Mechanical Properties and Transformation Behavior of NiTiNb Shape Memory Alloys

NiTiNb shape memory alloys have attracted much attention in pipe coupling or sealing system because of their large transformation hysteresis upon a proper pre-deformation. In order to clarify the effects of adding Nb on the mechanical properties as well as the transformation behavior of NiTiNb shape memory alloys, Ni₄₇Ti₄₄Nb₉ and Ni_49.8Ti_45.2Nb₅ alloys with different microstructures but with similar martensitic transformation start temperature, are prepared. Comparative studies on the microstructures, mechanical properties and transformation characteristics are conducted by means of scanning electron microscopy (SEM), phase transformation measurements and mechanical property tests. It is found that Ni₄₇Ti₄₄Nb₉ and Ni_49.8Ti_45.2Nb₅ alloys possess similar transformation hysteresis in the as-annealed state. However, the presence of Nb and its status exerts important effects on the mechanical properties, especially the yield strength and the yield behavior of the alloys. Ni_49.8Ti_45.2Nb₅ alloy exhibits remarkable increase in the yield strength than the Ni₄₇Ti₄₄Nb₉ alloy. The transformation hysteresis of both alloys under pre-deformation is characterized and the relative mechanism is discussed.     

NiTiAl Intermetallic Alloys Strengthened by Mo Replacement

 NiTiAl based alloys have attracted attention as potential high temperature structural materials. Alloying is an effective way to improve their mechanical properties. The microstructures and mechanical properties of Ni₅₀Ti_44-xAl₆Mo_x (x = 0, 0.5, 1, 1.5, 3) alloys have been investigated by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron mi-croscope (TEM) and compressive tests. It is found that Ni₅₀Ti_44-xAl₆Mo_x alloy is composed of NiTi (B2) and Ti₂Ni (FCC) phases when Mo content is no more than 1 at%. The effect of Mo solid solution hardening at room temperature has been indi-cated by the rise of yield strength and the fall of plasticity with increasing Mo content. For Ni₅₀Ti_42.5Al₆Mo_1.5 and Ni₅₀Ti₄₁Al₆Mo₃ alloys, a Mo solid solution appears and increases the yield strength by precipitation strengthening. The maxi-mum yield strength at 600 °C and elevated temperatures is presented in Ni₅₀Ti₄₃Al₆Mo₁ alloy not in alloys with a higher Mo content, which is possibly due to the softness of Mo-Ti-Ni solid solution phase.     

热轧对Ti50Ni50-xAlx(x=1,2,4)形状记忆合金微观组织、相变和力学性能的影响

形状记忆合金的相变和力学性能受机械加工过程影响明显.在850℃下将Ti50Ni50-xAIx(x=1,2,4)形状记忆合金轧制成为板材.采用扫描电镜、透射电镜和X射线衍射对合金的微观结构研究表明:合金由NiTi相和Ti2Ni相组成,Al元素在Ti2Ni相中的固溶度比在TiNi相中高.差示扫描量热分析和电阻-温度测试表明...     

TiNiFeNb形状记忆合金的组织结构及相变特性

通过向TisoNiso合金中加入Fe和Nb元素,制备出一种四元Ti49Ni50-xFexNb1形状记忆合金.采用X射线衍射和背散射电子衍射的方法,测试和分析了合金的相结构及微观组织形态,采用电阻法系统研究了合金的相变特性.结果表明:Nb元素的加入并没有改变TiNiFe合金的B2结构,仅导致了广极少量的富Nb相在基体中析...     

Nb对富钛TiNiAl金属间化合物强化机制的影响

 采用力学压缩试验、扫描电镜和X射线衍射方法研究了Nb元素对于Ti₅₀Ni₄₂Al₈合金的力学性能和微观组织结构的影响。结果表明，Nb能够显著提高合金从室温到700 ℃之间的强度。在600 ℃时，Ti₅₀Ni₄₀Al₈Nb₂合金的压缩屈服强度和比强度分别为1 237 MPa和216 MPa/(g·cm^-3)，超过Rene95高温合金。微观组织观察及成分分析表明，Nb元素在基体和强化相Ti₂Ni（Al）中固溶，促进强化相的弥散析出和细化，并导致高强富Nb相的析出，从而提高了合金的强度。     

