固溶处理对7075/6009层状复合材料组织与性能的影响

通过分析7075/6009铝合金层状复合板材内层显微组织与显微硬度分布,研究了固溶处理对板材内层显微组织与力学性能的影响。结果表明:在470～500℃范围内,随着固溶温度的升高,板材内层和过渡区的显微硬度值呈先升后降的趋势,在485℃时达到峰值,而外层显微硬度值呈上升趋势;内层显微组织在485℃时残留的颗粒相数量最少,而在500℃时发生"过烧"。在15～300 min内,板材内层和过渡区显微硬度值在30 min时达到峰值,而外层显微硬度值变化不明显;内层显微组织随着固溶时间的延长而变粗大,残留颗粒相数量在30 min后趋于平衡。通过T6热处理工艺:485℃固溶30 min+水淬+175℃时效8 h,7075/6009铝合金层状复合板材可获得较高的力学性能:抗拉强度为404 MPa,屈服强度为364 MPa,伸长率为15.3%;同比T6热处理的6009铝合金板材,其抗拉强度提高36%,屈服强度提高75%,但伸长率降低16%。     

Electrical and magnetic properties of NdCuPb compound

Electrical and magnetic properties of NdCuPb compound were investigated by means of electrical resistivity, magnetization measurements in the temperature range 1.5–100 K. Low-field dc susceptibility goes through a maximum at T_N=13.2  K, indicating a paramagnetic to an antiferromagnetic transition and then follows a sharp peak at T=5.9  K. The susceptibility data exhibits a Curie–Weiss like behavior in the paramagnetic regime and the effective moment per neodium atom is found to be 3.62μ_B from the data at temperatures above 42 K. This value is exactly equal to that for Nd³⁺, while at lower temperatures, the data yields a little bit less than its free ion value. The ratio M/H versus temperature T curves for different values of the magnetic field split into multiple branches at about T=42  K due to crystalline field effects.In addition, electrical resistivity in a magnetic field up to 120 kOe was also measured in the same temperature range. The resistivity gives non-metallic behavior. The antiferromagnetic transition is clearly discern by a “Cr-like” anomaly at about 13 K, followed by a sharp increase in the resistivity (like a jump) at T=5.9  K where the susceptibility gives similar effect. On other hand, the magnetic contribution to the resistivity begins to decrease at T=42  K at which M/H curves merges. All these behavior may be attributed to crystal-field-splitting of neodium atoms’levels.     

Effect of silver on the glass forming ability of MgCuGdY bulk metallic glasses

Bulk metallic glasses have been synthesized in the Mg_xCu_yAg_zGd₁₀ and Mg_xCu_yAg_zGd₉Y₂ systems by HF melting and injection in conical and cylindrical shape copper moulds in order to determine their critical diameter and to prepare samples for mechanical tests. Silver can be substituted either to the copper or to the magnesium leading to different alloys whose properties have been measured by differential scanning calorimetry (DSC) curves and X-ray analyses. Up to a high diameter (10 mm), they present the structure of bulk metallic glasses. The effect of silver is in general to reduce the glass transition region (ΔT = T_x − T_g) but with an increase of the critical diameter which is not in agreement with glass forming ability (GFA) criteria when choosing ΔT as so, however the γ = T_x/(T_g + T_l) parameter is in agreement with the GFA as the γ parameter is increasing when the critical diameter is increasing. The effect of silver in the gadolinium–yttrium-based alloys is to decrease the ΔT range from 69 K without silver to 40 K with 9 at.% of Ag. Mechanical measurements made by compression tests at room temperature show that these alloys stay brittle with an elastic limit above 800 MPa but with no improvement of plasticity due to silver.     

