Ti20Zr20Hf20Cu20Be20高熵非晶合金应力弛豫行为研究

高熵非晶合金力学弛豫行为的研究,对于理解玻璃转变、塑性变形、弛豫机理等科学问题和拓展其工程应用极为关键。本文采用应力分析方法对Ti20Zr20Hf20Cu20Be20高熵非晶合金条带进行了研究,旨在揭示高熵非晶合金应力弛豫行为。通过分析其在恒定应变下较宽时间窗口和温度窗口内的应力衰减过程,发现在低于Tg的玻璃态下,高熵非晶合金中存在着弛豫解耦现象,分别是慢弛豫和快弛豫过程。其中慢弛豫呈现扩展指数衰减模式,弛豫时间存在对温度的Arrhenius动力学依赖关系,与长程原子重排运动有关,快弛豫过程对应于微观局部内应力的逐步消散过程。不论该高熵非晶合金变形处于弹性阶段还是发生了屈服,应力弛豫过程受应变的影响都较小。本研究揭示了高熵非晶合金中新的弛豫解耦现象和与之相关的独特动力学机制,拓宽了我们对高熵非晶合金弛豫动力学过程以及其本征特性的认知。     

Thermal behaviour of Pd40Cu30Ni10P20 bulk metallic glass

The thermal behaviour of differently milled Pd₄₀Cu₃₀Ni₁₀P₂₀ bulk metallic glass through the glass transition has been investigated by in situ high-energy synchrotron X-ray diffraction. Repeated heating and cooling were performed between the glassy and the supercooled liquid state. The changes in positions and intensities of the first and second diffraction maxima of the as-milled powder indicate irreversible changes during first heating up to the glass transition temperature T_g due to structural relaxation. After annealing, reversible structural changes with temperature are observed upon heating and cooling in the glassy phase, and in the supercooled liquid state respectively. The shift in the position of the first maximum scales approximately with the linear thermal expansion for the glassy state; however, this relation does not hold for the supercooled liquid. The structural transition from the glass to the supercooled liquid at the glass transition temperature is reflected by the intensity of the diffraction maxima and by a reversed temperature dependence of the position of the second diffuse maximum below and above T_g. The changes of the glass structure for the decrease of free volume by annealing are found to be different from those observed for the reversible volume expansion or shrinkage by varying the temperature. Therefore, the shift of the first diffuse maximum position of bulk metallic glasses cannot be used as a measure of the change in free volume.     

Synthesis, thermal stability and properties of [(Fe1−xCox)72Mo4B24]94Dy6 bulk metallic glasses

A series of [(Fe_1−xCo_x)₇₂Mo₄B₂₄]₉₄Dy₆ (x = 0.1, 0.2, 0.3, 0.4 and 0.5 at.%) bulk metallic glasses (BMGs) in rod geometries with critical diameter up to 3 mm were fabricated by copper mold casting method. This alloy system exhibited good thermal stability with high glass transition temperature (T_g) 860 K and crystallization temperature (T_x) 945 K. The addition of Co was found to be effective in adjusting the alloy composition deeper to eutectic, leading to lower liquidus temperature (T_l). The [(Fe_0.8Co_0.2)₇₂Mo₄B₂₄]₉₄Dy₆ alloy showed the largest supercooled liquid region (ΔT_x = T_x − T_g = 92 K), reduced glass transition temperature (T_rg = T_g/T_l = 0.622) and gamma parameter (γ = T_x/(T_g + T_l) = 0.424) among the present system. Maximum compressive fracture strength of 3540 MPa and micro-Vickers hardness of 1185 kg/mm² was achieved, resulting from the strong bonding structure among the alloy constituents. The alloy system possessed soft magnetic properties with high saturation magnetization of 56.61-61.78 A m²/kg and coercivity in the range of 222-264.2 A/m, which might be suitable for application in power electronics devices.     