Al含量对空心阴极等离子烧结Ti/Ni等原子比TiNiAl合金组织和力学性能的影响

 采用空心阴极等离子烧结工艺制备了Ti/Ni等原子比的Ti_50-x/2Ni_50-x/2Al_x(x=0,3,6,9)合金,研究了Al含量对合金微观组织以及力学性能的影响。结果表明:未添加铝的合金微观组织主要由NiTi基体、强化相Ti₂Ni、Ni₃Ti及孔隙组成;随着Al含量的提高,合金中Ti₂Ni(Al)数量不断增多,孔隙数量和孔径不断增加,Ni₃Ti(Al)数量不断减少,在Ti_45.5Ni_45.5Al₉中还生成了少量Ni₂TiAl相;合金的抗弯强度随Al含量的提高而增加,并在Al含量为6%时达到最大值296.3 MPa;合金的硬度随铝含量的提高而增加,Ti_45.5Ni_45.5Al₉的硬度值为295.6 HV。     

Phase Transformation and Microstructure of Ti50Ni48-xFe2Ndx Shape Memory Alloys

The influences of rare earth element Nd on the phase transformation and the microstructure of Ti₅₀Ni_48−xFe₂Nd_x shape memory alloys are investigated by means of electrical resistivity, optical microscopy, electron probe microanalyzer, and X-ray diffraction. The results show that TiNiFe alloys with different Nd contents exhibit two-step martensitic transformation. The start temperature of martensitic transformation increases sharply from 212 K to 267 K when 0.1 at.% Nd is added in, and then decreases gradually if Nd content further increases. The microstructure of TiNiFeNd quaternary alloys consists of TiNi matrix, Ti₂Ni second phase, and Nd₃Ni intermetallic compound. The spherical Nd₃Ni precipitate-particles evenly disperse in the matrix.     

Yield anisotropy and tension/compression asymmetry of a Ni3Al based intermetallic alloy

In order to investigate the yielding behavior of the newly developed Ni 3 Al-based intermetallic alloy IC10, yield stresses have been measured in tension and compression with different orientations. The specimens were cut from a sheet with different angles inclined from the solidification direction. The inclined angles were taken to be 0 , 22.5 , 45 , 67.5 and 90 . All experiments were conducted at room temperature except for orientation 0 , whose deformation temperatures ranged from 298 to 1273 K. Experimental results show that the yield strength of alloy IC10 has the anomalous behavior which has been observed for other Ll 2 -long-range ordered intermetallic alloys, but it is less pronounced. The abnormalities show the following characteristics: (i) the yield strength increases as the temperature is raised below the peak temperature, (ii) yield strength anisotropy, (iii) tension/compression asymmetry. Compared to Ni 3 Al single crystals, the polycrystalline exhibits some different yielding behaviors which may be due to the high volume fraction of c phase.     

定向凝固Ni-44Ti-5Al-2Nb-1Mo合金的组织特征

采用液态金属冷却定向凝固方法制备了Ni-44Ti-5 Al-2 Nb-1 Mo(原子分数,％)合金,分析了加热温度为1 550℃,抽拉速率为0.3、1.2、3.0、6.0、18.0 mm/min时的定向凝固组织特征.结果表明,定向凝固没有改变合金的相组成,但改变了组成相的形态.定向凝固组织均由初生的NiTi相与晶间析出...     

Microstructure evolution of a hypereutectic Nb–Ti–Si–Cr–Al–Hf alloy processed by directional solidification

In this work, the Nb–14Si–24Ti–10Cr–2Al–2Hf–0.1Y alloy(at.%) was processed by the liquid–metal-cooled directional solidification(DS) at 1750 C with withdrawal rates of 1.2, 6,18 mm/min and post heat treatment(HT) at 1450 C for 10 h. The microstructures of the directionally solidified and heat treated samples were investigated. The results show that the microstructure of directionally solidified alloy mainly consists of petaloid Nbss+ Nb5Si3eutectics and Ti-rich Nbss+ Nb5Si3+ Cr2Nb eutectics. With the increase of withdrawal rate, the primary Nb5Si3is eliminated, Nbss+ Nb5Si3eutectic cells turn round and connected with the microstructure refinement and Nbss+ Nb5Si3+ Cr2Nb eutectics turn to a river-like morphology. After heat treatment,Nbss+ Nb5Si3+ Cr2Nb eutectics disappeared and petaloid Nbss+ Nb5Si3eutectics turn to a specific fiber-mesh structure gradually, which is promoted by higher withdrawal rates. Furthermore,both the volume fraction of Cr2Nb and the content of Cr in Nbssof Nbss+ Nb5Si3eutectics change regularly with the increase of withdrawal rate and heat treatment at 1450 C for 10 h.     