Effect of potassium doping on the structural, magnetic and magnetocaloric properties of La0.7Sr0.3−xKxMnO3 perovskite manganites

We investigate the effect of potassium doping on the structural, magnetic and magnetocaloric properties of La_0.7Sr_0.3−xK_xMnO₃ (x = 0.05, 0.1, 0.15 and 0.2) powder samples. Our polycrystalline compounds were synthesized using the solid-state reaction at high temperature. X-ray diffraction characterizations showed that all our studied samples crystallize in the distorted rhombohedral system with space group. With increasing potassium content, the unit cell volume exhibits a broad maximum around x = 0.15. Magnetization measurements versus temperature showed that all our samples exhibit a paramagnetic to ferromagnetic transition with decreasing temperature. The Curie temperature T_C is found to decrease from 365 K for x = 0 to 328 K for x = 0.2 as well as the saturated magnetization M_sp which shifts from 3.68 μ_B/Mn for x = 0 to 3.05 μ_B/Mn for x = 0.2. The critical exponent γ defined as M_sp (T) = M_sp(0)[1−(T/T_C)]^γ is found to remain almost constant and equal to 0.33 for all our samples. The maximum of magnetic entropy changes |ΔS_max| of La_0.7Sr_0.3−xK_xMnO₃ for x = 0.05 and 0.15 is found to be respectively, 1.37 and 1.2 J kg⁻¹ K⁻¹ under a magnetic field change of 1 T.     

Low-temperature thermoelectric properties of α- and β-Zn4Sb3 bulk crystals prepared by a gradient freeze method and a spark plasma sintering method

The low temperature thermoelectric properties of Zn₄Sb₃ samples prepared by the gradient freeze (GF) method and sintering have been characterized. With decreasing temperature a dramatic rise in the thermal expansion is observed associated with the structural transition from β- to α-phase; Δl/l=2.8×10⁻⁴ at T_s^GF=257.4 K for GF and Δl/l=1.6×10⁻⁴ at T_s^S=236.5 K for sintered samples. Enhancement is observed in electrical conductivity and p-type thermopower at T_s^GF and T_s^S, while a reduction is observed in the magnetic susceptibility. The GF sample exhibits higher thermoelectric performance than the sintered sample. The power factor of the α-phase in the GF sample is twice as large as that of the β-phase; it exceeds 20 μW/cm·K² between 120 and 240 K, indicating that the α-phase Zn₄Sb₃ is one of the prime candidates for thermoelectric materials for cryogenic use.     

Coexistence of localized magnetism and itinerant ferromagnetism in a Gd5Y2Pd3 single crystal

Experimental results of the single-crystal X-ray diffraction, XPS, ac-magnetic susceptibility (ac-χ), dc-magnetization M(T), and electrical resistivity (ρ) measurements for the hexagonal Th₇Fe₃-type Gd₅Y₂Pd₃ single crystal are presented. Anomalies in (ac-χ), (T) and M(T)-curves have allowed to establish that Gd₅Y₂Pd₃ undergoes a long-range ferromagnetic-type ordering at T_C = 263 K, followed by a spin-reorientation below 190 K. The magnetization data indicate that there is an excess of the magnetic moment for the Gd³⁺ ions. The observed XPS, magnetic and electrical resistivity behaviour points to the coexistence of localized magnetism from the magnetic Gd³⁺ ions and itinerant ferromagnetism from 4d- and 5d-electron bands. We discuss the magnetic behaviour of the Gd_7−xY_xPd₃ solid solutions in terms of three competitive mechanisms: RKKY-interaction, magnetic frustration and spin-fluctuation.     

The micro-nanoformability of Pt-based metallic glass and the nanoforming of three-dimensional structures

Pt_48.75Pd_9.75Cu_19.5P₂₂ amorphous alloy exhibits obvious glass transition behavior at a temperature of T_g=502 K, which is below the crystallization temperature T_x=588 K, and develops a supercooled liquid state in a wide temperature range of ΔT_x (=T_x−T_g)=86 K. The present paper investigates the macroscopic and microscopic deformation behavior of the material and the possibility of micro-nano forming as a fabrication method and material for nano-devices. On a macroscopic scale, the material exhibits a Newtonian viscous flow in a supercooled liquid state. An index of the microformability is also evaluated by the geometrical transferability of a die shape to the material. For this study, we fabricated V-grooved micro dies of (100) Si by a silicon process. The V-groove dies are from 100 nm to 1 μm wide. The material exhibits superior formability on micrometer and nanometer scales and may possibly be applied to micro-nanomaterials for micro-nano devices.     

