Ag对Al—Cu—Mg合金拉伸延性的影响

通过实验和理论计算研究Al-Cu-Mg-(Ag)合金的拉伸延性.研究表明:Al-Cu-Mg-(Ag)合金的拉伸延性与析出相的体积分数和尺寸有重要关系:析出相体积分数的增加将减小位错的有效滑移距离,从而降低合金的拉伸延性;在时效过程中,合金的拉伸延性首先随着时效时间的延长而降低,达到时效峰值后,拉伸延性随着时效时间的进一步延长而增大;Ag的加入可以提高Al-Cu-Mg合金中析出相的体积分数和强度,但降低合金的拉伸延性:通过控制时效时间虽然可以提高合金的拉伸延性,但同时将降低合金的屈服强度.     

Microstructural analysis of thermally induced and deformation induced martensitic transformations in Fe–12.5 wt.% Mn–5.5 wt.% Si–9 wt.% Cr–3.5 wt.% Ni alloy

Martensitic transformations induced by thermally and compression deformation at room temperature in Fe–12.5 wt.% Mn–5.5 wt.% Si–9 wt.% Cr–3.5 wt.% Ni alloy were studied in detail by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). From microstructural observations, it was seen that heat treated samples exhibited regular overlapping of stacking faults and ɛ martensite plates were formed parallel to each other. Also, TEM investigations showed that the orientation relationship between γ (fcc) and ɛ (hcp) phases corresponds to Shoji–Nishiyama type. With applied low plastic deformation rate, only ɛ martensite occurred in austenite grain. As a consequence, 4 and 25% plastic deformation at room temperature caused ɛ martensite formation in austenite phase and the new ɛ (hcp) and α′ (bcc) martensite formation in martensite phases, respectively. Orientation relationship between ɛ and α′ phases was found by the electron diffraction analysis.     

Interface structure and energy in Cu–71.8 wt.% Ag

The as-cast eutectic Cu–71.8 wt.% Ag was prepared. The orientation relationship and interface structure of Cu and Ag phases were investigated using high-resolution transmission electron microscopy. The interface energies and nucleation rates in the formation of different orientation relationships were discussed. The Cu and Ag eutectic lamellae can form the cube-on-cube and hetero-twin orientation relationships and the occurrence probabilities of the two orientation relationships are approximate in the solidification of the eutectic structure. There are misfit dislocations and monolayer steps at the interface between both phases having the cube-on-cube or hetero-twin relationship. The twin fault and lattice mismatch energies at the interface result only in an insignificant effect on the formation of different orientation relationships between both phases of the eutectic laminae.     

Effects of growth rate and temperature gradient on the microstructure parameters in the directionally solidified succinonitrile–7.5 wt.% carbon tetrabromide alloy

Succinonitrile (SCN)–7.5 wt.% carbon tetrabromide (CTB) alloy was unidirectionally solidified with a constant growth rate (V = 33 μm/s) at five different temperature gradients (G = 4.1–7.6 K/mm) and with a constant temperature gradient (G = 7.6 K/mm) at five different growth rates (V = 7.2–116.7 μm/s). The primary dendrite arm spacings, secondary dendrite arm spacings, dendrite tip radius and mushy zone depths were measured. Theoretical models for the microstructure parameters have been compared with the experimental observations, and a comparison of our results with the current theoretical models and previous experimental results have also been made.     

Nanoscale characterization of the transfer layer formed during dry sliding of Cu–15 wt.% Ni–8 wt.% Sn bronze alloy

The microstructure of the transfer layer, and the underlying severely plastically deformed layer (SPDL), formed during the dry sliding of a spinodally hardened Cu–15 wt.% Ni–8 wt.% Sn bronze against a stainless steel, is characterized at the nanoscale by conventional and analytical transmission electron microscopy, including energy-dispersive spectroscopy and electron energy loss spectroscopy. The SPDL consists of a Cu–Ni–Sn solid solution with elongated nanograins, due to extensive dislocation glide and twinning. In contrast, the transfer layer, 2–3 μm thick, is an equiaxed nanocomposite comprised of a Cu-rich metallic phase with a (Fe,Cr)₂O₃-based oxide precipitates, and forms as a result of the mechanical mixing and compaction of wear debris. The bronze in this layer has undergone dealloying, indicative of the importance of thermal effects. The dispersion of oxide in the transfer layer suggests a different type of forced mixing, possibly turbulent mixing. The transfer layer is observed to improve significantly the wear resistance of the bronze.     

