Fe含量对Al-1.04wt.%Mg-0.64wt.%Si-0.23wt.%Cu合金析出相、力学性能和腐蚀性能的影响

Fe含量对于铝合金的熔炼成本、成形性能和使用性能具有重要影响,本文研究了 Fe含量(质量分数)对AlMg1SiCu合金析出相、力学性能和腐蚀性能的影响,并探讨了 Fe含量对性能的影响机制.结果表明,随着Fe含量由0.27％增加至1.44％,试样中的主要富铁相变化规律为:针状β-Al9Fe2Si2 →汉字状α-Al17 Fe3.2 Mn0.8 Si2 →短棒状Fe2Al3Si3.Fe含量由0.27％增加到1.21％时,试样的平均晶粒尺寸由173 μm减小到122 μm,继续提高Fe含量则平均晶粒尺寸增大.在本文Fe含量范围内,试样的屈服强度基本不变,但抗拉强度和断后伸长率随着Fe含量的增加而显著增加,当Fe含量超过1.21％时略有下降,其原因与富铁相的变化规律有关.试样的腐蚀速率与试样中主要富铁相的种类和数量有关,试样的自腐蚀电位随着Fe含量的增加先增大后减小,腐蚀电流随着Fe含量的增加逐渐增大.综合考虑力学和腐蚀性能因素,AlMg1SiCu合金合适的Fe含量约为0.50％.     

变形程度对强变形Al-4%Cu合金退火组织性能的影响

为了研究变形程度对强变形Al-4%Cu合金退火行为的影响,通过透射电镜观察和拉伸试验,研究了人工时效Al-4%Cu合金经过不同变形量的多向压缩变形(MAC),退火(120℃/60 min)后的显微组织和力学性能.研究表明:试样中的第二相在MAC过程中破碎溶回基体后,会在后续退火过程中再次析出,且析出相回溶的程度对退火组织性能的影响很大.含θ″相试样和含θ’相试样经MAC变形后,析出相完全回溶于基体,在退火过程中有新的第二相析出,试样强度升高;同时试样的的塑性也得到了提高,这与再析出粒子对超细晶粒长大的阻碍作用有关.析出相未完全回溶的含θ相试样,退火后强度低于退火前,析出相回溶和再析出交替进行.析出相基本回溶态强韧化效果最佳.     

On the characterization of Al-0.11 wt.%Sc alloy thin film

The characterizations (i.e., film adhesion, optical reflectivity, illumination aging and anti-corrosion behavior) should be considered for the metallic thin film served as a candidate of autolamp materials. The characterizations of three candidates (i.e., sputtered Al-0.11 wt.% Sc, sputtered pure Al, and thermally evaporated pure Al) have been investigated and compared. The sputtered Al-0.11 wt.% Sc film performed much better than any of the other two regarding the characterizations concerned. Examining through AFM, the sputtered Al-0.11 wt.% Sc revealed better surface property (including flatness and smoothness) than the others. Excellence in surface properties and optical reflectivity for the Al-0.11 wt.% Sc may arise from grain refinement in the film. Under long-term illumination, the films led to grain growth and hillocks formation. This illumination aging degraded the surface properties and optical reflectivity of the films, especially for the pure Al films. The sputtered Al-0.11wt.%Sc revealed more corrosion-resistance than the other films in 5% NaCl solution regardless whether or not under illumination aging.     

Anelastic relaxation associated with the motion of domain walls in barium titanate ceramics

The mechanical loss (Q ^–1) and shear modulus (G) were measured as a function of temperature in the range 130 K < T < 430 K for BaTiO₃ ceramics by an inverted pendulum at low frequencies ranging between 1-0.01 Hz. The Q ^–1 (T) and G(T) curves exhibit the three phase transitions observed in BaTiO₃. Each phase transition induces a loss peak and an anomaly in the elastic modulus. Moreover, three other loss peaks (R) are located below the phase transition temperature. These peaks can be described by an Arrhenius relationship, and the activation energies are determined as 0.92, 0.68, and 0.45 eV for the peak located in the tetragonal-, orthorhombic-, and rhombohedral-phase, respectively. These peaks are due to the interaction between the domain walls and the diffusion of oxygen vacancies in the domains.     

