Papers by Keyword: Superplastic Deformation

Abstract: The isothermal constant strain rate tensile tests of the TC21 titanium alloy were conducted by SSAN-CMT4104 electronic tensile testing machine at different temperature. After superplastic deformation, the alloy was treated with double annealing heat treatment, and microstructure of the alloy was analyzed. The results show that dynamically recrystallization occurs during the superplastic tensile deformation. The primary α-grains aggregated and merged to form new crystal grains with irregular grain boundaries. The amount and morphology of primary α phase change gradually with the increasing of temperature. The alloy has duplex microstructure composed of primary α phase and different forms of β-transformed structure after superplastic deformation and double annealing. At deformation temperature of 930°C, the basket weave structure with equiaxed α phase appears.

Abstract: An earlier proposal is generalized to explain superplasticity in different classes of materials and grain size ranges. A definition of “superplasticity” as due to a unique physical mechanism, rather than in terms of extreme elongations and/ or strain rate sensitivity index, m, being more than or equal to 0.30 emerges.

Abstract: Grain refinement and superplastic deformation behavior of Zn-Al alloys were investigated in this study. To obtain fine grain size in Zn-0.3Al alloys, rolling and equal channel angular pressing (ECAP) were conducted at temperatures from 40 to 160°C after casting and homogenization heat treatment. Material processing maps for Zn-0.3Al alloy were constructed from a series of compression tests conducted at temperatures from RT to 200°C and strain rates from 3×10^-2 to 10¹ s^-1. Superplasticity of ECAPed specimens were evaluated at the temperature of 100°C under the strain rate of 2×10^-4 s^-1. After ECAP of the Zn-0.3Al alloy, elongation was dramatically increased up to 500%. The effects of ECAP on the texture and anisotropy in the superplastic deformation bebavior were found to be negligible.

Abstract: The hot deformation characteristics of Al-12.7Si-0.7Mg alloy were investigated on an Instron5500 electronic universal testing machine at strain rates ranging from 1.67×10^-4 s^-1 to 1.67×10^-3 s^-1 and testing temperatures ranging from 460 °C to 520 °C. The results show that strain rate had a great effect on the flow stress. The flow stress increased with increasing the strain rate. The true stress-strain curves of the Al-12.7Si-0.7Mg alloy were obtained, and the strain rate sensitivity and hot deformation parameters of deformation activation energy were calculated. The hot deformation constitutive equation with hyperbolic sine form was also obtained. The test calculations demonstrate that the constitutive equation describes the deformation features of the Al-12.7Si-0.7Mg alloy well.

Abstract: In order to study dynamic response of metal, this paper makes use of theoretical formula to investigate changes of temperature and grain size on steel target after the penetration of copper jet based on data gathered from the experiments. Deformed target penetrated by copper jet could be divided into superplastic deformation zone and normal deformation zone according to the different microstructure. Temperature distribution of each deformation zones is in turn calculated by two constitutive equations. The results indicate that areas with high temperature concentrate on the narrow zone near the penetrated channel. Then, the calculation of grain size conforms to the observation. It is obviously proven that the method used in this paper is trustworthy for calculating the changes of temperature and grain size of target caused by penetration.

Abstract: Equal channel angular pressing (ECAP) was conducted using the die with a 90° angled channel under routes A, B_C and C for hot extruded AZ91 magnesium alloy. Tensile tests were carried out at 300 °C and initial strain rates ranging from 2×10⁻⁴ to 5×10⁻³ s⁻¹. The experimental results show that different routes have obviously effect on elongation to failure. It is found that the highest elongation to failure is 410 % at a strain rate of 2×10⁻⁴ s⁻¹ for the ECAPed AZ91 alloy with route B_C. At the same strain rate, route B_C can bring the greater superplasticitic deformation compared with routes A and C. Moreover, the strain rate sensitivity coefficient m values of about 0.3 to 0.5 are attained for the ECAPed AZ91 alloys with different routes. For the ECAPed AZ91 alloys, the main superplastic deformation mechanism is the grain boundary sliding, while the main accommodation mechanism is the dislocation creep mechanism controlled by the grain boundary diffusion.

