第一性原理研究压力下NiTi合金B2相的结构、力学以及热力学性质（英文）

运用第一性原理研究0~40GPa下B2相NiTi合金的机械性能、电子性质以及热力学性能。计算发现,几何优化后NiTi晶体的晶格常数与实验值和其他文献提供的数值大体一致,表明随着压力的增加该型合金力学稳定且没有相变产生。NiTi合金的体模量B、剪切模量G和杨氏模量E以及B/G的值随压力增大呈线性增加,表明压力使其抗体积变形能力、抗剪变能力及塑性增强。研究发现,压力也会使NiTi合金的各向异性发生改变。对NiTi合金态密度的研究表明,该合金同时显现出共价性与离子性,并且压力对其电子性质无明显影响。此外,本文还研究了不同温度和压力下NiTi合金的热力学性能,包括德拜温度Θ_D,热容C_v和C_p的变化,为今后实验提供理论数据。     

基于第一性原理的高压下TiAl结构、力学性能及热力学性质研究

采用第一性原理计算方法,研究了压力与温度对TiAl合金结构、力学性能与热力学性质的影响.结果显示,随着外加压力的增加,TiAl体积比降低. 计算了不同压力下TiAl的弹性常数Cij , 所有Cij均力学稳定性判据, 表明不同压力下的模拟结果均满足力学稳定性条件. 通过弹性常数, 计算了体模量与剪切模量, 发现在0 Gpa下的计算值与文献值相吻合,表明计算的准确性. 体模量与剪切模量可以用来反映材料抵抗变形能力, 随着压力的增加, 其数值增加, 表明材料抵抗变形能力得到提升. 由B/G发现, 当压力在10-20 Gpa之间时, TiAl由脆性材料转变为延性材料. 借助准谐德拜模型, 研究了当温度在0-1 000 K、 压力在0-50 Gpa下压力与温度对TiAl体模量、 德拜温度、 线膨胀系数以及热容的影响, 这有助于研究温度与压力对热力学参数的影响. 最后, 研究了不同压力下TiAl的电子结构, 随着压力的增加, 材料的态密度强度降低, Ti原子成键相互作用减弱, Al原子成键相互作用增强, 材料的延性得到提升.     

基于第一性原理的高压下TiAl结构、力学性能及热力学性质研究（英文）

采用第一性原理计算方法,研究了压力与温度对TiAl合金结构、力学性能与热力学性质的影响。结果显示,随着外加压力的增加,TiAl体积比降低。计算了不同压力下TiAl的弹性常数C_(ij),所有C_(ij)均力学稳定性判据,表明不同压力下的模拟结果均满足力学稳定性条件。通过弹性常数,计算了体模量与剪切模量,发现在0GPa下的计算值与文献值相吻合,表明计算的准确性。体模量与剪切模量可以用来反映材料抵抗变形能力,随着压力的增加,其数值增加,表明材料抵抗变形能力得到提升。由B/G发现,当压力在10-20GPa之间时,TiAl由脆性材料转变为延性材料。借助准谐德拜模型,研究了当温度在0-1 000K、压力在0-50GPa下压力与温度对TiAl体模量、德拜温度、线膨胀系数以及热容的影响,这有助于研究温度与压力对热力学参数的影响。最后,研究了不同压力下TiAl的电子结构,随着压力的增加,材料的态密度强度降低,Ti原子成键相互作用减弱,Al原子成键相互作用增强,材料的延性得到提升。     

高压下β-Ti的结构稳定性、弹性和电子性能的第一性原理研究

应用第一性原理的方法, 研究了高压下β-Ti在0 K下的结构稳定性、 弹性和电子性能. 吉布斯自由能和电子态密度的研究表明, 随着压力的增加, β-Ti 的结构趋于稳定. β→β 的相变压强为 64.3 GPa, 这一计算结果与其它的理论结果 63.7 GPa 和实验结果 50 GPa相吻合. 弹性常数的计算表明, β-Ti 力学稳定的低压极限约为50 GPa, 该压力下的弹性模量约为30.01 GPa, 接近人体骨骼的弹性模量30 GPa. 另外, 讨论了不同压力下β-Ti体积模量B、 剪切模量G、 杨氏模量E、 泊松比β、 声速和韧性/脆性. 随着压力的增加, B, G, E增加, 但是β减小. B/G 计算表明, β-Ti具有良好的韧性.     