Effect of grain size upon flow and fracture in a precipitation-strengthened Ti-8 wt % Al-0.25 wt % Si alloy

The interaction of grain size and precipitation strengthening has been studied in a Ti-8 wt % Al-0.25 wt %Si alloy. Grain sizes varying from 9 to 90m were produced by warm-working and annealing the alloy in the single--phase field. A uniform distribution of the coherent ₂ particles in the matrix was produced by ageing the alloy in the two-phase ( + ₂) field. The yield strength Hall-Petch slopes of the alloys with and without the ₂ precipitates were found to be nearly equal, indicating that the precipitation and grain-boundary strengthening are linearly additive. While specimens containing no precipitates exhibited a high ductility for all grain sizes, the ductility of the specimens with the ₂ particles decreased drastically with increasing grain size. TEM examination of the specimens containing the precipitates revealed a highly planar, localized slip and SEM examination of the fracture surfaces of these specimens revealed a transition in fracture behaviour from highly dimpled to mixed cleavage and intergranular with increasing grain size.     

Pore filling in graphite particle compacts infiltrated with Al–12 wt.%Si and Al–12 wt.%Si–1 wt.%Cu alloys

The drainage curves (saturation versus applied pressure) that characterize pressure infiltration of Al–12 wt.%Si and Al–12 wt.%Si–1 wt.%Cu alloys into particulate preforms made out of carbon particles of three sizes have been determined. The results show that, no matter the alloy or the particle preform, saturation (pore filling) varies with pressure much faster at low than at high pressures. Besides, the drainage curves at high and low pressures, can be reasonably fitted with the semi-empirical model of Brooks and Corey, although with significantly different model parameters.     

Superplastic diffusion bonding of γ-TiAl-based alloy

With laser surface melting and conventional heat treatment, superplastic diffusion bonding of TiAl alloy samples was carried out. Three different microstructure, i.e. fully lamellar structure, fine dendritic structure and refined equiaxed structure are used and their effects on bonding quality were investigated, and the bond quality was assessed by shear test at room temperature. Sound bonds could be achieved at 900 °C by laser surface modification or by laser surface modification and pre-bond heat treatment at 1000 °C for 60 min, which is lower than conventional diffusion bonding temperature. The bonds were also post-bond heat treated at 1200 °C for 30 min, which improved the bond quality in all cases. The best shear strength of the bonds is greater than 80% of that of base metal.     

Solute diffusion in the γ′ phase of Ni based alloys

We systematically investigate the diffusion mechanisms of 3d (Ti–Cu), 4d (Zr–Ag) and 5d (Hf–Au) transition metal solutes in γ′-Ni₃Al phase using first-principles calculations. The results reveal that the diffusion of Ni-substituting solute is mainly controlled by the sublattice diffusion mechanism via Ni vacancies and the diffusion of Al-substituting solute is mostly governed by the formation of the anti-structure defects on the Ni sublattice with negligible contribution of the anti-structure bridge mechanism. The elements which occupy both the Al and Ni sites show a diffusion behavior similar to that of the Ni-substituting solutes. Our calculations show that larger atoms can move much faster than smaller atoms, which disprove the traditional view that larger atoms move slower than smaller atoms.     

The 3-dimensional morphology of dendrite during equiaxed solidification of an Al-5 wt.% Cu alloy

In a sample quenched during equiaxed solidification of an Al-5 wt.% Cu alloy, the multi-scales 3-dimensional morphology of equiaxed dendrite was observed. The slim primary stem and secondary branches constitute the frame of dendrite, and rows of dense tertiary branches further divide the 3-dimensional space. In the divided space, the quartic branches grow further. The dendritic branches,which are perpendicular to each other, can change their growth directions and coalesce into a whole. In the tertiary branches and quartic branches, the formation of double branch structures is induced by competitive growth. The branch that wins in the competitive growth will produce a cabbage-like structure by wrapping the failed branches. In addition, the side branch can also wrap the original parent branch to produce cabbage-like structures. Depending on the historical growth direction, the dendritic arms can form vein-like and spicate structures, and the shapes of single dendritic arm may be the cylinder, plate and trapezoid platform. According to the compositions and etching morphology, the single dendritic arm in the final solidification structures should coalesce from a fine porous structure. The porous structures at different length-scales are principally induced by the preferential growth. Based on 3-dimensional morphology of equiaxed dendrite, a new research object for the investigation of microsegregation was suggested.     

