Spraying (WPS) is applied for NiTi powders. Layers with a thickness of 150 μm are obtained from pre‐alloyed NiTi powders with a particle size of < 12 μm. Optimized process control for spraying and sintering was used. Microstructure and phase formation is characterized. The sheets with a porosity of 15 % show a high pseudoelastic flexibility at room temperature. Phase transformation temperatures are determined by Differential scanning calorimetry (DSC). The one way effect could be demonstrated.     

多孔NiTi形状记忆合金的制备及性能

采用球磨后的NiTi合金粉末为原料,添加尿素作为造孔剂,利用粉末烧结法制备多孔NiTi形状记忆合金.研究烧结温度、保温时间和预成型压力等条件对制备的多孔NiTi合金组织结构和力学性能的影响.结果表明:相对于传统的Ni粉和Ti粉近等原子比混合烧结方法,此方法制备的多孔NiTi合金的相组成更加纯净.且随烧结温度升高,多孔N...     

Preparation of nitinol by non-conventional powder metallurgy techniques

The aim of the present paper was to compare the evolution of Ni–Ti intermetallics in two non-conventional production techniques for the synthesis of NiTi shape memory alloy. Short term ultrahigh energy mechanical alloying is proposed to be able to describe the early stages of the milling process, which was not described in the literature previously, and to obtain intermetallics in shorter process durations. The reactive sintering using high heating rate (>300°C?min^??1) is a process designed to suppress the formation of secondary intermetallics and to reduce the porosity of the product. The same phases' formation sequence was determined for both processes. The detrimental Ti₂Ni phase forms preferentially, and therefore, its presence cannot be avoided in any of the investigated techniques.     

Characterization of porous, net-shaped NiTi alloy regarding its damping and energy-absorbing capacity

Porous NiTi alloys are highly attractive for energy absorbers, damping devices and biomedical implants. In the present work, metal injection moulding (MIM) in combination with the application of a suitable space holder material was used for the production of NiTi parts with well defined pore sizes and porosities in the range of 30-70 vol.%. For comparing the properties, porous titanium and Ti-6Al-4V samples were prepared in the same manner.Focus of the present work was a detailed investigation of the mechanical properties of porous NiTi to estimate its potential regarding the abovementioned applications. For a Ni-rich NiTi alloy with a porosity of 50 vol.%, fully pronounced pseudoelasticity after 6% compression was demonstrated. An energy dissipation of 1.5 MJ/m³ was measured, which could be directly related to the reversible austenite-martensite phase transformation. At higher deformations, pseudoelasticity becomes more and more superposed by the onset of plastic deformation. Nevertheless, even at deformations of up to 50%, a clearly pronounced amount of pseudoelastic shape recovery still remained. Fatigue of pseudoelasticity was investigated by conducting of up to 230,000 load cycles to 4% compression at a frequency of 1 Hz.     

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 Ni-Ti 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.     

Fabrication and characterization of NiTi shape memory alloy synthesized by Ni electroless plating of titanium powder

In this work NiTi shape memory alloy was fabricated from mixed elemental powders, Ni plated titanium powder and Ni heated/plated titanium powder by Ar-sintering. Electroless plating process was utilized to fabricate Ni plated titanium powder. For this purpose titanium powder was plated in an electroless Ni bath for 225?min and hydrazine hydrate was used as a reductant to deposit pure nickel on the titanium particles. Ni plated titanium powder was heat treated under an argon atmosphere at 1000?°C to prepare Ni heated/plated titanium powder. Finally, the three sample powders were pressed by CIP followed by sintering at 980?°C for 8?h to manufacture NiTi shape memory alloy. The prepared powders, as well as sintered samples, were characterized by scanning electronic microscopy (SEM), energy dispersive spectrometer analysis (EDS), X-ray fluorescence (XRF), X-ray diffraction (XRD) and differential scanning calorimetric (DSC). The results indicated the presence of NiTi phase and also non-transformable phases (NiTi₂ and Ni₃Ti) in the heated/plated Ti powder and sintered samples. NiTi compound was dominated phase in the heated/plated sintered sample. All three sintered samples, as well as heated/plated powder, showed one-step phase transformation (B2???B19′).     

Phase Development and Crystallization Kinetics of NiTi Prepared by Mechanical Alloying

NiTi alloy is produced by mechanical alloying(MA). It becomes amorphous after milling for enough time, such as 100 h in this paper. DSC measurement shows that the crystallization temperature is 676 K for the amorphous powder. Activation energy of crystallization is 199.98kJ/mol for MA powder, which is lower than that of amorphous prepared by magnetron sputtering.Avrami parameter of crystallization is 1.07.     

