1,721,434 research outputs found
Load relaxation behavior of superplastic Al alloys
No full textThe high temperature deformation behavior of superplastic Al alloys, such as 8090 AI-Li and 7475 Al, has been studied experimentally within the framework of an internal variable theory of inelastic deformation. A simple rheological model including the grain boundary sliding (GBS) has been developed and used to interpret the plastic flow behavior of these alloys at high temperatures. A series of load relaxation tests has been carried out for fine grained specimens of superplastic Al alloys at various temperatures. The effect of grain size has also been examined. A Hall-Fetch type relation has been found between the grain size and an internal strength variable (sigma*), instead of generally used flow stress (sigma). Based on the experimental observation, a necessary condition for superplastic deformation has finally been proposed in terms of sigma* and the static friction stress for GBS (Sigma(g)) as Sigma(g) less than or equal to sigma less than or equal to sigma*.X112sciescopu
Effects of temperature and microstructure on the superplasticity in microduplex Pb-Sn alloys
No full textSuperplastic deformation behavior of microduplex Pb-Sn alloys has been investigated in this study. The effects of test temperature and microstructure were examined within the framework of the internal variable theory of structural superplasticity. A series of load relaxation and tensile tests were conducted at room temperature for the alloys with various microstructures such as as-cast eutectic, equiaxed eutectic, hypoeutectic and hypereutectic alloys. For eutectic alloys, load relaxation tests were carried out from room temperature to 140 degreesC. The flow curves obtained from load relaxation tests on the superplastic Pb-Sn alloys were shown to consist of the contributions from interface sliding (IS) and the accommodating plastic deformation. The IS behavior could be described as a viscous flow process characterized by the power index value of M-g = 0.5, suggesting the onset of intense phase boundary sliding (PBS). Superplasticity of hypoeutectic alloy was comparable to that of superplastic eutectic alloys and superior to that of hypereutectic alloy. As the test temperature increased, contribution from IS appeared to be exhibited in lower stress and faster strain rate region.X111sciescopu
An internal variable approach to creep behavior of pure magnesium poly and single crystals
No full textA new interpretation for the creep behavior of pure magnesium single crystals as well as poly crystals with 99.94 mass % purity has been made by using an internal variable approach. A series of load relaxation tests and creep tests for magnesium single and poly crystals were performed at elevated temperature. The single crystals used in this study were grown from the melt using a modified Bridgman technique. The creep behavior has been well described by the internal variable theory based on dislocation dynamics, consisting of two deformation modes; dislocation glide and dislocation creep deformation modes. The flow curves obtained from the load relaxation tests at elevated temperature could be resolved into a dislocation glide and a creep component effectively by the internal variable theory. The dislocation creep component above 523 K for single crystal magnesium shifted toward the faster strain rate regime with increasing temperature. This could be due to activation of the cross-slip of dislocations from basal to prismatic planes above 523 K.X115sciescopu
The effect of grain size and temperature on the superplastic deformation behavior of a 7075 Al alloy
No full textThe high-temperature deformation behavior of a 7075 Al alloy has been investigated within the framework of a recently proposed internal-variable theory for structural superplasticity (SSP). The flow curves were obtained by performing a series of load relaxation tests for specimens with various grain sizes, at temperatures ranging from 445 degrees C to 515 degrees C. The overall flow curves were then separated into two parts, according to the respective physical mechanisms, viz., the grain-boundary sliding (GBS) and the accommodating dislocation glide processes, contrary to the conventional approach which uses a single power-law relation. These individual curves were then analyzed based on the internal-variable theory. Much valuable information has been obtained in this way, providing new physical insight as well as a more comprehensive understanding of SSP. The GBS curve could be described as a Newtonian viscous flow, signified by the power-index value of M-g = 1.0 for this alloy. The unresolved issue of threshold stress is also clarified and identified as a critical stress required for the GBS. The role of grain refinement is found to shift the gain-matrix deformation (GMD) curve into a higher stress and strain-rate region, while the GBS curve into a lower stress and higher strain-rate region along the respective characteristic scaling line to bring both curves into a common flow-stress region, in which the GMD and GBS can operate simultaneously, resulting in the usual superplastic deformation behavior.open1114sciescopu
On the role of constituent phases in superplastic deformation of a Pb-Sn eutectic alloy
No full textX114sciescopu
Load relaxation and creep behavior of a spray cast hypereutectic Al-25Si-2Cu-1Mg alloy
No full textWe studied the high temperature deformation behavior of a spray cast Al-25Si-2Cu-1Mg alloy within the framework of an internal variable theory for inelastic deformation. We then carried out a series of load relaxation tests at elevated temperatures to obtain flow curves between flow stress and strain rate. The results were analyzed based on constitutive relations prescribed by an internal variable theory. The hypereutectic Al-25Si-2Cu-1Mg alloy exhibited two distinctly different deformation modes, viz. plastic deformation induced by dislocation glides and creep deformation by dislocation climbs at high temperatures. These load relaxation test results were then compared with those obtained from a series of creep tests performed at elevated temperatures under various stress levels. They were found to agree well with each other, providing a means to replace the time-consuming creep test by a simple load relaxation test in order to obtain high temperature creep data.X1111sciescopuskc
An internal variable approach to deformation behavior of a Pb-Sn eutectic alloy
No full textX1118sciescopu
MACROSEGREGATION BEHAVIOR IN CONTINUOUSLY CAST HIGH-CARBON STEEL BLOOMS AND BILLETS AT THE FINAL STAGE OF SOLIDIFICATION IN COMBINATION STIRRING
No full textThe influence of various Electro-Magnetic Stirring (EMS) modes on reducing macrosegregation has been investigated by conducting a series of plant tests using a continuous bloom easter and a billet easter of POSCO as well as the relevant laboratory experiment. The emphasis was put on the reduction mechanism of macrosegregation due to a combination stirring including a final EMS (FEMS) in continuous casting high carbon steel blooms and billets. The variation of mushy zone shapes and crater end positions depending on EMS modes was also examined through a Pb addition test providing a close relationship between them and macrosegregation behavior. The optimum stirring pool thicknesses at final EMS (FEMS) have been determined as a result for each carbon steel tested providing a valuable production line data for plant operation and their relation with carbon content are also discussed in view of the theoretical calculation from a solidification model and a relative permeability measurement using a Tamman furnace. Finally, the reduction mechanism of macrosegregation has been clarified as due to the enhanced solidification rate, finely distributed segregation spots in the whole centre region, and narrowed width of mushy zone during the final stage of solidification under a combination stirring mode.X1138sciescopu
Effects of nitrogen on the deformation behavior of duplex stainless steel
No full textThe effects of nitrogen on the deformation behavior of duplex stainless steel have been studied. The variation of strength was correlated with the characteristic microstructures. Analysis based on Hall-Petch relation confirmed that nitrogen enhances phase-boundary strengthening effect. The evolution of dislocation structure, slip traces, and misorientation distribution during deformation were also characterized to elucidate the effect of nitrogen on inelastic deformation mechanism.X112sciescopu
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