Temperature dependent tensile and fracture behaviors of Mo–10 wt.% Cu composite

Uniaxial tension tests are carried out for the Mo–10 wt.% Cu (Mo–10Cu) composite under a scanning electron microscope (SEM) at a temperature range from 25 °C to 725 °C. The stress–strain curves are obtained with both the tensile strength and the fracture strain peaked at 500 °C. Further raise of temperature would reduce the tensile strength and the fracture strain. In-situ SEM observations reveal the microstructure characteristics for Mo–10Cu composite at different temperatures. The fracture is of brittle inter-granular type when uni-axially tensioned at room temperature. As the temperature increases, formation of slip bands and linkage of micro-voids via plastic shear are observed. The fracture is characterized by mixed inter-granular fracture and plastic shear. The fracture is of predominantly plastic shear when uni-axially tensioned at 500 °C. Under uniaxial tension at temperatures higher than 650 °C, Mo–10Cu composite embrittles due to the insolubility of molybdenum and copper, and the activated grain boundary diffusion of Cu. These results are of importance for the basic understanding of the microstructure–mechanical properties relationship, as well as for the evaluation of Mo–Cu composites in practical applications.     

Effects of Zr additions on structure and tensile properties of an Al-4.5Cu-0.3Mg-0.05Ti (wt.%) alloy

The effects of different Zr additions(0.05wt.%-0.5wt.%)on the structure and tensile properties of an Al-4.5Cu-0.3Mg-0.05Ti(wt.%)alloy solidified under a high cooling rate(18℃·s^-1),in as-cast and T6 heat-treated conditions were studied.The as-cast structure of the alloy consists of equiaxed grains ofα-Al with an average size of 64μm which is unaffected by the Zr additions,indicating the ineffectiveness of Zr in the grain refinement of the alloy.Scanning electron microscopy,along with X-ray diffraction analysis revealed the presence of elongatedθ-Al2Cu at the grain boundaries;in addition,coarse Al3Zr particles exist in the intergranular regions of the 0.5wt.%Zr-containing alloy.After the T6 heat treatment,the elongatedθparticles were fragmented;however,the coarse Al3Zr particles remained unchanged in the microstructure.Also,the formation of fineβ’-Al3Zr andθ’’-Al3Cu/θ’-Al2Cu phases during T6 heat treatment was revealed by transmission electron microscopy.The results of the tensile tests showed that the Zr additions increase the strength of the alloy in both as-cast and T6 heat-treated conditions,but reduce its elongation,especially with 0.5wt.%Zr addition.The 0.3wt.%Zr-added alloy in the T6 heat-treated condition has the highest quality index value(249 MPa).Fractography of the fracture surfaces of the alloys revealed ductile fracture mode including dimples and cracked intermetallic phases in both conditions.     

Precipitation processes in an Al-2.5Cu-1.5Mg (wt. %) alloy microalloyed with Ag and Si

We have produced precipitation on and along both the <100>_α and the {111}_α crystal systems by microalloying an Al-2.5Cu-1.5Mg (wt. %) base alloy with Ag and Si. Using electron microscopy and atom probe field ion microscopy, we have demonstrated that Ag+Si additions produce multi-component clustering reactions. When compared to comparable quaternary compositions, the quinary alloy exhibits a higher hardness. The first precipitates observed were GPB zones, rich in Cu and Mg and containing traces of both Si and Ag. Following ageing at 200 °C, the peak hardness microstructure consisted of rod-shaped GPB zones along <100>_α together with X′ and Ω precipitation, both as plates on the {111}_α planes. The Ω phase was less stable than the X′ phase and dissolved soon after ageing to peak hardness. This was associated with a rapid drop in hardness. The overaged microstructure was dominated by S phase.     