高压下β-Ti的结构稳定性、弹性和电子性能的第一性原理研究（英文）

应用第一性原理的方法,研究了高压下β-Ti在0K下的结构稳定性、弹性和电子性能。吉布斯自由能和电子态密度的研究表明,随着压力的增加,β-Ti的结构趋于稳定。β→β的相变压强为64.3GPa,这一计算结果与其它的理论结果63.7GPa和实验结果50GPa相吻合。弹性常数的计算表明,β-Ti力学稳定的低压极限约为50GPa,该压力下的弹性模量约为30.01GPa,接近人体骨骼的弹性模量30GPa。另外,讨论了不同压力下β-Ti体积模量B、剪切模量G、杨氏模量E、泊松比β、声速和韧性/脆性。随着压力的增加,B,G,E增加,但是β减小。B/G计算表明,β-Ti具有良好的韧性。     

NiTi气雾化制粉工艺对选区激光熔化成型性、制件超弹性的影响

选区激光熔化成型具有外界温度感知能力的NiTi形状记忆合金是4D打印金属材料技术的基础研究,根据选区激光熔化技术对粉末性能的要求,研究NiTi形状记忆合金不同气雾化制粉工艺对选区激光熔化成型性及制件超弹性的影响规律具有重要意义。通过对比分析真空惰性气体雾化(VIGA)、电极感应熔炼气雾化(EIGA)制粉工艺对NiTi合金粉末杂质含量、流动性、球形度等性能的影响,发现VIGA制粉工艺由于采用坩埚熔炼,导致合金杂质元素增加、粉体性能恶化,粉末粒度分布偏向细粉侧,极易形成卫星粉,导致粉末流动性差,在打印过程中铺粉困难而难以成型,并且氧含量的增加导致打印过程中易发生球化、开裂等现象,使得VIGA工艺制备的NiTi合金粉末SLM成型性较差。而采用EIGA工艺制备的粉末粒度分布均匀、流动性好、氧含量低,满足选区激光熔化技术对NiTi合金粉末的特性要求。并对比分析两种工艺制备的粉末打印样品的表面形貌,成型了具有完全回复性能的超弹NiTi形状记忆合金样件。     

NiTi基体上沉积铁电陶瓷PZT复合材料的振动响应特性

用溶胶-凝胶法在NiTi形状记忆合金冷轧薄片基体上沉积铁电陶瓷PZT薄膜,再通过650℃烧结处理而得到NiTi/PZT复合材料,采用动态热机械分析方法测试了该复合材料及相同温度时效处理的NiTi形状记忆合金的内耗随温度及振动频率的变化曲线,比较了两种材料内耗曲线特征的差异;通过分析内耗曲线随温度、振动频率变化规律研究了材料的振动响应特性.结果表明:对于NiTi/PZT复合材料,响应振动载荷的能力相对于单纯NiTi形状记忆合金有明显改善,响应范围从10 Hz以内提高到33 Hz以内;tanδ-t曲线特征凸起峰高度值(材料的响应程度)从3%增加到5%,提高60%以上.     

Mg2Si和MgCu2相电子结构、弹性性能第一性原理研究

本文通过第一性原理计算研究了MgCu2和Mg2Si相的结构稳定性、电子结构和弹性性能等.计算的结构参数与实验理论结果非常吻合.合金形成热和结合能计算结果表明,MgCu2具有较强的结构稳定性,Mg2Si具有较强的合金化能力.计算的剪切模量G、体模量B和杨氏模量E显示:MgCu2属于延性材料,Mg2Si属于脆性材料,Mg2Si的刚度较大.态密度(DOS)和Mulliken布居数的计算表明,Mg2Si离子键较强.     

粉末冶金法制备多孔Ni-Ti合金的烧结过程研究

研究粉末冶金法固相烧结多孔体的原理及工艺参数,并对多孔Ni-Ti合金烧结进行实验研究,结果表明,在140MPa压力下压制,在950℃下粉末冶金固相烧结制备的Ni-Ti合金,具有以NiTi为主相的、合适孔隙度的合金.     