新型Co基合金Co-9Al-(9-x) W-xMo-2Ta-0.02B的显微组织与高低温力学性能

以新型Co基合金Co-9Al-9W-2Ta-0.02B成分为基础,分别添加了原子分数为2％、4％、6％、9％的Mo元素替代W（分别称2Mo、4Mo、6Mo、9Mo合金,W+Mo的原子分数为9％,无Mo添加的称为0Mo合金）,研究了Mo元素对合金相组成、显微组织形貌、维氏硬度和高低温压缩强度的影响.结果表明铸态合金由Co...     

Nb-10Si合金室温断裂韧性研究

利用真空电弧熔炼制备了处于亚共晶区的Nb-10Si-xMo(x=5,15)复合材料,采用扫描电子显微镜(SEM)和X射线衍射(XRD)分析了在1200℃退火100h后复合材料的微观组织形貌、相组成和断口形貌,并测定了Nb-10Si-Mo复合材料的显微硬度。用单边切口悬臂梁法(SENB)研究了加入合金元素后复合材料室温韧性的变化。深入分析了合金元素对材料韧性相变形行为和复合材料室温断裂韧性的影响。     

Nb-Si-Mo三元系合金相图

 Nb/Nb5Si3原位复合材料是一种有望替代镍基超合金用做飞机发动机叶片的新一代超高温结构材料。采用非自耗电弧熔炼技术制备出不同成分的Nb/Nb5Si3原位复合材料,利用X射线衍射仪、扫描电镜和能谱仪对Nb-Si-Mo三元系合金的相组成和微观组织进行研究。结果表明：Nb-Si-Mo三元系部分液相面投影图（<37.5 at% Si）含有4个初生相区,其立体相图中存在2个液固四相平衡反应和1个固态四相平衡反应。在2个液固四相平衡反应面之间存在一个三相共晶平衡棱柱,成分位于该棱柱内的合金会通过L→Nbss+β-Nb5Si3反应凝固生成Nbss/β-Nb5Si3片层共晶。     

Effects of B on the Microstructure and Oxidation Resistance of Nb-Ti-Si-based Ultrahigh-temperature Alloy

Nb-Ti-Si-based ultrahigh-temperature alloys concocted with boron ranging from 0 to 2 at% are prepared by arc-melting technology. The effects of adding boron on their as-melted microstructure and oxidation resistance are analyzed. The (Nb,Ti)ss, β-(Nb,Ti)₅Si₃ and γ-(Nb,Ti)₅Si₃ exist in Nb-22Ti-16Si-6Cr-3Al-4Hf alloy, while (Nb,Ti)ss, α-(Nb,Ti)₅Si₃ and γ-(Nb,Ti)₅Si₃ are present in Nb-22Ti-16Si-6Cr-3Al-4Hf-1B and Nb-22Ti-16Si-6Cr-3Al-4Hf-2B alloys. The oxidation of Nb-Ti-Si-based ultrahigh-temperature alloys is dominated by the diffusion of oxygen through (Nb,Ti)ss. Compared to boron-free alloys, the boron-containing alloys have significantly lower oxidation rate when oxidized at 1 200 °C for less than 50 h, but, for more than 50 h, their oxidation resistance deteriorates.     

Cr对Nb-16Si-22Ti-2Al-2Hf合金显微组织与高低温力学性能的影响

 以Nb-16Si-22Ti-2Al-2Hf成分为基础,分别添加了2%和17%(原子分数,后同)的Cr元素替代Nb(分别称2Cr和17Cr合金),研究了Cr含量对Nb-16Si-22Ti-2Al-2Hf合金相组成、显微组织形貌、室温断裂韧性和高温强度的影响,分析了高低温失效机制。结果表明,铸态和1 375 ℃×100 h热处理后2Cr合金由Nbss和Nb₅Si₃两相组成;当Cr含量为17%时,出现了具有C15结构的Laves Cr₂Nb相,合金由Nbss、Nb₅Si₃和Cr₂Nb三相组成,热处理后在Nb₅Si₃中还析出了球状Cr₂Nb相。随着Cr含量由2%提高到17%,热处理合金的室温断裂韧性K_Q由14.32 MPa·m^1/2下降到10.30 MPa·m^1/2。合金强度与Cr含量的关系受温度影响,随Cr含量提高,室温和1 150 ℃时合金的硬度或强度增高,而1 250 ℃和1 350 ℃时合金强度降低。如1 150 ℃时2Cr和17Cr合金的屈服强度σ_0.2分别为349 MPa和387 MPa;1 350 ℃时分别为306 MPa和74 MPa。     