Phase transition-induced high electromechanical activity in [(K0.5Na0.5)1−xLix](Nb0.8Ta0.2)O3 lead-free ceramic system

A high electromechanical activity is observed in the [(K_0.5Na_0.5)_1−xLi_x](Nb_0.8Ta_0.2)O₃ (x = 0, 0.02, and 0.03) lead-free ceramic system at and around the orthorhombic (O)–tetragonal (T) phase transition temperature (T_O–T). This activity is found to originate from an O–T phase transition region at ambient temperature rather than a classical morphotropic phase boundary (MPB) region intrinsic in the Pb(Zr_1−xTi_x)O₃ (PZT) lead-based ceramic system. Li modification enables a large decrease in T_O–T instead of constituting a classical MPB. In contrast to the nearly temperature-independent classical MPB behavior in the PZT system, the strong temperature-dependent phase transition behavior in the system may impose a challenge to temperature demanding applications.     

Structural and electrical properties of Gd(Ni1/2Zr1/2)O3

The Gd(Ni_1/2Zr_1/2)O₃ (GNZ) ceramic is synthesized by the solid-state reaction technique. The X-ray diffraction pattern of the sample shows monoclinic phase at room temperature. The dielectric dispersion of the material is investigated in the temperature range from 303 K to 673 K and in the frequency range from 100 Hz to 1 MHz. The relaxation peak is observed in the frequency dependence of the loss tangent. The relaxation time at different temperatures is found to obey Arrhenius law having activation energy of 1.1 eV which indicates the hopping of ions at the lattice site and may be responsible for the dielectric relaxation of GNZ. The scaling behaviour of loss tangent suggests that the relaxation mechanism is temperature independent. The frequency dependent conductivity spectra follow the power law. In the impedance formalism, the Cole-Cole model is used to study the relaxation mechanism of GNZ.     

Magnetic and electrical transport properties of nanostructured La0.67Ca0.33MnO3 networks

Contiguously nanostructured networks of La_0.67Ca_0.33MnO₃ (LCMO) are fabricated successfully by pulsed electron deposition (PED) onto the surface of porous anodic alumina oxide (AAO). The Curie temperature (T_C) is about 250 K. The metal–insulator transition temperature (T_p) is about 145 K without the magnetic field. The magnetoresistance can reach 84% near the peak temperature, which suggesting a strong magnetic correlated electronic transportation process. Zero field cooling (ZFC) and field cooling (FC) are split below the paramagnetic–ferromagnetic transition. The ZFC curves exhibit a typical blocking process. The observed spin glass and weak localization phenomena are due to the size effect, and it is found that the resistance dependent temperature curve above metal–insulator transition temperature (T_p) is more suitable to small polaron hopping (SPH) model.     

Hf对Zr-Al-Ni-Cu金属玻璃弛豫行为的影响

通过测定快淬态和退火态比热与温度的关系,研究了Hf元素对Zr₇₀Al_7.5Ni₈Cu_14.5（70Zr）、Zr₅₅Al₁₀Ni₅Cu₃₀（55Zr）和(Zr_0.75Hf_0.25)₆₅Al_7.5Ni₁₀Cu_17.5（65Zr_0.75Hf_0.25）厘米级金属玻璃焓和硬度弛豫的影响。结果显示：70Zr和55Zr合金的结构弛豫表现为单峰现象,在退火温度（T_a）接近玻璃转变温度（T_g）时出现弛豫峰;而65Zr_0.75Hf_0.25合金的结构弛豫表现为双峰现象,分别在523和648 K（接近T_g）时出现焓弛豫峰。70Zr和55Zr合金在T_g温度附近出现明显的单峰弛豫行为,表明合金在T_g之前的整个温度范围内具有较高的抵抗退火诱导结构弛豫的能力;另一方面,65Zr_0.75Hf_0.25合金在523 K附近出现一个弛豫子峰,可能是由于Zr-Hf原子对的键合性较弱,混合焓接近于零,随后在T_g附近出现明显的主弛豫。65Zr_0.75Hf_0.25和(Zr_0.5Hf_0.5)₆₅Al_7.5Ni₁₀Cu_17.5（65Zr_0.5Hf_0.5）合金的硬度也表现出类似的双峰现象,导致第一峰的T_a与焓弛豫峰的T_a一致。结果表明,满足非晶形成三原则的Zr、Al、Ni和Cu组成的类二十面体中程有序结构在低T_a温度下保持稳定;只有背离非晶形成三原则的65Zr_0.75Hf_0.25和65Zr_0.5Hf_0.5金属玻璃焓弛豫和硬度弛豫出现了双峰,表明形成大块金属玻璃非必要组元的增加将导致合金结构在低温退火过程中的不稳定性。     

