Reflection and refraction of acoustic waves at the boundary between the ferromagnetic Heusler alloy and liquid

Reflection and refraction of quasi-longitudinal and quasi-transverse acoustic waves at a flat interface between a liquid and a ferromagnetic crystal of the Heusler alloy Ni_{2 + x + y} Mn_{1 ? x} Ga_{1 ? y} (in the region of its premartensitic and martensitic phase transitions) is considered. The directions of propagation and polarization and the amplitude of reflected quasi-longitudinal and quasi-transverse and of transmitted longitudinal waves in the (110) plane of the crystal are determined. In a wide region of phase transformations of this alloy, the possibility is shown of using temperature to efficiently control the angles of reflection and refraction of wave modes and the coefficients of their conversion due to a colossal acoustic anisotropy of the crystal. Beginning from a certain critical angle of the quasi-transverse wave, the quasi-longitudinal wave arising upon reflection acquires the nature of the accompanying surface vibration and at a large proximity to phase transition can be emitted into the bulk of the crystal. On the basis of the available experimental data for Ni₂MnGa crystals, numerical estimations of the above acoustic effects have been carried out.     

Reflection and refraction of acoustic waves at the boundary between a Heusler ferromagnetic alloy and dielectric

Reflection and refraction of longitudinal and transverse acoustic waves at a planar boundary between a dielectric and a ferromagnetic Heusler alloy Ni_2+x+y Mn_1?x Ga_1?y in the range of premartensitic and martensitic phase transitions is considered. We show the possibility of efficiently controlling the angles of reflection and refraction of both longitudinal and, especially, transverse waves, as well as all four coefficients of conversion of the wave type, by the variation of temperature which induces strong anisotropy of elastic properties of the crystal. Conditions for the appearance of critical angles and accompanying surface vibrations as well as for total reflection are discussed. The possibility is analyzed of emission of the arising wave gliding along the interface between the two media into the bulk of the material with the development of a new complex damped dissipationless wave process in the vicinity of the phase transition. Based on Trivisonno’s experimental data on the temperature dependences of the sound velocity in an Ni₂MnGa single crystal, the conclusions of the theory developed are illustrated by numerical calculations for a concrete Ni₂MnGa-quartz structure.     

Order-disorder transformations in the heusler alloy Cd2AgAu

A sequence of order-disorder phase transformations was studied in the alloy Cd-Ag-Au at or near the Heusler alloy stoichiometry Cd₂AgAu. Total energy calculations based on an LMTO-ASA Hamiltonian were performed for a series of bcc compounds, from which equilibrium formation enthalpies were computed. A set of effective cluster interactions were fit to these results and then used as input to ternary cluster variation method calculations. The predictions for the B2 to Heusler alloy transition agreed well with experiment, as did the low-temperature portion of an isoplethal section of the ternary phase diagram. Predictions of sublattice occupations indicated a preference of Ag for the Cd sublattice in the phase with B2 symmetry.     

Order-disorder transformations in the heusler alloy Cd2AgAu

A sequence of order-disorder phase transformations was studied in the alloy Cd-Ag-Au at or near the Heusler alloy stoichiometry Cd₂AgAu. Total energy calculations based on an LMTO-ASA Hamiltonian were performed for a series of bcc compounds, from which equilibrium formation enthalpies were computed. A set of effective cluster interactions were fit to these results and then used as input to ternary cluster variation method calculations. The predictions for the B2 to Heusler alloy transition agreed well with experiment, as did the low-temperature portion of an isoplethal section of the ternary phase diagram. Predictions of sublattice occupations indicated a preference of Ag for the Cd sublattice in the phase with B2 symmetry.     

Effect of external stress and bias magnetic field on transformation strain of heusler alloy Ni-Mn-Ga

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Strain-rate effect on dynamic recrystallization at grain boundary in Cu alloy bicrystal

The strain-rate dependence of the preferential occurrence of dynamic recrystallization (DRX) at grain boundary was investigated at 1023 K using orientation controlled Cu–Si bicrystals. Preferential DRX nucleation at the grain boundary was observed at all the strain rates. However, such preference of DRX nucleation at the grain boundary became less significant as the strain rate became higher. The strain-rate dependence of DRX nucleation at grain boundary will be discussed in relation to grain-boundary sliding, grain-boundary faceting and grain-boundary migration.     