Thermodynamic and kinetic studies of Mg–Fe–H after mechanical milling followed by sintering

The transition metal complex hydride Mg₂FeH₆ has been successfully synthesized utilizing mechanical milling of a 2Mg–Fe mixture followed by heating at 673 K under 6 MPa of hydrogen pressure, without pressing step. The obtained yield of Mg₂FeH₆ was about 50%. Hydrogen storage properties of the Mg–Fe–H system, i.e. capacities, absorption/desorption kinetics and thermodynamic parameters, were examined. The pressure-composition isotherm (PCI) measurements of the samples at 548–673 K showed that the alloys possessed good cyclic stability and reversibility. Enthalpies and entropies of decomposition of the Mg–Fe–H system were evaluated by van’t Hoff plots. The absorption/desorption rates at 573 K were very fast in comparison with the reported data. The non-isothermal desorption of hydrogen was found greatly dependent on the thermal history of the sample.     

Mechanical and microstructural behaviour of 2024–7075 aluminium alloy sheets joined by friction stir welding

The aim of the present work is to investigate on the mechanical and microstructural properties of dissimilar 2024 and 7075 aluminium sheets joined by friction stir welding (FSW). The two sheets, aligned with perpendicular rolling directions, have been successfully welded; successively, the welded sheets have been tested under tension at room temperature in order to analyse the mechanical response with respect to the parent materials. The fatigue endurance (S–N) curves of the welded joints have been achieved, since the fatigue behaviour of light welded sheets is the best performance indicator for a large part of industrial applications; a resonant electro-mechanical testing machine load and a constant load ratio R=σ_min/σ_max =0.1 have been used at a load frequency of about 75 Hz. The resulted microstructure due to the FSW process has been studied by employing optical and scanning electron microscopy either on ‘as welded’ specimens and on tested specimen after rupture occurred.     

Mechanical Properties of Laminated Aluminum-Based Plates with Gradient Interface

Two laminated composites, 2024/3003 and 7075/6009 aluminum alloys were prepared by double-stream-pouring continuous casting (DSPCC) followed by plastic deformation and heat treatment. The interface characteristics between the external and internal layers of the composites in the as-cast and plastic deformation conditions were analyzed. The results show that the macrostructure difference between the external and internal layers of both composite ingots in the as-cast condition can be clearly clarified but th...     

Investigations of mechanically alloyed Ni–Zr–Ti–Si amorphous alloys with significant supercooled regions

This study examined the amorphization behavior of Ni₅₇Zr₂₀Ti_23−xSi_x (x=0, 1, 3) alloy powders synthesized by mechanical alloying technique. According to the results, after 5 h of milling, the mechanically alloyed powders were amorphous at compositions of Ni₅₇Zr₂₀Ti_23−xSi_x (x=0, 1, 3). The amorphization behavior of Ni₅₇Zr₂₀Ti₂₀Si₃ was examined in details. The conventional X-ray diffraction and synchrotron EXAFS results confirm that the fully amorphous powders formed after 5 h of milling. The thermal stability of the Ni₅₇Zr₂₀Ti_23−xSi_x amorphous powders was investigated by differential scanning calorimeter (DSC). As the results demonstrated, the amorphous powders were found to exhibit a large supercooled liquid region before crystallization. The supercooled liquid regions, defined by the difference between T_g and T_x, (i.e. ΔT=T_g−T_x), are 95 K, 66 K, and 88 K, for Ni₅₇Zr₂₀Ti₂₃, Ni₅₇Zr₂₀Ti₂₂Si₁, and Ni₅₇Zr₂₀Ti₂₀Si₃, respectively.     