-NMRC-NMR study on α-(BEDT-TTF)2MHg(SCN)4

The low temperature electronic states of the organic conductors, α-(BEDT-TTF)₂MHg(SCN)₄ [M = K, Rb], have been studied by -NMR¹³C-NMR with the use of ¹³C labeled samples. We observed anomalies in the nuclear spin-lattice relaxatin rate, T₁, and shift of the NMR line below 8 K for the K salt and below 12 K for the Rb salt. These indicate some phase transition. However, absence of critical fluctuation near the transition temperature and absense of splitting or broadening of the NMR line is against the static magnetic ordering as found in usual SDW systems such as the TMTSF compounds. The present results imply that the electronic density of states decreases to a half below the transition.     

Elastic behavior of La0.67Ca0.33MnO3:ZrO2 composites

A series of Colossal Magneto Resistance materials, with compositional formula (1 − x) La_0.67Ca_0.33MnO₃ + xZrO₂ (where x = 0%, 10%, 20%, 40%, 60%, 80%) were prepared by sol–gel technique. When characterized structurally by X-ray diffraction they are found to have cubic structure. After measuring their bulk densities, the ultra sonic longitudinal (V_l) and shear velocities (V_s) were measured at room temperature using the pulse transmission technique. Using the ultrasonic data, the values of Young's and rigidity moduli along with Poisson's ratio and Debye temperatures have been calculated. As the materials are porous, zero porous elastic moduli have also been arrived at using a well-known model. The observed variation of elastic moduli with varying ZrO₂ concentration has been explained qualitatively.     

Mechanical study of metallized polyethylene terephthalate (PET) films

Mechanical properties such as Young's modulus (Y), storage modulus (E′), glass transition temperature (T_g), tensile strength (σ), and yield strength (σ_y) of metallized polyethylene terephthalate (PET) films have been measured using Dynamic Mechanical Analyser (DMA). Commercially available PET film thickness of 20 μm has been used for metallization. Aluminium (Al) and lead (Pb) have been coated separately on PET films by vacuum thermal evaporation method to form metallized PET films. The T_g of Al-PET and Pb-PET films has been found to be 112.3 °C and 111.2 °C respectively whereas T_g of commercial PET film is 89.7 °C. The stress–strain curves of metallized PET films at room temperature and at different elevated temperatures have been taken which show significant temperature dependence. In particular, the yield strength shows a decrement with increasing temperature.     

FRICTION WELDING OF Zr55Al10Ni5Cu30 BULK METALLIC GLASS

采用摩擦焊对Zr₅₅Al₁₀Ni₅Cu₃₀块体金属玻璃进行了焊接, 当焊机主轴转速为4.0×10³---5.0×10³ r/min, 摩擦压力为80---100 MPa, 摩擦时间为0.2---0.4 s, 顶锻压力和保压时间分别为200 MPa和2 s时, 能够成功实施Zr₅₅Al₁₀Ni₅Cu₃₀金属玻璃的焊接. 用SEM, XRD和TEM观察分析未检测到晶化相, 焊缝处金属仍保持非晶状态. 金属玻璃的塑性在玻璃转变点T_g附近随温度变化很大, 在T_g以上具有良好的塑性变形能力, 这是实施摩擦焊焊接的重要基础.     

Microstructure and properties of cold consolidated amorphous ribbons from (NiCu)ZrTiAlSi alloys

Amorphous ribbons of compositions (Ni₅₆Cu₂)Zr₁₈Ti₁₃Al₆Si₅ and (Ni₃₆Cu₂₃)Zr₁₈Ti₁₄Al₅Si₄ were consolidated by high pressure torsion (HPT) at room temperature. In the HPT experiments a 6 GPa pressure and two turns were applied. Samples in the form of discs, 6–7 times thicker than the ribbons and about 10 mm in diameter were achieved. The minimal deformation for the homogenous consolidation was estimated to be in the range of 400%. XRD showed that the microstructure was dependent on the composition. The sample with high Cu content remained amorphous while the sample with low Cu content revealed some crystallization. DSC experiments allowed a comparison of the glass transition temperature T_g and crystallization process of the amorphous ribbon and HPT sample which were different. The glass transition temperature T_g of the amorphous HPT sample of (Ni₃₆Cu₂₃)Zr₁₈Ti₁₄Al₅Si₄ composition decreased. For both alloys the nanohardness and the elastic modules showed decrease for cold consolidated samples in comparison to the ribbons.     

