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Uniaxial stress-controlled ratcheting behaviors of expanded PTFE (ePTFE) under cyclic compressive loads were tested. The effects of temperature, stress rate and mean stress on the ratcheting behaviors of ePTFE considering multiple load paths were discussed in detail. Results present that the steady ratcheting strain is rate-independent when the stress rate is less than about 0.1 MPa/s, while it approximately linearly decreases with increasing the stress rate for greater stress rate. Additionally, the steady ratcheting is temperature-independent when the temperature is greater than about 150 °C, but it nearly linearly increases with enhancing the temperature for lower temperature. Especially, the stress rate almost has little effect on the ratcheting strain of ePTFE at 200 °C. Moreover, the accumulated ratcheting strain enhances rapidly in about the first 80 cycles, and subsequently tends to shakedown in the subsequent cycles for each load path. Furthermore, if a higher stress is used in the prior cycling, the greater ratcheting strain may be produced, and a negative ratcheting strain rate can be obtained in the subsequent cycling with lower mean stress due to the greater strain hardening and deformation resistance produced by the previous higher stress. [ABSTRACT FROM AUTHOR]

The limited slip systems of magnesium (Mg) and its alloys hamper their widespread applications in key areas. Rational design of such lightweight alloys requires fundamental knowledge of their microscopic plasticity mechanisms which, however, remain partially unresolved. Here, to obtain a better understanding of the plastic deformation mechanisms of Mg alloys, we performed tensile straining over a wide range of strain rates from 10−5 s−1 up to 2000 s−1, revealing for the first time the occurrence of a rate-dependent transition of the dislocation mechanisms. Such a shift of plasticity mechanisms is identified by two distinct activation volumes. Systematic experimental characterizations, such as transmission electron microscopy under two-beam conditions and synchrotron X-ray diffraction analysis, were employed to analyze both qualitatively and quantitatively the characteristics of dislocations at changing strain rates, revealing that the rate-dependent dislocation mechanisms are accompanied by the change of dislocation activities from easy-glide 〈 a 〉 dislocations to glissile 〈 c + a 〉 dislocations with increasing strain rates. Specifically, temporarily glissile 〈 c + a 〉 dislocations, enabled by a thermally activated transformation of dislocation cores from the dissociated configuration to the compact one, govern the plasticity at high strain rates. This is in stark contrast to the dominance of the easy-glide 〈 a 〉 dislocations at low strain rates. Meanwhile, it is found that abundant 〈 c + a 〉 dislocations do not necessarily lead to enhanced ductility, contrary to the common belief. We expect that these results will contribute to a further understanding of the plasticity mechanisms of Mg alloys. [Display omitted] [ABSTRACT FROM AUTHOR]

The present work, for the first time, quantitatively studies the evolution of both dislocations and deformation twins with various strain rates in an ATZ311 Mg alloy. Interrupted tensile tests were performed at the strain rates of 0.001, 1 and 600 s−1. Deformation twins were characterized by microscopy techniques and evaluated using the fractions of twin boundaries and twinned area. The average dislocation density and the relative fractions of dislocations with different Burgers vectors were determined from neutron diffraction data. It was found that the twin fraction increases remarkably and becomes saturated at lower strain rates (0.001-1 s−1), while the dislocation density only obtains an obvious enhancement at a much higher strain rate (600 s−1). Besides, the distinctively high flow stress at high strain rate leads to a substantial increase in the density of the hard-to-activate 〈 c + a 〉 -type dislocations, after the 〈 a 〉 -type dislocation density reaches rapidly its saturated value at a small strain level. The twinning modes with opposite polarities are able to coexist in the same grains because of the high local stress concentration at strain rates above 1 s−1. Image, graphical abstract [ABSTRACT FROM AUTHOR]