AlCrFeCuNi高熵合金力学性能的分子动力学模拟

高熵合金具有传统合金无法比拟的高强度、高硬度和高耐磨耐腐蚀性,具有广阔的应用前景。为研究AlCrFeCuNi高熵合金(High entropy alloy,HEA)在轴向载荷作用下的力学性能,采用分子动力学方法,模拟高熵合金的实验制备过程并建立原子模型,研究温度和Al的含量对AlCrFeCuNi高熵合金力学性能的影响,从材料学角度分析了变形过程及其具有高塑性的原因。模拟结果表明,AlCrFeCuNi高熵合金在拉伸载荷作用下依次经历弹性、屈服、塑性3个变形阶段。在屈服阶段,开始出现孪晶和层错,孪晶和层错的产生和生长是合金产生不均匀塑性变形的主要原因之一。高熵合金的杨氏模量和屈服应力随着Al含量的增加近似线性降低,同时具有很强的温度效应,温度越低,Al含量越小,其杨氏模量和屈服应力的下降幅度越大。     

Effect of gamma irradiation on high temperature hardness of low-density polyethylene

Gamma irradiation can cause the change of microstructure and molecular structure of polymer, resulting in the change of mechanical properties of polymers. Using the hardness measurement, the effect of gamma irradiation on the high temperature hardness of low-density polyethylene (LDPE) was investigated. The gamma irradiation caused the increase in the melting point, the enthalpy of fusion, and the portion of crystallinity of LDPE. The Vickers hardness of the irradiated LDPE increases with increasing the irradiation dose, annealing temperature, and annealing time. The activation energy for the rate process controlling the reaction between defects linearly decreases with the irradiation dose. The process controlling the hardness evolution in LDPE is endothermic because LDPE is semi-crystalline.     

溅射压强对TiN/SiNx纳米多层膜微观结构及力学性能的影响

利用磁控溅射方法在不同溅射压强条件下制备了TiN/SiN_x纳米多层膜.多层膜的微观结构及力学性能分别用X射线衍射仪、原子力显微镜及纳米压痕仪来表征.结果表明随着溅射压强的增大,多层膜的界面变模糊,TiN层的择优取向由(200)晶面过渡到(111)晶面.与此同时,多层膜的表面粗糙度增大,硬度和弹性模量随溅射压强的增大而减小.多层膜力学性能的差异主要是由于薄膜的周期性结构及致密度存在差异所致. 关键词： x多层膜')" href="#">TiN/SiN_x多层膜 界面宽度 表面形貌     

Effect of complex inter-site couplings on the excitation energy transfer in the FMO complex

The laser irradiation effects on surface, structural and mechanical properties of zirconium (Zr) have been investigated. For this purpose, Zr samples were irradiated with Excimer (KrF) laser (λ ≈ 248 nm, τ ≈ 18 ns, repetition rate ≈ 30 Hz). The irradiation was performed under the ambient environment of oxygen gas at filling pressure of 20 torr by varying laser fluences ranging from 3.8 to 5.1 cm^-2. The surface and structural modification of irradiated targets was investigated by scanning electron microscope (SEM) and X-ray diffractometer (XRD). In order to explore the mechanical properties of irradiated Zr, the tensile testing and Vickers micro hardness testing techniques were employed. SEM analysis reveals the grain growth on the irradiated Zr surfaces for all fluences. However, the largest sized grains are grown for the lowest fluence of 3.8 J cm^?2. With increasing fluence from 4.3 to 5.1 J cm^?2, the compactness and density of grains increase whereas their size decreases. XRD analysis reveals the appearance of new phases of ZrO₂ and Zr₃O. The variation in the peak intensity is observed to be anomalous whereas decreasing trend in the crystallite size and residual stresses has been observed with increasing fluence. Micro hardness analysis reveals the increasing trend in surface hardness with increasing fluence. The tensile testing exhibits the increasing trend of yield stress (YS), decreasing trend of percentage elongation and anomalous behaviour of ultimate tensile strength with increasing fluence.     

Alternate stresses and temperature variation as factors of influence of ultrasonic vibration on mechanical and functional properties of shape memory alloys

It is known that the main factors in a variation in the shape memory alloy properties under insonation are heating of the material and alternate stresses action. In the present work the experimental study of the mechanical behaviour and functional properties of shape memory alloy under the action of alternate stresses and varying temperature was carried out. The data obtained had demonstrated that an increase in temperature of the sample resulted in a decrease or increase in deformation stress depending on the structural state of the TiNi sample. It was shown that in the case of the alloy in the martensitic state, a decrease in stress was observed, and on the other hand, in the austenitic state an increase in stress took place. It was found that action of alternate stresses led to appearance of strain jumps on the strain–temperature curves during cooling and heating the sample through the temperature range of martensitic transformation under the constant stress. The value of the strain jumps depended on the amplitude of alternate stresses and the completeness of martensitic transformation. It was shown that the heat action of ultrasonic vibration to the mechanical behaviour of shape memory alloys was due to the non-monotonic dependence of yield stress on the temperature. The force action of ultrasonic vibration to the functional properties was caused by formation of additional oriented martensite.     