粉末冶金多孔高氮奥氏体不锈钢的制备及性能

采用粉末冶金方法制备了多孔高氮奥氏体不锈钢并研究其力学性能和耐腐蚀性能。结果表明,高温气固渗氮能促进双相不锈钢向奥氏体不锈钢的转变,在其显微组织中出现了细条状和颗粒状CrN相析出物。随着造孔剂含量的提高孔隙率随之提高,而力学性能和耐腐蚀性能降低。与普通的多孔不锈钢相比,这种多孔高氮奥氏体不锈钢的力学性能更加优越,源于N的固溶强化和CrN等析出物的强化机制。随着孔隙率的提高多孔高氮奥氏体不锈钢的腐蚀倾向和腐蚀速率逐渐增大,造孔剂含量(质量分数)为10%的试样具有最佳的耐腐蚀性能。提高烧结温度有利于烧结块体的致密化,使腐蚀速率明显下降。     

Powder Metallurgical Near‐Net‐Shape Fabrication of Porous NiTi Shape Memory Alloys for Use as Long‐Term Implants by the Combination of the Metal Injection Molding Process with the Space‐Holder Technique

A new method was developed for producing highly porous NiTi for use as an implant material. The combination of the space‐holder technique with the metal injection molding process allows a net‐shape fabrication of geometrically complex samples and the possibility of mass production for porous NiTi. Further, the porosity can be easily adjusted with respect to pore size, pore shape, and total porosity. The influence of the surface properties of powder metallurgical NiTi on the biocompatibility was first examined using human mesenchymal stem cells (hMSCs). It was found that pre‐alloyed NiTi powders with an average particle size smaller than 45 μm led to the surface properties most suitable for the adhesion and proliferation of hMSCs. For the production of highly porous NiTi, different space‐holder materials were investigated regarding low C‐ and O‐impurity contents and the reproducibility of the process. NaCl was the most promising space‐holder material compared to PMMA and saccharose and was used in subsequent studies. In these studies, the influence of the total porosity on the mechanical properties of NiTi is investigated in detail. As a result, bone‐like mechanical properties were achieved by the choice of Ni‐rich NiTi powder and a space‐holder content of 50 vol% with a particle size fraction of 355–500 μm. Pseudoelasticity of up to 6% was achieved in compression tests at 37 °C as well as a bone‐like loading stiffness of 6.5 GPa, a sufficient plateau stress σ₂₅ of 261 MPa and a value for σ₅₀ of 415 MPa. The first biological tests of the porous NiTi samples produced by this method showed promising results regarding proliferation and ingrowth of mesenchymal stem cells, also in the pores of the implant material.     

多孔NiTi合金的化学氧化处理表面改性研究

在H2O2+HCl溶液中对多孔NiTi合金进行了化学氧化处理表面改性。利用EPMA、SEM、原子吸收光谱对合金表面氧化膜的组成、形貌及化学氧化处理前后的Ni离子释出进行了研究。结果表明,经过H2O2+HCl溶液化学氧化表面处理后合金表面Ni含量显著降低,在生理盐水中的Ni释放速率明显下降;H2O2+HCl溶液浓度和氧化时间均对合金表面氧化膜的生长有较大影响。     

Densification and microstructural evolution of spark plasma sintered NiTi shape memory alloy

The effect of particle size and sintering temperature on the densification and microstructural characteristics of nickel-titanium shape memory alloy (NiTi-SMA) has been investigated using spark plasma sintering (SPS) process. The Ni and Ti elements in different particle sizes were alloyed in the composition of Ni_50.6Ti_49.4. The milled NiTi powders were consolidated using SPS process in a temperature range of 700–900?°C. The densification was characterized by plotting temperature, current and relative displacement of punch as a function of holding time. The results showed that a maximum relative density of ～98% can be achieved for NiTi-SMA with an average particle size of 10?µm at a sintering temperature of 900?°C. The microstructure of the sintered NiTi-SMA was examined using scanning electron microscope (SEM) and composition of NiTi alloy was analyzed using energy dispersive spectroscopy (EDS) analysis. The effect of sintering temperature on the microstructural evolution and transformation was also studied.     

NiTi形状记忆合金基本驱动特性

研究了预应变ＮｉＴｉ形状记忆合金的基本驱动特性，结果表明，加热和冷却过程回复力存在滞后行为；卸载过程中应力与应变具有与加热、冷却过程相关的非线性关系；卸尖力随卸载应变速率增加而降低，此外研究了预应变ＮｉＴｉ合金丝产生回复力后对外应力的响应行为。     

材料状态对NiTi形状记忆合金相变行为的影响

用金相显微镜观察了冷加工和固溶状态的显微组织形貌,用示差热量扫描法(DSC)系统研究了冷加工、固溶和时效处理对近等原子比的NiTi形状记忆合金的相变温度的影响。试验结果表明,冷加工态NiTi合金组织形态呈纤维状,固溶处理后组织形态呈等轴状。冷加工带来的大量变形缺陷抑制了热弹性马氏体的相变;冷加工态NiTi合金直接进行时效发生了P→M相变;经固溶处理后再进行时效则发生了P→R→M相变。NiTi合金在不同的热处理条件下发生了不同类型的热弹性马氏体相变。分析认为,应力、位错密度及析出相对NiTi合金热弹性马氏体的相变行为有重要的影响。     

