Search

ZnAl eutectoid superplastic alloy is a typical superplastic alloy, but because its strength and corrosion resistance at room temperature are relatively low, its use is restricted. The present study was conducted to improved the performance of the alloy by adding some elements, seven alloys being prepared by the addition of copper, magnesium, manganese, bismuth, etc. The values of yield stress, σb elongtion at fracture, δ, and strain-rate sensitivity index, m, were measured. Various corrosion tests ofeach alloy were carried out employing steam corrosion and salt-bath corrosion. The performance of Zn-22%Al eutectoid alloy with the addition of alloying elements (Cx) and without the addition of alloying elements (C1) was compared, the test results showing that the strength and corrosion resistance of all seven alloys was enhanced. The strength of one of the seven alloys was greater by a factor of 2.5 compared to the basic Zn-22%Al eutectoid alloy, δ values for this alloy reaching 1500 percent, its corrosion resistance being increased also. The resistance to intercrystalline corrosion of these alloys increased markedly, the best being a more than 10-fold increase. The effect of the addition of elements on the performance of the Zn-22%Al eutectoid superplastic alloys is discussed.

It's a hot topic of safety risk evaluation of buildings and structures which influenced by construction of adjacent underground engineering, however, the work hasn't been standardized yet, which can't satisfy the need of engineering construction. Based on deep research and practice, the concept, purposes, objects, bases, procedures, tasks, methods and achievements of safety risk evaluation are proposed. Furthermore, typical case is adopted to illuminate the whole details. We aim to provide some ideas for evaluation of adjacent construction engineering and provide some references for systematically developed of safety risk evaluation work of present buildings and structures.

The superplasticity of high strength 7075 aluminium alloy has been improved to a great extent by the new thermomechanical treatment proposed. This treatment (TMPA) includes solution treatment, overageing, warm-rolling deformation, recrystallization and an artificial ageing process. The maximum elongation may be up to 2100% under deformation at an initial strain rate of 8.33×10−4s−1 and a temperature of 510 °C, which is much higher than reported before. Observation of the microstructure changes revealed that the excellent superplastic elongation of the alloy seems mainly to be due to a decrease in the grain growth rate of the alloy and a reduction in the number of cavities nucleated during superplastic deformation.

The superplastic 7075A1 alloy was tested over a range of strain rates 10−2−10−4s−1 at a temperature range 430–510°C using specimens machined with the rolling direction parallel and perpendicular to the tensile axis. It is shown that the mechanical properties of the alloy, including the elongations to failure, are essentially identical. Microstructural observations show that the cavities tend to form in stringers and these stringers are always oriented along the tensile axis regardless of the rolling direction. The cavities are not nucleated primarily at large Fe-rich or Si-rich particles, nor do they grow from pre-existing microvoids which may be introduced during thermomechanical processing. The cavities are nucleated preferentially at small particles or some irregularities in the grain boundary during superplastic deformation.

A simple thermomechanical treatment for grain refinement of 7475 aluminium alloy is reported. The process includes solution treatment, rolling deformation, recrystallisation, and artificial aging. This treatment procedure is more suitable than traditional thermomechanical processes for application in a modern plant. The average grain size produced is approximately 10 μm. When the alloy is deformed at a temperature of 510°C and a strain rate of 8·33×10−4 s−1, a maximum elongation of 1700% can be obtained.MST/1660

A simple thermomechanical treatment («TMPA») for grain refinement in 7475Al alloy is reported. The process includes solution treatment, rolling deformation, recrystallization and an artificial aging process. This procedure is more suitable for application in a modem plant. The resulting average grain size is approximately 10 μm. When the alloy is deformed at 510 o C at a strain rate of 8.33.10 -4 s -1 a maximum elongation of 1700% can be obtained, which is higher than reported in the literature

Based on previous theoretical models of cavity nucleation during creep deformation, a new cavity nucleation model is proposed in this paper. The basic idea of this new model is that cavity nucleation during creep deformation depends not only on the stress concentration of a piled-up group of dislocations but also vacancy clustering. The new equation for the critical radius of cavity nucleation is as follows, rc = (2γ/σ∞) − (2d/3E) σ∞ where rc is the critical radius of cavity nucleation, γ is the surface energy, σ∞ is the remote applied stress, d is the grain size and E is Young's modulus. When the applied stress is small enough, the equation can be simplified to rc = 2γ/σ∞, which was proposed by Raj et al. In the limit, when the applied stress σ∞ is large enough, the critical radius goes to zero and the upper bound stress for cavity nucleation can be obtained, which is very similar to that proposed by Smith et al.

Cavity nucleation during superplastic deformation of a high strength aluminium alloy has been studied using a high voltage electron microscope and an optical microscope. The results show that cavity nucleation is only owing to superplastic deformation and not to pre-existing microvoids which may be introduced during thermomechanical processing. The main reason for cavity nucleation is the high stress concentration at discontinuities in the plane of the grain boundary due to grain boundary sliding.

A fractal model for cavity damage and fracture of materials during superplastic deformation is proposed in this paper. The formula which shows the effect of gauge dimensions and grain size on total superplastic elongation was inferred. The formula shows that the finer the grain size, the larger the elongation of specimen and a larger elongation can be obtained by increasing the gauge width and thickness or by decreasing the gauge length of specimen under the same deformation condition. The prediction of the new formula is consistent with most experimental results observed up to now.

A fine grained 7475Al alloy has been studied for its cavity growth behavior in superplastic tensile deformation. A simple formula for cavity growth rate considering cavity interlinkage during superplastic deformation has been given. This new formula has a wider application than the power-law growth formula, and is consistent with the cavity growth behavior of 7475Al alloy during superplastic deformation.

Generally, the elimination of the voids in bond interface is considered to be the rate-controlling mechanism for diffusion bonding. In an earlier paper, the author proposed a new theoretical model for solid-state diffusion bonding of similar matals, in which the grain growth across the bond interface was considered to be the governing mechanism for diffusion bonding. The aim of the model is to understand how the joint strength is affected both by process variables (temperature, pressure, time) and by the microstructures of the materials being joined. In this paper, the mechanism of grain growth and recrystallization is discussed, and the calculation of an appropriate kinetic equation is made based on a diffusion bonding experiment on a hot-rolled 7075 aluminum alloy. The calculated results basically agree with those of the experiment.

Cavity nucleation during superplastic deformation of a high strength aluminium alloy has been studied using a high voltage electron microscope and an optical microscope. The results show that cavities nucleation is due only to superplastic deformation and not to pre-existing microvoids which may be introduced during thermomechanical processing. The main reason for cavity nucleation is the high stress concentration at discontinuties in the plane of the grain boundary due to grain boundary sliding.

Because of the presence of surface oxide films, aluminum alloys are very difficult to join by diffusion bonding processes when there are no interlayers between mating surfaces. In the present investigation, the diffusion bonding of superplastic 7075 aluminum alloy has been carried out using a special method for surface treatment in the temperature range 500∼520°C, at pressures of 2.07∼3.OMPa, times of 90∼150min and a vaccum of 1×10−5torr. The joint strength obtained is the same as the base metal strength of the alloy, and the joint microstructure is indistinguishable from that of the base metal. In this paper, the effect of the superplastic treatment of the alloy on the bonding process is discussed, and it is proposed that the bonding mechanism is the migration of the initial bond interface caused both by the diffusion of atoms and by the growth of grains during the diffusion bonding process.