Numerical simulation of flat-tip micro-indentation of glassy polymers: Influence of loading speed and thermodynamic state

Flat-tip micro-indentation tests were performed on quenched and annealed polymer glasses at various loading speeds. The results were analyzed using an elasto-viscoplastic constitutive model that captures the intrinsic deformation characteristics of a polymer glass: a strain-rate dependent yield stress, strain softening and strain hardening. The advantage of this model is that changes in yield stress due to physical aging are captured in a single parameter. The two materials studied (polycarbonate (PC) and poly(methyl methacrylate) (PMMA)) were both selected for the specific rate-dependence of the yield stress that they display at room temperature. Within the range of strain rates experimentally covered, the yield stress of PC increases linearly with the logarithm of strain rate, whereas, for PMMA, a characteristic change in slope can be observed at higher strain rates. We demonstrate that, given the proper definition of the viscosity function, the flat-tip indentation response at different indentation speeds can be described accurately for both materials. Moreover, it is shown that the model captures the mechanical response on the microscopic scale (indentation) as well as on the macroscopic scale with the same parameter set. This offers promising possibilities of extracting mechanical properties of polymer glasses directly from indentation experiments.     

Deformation hardening and temperature dependence of mechanical properties in ternary ordered alloys based on Ni3Fe I

The variation of hardening factor and resistance to deformation with the quenching and testing temperatures for ordered alloys based on Ni₃Fe was investigated. It was shown that the yield point _0.2 and flow stresses are independent of the quenching temperature right up to about 450 ° C. At higher quenching temperatures there are increases in _0.2 and in the flow stresses corresponding to small degrees of deformation ( < 10%); the highest mechanical properties are found when the quenching temperature is near the Curie point. In quenching from a temperature above the Curie point the mechanical properties and hardening factor fall suddenly to values corresponding to the disordered state of the alloys. The temperature variation of _0.2 is similar to that mentioned above. The deformation stress at high values and the hardening factor decrease with increase in testing temperature, and below the Curie point they reach values corresponding to the disordered state. The temperature variation of mechanical properties for specimens in which the formation of long-range order is suppressed by special treatment is characterized by the absence of substantial change near the Curie point. The results obtained are discussed in relation to modern dislocation theories of hardening of alloys with long-range order.     

Mechanical Characterization of Protein Crystals

The mechanical properties (critical stress intensity factor, hardness and Young's modulus) of 4 crystalline materials (two proteins, lysozyme and glucose isomerase and two non‐proteins, glutamic acid and potassium sulphate) were measured with an indentation technique. It was found that the mechanical properties of lysozyme crystals depend on their state – dried, partly dried and moisture saturated – and their surroundings. The hardness, Young's modulus and the critical stress intensity factor of lysozyme crystals were observed to be much lower than those for the tested non‐proteins, leading to the conclusion that crystalline lysozyme is comparatively more fragile and softer. In combination the mechanical properties of lysozyme and the non‐proteins indicated that these materials were fairly brittle. Mechanical properties for crystals of the other protein, glucose isomerase, could not be quantified by indentation. However, qualitatively crystalline glucose isomerase was found to be more ductile and less fragile than crystalline lysozyme. The experimental findings were interpreted in terms of relative susceptibility to attrition and secondary nucleation in stirred industrial crystallizers.     

Self-Reinforced High-Density Polyethylene Prepared by Low-Frequency,Vibration-Assisted Injection Molding. 1. Processing Conditions and Physical Properties

Using a low-frequency, vibration-assisted injection molding (VAIM) device, the effects of vibration variables (frequency and amplitude) on mechanical properties and thermal softening temperature of high-density polyethylene (HDPE) injection moldings were investigated. For VAIM-processed samples, the mechanical properties can be improved by changing vibration frequency and vibration pressure amplitude. Injected at a constant vibration pressure amplitude, a low range of frequency (below 0.7 Hz) was favorable for increasing yield strength; in the high range of frequency (0.7 Hz < f < 2.33 Hz) the yield strength remained at a plateau. Injected at a constant frequency (0.7 Hz) the yield strength increased sharply with decreased elongation when applying large vibration pressure amplitude. The maximal yield strength and Young's modulus were 60.6 MPa and 2.1 GPa for a VAIM sample compared with 39.8 MPa and 1.0 GPa for a conventional injection-molded (CIM) sample, respectively; there was also a 10°C increase in Vicat softening point temperature.     