The effect of direct electric current on precipitation in a bulk Al-4 wt % Cu alloy

The effect of the stress of direct electric current on precipitation in a quenched Al-4.15 wt % Cu alloy is investigated by measurement of the change in electrical resistance of a bulk specimen as a function of current density up to about 3000 A cm^–2 at a given annealing time. A similar effect of d.c. stress is observed at 50, 75 and 100 C. For a lower current density, a constant or slightly increasing resistance is observed, while for a current density higher than a critical value, the resistance decreases with an increase in density. On the assumption that the temperature increase of a specimen due to the Joule heating is proportional to the square of the current density, it is concluded that there exists some effect of d.c. stress per se to prevent the precipitation reaction, aside from the effect of the temperature increase due to the Joule heating, and it is suggested that this retarding effect is related to the sweeping out of quenched-in excess vacancies into grain boundaries by electromigration.     

Room temperature instability of an Al-4%Cu super saturated solid solution in a nano-crystalline alloy produced by SPD

The long-term stability of a severely deformed aluminium alloy has been investigated when copper in solution is used to inhibit recovery. A grain thickness of only 70 nm was obtained in an Al-4 wt% Cu alloy, after processing by equal channel angular extrusion to a strain of ε_eff ~ 15, resulting in a lamellar nano-grain structure. However, the severely deformed solid solution was found to be unstable at room temperature and copious precipitation of θ occurred at new grain boundaries within the deformed state. The solute level fell to equilibrium within ~9 months. The precipitation kinetics were shown to occur at many orders of magnitude higher than can predicted by classical theory.     

Crystal growth of the primary silicon in an Al-16 wt % Si alloy

Studies on the crystallographic growth habit of primary silicon crystals in an Al-16 wt% Si alloy were carried out by X-ray micro focus Laue analysis and ECP (electron channelling pattern) analysis. The plate-like primary silicon crystals grow by the same mechanism as that for germanium dendrites, i.e. the TPRE (twin plane re-entrant edges) mechanism. The spherical primary silicon crystal in sodium treated melts is composed of several pyramidal grains with tops at the centre of the sphere. Many of these grains have a twin relation to each other. The sodium enriched regions are found at the boundaries of these pyramidal silicon grains. The external surfaces of the spherical primary crystals exhibit regular crystal facets. The surface facets are most frequently parallel to {111} plane but there are also some facets parallel to other less densely packed planes such as {100}, {211} and so on.     

Characterization of microstructural changes in an Al-6.8 wt.% Mg alloy by electrical resistivity measurements

Microstructural changes in an AlMg6.8 alloy after different thermo mechanical (TMT) and sensitization treatments were investigated by electrical resistivity measurements. The electrical resistivity was most affected by the content of Mg solute atoms in the α-Al matrix, due to β-phase precipitation, while contribution of the dislocation density to the resistivity of the AlMg6.8 alloy was less pronounced. The amount and distribution of the β-phase precipitated during sensitization were found to be strongly affected by the microstructure developed under the previously applied TMTs, i.e. by the dislocation density and the primary β-phase particles in the dual (α + β) phase structure. During sensitization of the specimens with a recovered/recrystallized dual (α + β) phase structure, precipitation of randomly distributed, globular β-phase particles occurred. Sensitization of cold deformed and recrystallized single α-Al structures induced β-phase precipitation in the form of a continuous layer along the shear bands/grain boundaries.     