An intrinsic effect of hydrogen on cyclic slip deformation around a {1 1 0} fatigue crack in Fe–3.2 wt.% Si alloy

The effect of gaseous hydrogen on cyclic slip behavior around a fatigue crack tip introduced along the {1 1 0} plane in a Fe–3.2 wt.% Si alloy is precisely investigated by cross-sectional transmission electron microscopy and fractography. The results clearly suggest that the fatigue crack growth rate is promoted by hydrogen, whereas the number of dislocations emitted per load cycle is reduced. In addition, dislocation distribution is localized around the crack, causing quasi-brittle crack morphology. A sustained load test confirms that no subcritical crack growth caused by cleavage or micro-void coalescence exists along the {1 1 0} plane, which indicates that the observed increase in the fatigue crack growth rate is correlated solely to the intrinsic effect of hydrogen on the cyclic slip-off process around the crack tip.     

Pitting corrosion of rheocast A356 aluminium alloy in 3.5 wt.% NaCl solution

In this study, the microstructure and corrosion behaviour of rheocast and gravity-cast A356 aluminium alloys were examined and compared. Scanning Kelvin probe force microscopy (SKPFM) results proved that large potential differences between iron-containing intermetallics and the α-Al matrix were responsible for the initiation of the attack at the intermetallics/α-Al interfaces. For longer immersion times, corrosion attack proceeded through the eutectic areas. Semisolid processing refined the eutectic silicon and iron-intermetallics and reduced the potential difference between secondary phases and the matrix. This resulted in improved pitting corrosion resistance of the rheocast A356 aluminium alloy.     

The dependence of lamellar spacings and microhardness on the growth rate in the directionally solidified Bi-43 wt.% Sn alloy at a constant temperature gradient

Bi-43 wt.% Sn eutectic samples were directionally solidified upward with five different growth rates (V = 8.3–164.8 μm/s) at a constant temperature gradient (G = 3.55 K/mm) in a Bridgman type directional solidification furnace. The lamellar spacings (λ) and microhardness values (HV) were measured from the quenched samples; the directionally solidified Bi-43 wt.% Sn eutectic alloy and the minimum undercoolings (ΔT) were determined from the Jackson-Hunt eutectic theory. The dependency of lamellar spacings (λ), microhardness (H_ν) and undercooling (ΔT) on the growth rate (V) were analyzed. According to these results, it has been found that the value of λ decreases with the increasing value of V and that the values of HV and ΔT increase for a constant G. The values of λ²V, λΔT and ΔT V^−0.5 were determined by using the values of λ, ΔT and V. The results obtained in the present work have been compared with those predicted by the Jackson-Hunt eutectic theory and with similar experimental results.     

Investigation on microstructure characterizations and phase compositions of rapidly solidification/powder metallurgy Mg-6 wt.% Zn-5 wt.% Ce-1.5 wt.% Ca alloy

Rapidly solidification/powder metallurgy (RS/PM) Mg-6Zn-5Ce-1.5Ca (wt.%) alloy in form of rods has been prepared by hot extrusion with RS powders, produced via atomizing the alloy melt and subsequent splat-quenching on the water-cooled copper twin-rollers. Microstructure characterizations and phase compositions of the alloy have been investigated. The results showed that the RS/PM Mg-6Zn-5Ce-1.5Ca alloy was the characteristics of very fine grains with the size ranging from 200 to 650 nm and was composed of α-Mg, Mg_xZn_yCe_z phase with a few Ca (about 1 at.%) shortened as the Mg_xZn_yCe_z-(Ca) phase, and a small quantity of Mg₅₁Zn₂₀ phases. The melting point of the Mg_xZn_yCe_z-(Ca) phase in the alloy was a little higher than that of the Mg_xZn_yCe_z phase in RS/PM Mg-6Zn-5Ce alloy, which may be due to the dissolving of Ca in it. Moreover, the atomic percentage ratio of Zn to Ce in the Mg_xZn_yCe_z-(Ca) phase around the grain boundary was close to 1.5.     

Preparation by gravity casting and hydrogen-storage properties of Mg–23.5 wt.%Ni–(5, 10 and 15 wt.%)La

Mg–23.5 wt.%Ni–(5, 10 and 15 wt.%)La alloys were prepared by gravity casting and their hydrogen-storage properties were examined after pulverizing. The gravity cast Mg–23.5Ni–(5, 10 and 15)La alloys consist of α-Mg, Mg₂Ni and Mg₁₇La₂ phases. The activated Mg–23.5Ni–10La alloy has the highest hydrogen-storage capacity of 4.96 wt.%H (from P–C–T curve) and the highest initial hydriding rate (hydrogen content 3.83 wt.%H at 10 min) with an initial hydrogen pressure in the channel of 11 bar H₂ at 573 K. This is attributed to its containing the largest amount of the Mg₁₇La₂ phase, which is easily dissociable during the hydriding reaction.     