Fe-40Ni-Ti合金中晶界高温迁移特征

利用金相观察、定量统计及透射电子显微镜分析,研究了含Ti和无Ti的Fe-40Ni合金在高温等温过程中的晶界迁移行为。结果表明:Fe-40Ni-Ti合金原始试样中存在两种析出,分别是凝固过程中形成的尺寸较大的Ti N颗粒及锻压过程中形成的应变诱导析出相,尺寸为10 nm左右。在1100℃等温30 min后,没有发现应变诱导析出相存在。直接观察结果则显示粗大的Ti N颗粒对晶界迁移有明显的阻碍作用,以类似于位错绕过析出颗粒的Orowan机制绕过Ti N颗粒,其主要原因可能是高温时晶界自由能明显各向同性,只有直接面对析出相的局部晶界受到析出相的阻碍,能够发生大曲率弯曲,晶界表现出较强的"柔性",此时析出粒子必然无法对其运动产生强烈的阻碍作用。     

Analysis of modal acoustic emission signals of LY12CZ aluminum alloy at anodic and cathodic polarization

Modal acoustic emission technique was used to analyze corrosion signals produced by polarization corrosion of LY12CZ aluminum alloy. The results showed that there were two-type waveforms for both anodic and cathodic polarization. Only extensional signals appeared within 1800 s, but extensional and flexural signals both appeared beyond 1800 s. The frequency spectrums were presented and analyzed, and potential source identification was discussed.     

The determination for the adhesion of film and substrate by surface acoustic waves

Layered film structures are widely applied in industry. The adhesion of thin film, which controls the product quality, is a key factor to be known before being used. The possibility of applying the surface acoustic waves (SAWs) technique to determine the adhesion of film and substrate is explored by a theoretical approach. The adhesion of interface is modeled by a spring boundary model. A commonly used structure of low-k film deposited on Si substrate in semiconductor industry is taken as an example. Theoretical results show that the SAWs dispersion curves are well separated for the distributed spring constants from 0.05 TPa to 5 TPa, which correspond to the poorly bonded and perfectly bonded interface, respectively. The results confirm that the SAWs technique has a high resolution on determining the adhesion property of film and substrate nondestructively.     

Grain boundary diffusion in Cr-doped NiO and the oxidation of Ni-Cr alloy

We have measured the diffusion of ⁶³Ni radiotracer into polycrystalline NiO, nominally doped with 0.1% Cr (Cr/Ni ratio) in the temperature range 600 to 900°C. The experiments show that Cr doping increases diffusion of Ni in the oxide lattice, but decreases diffusion of Ni along grain boundaries provided that the grain boundary Cr/Ni ratio is sufficiently large (greater than about 1%). This is believed to be due to the formation of immobile Cr-vacancy pairs which block fast boundary diffusion. The oxidation rate of Ni 0.1% Cr alloy is, however, slightly faster than that of pure Ni at 700°C, at which temperature grain boundary diffusion should be dominant. This apparent discrepancy between oxidation rates and diffusion studies, it is argued, is due to the complicating effects of low Cr mobility, a duplex film structure and inward oxygen transport during oxidation of the alloy.     

Cavitation and grain boundary sliding during creep of Mg-Y-Nd-Zn-Mn alloy

Creep of squeeze-cast Mg-3Y-2Nd-1Zn-1Mn alloy was investigated at the constant load in the stress range of 30-80 MPa. Tensile creep tests were performed at 300℃up to the final fracture.Several tests at 50 MPa were interrupted after reaching the steady state creep;and another set of creep tests was interrupted after the onset of ternary creep.Fraction of cavitated dendritic boundaries was evaluated using optical microscopy.Measurement of grain boundary sliding by observation of the offset of marker lines ...     

Measuring dispersion curves of acoustic waves using PVDF line-focus transducers

This paper presents a new method for measuring dispersion curves of elastic waves propagating in thin plates and layered media. The measurement is based on line-focus PVDF transducers that have been used successfully to measure surface acoustic wave velocities for both isotropic and anisotropic bulk materials. In this study, the transducer has been further applied to thin plates and layered media for measuring their wave velocity dispersion. In order to obtain the dispersion relation, a new waveform processing method is introduced. A theoretical analysis is given to explain how this method can be used for the determination of velocity dispersion curves. Experimental testing on two samples, a thin glass plate and a glass/silicon layered medium, were carried out using the line-focus PVDF transducer and the new waveform processing technique. The experimental results when compared to theoretical values indicate that the wave dispersion curves can be determined accurately. Discussions on the measurement system and its future application will be detailed.     

A new technique for measuring the acoustic nonlinearity of materials using Rayleigh waves

This note presents a procedure to generate nonlinear Rayleigh surface waves without having to drive the transmitting piezoelectric transducer at high voltages; driving at low voltages limits the excitation of the intrinsic nonlinearity of the piezoelectric transducer element, and enables an efficient measurement procedure to isolate inherent material nonlinearity. The capabilities of this proposed technique are demonstrated by measuring the material nonlinearity of aluminum alloy 2024 and 6061 plates with Rayleigh surface waves.     