Correlation between mechanical,thermal and electronic properties in Zr–Ni,Cu amorphous alloys

We show that mechanical properties (stiffness and hardness) of Zr–Ni, Cu amorphous alloys increase linearly with Ni, Cu content over a wide composition range (22?≤?x _Ni,Cu?≤?65 at%). This correlates with the observed increase in the Debye temperatures and densities with x and shows that the strength of interatomic bonding increases with x in these alloys. Accordingly, the thermal stability (e.g. the crystallization and glass transition temperatures) of these alloys also increases with x. Since the electronic density of states at the Fermi level decreases linearly with x within the same x-range, a very simple relationship exists between the electronic structure and mechanical and thermal properties. We also deduce the mechanical properties of hypothetic amorphous Zr and briefly discuss the possibility of its preparation.     

Influence of stress and temperature on damping behavior of amorphous Pd₇₇.₅Cu₆.₀Si ₁₆.₅ below Tg

In this work we analyzed the mechanical damping behavior of amorphous Pd(77.5)Cu(6.0)Si(16.5) below the glass transition temperature (T(g)) with creep/recovery measurements. Here a correlation between temperature stimulation and external stress is found in an exponential, multiplicative way. This demonstrates that not only is the yield stress of the material influenced by temperature variation (mechanical melting) but also the secondary relaxation is modified under stress and temperature.     

Microstructure and mechanical properties of a partially crystallized La-based bulk metallic glass

The evolution of microstructure upon partial crystallization and its influence on the mechanical properties such as hardness, elastic modulus and viscosity in a La 55 Al 25 Cu 10 Ni 5 Co 5 bulk metallic glass alloy are studied. Specimens with various volume fractions of crystalline phases were obtained by annealing the as-cast amorphous alloy above its glass transition temperature and were characterized by transmission electron microscopy. Microscopic examination of the heat-treated samples shows short-range-ordered domains prior to nanocrystallization within the amorphous matrix, followed by the growth and impingement of the crystallites. Whereas the hardness of the annealed samples increases linearly with increasing crystallinity, the elastic modulus and the viscosity both increase abruptly when the crystalline volume fraction is about 40 vol.%, with a only minor variation on either side of this range. The sudden rises in the modulus and viscosity are similar to those in the literature data on the fracture strength of partially crystallized bulk amorphous alloys that shows a steep drop in strength at 30-50 vol.% crystallinity. On the basis of the microscopic observations, it is suggested that the interaction and formation of rigid networks of crystalline phases upon the attainment of a critical second-phase volume fraction may be the possible reason for the sudden change in mechanical properties. Percolation theory is utilized in further substantiating this hypothesis.     

Microstructure, hardness and stress in melted zone of 42CrMo steel by wide-band laser surface melting

Using laser surface melting (LSM) of a roller, to obtain the desired distribution of the microstructure, hardness and residual stresses with minimum distortion, is essential in order to improve machining efficiency and to achieve reliable service performance. In this study, a 3D finite element model has been developed to simulate the wide-band LSM process and predict the thermal and mechanical properties in the melted zone. The microstructure evolution, hardness distribution and stress field in the melted zone with different laser power were simulated. With the increase of the laser power from 3000 to 3800 W, the width and the depth of the laser melted layer increase, while the laser power has a little effect on the martensite contents, which exceed 90% in the melt-hardened zone. It greatly affects the mechanical properties in the melt-hardened zone with its volumetric expansion effect and the hardness increases by 2-3 times. The residual stress distributed within the melt-hardened zone is always of the compressive type. The amplitude of compressive stress exists in the transition region, and the amplitude of von Mises stress within the heat affected-zone (HAZ) decreases with the increase in laser power. The accuracy of the developed finite element simulation strategy is validated for phase proportion and hardness distributions through the wide-band LSM on roller steel with proper instrumentation for data measurement. This agreement is encouraging.     

Computational and experimental study of a melt-hardened zone on a roller modified by wide-band laser treatment

A three-dimensional finite element model has been developed to simulate the wide-band laser remelting process and predict the thermal and mechanical properties in the melt-hardened zone. The simulation of the laser remelting process was performed using the nonlinear thermo-mechanical properties, based on a wide-band heat source model. The temperature fields, phase transformations, hardness and residual stress distributions in the melt-hardened zone were analyzed. In the remelting zone, the transformed volumetric percentage of martensite is beyond 80% and the excessive transformed martensitic structure greatly affects the mechanical properties in the melt-hardened zone with its volumetric expansion effect. After remelting, the hardness can be improved and the residual stress distributed within the melted zone is mainly of the compressive type, while the tensile stress in the heat-affected zone (HAZ) may cause the initiation of cracks. The computational results are in good agreement with experimental measurements.     