Investigation of α platelet boundaries in a near-α titanium alloy

Transmission electron microscopy (TEM) of a bimodal near-α titanium alloy revealed the existence of retained β phase layers and silicide precipitates at the α platelet boundaries inside transformed β grains. The β to α phase transformation accompanied by the precipitation of silicide resulted in the formation of a large number of dislocations at α platelet boundaries. Orientation relationships between silicide, β phase and α phase were also identified. However high-resolution TEM (HRTEM) revealed crystal mismatches between these phases generating high strains at α platelet boundaries. The strengthening effects of the platelet boundaries are discussed in terms of dislocations slip across the boundaries. The mechanism that governs the β to α phase transformation is also discussed.     

Comparative determination of the α/β phase fraction in α+β-titanium alloys using X-ray diffraction and electron microscopy

A comparison is made between the measured α/β phase fractions in Ti-6246 using X-ray diffraction (XRD) and electron microscopy. Image analysis of SEM and TEM images was compared to the phase fraction estimate obtained using electron backscattered diffraction, lab and high-energy synchrotron XRD. There was a good agreement between the electron microscopic and diffraction techniques, provided that the microstructural parameters of grain size and texture are estimated correctly when using quantitative Rietveld refinement.     

Calculation of efficiency functions in 4π β-γ counting

An attempt is made to calculate an efficiency function applicable to 4πβ-γ coincidence measurements as the first stage of evaluating the order of the polynomial of the fitting function. For this purpose, the β-ray energy spectra and self-absorptions of spherical particle sources are calculated by the Monte Carlo simulation under the continuous slowing down approximation. On the other hand, it is shown that three sets of absorption coefficients and partial intensity ratios corresponding to a β-ray group give analytically the self-absorption for the same particle sources. Finally, we show that the efficiency functions applicable to ⁵⁹Fe and ¹³⁴Cs are easily obtained by using the energy spectra or self-absorptions.     

β to ω phase transformation due to aging in a Ti–Mo alloy deformed in impact compression

We investigated the roles of vacancies and their clusters introduced in a Ti–20mass% Mo alloy by high-speed compression in the formation of aged ω-phase crystals. Specimens were deformed by a static compression mode and a high-speed compression mode, and were then aged. The relationships between morphology of aged ω-phase crystals and deformation modes are discussed along with the roles of vacancies and their clusters in the nucleation and growth of aged ω-phase crystals. Aged ω-phase crystals were found to be smaller but of higher density in a high-speed deformation specimen. These results suggest that vacancies and their clusters easily become nucleation sites of aged ω-phase crystals. Several aged ω-phase crystals in a high-speed deformation specimen were of string-like shape. High-resolution electron microscopy confirmed that the string-like crystals have the ω-phase crystal structure. One of the roles of vacancies of and their clusters introduced by high-speed deformation is considered to be relief of compressive stress, which is predicted to arise in the course of transformation.     

Effect of heat treatments on discontinuous precipitation kinetics in Al-30 wt.% Zn alloy

The discontinuous precipitation kinetics in Al-30% wt. Zn alloy have been investigated at temperatures ranging from 348 to 503 K (75 to 230 °C) by using an optical microscopy, X-ray diffraction, differential dilatometer, differential scanning calorimetry and microhardness measurement. We have found that at all temperatures less than 180 °C the supersaturated solid solution of quenched alloy was observed to decompose completely by a cellular precipitation reaction. Quantitative metallography methods have been applied to measure the corresponding transformed volume fractions at different temperatures and times of precipitation. The variation of the heating rate and the application of different methods have allowed us to calculate two kinetic parameters of precipitation: the activation energy of the process and the Avrami exponent.     

Some metallurgical aspects of chip formation in cutting Ti-6wt.%Al-4wt.%V alloy

Adiabatic shear banding is a phenomenon observed in machining titanium and some other metals and alloys as well. The shear localized chips are important in the chip disposal process, machined surface integrity and the automation of machining operations. It is therefore necessary to understand the mechanisms of metal cutting such as the formation of shear banding in the produced chips. In this paper, the behaviour of the chip formation is investigated using various metallurgical analysis techniques. It was found, in cutting Ti-6wt.%Al-4wt.%V titanium alloy, that some non-diffusional phase transformation took place in the shear localized chips. The process of chip formation with shear banding and the effects of cutting conditions on shear banding frequency were also studied. The results of these tests are presented in this paper.