退火温度对NiTi/Si膜结晶特性及力学性能的影响

采用射频磁控溅射法在NiTi膜表面沉积一层非晶硅膜而形成NiTi/Si膜,并对该膜进行了不同温度的退火处理;用XRD、SEM和纳米压痕仪研究了退火温度对NiTi/Si膜的结晶特性和力学性能的影响。结果表明:NiTi/Si膜在600℃下退火后其中的硅仍为非晶态;当退火温度在650℃后开始出现明显的结晶硅衍射峰,且随着温度升高,硅的结晶度增大,晶粒逐渐长大;而该膜的硬度和抗压痕蠕变能力逐渐降低;该膜弹性模量在650℃退火后最大。     

工艺参数对磁控溅射碳化锆薄膜的微结构与力学性能影响

采用碳化锆靶通过磁控溅射方法制备一系列碳化锆薄膜,采用XRD、SEM、AFM和微力学探针研究溅射气压和基于温度对碳化锆薄膜的微结构和力学性能的影响.结果表明:低溅射气压下,薄膜呈现结品良好的柱状品结构,其硬度和弹性模量分别为30.1GPa、256GPa.溅射气压提高到2.0Pa以上后,薄膜的柱状晶结构受到破坏,品粒减小...     

Combined ultrasonic elastic wave velocity and microtomography measurements at high pressures

Combined ultrasonic and microtomographic measurements were conducted for simultaneous determination of elastic property and density of noncrystalline materials at high pressures. A Paris-Edinburgh anvil cell was placed in a rotation apparatus, which enabled us to take a series of x-ray radiography images under pressure over a 180° angle range and construct accurately the three-dimensional sample volume using microtomography. In addition, ultrasonic elastic wave velocity measurements were carried out simultaneously using the pulse reflection method with a 10° Y-cut LiNbO(3) transducer attached to the end of the lower anvil. Combined ultrasonic and microtomographic measurements were carried out for SiO(2) glass up to 2.6 GPa and room temperature. A decrease in elastic wave velocities of the SiO(2) glass was observed with increasing pressure, in agreement with previous studies. The simultaneous measurements on elastic wave velocities and density allowed us to derive bulk (K(s)) and shear (G) moduli as a function of pressure. K(s) and G of the SiO(2) glass also decreased with increasing pressure. The negative pressure dependence of K(s) is stronger than that of G, and as a result the value of K(s) became similar to G at 2.0-2.6 GPa. There is no reason why we cannot apply this new technique to high temperatures as well. Hence the results demonstrate that the combined ultrasonic and microtomography technique is a powerful tool to derive advanced (accurate) P-V-K(s)-G-(T) equations of state for noncrystalline materials.     

Energy dissipation of materials at high pressure and high temperature

We report an experimental method to study the anelastic properties of materials at high pressure and high temperature. The multianvil high pressure deformation device, used to apply a cyclic loading force onto the sample, can reach 15 GPa and 2000 K. A synchrotron x-ray radiation source provides time resolved images of the sample and reference material. The images yield stress and strain as a function of time; stresses are derived from the reference material, and strains from the sample. This method has been tested by applying a sinusoidal stress at megahertz to hertz frequency on a San Carlos olivine specimen at 5 GPa and up to 2000 K. Strain as small as 10(-5) can be resolved. We have obtained experimental results which exhibit resolvable attenuation factor (Q(-1)) and shear modulus (M) at deep Earth conditions. These results are in quantitative agreement with previously reported lower pressure data and suggest that temperature and grain size have dominating effect on these properties.     