Hot deformation behavior of Mg-7.22Gd-4.84Y-1.26Nd-0.58Zr magnesium alloy

The behavior evolvement of Mg-7.22Gd-4.84Y-1.26Nd-0.58Zr(GWN751K) magnesium alloy during the hot deformation process was discussed.The flow stress behavior of the magnesium alloy over the strain rate range of 0.002 to 2.000 s-1 and in the temperature range of 623 to 773 K was studied on a Gleeble-1500D hot simulator under the maximum deformation degree of 60%.The experimental results showed that the relationship between stress and strain was obviously affected by strain rate and deformation temperature.The flow stress of GWN751K magnesium alloy during high temperature deformation could be represented by the Zener-Hollomon parameter in the hyperbolic Arrhenius-type equation.The stress exponent n and deformation activation energy Q were evaluated by linear regression analysis.The stress exponent n was fitted to be 3.16.The hot deformation activation energy of the alloy during hot deformation was 230.03 kJ/mol.The microstructures of hot deformation were also influenced by strain rate and compression temperature strongly.It was found that the alloy could be extruded at 723 K with the mechanical properties of σ0.2 = 260 MPa,σb = 320 MPa,and δ = 18%.     

Effect of Pt on the superconducting and magnetic properties of ErNi2B2C

We show that the variation of T_c in Er(Ni_1−xPt_x)₂B₂C (T_c  10.6 K and T_N  5.7 K for x = 0) as a function of x proceeds in two steps: strong decrease of T_c for initial values of x (0 ≤ x < 0.10, T_c = 7.3 K at x = 0.1) and, thereafter, a relatively much weaker drop (almost a plateau) of T_c with further increase of x. T_N exhibits a slight, almost linear, decrease over the entire range of x studied here; T_N = 4.7 K for x = 0.2. Our results for x = 0.10 are in sharp disagreement with the results, namely, T_c < T_N, as reported by Felner et al. [I. Felner, D. Schmitt, B. Barbara, C. Godart, E. Alleno, J. Solid State Chem. 133 (1997), 5].     

Influence of heat treatment on the microstructure and mechanical properties of DZ951 alloy

DZ951 directionally solidified nickel-base superalloy is mainly strengthened by y phase.Regularly aligned cuboidal and bimodal γ precipitates were attained by two heat treatments.The effect of microstructure on the mechanical properties of DZ951 alloy has been investigated.The results indicate that MC carbide changes to little blocks during aging treatment at 1050℃ (HT1).MC carbide partly degrades into M23c6 and there is a layer of γ around the carbide during aging treatment at 115℃ (HT2),which is beneficial to the elongation of DZ951 alloy.Small γ volume fraction and the uneven deformation structure are contributed to low mechanical propexties of the as-cast alloy.HT1 alloy has a better stress rupture life at 1100℃50 MPa and yield stress at 20℃,800℃ and 1100℃,which is attributed to regularly aligned cuboidal γ phase and even deformation structure.HT2 alloy has a good combination of strength and ductility.This arises fi'om the bimodal γ precitates and the degeneration of MC carbide.     

Structural, magnetic and electrical properties of (La0.70−xNdx)Sr0.30Mn0.70Cr0.30O3 with 0 ≤ x ≤ 0.30

The structural, magnetic and electrical properties of (La_0.70−xNd_x)Sr_0.30Mn_0.70Cr_0.30O₃ perovskites (0 ≤ x ≤ 0.30) prepared by the usual ceramic procedure were investigated. Structural Rietveld refinement revealed that these compounds crystallize in a rhombohedral perovskite structure when x = 0, 0.10 and 0.20, while for x = 0.30 the structure becomes orthorhombic (Pbnm). It was found that the substitution of La by Nd reduces the Curie temperature (T_C). The FC, ZFC, M(H) and AC susceptibility measurements show typical canted-antiferromagnetism for the Nd-doped samples, in which a ferromagnetic component coexists with predominant antiferromagnetic interactions. The values of the magnetization (M(H)) decrease very slightly when increasing the Nd content, compared to the undoped sample (M_S values at 5 T and 2 K are, respectively, 47.9, 47.3 and 47.5 emu/g for x = 0.10, 0.20 and 0.30, compared to 48.2 emu/g for x = 0), indicating that the Nd³⁺ contribution is negligible compared to the total moment of the ferromagnetic (Mn/Cr) network. The resistivity increases by several orders of magnitude with Nd-doping and the semi-conducting behaviour persists in the whole temperature range. The interaction between Mn⁴⁺–O–Cr³⁺and Cr³⁺–O–Cr³⁺ is responsible for the semi-conducting state.     