High temperature oxidation of AZ31 + 0.3 wt.%Ca and AZ31 + 0.3 wt.%CaO magnesium alloys

Magnesium alloys of AZ31 + 0.3 wt.%Ca and AZ31 + 0.3 wt.%CaO were cast and oxidized between 450 and 650 °C in atmospheric air. The initially added Ca and CaO enabled to cast the alloys in air without using environmentally hazardous SF₆ gas, by forming a thin CaO-rich barrier layer at the surface during casting. A thin CaO-rich barrier layer was also formed at the surface during oxidation in air, thereby increasing the oxidation resistance of the AZ31 alloy considerably. The initially added Ca and CaO reacted with Al to become Al₂Ca along the grain boundaries of the AZ31 alloy during casting.     

The effect of submicron-sized initial tungsten powders on microstructure and properties of infiltrated W-25 wt.% Cu alloys

In the present work, several W-25 wt% Cu alloys have been prepared through combined processes of high-energy ball-milling, liquid-phase sintering and infiltration, using the precursors of industrial copper powders with an average particle size of 50 μm and tungsten powders with alternative average particle size of 8 μm, 800 nm, 600 nm or 400 nm. Microstructure characteristics, relative density, hardness and electrical conductivity of the WCu alloys were investigated to elucidate the effect of initial particle size of tungsten powders. EBSD was further utilized to reveal the orientation and grain size distribution in the WCu alloys prepared by 8 μm and 400 nm-sized tungsten powders. The results showed that the WCu alloy made by 400 nm-sized tungsten powders exhibited excellent homogeneity for both sintered tungsten powders and grains, together with the highest relative density of 98.9%, the highest hardness of 230 HB, and good electrical conductivity of 48.7% IACS. Moreover, it also showed highly improved arc erosion and mechanical wear resistances.     

Effects of exposure at 700 °C on RT tensile properties in a PM γ-TiAl alloy

Gas atomized powder compacts of a Ti–47Al–2Cr–2Nb alloy have been used to assess their mechanical performance after long-term exposure at potential service temperatures. Unexposed test specimens were prepared with different machining conditions including turning, grinding and electro-polishing and the results revealed that the room-temperature tensile properties were not really affected by the machining conditions. On the other hand, tensile test specimens, when exposed to a temperature of 700 °C up to 400 h, showed a pronounced reduction in tensile elongation irrespective of the machining procedure. A number of techniques including Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD) experiments and surface roughness measurements, were used to identify the origins of such an embrittlement.     

Influence of nitridation and temperature cycling on oxidation of Ti-0.96 wt % Si alloy

The oxidation kinetics of prenitrided, and subsequently oxidized in either air or oxygen, Ti-0-96 wt % Si alloy have been investigated in the temperature range of 900–1100°C. The activation energies for various gas-alloy reactions are reported. Comparative kinetics data are given for oxidation of the alloy in oxygen or air without prenitridation. The oxidation resistance of nitrogen pretreated alloy is about the same as its resistance to oxidation in air. Temperature cycling during oxidation does not exert a beneficial effect on the oxidation characteristics of the alloy. X-ray and electron microprobe analyses reveal the presence of TiO₂, Ti₂O, -TiN, and -TiN in the scale and the concentration profiles of Ti, Si, O, and N in the alloy.     

Enhancement of corrosion resistance of Mg-9 wt.% Al-1 wt.% Zn alloy by a calcite (CaCO3) conversion hard coating

A calcite (CaCO₃) coating on Mg alloy, formed by chemical conversion treatment, was investigated. Aqueous with Ca²⁺ concentration of ∼220 ppm was employed in the chemical conversion treatment. Cross-sectional microstructures of the coated sample after 2 h of treatment revealed a two-layer coating structure. The corrosion current density (I_corr) of the coated sample was approximately two orders of magnitude lower than that of the untreated sample. Electrochemical impedance spectroscopy (EIS) and an appropriate equivalent circuit suggested that each of the layers of the two-layer coating effectively protects Mg alloy against corrosion.