The dielectric spectra of methyl m-toluate (MMT) in supercooled liquid and glassy states were measured over wide ranges of temperature T at ambient and elevated pressures P. We found that the frequency dispersion of the loss peak contributed by the structural α-relaxation is invariant to changes of P and T, while keeping the loss peak frequency fα(T,P) constant. This isochronal superposition property of the α-relaxation holds for different choices of fα(T,P). The invariant frequency dispersions for the same fα(T,P) are also indicated by the fractional exponent βKWW in the Fourier transform of the Kohlrausch–Williams–Watts (KWW) function. Similarly, the fragility m index of MMT keeps approximately constant on varying pressure, largely different from H-bonded glass formers. The secondary β-relaxation at a frequency higher than fα(T,P) is found to shift to lower frequencies by elevating pressure in concert with the α-relaxation. The ratio τα(T,P)/τβ(T,P) is approximately unchanged to variations of T and P while keeping τα(T,P) constant. These properties observed in MMT offer experimental evidence of the dynamic correlation between α- and β-relaxations in pure small-molecule glass-formers.
Two glass-transitions have been observed in some miscible molecular mixtures with notable differences in geometry or chemistry of constituents. The explanation of the phenomena has been puzzling with diverse structural models. Here, we present detailed studies on two glass-transition mixtures composed of tripropyl phosphate (TPP) and polystyrene (PS) by using calorimetric and dielectric measurements. We found that ageing between the two transitions always generates endothermic peaks at temperatures ∼4 K higher than the ageing temperatures and, subsequent thermal cycles around the peaks can remove the ageing effect and restore the systems, confirming the co-existence of nonequilibrium and equilibrium states in the regions. We also found that the broad glass transition thermogram is associated with highly stretched relaxation dynamics. The results allow us to draw a conclusion of continuous mobility gradient spanning the two TPP-PS glass-transitions, rather than complete phase separation.
After undergoing 75% hot deformation and 40% cold deformation, a variant of TWIP (Twin Induced Plasticity) steel with a composition of Fe−22Mn−2.7Al−0.95C−0.75Si (wt.%) was subjected to annealing at various temperatures (850°C, 650°C, 550°C) for 20min. The recovery annealed C550 sample exhibited a yield strength of 1289.20 MPa, a tensile strength of 1442.29 MPa and an elongation of 30.78%. Characterization and comparison of the samples were conducted using XRD, SEM, EBSD, TEM and uniaxial tensile tests. The yield strength of the sample was assessed through different strengthening mechanisms to analysis the exceptional properties of the recovery annealed sample. Experimental findings indicate that the high-density dislocation and the refinement of slip bands in the recovery annealed C550 samples contribute to their superior performance compared to the partially and fully recrystallized samples.
Microstructures of friction stir welding joint of 7075 aluminum alloy and H68 brass were observed by using optical microscopy(OM).The phase structure of the joint was determined by X-ray diffraction(XRD).Meanwhile,the micro-hardness of the welding joint was determined by microhardness tester.The results show that good weld joint is obtained when rotating speed is 1100 r/min and the welding speed is 80mm/min.7075 aluminum alloy and H68 brass are interweaving each other with river pattern and vortexes shape,intermetallic compounds are not formed in the lap joint.The weld nugget zone is mainly composed of recrystallization equiaxed grain.Meanwhile,the grain in nugget zone is refined and the micro-hardness is increased obviously.
In our recent study [T. Wu et al., J. Chem. Phys. 147, 134501 (2017)], an alkyl phosphate glass former was studied and it suggested that the enthalpy relaxation involving the motions of all parts of the molecule is global, while the dielectric relaxation detects the local rotation of the polar core. In this work, we study a series of trialkyl phosphates using calorimetric and dielectric measurements over a wide temperature range. The results indicate a departure of the dielectric fragility indexes from the enthalpic ones as the length of the branch chain increases in the trialkyl phosphates. The Kirkwood correlation factor (gk) is found to coincide at ∼0.6 at glass transition temperature (Tg) from triethyl phosphate to tributyl phosphate, indicating a similar structural alignment. The enthalpic relaxation serving as the more fundamental relaxation relevant to the structural relaxation is confirmed. Strikingly, we observed the relation of Tg to the chain length in alkyl phosphates, revealing a minimum Tg behavior, and its explanation assists in the understanding of the glass transition in relation to the structure of the glass-formers.
Found in our recent dielectric study of a planar and rigid glass-former, 1-methylindole (1MID), is an unusual secondary relaxation unrelated in its dynamic properties to the structural α-relaxation. We speculated that it originates from the in-plane motion of the molecules, and the supposedly universal Johari-Goldstein (JG) β-relaxation with strong connection to the structural α-relaxation in rigid glass-formers is not resolved [X. Q. Li et al. J. Chem. Phys. 143, 104505 (2015)]. In this work, dielectric measurements are performed in binary mixtures of 1MID with two aromatics of weak polarity, ethylbenzene (EB) and triphenylethylene (TPE), in the highly viscous regimes near glass transition. EB and TPE have smaller and larger molecular sizes and glass transition temperatures Tg than 1MID, respectively. Strikingly, the results show that the resolved secondary relaxations of 1MID in the two mixtures share the same relaxation time and their temperature dependence as pure 1MID, independent of the mode and degree of dilution. The results indicate that the unusual secondary relaxation is not directly coupled with the α-relaxation, and support the in-plane-rotation interpretation of its origin. On the other hand, the supposedly universal and intermolecular JG β-relaxation coming from the out-of-plane motion of the planar molecule has weaker dielectric strength, and it cannot be resolved from the more intense in-plane-rotation secondary relaxation because the dipole moment of 1MID lies on the plane.
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