粉末冶金泡沫铝的研究进展

泡沫铝是一种新型的超轻多功能材料。粉末冶金发泡法因能制备异型泡沫铝和泡沫铝芯三明治复合结构．可满足汽车、航天、航空和军工等领域的需求，已成为当前泡沫铝研究的热点方向.本文介绍了国内外粉末冶金泡沫铝方面的研究现状，指出了现存问题。并展望了今后的发展趋势。     

Porous NiTi by creep expansion of argon-filled pores

NiTi powders are densified in the presence of argon gas, whose initial pressure is varied between 1 and 33 atm, to create NiTi billets containing isolated Ar-filled pores. Upon vacuum annealing, the pressurized pores expand by creep of the surrounding NiTi matrix at rates which are in agreement with a simple analytical model up to 16% porosity. Beyond this porosity, foaming becomes very slow, as pores connect with each other and with the specimen surface where the gas escapes. This is due to failure of previous NiTi powder boundaries weakened by oxides insoluble in NiTi; this mechanism does not occur in Ti foams which dissolve their oxides at high temperature, allowing higher levels of pore expansion and foam porosity. NiTi with 10–16% porosity exhibits Young's moduli of 48–57 GPa, and may be useful for high-strength, low stiffness biomedical implants with superelastic or shape-memory properties.     

Property Change During Fixtured Sintering of NiTi Memory Alloy

Fixtured sintering of up to 5 hours at 1223 to 1323 K is successfully used to diminish the dimensional change of NiTi memory alloys produced from mixtures of elemental Ni and Ti powders packed and pressed at room temperature under 400, 500, and 600 MPa pressure. The improvements obtained can help near-net-shape technology via powder metallurgy that overcomes the traditional casting problems such as oxygen, nitrogen, hydrogen, and carbon absorption and intermetallic compound precipitation, which lower the workability and the homogeneity of the final alloy. The effects of compaction pressure, sintering time, and sintering temperature on dimensional change, porosity, hardness, and morphology of the produced TiNi intermetallic alloy are investigated. Phase transformation temperatures and mechanical properties of the sintered samples were determined and compared for the influence of compaction pressure and sintering conditions. It was concluded that suitable thermoelastic and superelastic effects are achievable via appropriate selection of fixtured sintering conditions.     

Effect of rotation speed and milling time to attain Nb-10Hf-1Ti alloy powders on morphology and microstructure of Nb-10Hf-1Ti powder

The mechanical alloying process was employed to produce C103 alloy with Nb-10% Hf-1% Ti (wt.%) composition using Nb, Hf and Ti powders. The mechanical alloying process was performed in an argon atmosphere in the chamber and bullets of tungsten carbide with a ball-to-powder weight ratio (BPR) of 20:1 at rotation speed of 200, 300 and 400 rpm for 2, 5 and 8 h. At rotation speeds of 200 and 300 rpm particle size decreased and became more spherical during MA. While increasing milling time at 400 rpm caused agglomeration of particles. XRD results showed that increasing milling time at a constant rotation speed has no considerable effect on reduction of crystallite size, but the lattice strain is strongly affected by it and increased obviously with further rotation speed. The results showed that the optimum milling time and rotation speed to attain Nb-10Hf-1Ti alloy powders with the least amount of contamination and appropriate morphology are 5 h and 300 rpm, respectively.     

Effect of Different Amounts of Graphite on the Sintering and Transverse Rupture Properties of Powder Metal Parts

Iron powders were mixed with graphite powders by 1-15 wt pct to produce block samples using powder metallurgy technique. The powders were blended in a three dimensional blender for 20 min and compacted in a die under 500 MPa by using a one directional press. Sinterability and mechanical properties of the samples with different carbon content were investigated. Sintering process was carried out on a belt furnace with flame curtain in dissociated ammonia atmosphere. Block samples produced were suitable for ASTM B-312 transverse rupture strength test apparatus and were tested with 0.050 kN/s velocity in a press under 100 g load. It was found that graphite amount up to 2 wt pct increased the contact area of particles and acted as a lubricant to affect the sintering behaviour positively. The results indicated that the samples containing up to 5 wt pct showed good sintering behaviour and also good hardness due to an increase in pearlite amount. However, the samples containing higher amount of graphite （more than 5 wt pct） affected the sintering behaviour negatively due to the settlement of free graphite around the Fe powders, which resulted in a decrease in the hardness and transverse rupture strength.