Investigation of glass forming ability and crystallization kinetics of Zr63.5Al10.7Cu10.7Ni15.1 bulk metallic glass

The glass forming ability, thermal stability and non-isothermal crystallization kinetics of Zr_63.5Al_10.7Cu_10.7Ni_15.1 glass forming alloy were investigated. Its maximum glass forming dimension is up to 6 mm and its critical cooling rate is less than 40 K s⁻¹. It manifests two crystallization procedures and the second crystallization peak is more sensitive to heating rate than the first crystallization peak. The glass transition and crystallization both have remarkable kinetics effects. The ms fitted by Arrhenius and VF equations are consistent with each other. Small m value about 17 indicates better thermodynamic stability and GFA of Zr_63.5Al_10.7Cu_10.7Ni_15.1.     

Crystallization kinetics of new compound of V2O5–PbO–Li2O–Fe2O3 glass using differential thermal analysis

Different glasses with nominal compositions of (70 − x)V₂O₅–xPbO–20Li₂O–10Fe₂O₃ (0 ≤ x ≤ 12.5 mol.%) were successfully obtained by the melt quenching technique. Crystallization kinetics of these glasses was studied under non-isothermal conditions using the formal theory of transformations for heterogeneous nucleation. The procedure was applied to the experimental data obtained by differential thermal analysis, using several measurements at different heating rates. In addition, from the heating rate dependence of the glass transition temperature, the glass transition activation energy was derived. The crystallization results are analyzed and both the activation energy of crystallization process and the crystallization mechanism are characterized in terms of Gao–Wang method. The phases at which the glass crystallizes after the thermal process have been identified by X-ray diffraction. The diffractogram of the transformed material indicates the presence of microcrystallites of LiV₁₅O_35.5, Pb₃(VO₄)₂ and PbO₂ beside the remaining an additional amorphous matrix.     

Dielectric and piezoelectric properties of Sb doped (NaBi)0.38(LiCe)0.05[]0.14Bi2Nb2O9 ceramics

Sb⁵⁺-doped (NaBi)_0.38(LiCe)_0.05[]_0.14Bi₂Nb₂O₉ (represented as NBNLCS-x, where [] represents A-site vacancies) ceramics were prepared by the conventional solid-state route. The ceramics well sintered to approach ∼98.5% theoretical density and the tetragonality of crystal structure increased with Sb⁵⁺ additions. However, the Curie temperature (T_C) and the piezoelectric coefficient (d₃₃) of Sb⁵⁺-modified ceramics gradually decreased. The 3 mol% Sb⁵⁺-doped samples exhibited optimum properties with a d₃₃ value of ∼22 pC/N planar electromechanical coupling factor (k_p) of ∼11.2% and relatively high T_C of ∼765 °C. These results indicate that NBNLCS-x material is a promising candidate for high-temperature piezoelectric applications.     

Embrittlement of Zr-based bulk metallic glasses

Embrittlement of Zr_46.75Ti_8.25Cu_7.5Ni₁₀Be_27.5 and Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 bulk metallic glasses (BMGs) is studied after annealing at temperatures below and above the glass transition temperature T_g for time scales comparable with structural relaxation and crystallization. The effect of annealing on the bending ductility, the isoconfigurational elastic constants, the structure and the thermal stability is examined. The embrittlement during sub-T_g annealing originates from structural relaxation and can be reversed by subsequently annealing for a short duration above T_g. The embrittlement kinetics correlate with the structural relaxation. However, only a fraction of relaxation time at a given temperature (<T_g) is sufficient to embrittle the BMG significantly. Above T_g, plasticity is retained for annealing far beyond the relaxation time but, instead, embrittlement is caused by crystallization. The magnitude of the decrease in Poisson’s ratio is insufficient to explain the severe embrittlement within the framework of a critical value as previously suggested.     