Effect of W addition on phase transformation and microstructure of powder metallurgic Ti-22Al-25Nb alloys during quenching and furnace cooling

Powder metallurgic Ti2 AlNb alloys with W addition are sintered at 900, 1000, 1070 °C,and 1150 °C(i.e., in the O + B2, a_2+ B2 + O, a_2+ B2, and single B2 phase regions, respectively)for 12 h, followed by water quenching and furnace cooling. Comparisons of phase and microstructure between quenched and furnace-cooled W-modified alloys are carried out to illustrate the phase transformation and microstructure evolution during the cooling process. Furthermore, a comparison is also made between W-modified and W-free alloys, to reveal the function of the W alloying.W addition accelerates the solutions of a_2 and O phases during the high-temperature holding, and a Widmannsta¨tten B2 + O structure, which contributes to the properties, is induced by furnace cooling from all the phase regions. The Widmannsta¨tten structure includes a B2 matrix, primary O, and secondary O precipitates. However, W alloying refines the Widmannsta¨tten structure only when the alloys are solution-treated and then cooled from the single B2 phase. Although the hardness of the W-modified alloys is lower than that of the W-free alloys sintered in the same phase region, an enhancement of hardness, 489 ± 18 HV, is obtained in the alloy solution-treated in the single B2 phase region for only 0.5 h.     

塑性变形提高形状记忆合金相变滞后机制的探讨

形状记忆合金经适当温度下的塑性变形可以有效地提高马氏体的稳定性,从而使相变滞后得以大幅度提高.本文根据马氏体相变热力学和动力学,并结合相关的实验结果,研究了弹性应变能弛豫和相变滞后的关系.结果表明,塑性变形产生的位错以及变形的第二相颗粒对逆马氏体相变温度的提高具有一定作用,但塑性变形导致应变能释放才是形变提高形状记忆合金相变滞后的主导因素.     

Liquid-Solid Phase Equilibria of Nb-Si-Ti Ternary Alloys

To determine the liquid-solid phase equilibria of the Nb-Si-Ti ternary system, Nb-Si-Ti alloys of different compositions are prepared. By means of scanning electron microscopy （SEM）, X-ray diffraction （XRD） and electron probe microanalysis （EPMA）, the phases in the alloys, such as Si-based solutions, Ti（Nb）Si, Ti（Nb）Si2, Nb（Ti）Si2, Ti（Nb）5Si4, Nb（Ti）5Si3, Ti（Nb）5Si3, Nb（Ti）3Si and Nb-based solutions are identified, and the phase evolution is analyzed. As a result, the microstmctural and microchemical evidence provides a clear definition of the Nb-Si-Ti liquidus surface projection and indicates that the ternary phase diagram has seven transition reactions.     

Phase Equilibria in Nb-Si-Mo Ternary Alloys at 1 273 K and 2 073 K

Phase equilibrium in Nb-Si-Mo ternary alloys （〈37.5 at.% Si） at 1 273 K and 2 073 K is investigated by using X-ray diffraction （XRD） analysis, scanning electron microscopy （SEM） and energy dispersive spectroscopy （EDS）. The partial isothermal section at 1 273 K, which contains four single-phase regions, five two-phase regions and two three-phase regions, is basically the same as that at 1 973 K. However, when the temperature increases to 2 073 K, the three-phase region of Nbss＋ct-（Nb（Mo））sSi3＋13-（Nb,Mo）sSi3 obviously moves towards the Nb-rich comer. This suggests that Nb-Si-Mo ternary alloys remain stable at least up to 1 973 K.     

Ultrasonic constitutive model and its application in ultrasonic vibration-assisted milling Ti3Al intermetallics

Ultrasonic vibration-assisted technology is widely utilized in the performance research and manufacturing process of metallic materials owing to its advantages of introducing highfrequency acoustic systems. However, the acoustic plasticity constitutive model and potential mechanism, involving Ti₃Al intermetallic compounds, have not yet been clarified. Therefore, the Ultrasonic-K-M hybrid acoustic constitutive model of Ti₃Al was established by considering the stress superpos...