Investigation of the effect of diffusion bonding parameters on microstructure and mechanical properties of 7075 aluminium alloy

In the present study, diffusion bonding of aluminium alloy (AA7075) sheet materials which are used especially in the automobile and aerospace industry has been investigated at temperatures of 425 and 450 °C and pressures of 2 and 3 MPa for 180 min in argon atmosphere. The microstructural and mechanical properties of bonding have been characterized with different welding parameters such as bonding temperature and pressure. The microstructure was characterized by light optical microscope, scanning electron microscope and energy dispersive spectroscopy, while the mechanical properties were determined by tensile-shear tests and microhardness tests. The results obtained are discussed from both the microstructural and mechanical points of view. It was observed in the microstructural investigations that the interfacial oxide layer decreased with increasing of the bonding temperature and pressure. The maximum shear strength was found to be 131 MPa for the Al 7075 sample bonded at 450 °C and 3 MPa for 180 min. It is shown that in certain extent, the bonding temperature and bonding pressure have great effect on the joint shear strength. With the increasing of bonding temperature and pressure, the shear strength of the joints increases due to diffusion of atoms in the interface. The strength achieved after bonding were dependent on interface grain boundary migration and on grain growth during the bonding process. The maximum hardness value of the Al 7075 sample bonded at 450 °C, 3 MPa for 180 min is 92.5 HV_0.2. Increasing hardness with increasing temperature can be attributed to the formation of metallic bond at high temperatures and pressures.     

Chemical Structure and Micro-Mechanical Properties of Ultra-Thin Films of Boron Carbide Prepared by Pulsed-Laser Deposition

Ultra-thin boron carbide films with a thickness of about 40 nm were deposited on silicon substrates by means of pulsed-laser ablation of a sintered B₄C target in vacuum. Together with the determination of the film composition by X-ray photoelectron spectroscopy (XPS) and the observation of the surface topography by atomic force microscopy (AFM), the chemical structure of the films was studied by Fourier transform infrared (FTIR) spectroscopy. Mechanical characterization of the films was performed on the micron and sub-micron scales by means of nano-indentation and micro-scratch tests, from which the hardness, Young's modulus and micro mar resistance of the films were determined. The optimal values were obtained for the films prepared at elevated temperature of 600 °C, with hardness of 39 GPa, Young's modulus of 348 GPa and micro mar resistance (MMR) of 5.0 × 10³ GPa, in comparison with those of 23, 252, and 7.1 × 10² GPa, respectively, for the films prepared at room temperature.     

Simultaneous structure and elastic wave velocity measurement of SiO2 glass at high pressures and high temperatures in a Paris-Edinburgh cell

An integration of multi-angle energy-dispersive x-ray diffraction and ultrasonic elastic wave velocity measurements in a Paris-Edinburgh cell enabled us to simultaneously investigate the structures and elastic wave velocities of amorphous materials at high pressure and high temperature conditions. We report the first simultaneous structure and elastic wave velocity measurement for SiO(2) glass at pressures up to 6.8 GPa at around 500°C. The first sharp diffraction peak (FSDP) in the structure factor S(Q) evidently shifted to higher Q with increasing pressure, reflecting the shrinking of intermediate-range order, while the Si-O bond distance was almost unchanged up to 6.8 GPa. In correlation with the shift of FSDP position, compressional wave velocity (Vp) and Poisson's ratio increased markedly with increasing pressure. In contrast, shear wave velocity (Vs) changed only at pressures below 4 GPa, and then remained unchanged at ~4.0-6.8 GPa. These observations indicate a strong correlation between the intermediate range order variations and Vp or Poisson's ratio, but a complicated behavior for Vs. The result demonstrates a new capability of simultaneous measurement of structures and elastic wave velocities at high pressure and high temperature conditions to provide direct link between microscopic structure and macroscopic elastic properties of amorphous materials.     

Rolling contact fatigue of alumina ceramics sprayed on steel roller under pure rolling contact condition

The rolling contact fatigue of sprayed alumina ceramics with a nominal composition of Al₂O₃–2.3 mass% TiO₂ was studied with a two-roller test machine under a pure rolling contact condition with oil lubricant. The influence of undercoating of sprayed Ni-based alloy on the rolling contact fatigue was investigated. The failure mode of all sprayed rollers was spalling caused by subsurface cracking. The undercoating did not contribute to the improvement of the rolling contact fatigue life. The elastic modulus of the alumina sprayed layer evaluated with the nano-indentation method was around 85 GPa. The depths of the observed subsurface cracks corresponded approximately to the depths where the orthogonal shear stress or the maximum shear stress calculated with two-dimensional FEM became maximum.