Abstract: Uni-axial hot tensile tests were used to investigate the transverse superplastic deformation of laser welded Ti-6Al-4V sheet. Microstructures of the weld bead before and after superplastic deformation were observed by optical microscope. The maximal deformation of 46% was obtained on weld joint at strain rate of 10−2s−1 and temperature of 1173K. A coefficient K was introduced to evaluate the non-uniform deformation of weld joint. After superplastic deformation the microstructure of weld bead transformed into lamellas from original acicular structure.

Abstract: Optimum variable strain rate forming paths based on two multiscale deformation-based stability criteria are developed. The first criterion is based on Hart’s linear stability analysis while in the second criterion; we introduce a modified one dimensional nonlinear long wavelength analysis introduced by Hutchinson and Neale [7] based on the well known 2-D Marciniak-Kuczynski criterion. The stability criteria are calibrated for the AZ31 Mg alloy at 400 °C yielding two different variable strain rate forming paths. These paths show that the nonlinear wavelength analysis is more sensitive to strain rate sensitivity and results in larger attainable uniform strains than Hart’s approach especially at low strain rates. This result is demonstrated through finite element simulations of a deep rectangular box using pressure profiles derived from the two variable strain rate forming paths. The FE results clearly illustrate that Hart’s approach underestimates the amount of uniform deformation and therefore prolongs the forming time to prevent failure compared to the nonlinear analysis.

Abstract: According to the characteristic of appearing cavitation in the metals during superplastic deformation, the influence of strain rate on cavity evolvement, the influence of cavity on superplastic deformation capability, and the formation, development process of cavity were investigated for Al-Cu-Mg alloy (i.e. coarse–grained LY12). The results show that: ①The pore nucleation occurs not only at triangle grain boundaries, but also along nearby the second phase particles, and even within grains. The cavities at the triangle grain boundaries are present in V-shape, others near the second phase particles and within grains are present in O-shape. These cavities may result from disharmony slippage of grain boundaries. ②The tendency of cavity development decreases with increasing of strain-rate. In lower strain-rate condition, though Al-Cu-Mg alloy has better superplasticity, many big cavities in the specimen may reduce the room temperature properties of the alloy. In higher strain-rate condition, Al-Cu-Mg alloy has certain superplasticity and room temperature properties as well as few cavities forming. By analyzing, viscous layer on grain boundaries is very thin and grain sizes can be refined during their extruding and rotating each other in higher strain-rate superplastic deformation condition. ③Growth and coalescence of cavity are the main reason of the superplastic fracture of Al-Cu-Mg alloy. And small and a certain amount of cavities with dispersion and independence state are very useful to crystal boundary slippage.

Abstract: Previous studies of superplasticity using surface markers have tended to use either diamond paste scratches, with which it is difficult to make quantitative measurements, or regular grids with spacing greater than the grain size, which prevents measurements of any deformation occurring within the grains. This paper reports the etching of regular marker grids with submicron line spacings using Focused Ion Beams (FIB), as well as mesoscopic grids with line spacings many grains across. These have enabled us to make quantitative measurements of deformation within the surface grains, and at length scales up to 0.5 millimeters. Further refinements used include the use of Interferometric Surface Profilometry to measure the out-of-surface displacement during deformation. Results on Sn-Pb eutectic alloy tested in tension in Regions I and II showed that there was almost no intragranular deformation anywhere in any of the surface grains. Grain boundary sliding took place, particularly at Sn-Sn grain boundaries, and the accommodation mechanism was separation of interfaces normal to the tensile axis where sliding was blocked, particularly Pb-Pb grain boundaries. It is argued that this is a surface artifact not readily available in the bulk thus the case is made for testing under a different strain condition.