外浇包熔体温度对7075/6009铝合金铸锭组织和硬度分布的影响

采用双流浇注连续铸造技术制备了7075/6009铝合金铸件,重点研究了外浇包熔体温度对该复合材料铸锭的组织及铸锭横截面上成分和硬度分布的影响.结果表明:在其他工艺参数一定的情况下,外浇包熔体温度从720℃上升到760℃时,铸锭外层合金的平均厚度减少了14.3%,铸锭内层合金的平均二次枝晶间距由约15μm增加到20μm,铸锭外层合金的平均二次枝晶间距由约80μm增大到100μm,铸件内层合金和外层合金的硬度分别降低了8.5%和9.3%.     

Thermoelectric properties of hot-pressed Zn4Sb3−xTex

A series of samples have been fabricated through vacuum melting method followed by hot-pressing for Zn₄Sb_3−xTe_x (x = 0.02–0.08), XRD patterns indicated that all the samples were single-phased β-Zn₄Sb₃. Electrical conductivity and Seebeck coefficient were evaluated in the temperature range of 300–700 K, showing p-type conduction. The thermoelectric figure of merit (ZT) was increased with the increase of Te content. ZT values of 0.8 and 1.0 were obtained at 673 K for Zn_4.08Sb₃ and Zn₄Sb_2.92Te_0.08, respectively.     

Investigation of the dependency of the upper critical magnetic field on the content x in Y1−xYbx/2Gdx/2Ba2Cu3O7−y superconducting structures

The Y_1−xYb_x/2Gd_x/2Ba₂Cu₃O_7−y superconducting samples for x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0 were prepared by using the solid-state reaction technique. Resistivity measurements of the samples were performed in QD–PPMS system under different magnetic fields up to 5 T in zero fields cooling regime. Using the resistivity data, the upper critical magnetic field H_c2(0) at T = 0 K for 50% of R_n was calculated by the extrapolation H_c2(T) to the temperature T = 0 K. The coherence length in T = 0 K were calculated from H_c2(0) and the effects of x in the composition on both the coherence length and the upper critical magnetic field were examined. The results showed that H_c2(0) varied from 84.05 to 122.26 T with the content x. The upper critical magnetic field in the temperature T = 0 K slightly decreased with increasing the content x. Using the content x, the upper critical magnetic field can be controlled and this can be used in the superconductivity applications.     

Effects of strain accumulation on the superplastic deformation behavior of 7075 Al alloy

The superplastic deformation behavior of a fine-grained 7075 Al alloy has been investigated within the framework of an internal variable theory for inelastic deformation. The theory takes the dislocation glide process within and across the grain boundaries (grain matrix deformation (GMD)) as the major accommodation mechanism for the grain boundary sliding (GBS). The flow curves were obtained by performing a series of load relaxation tests at the various prestrain values to examine the effects of accumulated strain on the superplastic deformation behavior. The most significant result obtained in this study is that the grain boundary characteristics change gradually with the strain accumulation from an initially Newtonian viscous flow signified with the power index value of M _g =1 to a non-Newtonian flow with the value of M _g =0.5 commonly observed in the various microduplex alloys such as Ti-6Al-4V. The variation of GBS characteristics with the prestrain is then examined by observing the microstructural evolution with the strain through the use of transmission electron microscopy (TEM).     

CoCrFeMnNi high entropy alloy matrix nanocomposite with addition of Al2O3

High Entropy Alloy Matrix nano-Composite (HEAMnC) based on CoCrFeMnNi with 5% additions of α-Al₂O₃ nano-particles was produced by mechanical alloying and consolidation in Hot Isostatic Press (HIP). Its microstructure consisted of spherical grains with average size 30–150 nm, among which 5–35 nm nano-particles of Al₂O₃ were identified. X-ray analysis and transmission electron microscopy studies with micro-analysis confirmed predominant presence of face-centered cubic solid solution with alpha aluminum oxide and small amount of M₂₃C₆ carbides. In comparison with CoCrFeMnNi HEA without additions, composite hardness increased up to 545 ± 13 HV, while yield strength grew from 1180 MPa to 1600 MPa, accompanied by decrease in plasticity and compressive strength.