GH3230合金高温拉伸特性及声发射现象

GH3230作为一种塑性较好的固溶强化型合金,力学性能参数是工程应用中需要重点关注的内容.研究了不同温度下的拉伸曲线,结果表明,在500℃、600℃拉伸过程中表现出明显的声发射现象,拉伸曲线出现力值抖动、锯齿型形状.700℃、800℃的拉伸过程中声发射局部发生,拉伸曲线局部出现锯齿状;900℃、950℃的拉伸过程中声发...     

Nonlinear acoustic characteristics of Fatigue microcracks in al alloy plate

A system consisting of piezoelectric transducers, arbitrary waveform generator, power amplifier, laser vibrometer, digital oscilloscope, and a computer has been constructed to study the nonlinear acoustic characteristics of fatigue microcracks of 2024-T351 aluminum alloy plates. Using two different frequency signals (f_H=810 kHz, f_L=100 kHz), the intact and microcracked samples are experimented, respectively. The experimental results show that in the intact samples there are only two fundamental frequencies and no harmonics and sidebands occur. In the microcracked samples, there is an abundance of the harmonics and sidebands, and the frequencies f_H±f_L and f_H±2f_L rise. These new frequency components can be used to indicate the presence of a microcrack or damage.     

Modelling of acoustic waves propagating in nesting Fibonacci super-lattice phononic crystal

Herein, we report construction of one kind of nesting-Fibonacci-super-lattice phononic crystal, in which the super-lattice cell is a well-defined Fibonacci generation sequence. We present a comparative study on band-gap structures of acoustic waves propagating in one-dimensional, nesting Fibonacci-periodic structure and simple-periodic structure. We find that there are more band gaps in nesting Fibonacci super-lattice models, and that they present behavior different from the split-up of band gaps with different generation numbers. With the increase of generation number, more band gaps split and occur. Additionally, when generation number becomes larger, Bragg scattering becomes more significant: the characteristic curves become flatter and band gaps become wider. Furthermore, we study the effect of various parameters such as density, thickness and defects on band-gap structures. Our work is significant both for understanding the intrinsic physical properties of nesting Fibonacci sequences and for providing flexible choices to meet real engineering requirements.     

Determination of magnetic flux density at the nanometer-scale antiphase boundary in Heusler alloy Ni50Mn25Al12.5Ga12.5

Understanding magnetic degradation in interface regions is important for materials science and technologies. An essential problem is the significant depression of ferromagnetism observed in an antiphase boundary (APB), which induces material functionalities/anomalies such as pinning of magnetic domain walls, etc. However, magnetization analysis on APB remains challenging due to the difficulty of collecting magnetic information from such a nanometer-scale interface region. We here use electron holography in order to obtain magnetic information from the narrow APB region in a Heusler alloy Ni₅₀Mn₂₅Al_12.5Ga_12.5. The magnetic flux density in APB (～5.6 nm in breadth) was determined at 0.04 T, and was only 12% of the value for the APB-free matrix. This magnetic depression could be explained by atomic disordering in the APB region, ascertained by high-resolution atomic-column imaging. It is expected that our microscopic approach can be used to understand anomalous interface magnetism observed in various magnetic compounds and/or spin devices.     

铝合金疲劳裂纹扩展声发射监测

采用声发射(acoustic emission,AE)技术对7N01铝合金单边缺口三点弯曲试样不同应力比、不同峰值载荷下疲劳裂纹扩展过程中声发射信号进行了监测,建立了裂纹扩展速率、声发射计数(count)与应力强度因子之间的关系.结果表明,大部分的声发射信号主要产生于疲劳循环载荷的低应力阶段,这主要是低应力阶段的声发射活动主要与裂纹尖端的塑性变形和裂纹闭合现象有关,声发射计数与应力强度因子之间呈指数增长的关系.基于所建立的声发射计数率与裂纹扩展速率的关系,可以预测疲劳损伤结构的剩余寿命.     

Grain boundary grooving at the singular surfaces

The unusual topographies of the grain boundary thermal grooves in Ni-rich NiAl were observed after annealing at 1400°C. One of the surfaces forming the grain boundary groove exhibited no curvature measurable in the atomic force microscope, thus indicating its singular character. The theory of grain boundary grooving at singular surfaces was developed in the small-slope approximation and under assumption of negligible diffusivity on these surfaces. The calculated groove shapes are in good agreement with the experimental data and differ considerably from the shapes predicted by the classical Mullins grooving theory for isotropic surfaces.