First-Principle Calculations for Transition Phase and Elastic Properties of SrS

The transition phase and elastic properties of SrS from NaCl structure （B1） to CsCl structure （B2） are investigated by ab initio plane-wave pseudopotential density functional theory method and by the quasi-harmonic Debye model. The transition pressure varies non-linearly with temperature, and the pressure of the mechanical instability increases linearly with increasing temperature. It is shown that the B1 structure SrS is a most elastically anisotropic minerat any pressure. The Debye temperature, the heat capacity, thermal expansion and Gruneisen parameter over a wide range of pressures and temperatures are also obtained.     

Deformation mechanisms of ultra-thin Al layers in Al/SiC nanolaminates as a function of thickness and temperature

The mechanical properties of Al/SiC nanolaminates with layer thicknesses between 10 and 100 nm were studied by nanoindentation in the temperature range 25 to 100 °C. The strength of the Al layers as a function of the layer thickness and temperature was obtained from the hardness of the nanolaminates by an inverse methodology based on the numerical simulation of the nanoindentation tests by means of the finite element method. The room temperature yield stress of the Al layers showed a large ‘the thinner, the stronger’ effect, which depended not only on the layer thickness but also on the microstructure, which changed with the Al layer thickness. The yield stress of the Al layers at ambient temperature was compatible with a deformation mechanism controlled by the interaction of dislocations with grain boundaries for the thicker layers (>50 nm), while confined layer slip appeared to be dominant for layers below 50 nm. There was a dramatic reduction in the Al yield stress with temperature, which increased as the Al layer thickness decreased, and led to an inverse size effect at 100 °C. This behavior was compatible with plastic deformation mechanisms controlled by grain boundary and interface diffusion at 100 °C, which limit the strength of the ultra-thin Al layers.     

SrS相变和弹性性质的第一原理计算

利用平面波赝势密度泛函方法和准谐德拜模型研究了SrS从NaCl结构到CsCl结构的相变以及弹性性质.在零温下,我们计算的相变压强为17.9 GPa,这与实验值和其他作者的计算值符合很好.研究还表明:相变压强随温度增加而非线性地增加,然而力学不稳性的压强随温度增加而线性地增加.     

烧结压力对碳化钽陶瓷维氏硬度的影响

为了探究烧结压力对不同晶粒尺寸碳化钽(TaC)力学性能的影响,通过高温高压技术对纳米、微米尺寸TaC粉末进行高温高压烧结,制备不同烧结条件下的块状TaC陶瓷。利用X射线衍射等表征方法对烧结样品的物相、元素分布、压痕形态进行表征,结果表明:TaC在烧结过程中物相稳定,且无杂质渗入。利用维氏硬度计对不同烧结压力(3.0、4.0和5.5 GPa)条件下的3种陶瓷样品进行维氏硬度测试,并进行微观结构分析,结果表明:随着烧结压力由3.0 GPa提升到5.5 GPa,微米尺寸TaC的维氏硬度(21.0 GPa)优于3.0、4.0 GPa下的纳米尺寸TaC维氏硬度(17.5、19.2 GPa)。此外,研究发现,测试维氏硬度时,3.0 kg应用载荷对测试TaC维氏硬度更加精确。研究结果对结构陶瓷烧结和超高温陶瓷硬度研究具有指导意义。     

Mechanical properties of thin Ag films on a silicon substrate studied using the nanoindentation technique

The mechanical properties of thin Ag films of equal thickness containing grains of various sizes were studied. The film hardness was measured using the Oliver-Pharr techniques based on indentation work calculations or on direct measurements of the area of pyramid imprints in AFM images. In order to avoid the influence of a substrate on the measured hardness, a technique was developed to determine the true values of the film hardness. It was established that the hardness of Ag films decreases with an increase in mean grain size, whereas the elastic modulus remains almost unchanged. It was shown that the dependence of the yield stress of Ag films on grain size does not obey the classical Hall-Petch law.     

Pressure‐induced incompressibility and point singularity of mechanical properties of TaC in NiAs‐type structure

The structural and elastic properties of TaC in NiAs‐type structure under high pressure have been investigated using first principles calculations based on density functional theory. Results indicate that the incompressibility along the c‐axis of TaC exceeds that of diamond under higher pressure. Particularly, an interesting point singularity exists in its mechanical properties as the pressure increases from 20 GPa to 40 GPa. The minimal shear modulus, Young's modulus, Debye temperature, and maximum Poisson ratio of TaC are simultaneously obtained at 28 GPa. The calculations of hardness indicate that the NiAs‐type TaC crystal possesses excellent mechanical properties. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)