Synthesis, structural, dielectric and electrical impedance study of Pb(Cu1/3Nb2/3)O3

Lead copper niobate Pb(Cu_1/3Nb_2/3)O₃ (PCN) prepared by the columbite precursor method and structurally characterized using X-ray diffraction analysis. The final phase X-ray diffraction diagram shows that final phase material has perovskite structure with pyrochlore phase. Temperature dependent of ′ and ″ shows that the compound exhibit dielectric anomaly in the studied temperature range. Impedance spectroscopy used to characterize the electrical behaviour. ac impedance spectrum results indicate that the relaxation mechanism of the material is temperature dependent and has dominant bulk contribution in different temperature ranges. Frequency dependence of the real (′) and imaginary (′′) part of the dielectric permittivity shows typical characteristic of ferroelectric materials. Temperature dependent of dc resistively shows that resistance decreases with the increase in temperature and follows Arrhenius behaviour in different temperature regions.     

Ferroelectric relaxor behavior and dielectric spectroscopic study of 0.99(Bi0.5Na0.5TiO3)-0.01(SrNb2O6) solid solution

0.99(Bi_0.5Na_0.5TiO₃)-0.01(SrNb₂O₆) was prepared by simple solid state reaction route. Material stabilized in rhombohedral perovskite phase with lattice constants a = 3.9060 Å, α = 89.86° and a_h = 5.4852 Å, c_h = 6.7335 Å for hexagonal unit cells. Density of material was found 5.52 g_m/cm³ (92.9% of theoretical one) in the sample sintered at 950 °C. The temperature dependent dielectric constant exhibits a broad peak at 538 K (?_m = 2270) at 1 kHz that shows frequency dependent shifts toward higher temperature - typical relaxor behavior. Modified Curie-Weiss law was used to fit the dielectric data that exhibits almost complete diffuse phase transition characteristics. The dielectric relaxation obeys the Vogel-Fulcher relationship with the freezing temperature 412.4 K. Significant dielectric dispersion is observed in low frequency regime in both components of dielectric response and a small dielectric relaxation peak is observed. Cole-Cole plots indicate polydispersive nature of the dielectric relaxation; the relaxation distribution increases with increase in temperature.     

Low field magnetoresistance properties of (La0.75Sr0.25)1.05Mn0.95O3 polycrystalline thin films on a-SiO2/Si substrates prepared by ex-situ solid phase crystallization

We report the low field magnetoresistance (LFMR) properties of (La_0.75Sr_0.25)_1.05Mn_0.95O₃(LSMO) films on a-SiO₂/Si substrates, prepared by ex-situ solid phase crystallization of amorphous films deposited by dc-magnetron sputtering at room temperature. The average grain size of the LSMO films was gradually increased with increasing annealing temperature (T _an ) and film thickness. High T _an also caused the growth of an amorphous inter-diffusion layer between a-SiO₂ and LSMO. The highest LFMR values of 16 and 1.0 % were achieved at 100 K, 1.2 kOe and 300 K, 0.5 kOe, respectively, from an LSMO film of 200 nm thickness annealed at 900 °C. In accordance with a modified brick layer model, grain boundary areal resistance gradually increased with increasing T _an and decreasing film thickness due to the penetration of the amorphous inter-diffused phase into the LSMO grain boundary. Improved LFMR values are attributable to modification of the LSMO grain boundary into a more effective spin-dependent scattering center.     

Electrical anisotropy of α-(BEDT-TTF)2KHg(SCN)4

The transverse magnetoresistance of α-(BEDT-TTF)₂KHg(SCN)₄ was investigated with the magnetic field rotated within a conducting ac-plane. It was found that the magnetic-field-orientation dependence of the magnetoresistance in the weak-field limit, ΔR(B,θ), has the form ΔR(B,θ) = B²(psin²(θ − θ_min) + qcos²(θ − θ_min), where θ is the angle betweeen a-axis and magnetic field direction and (p, q, θ_min) are temperature-dependent parameters. By examining the results based on the classical theory of magnetoresistance, it was concluded that the electrical anisotropy within be-plane is 3.5 4.5 and 2.5 3.0 above and below the phase transition at 10 K, respectively.