Strengthening mechanisms in thermomechanically processed NbTi-microalloyed steel

The effect of deformation temperature on microstructure and mechanical properties was investigated for thermomechanically processed NbTi-microalloyed steel with ferrite-pearlite microstructure. With a decrease in the finish deformation temperature at 1348 K to 1098 K (1075 °C to 825 °C) temperature range, the ambient temperature yield stress did not vary significantly, work hardening rate decreased, ultimate tensile strength decreased, and elongation to failure increased. These variations in mechanical properties were correlated to the variations in microstructural parameters (such as ferrite grain size, solid solution concentrations, precipitate number density and dislocation density). Calculations based on the measured microstructural parameters suggested the grain refinement, solid solution strengthening, precipitation strengthening, and work hardening contributed up to 32 pct, up to 48 pct, up to 25 pct, and less than 3 pct to the yield stress, respectively. With a decrease in the finish deformation temperature, both the grain size strengthening and solid solution strengthening increased, the precipitation strengthening decreased, and the work hardening contribution did not vary significantly

Thermo-responsive Diblock Copolymer Worm Gels in Non-polar Solvents

Benzyl methacrylate (BzMA) is polymerized using a poly(lauryl methacrylate) macromolecular chain transfer agent (PLMA macro-CTA) using reversible addition–fragmentation chain transfer (RAFT) polymerization at 70 °C in n-dodecane. This choice of solvent leads to an efficient dispersion polymerization, with polymerization-induced self-assembly (PISA) occurring via the growing PBzMA block to produce a range of PLMA–PBzMA diblock copolymer nano-objects, including spheres, worms, and vesicles. In the present study, particular attention is paid to the worm phase, which forms soft free-standing gels at 20 °C due to multiple inter-worm contacts. Such worm gels exhibit thermo-responsive behavior: heating above 50 °C causes degelation due to the onset of a worm-to-sphere transition. Degelation occurs because isotropic spheres interact with each other much less efficiently than the highly anisotropic worms. This worm-to-sphere thermal transition is essentially irreversible on heating a dilute solution (0.10% w/w) but is more or less reversible on heating a more concentrated dispersion (20% w/w). The relatively low volatility of n-dodecane facilitates variable-temperature rheological studies, which are consistent with eventual reconstitution of the worm phase on cooling to 20 °C. Variable-temperature 1H NMR studies conducted in d26-dodecane confirm partial solvation of the PBzMA block at elevated temperature: surface plasticization of the worm cores is invoked to account for the observed change in morphology, because this is sufficient to increase the copolymer curvature and hence induce a worm-to-sphere transition. Small-angle X-ray scattering and TEM are used to investigate the structural changes that occur during the worm-to-sphere-to-worm thermal cycle; experiments conducted at 1.0 and 5.0% w/w demonstrate the concentration-dependent (ir)reversibility of these morphological transitions

Polymerization-Induced Self-Assembly of Block Copolymer Nano-objects via RAFT Aqueous Dispersion Polymerization

In this Perspective, we discuss the recent development of polymerization-induced self-assembly mediated by reversible addition–fragmentation chain transfer (RAFT) aqueous dispersion polymerization. This approach has quickly become a powerful and versatile technique for the synthesis of a wide range of bespoke organic diblock copolymer nano-objects of controllable size, morphology, and surface functionality. Given its potential scalability, such environmentally-friendly formulations are expected to offer many potential applications, such as novel Pickering emulsifiers, efficient microencapsulation vehicles, and sterilizable thermo-responsive hydrogels for the cost-effective long-term storage of mammalian cells

Self-regulated growth of tilted superlattices by atomic layer epitaxy

We report on a self-regulated method for the growth of tilted superlattices. It relies on the reconstructed surfaces alternatively stabilized during the atomic layer epitaxy (ALE) of compound semiconductors. The c(2×2)+(2×1) Cd-stabilized and the (2×1) Te-stabilized surfaces alternatively formed during the ALE of CdTe and CdMn(Mg)Te ensure a self-regulation of the growth at 0.5 monolayer deposited per ALE cycle for both CdTe and CdMn(Mg)Te. We are thus able to overcome the problem of precise flux control inherent to tilted superlattices

Atomic layer epitaxy of CdTe and MnTe

Atomic deposition techniques are investigated for binary semiconductors of the telluride family, namely CdTe and MnTe. An original method for directly determining the CdTe atomic layer epitaxy (ALE) growth rate—in monolayers/cycle—is proposed, consisting in monitoring the reflection high-energy electron diffraction (RHEED) sublimation intensity oscillations of an ALE grown CdTe layer deposited on a MgTe buffer layer. The ALE CdTe autoregulated growth rate at 0.5 monolayer/cycle (in the substrate temperature domain between 260 and 290 °C) is accounted for on the basis of an atomic model which relies on the alternating c(2×2) Cd and (2×1) Te surface reconstructions during the ALE cycle. RHEED studies on MnTe atomic deposition, together with x-ray diffraction and transmission electron microscopy on ALE grown CdTe/MnTe superlattices reveal that all deposited Mn atoms are incorporated so that no autoregulated growth can be achieved. Furthermore, less than one or just one monolayer of Mn must be sent on the surface per ALE cycle to obtain well controlled superlattices with abrupt interfaces

Growth of CdTe/MnTe tilted and serpentine lattices on vicinal surfaces

We have grown by atomic layer epitaxy CdTe/MnTe tilted and serpentine superlattices. These heterostructures are formed by depositing in the step-flow growth mode fractional monolayer superlattices (CdTe)m(MnTe)n, with p=m+n∼1, onto 2 °A and 2 °B Cd0.95Zn0.05Te vicinal substrates. Transmission electron microscopy images reveal a good in-plane CdTe/MnTe separation and a uniform short-range superlattice period. The very existence of those superlattices imply that Te-based vicinal surfaces present a regular array of monomolecular steps, with no important step meandering and no step bunching