Zhida Wang

ESEM observations of SCC initiation for 4340 high strength steel in distilled water

The stress corrosion cracking (SCC) initiation process for 4340 high strength steel in distilled water at room temperature was studied using a new kind of instrument: an environmental scanning electron microscope (ESEM). It was found that the applied stress accelerated oxide film formation which has an important influence on the subsequent SCC initiation. SCC was observed to initiate in the following circumstances: (1) cracking of a thick oxide film leading to SCC initiation along metal grain boundaries, (2) the initiation of pits initiating SCC in the metal and (3) SCC initiating from the edge of the specimen.All these three SCC initiation circumstances are consistent with the following model which couples SCC initiation with cracking of a surface protective oxide. There is a dynamic interaction between oxide formation, the applied stress, oxide cracking, pitting and the initiation of SCC. An aspect of the dynamic interaction is cracks forming in a protective surface oxide because of the applied stress, exposing to the water bare metal at the oxide crack tip, and oxidation of the bare metal causing crack healing. Oxide crack healing would be competing with the initiation of intergranular SCC if an oxide crack meets the metal surface at a grain boundary. If the intergranular SCC penetration is sufficiently fast along the metal grain boundary, then the crack yaws open preventing healing of the oxide crack. If intergranular SCC penetration is not sufficiently fast, then the oxidation process could produce sufficient oxide to fill both the stress corrosion crack and the oxide crack; in this case there would be initiation of SCC but only limited propagation of SCC. Stress-induced cracks in very thin oxide can induce pits which initiate SCC, and under some conditions such stress induced cracks in a thin oxide can directly initiate SCC.

Control on the Morphology of ABA Amphiphilic Triblock Copolymer Micelles in Dioxane/Water Mixture Solvent

This work offers a typical understanding of the factors that govern the nanostructures of poly(4-vinyl pyridine)-b-polystyrene-b-poly(4-vinyl pyridine) (P4VP-b-PS-b-P4VP) block copolymers (BCs) in dioxane/water, in which water is a selective solvent for the P4VP block. It is achieved through an investigation of the amphiphilic triblock copolymer micelles by variation of three different factors, including water content (above CWC but under the immobile concentration), temperature (ranging from 20 °C to 80 °C), and copolymer composition (low and high PS block length). Transition of bead-like micelles to vesicles is observed with the increase of water content due to the increase of interfacial energy between the copolymer and the solvent. Effect of temperature superposed on that of water content results in various morphologies, such as beads, fibers, rods, capsules, toroids, lamellae, and vesicles. The interfacial tension between the BC and the solvent increases with the increase of water content but decreases with the increase of temperature, indicating that the micellar morphologies are resulted from the competitive interplay between the temperature and the water content and always change in a direction that decreases the interfacial energy. Based on the micellar structures obtained in this work and the effects of temperature superposed on water concentration, a diagram of phase evolution of different micellar morphologies is illustrated here, covering the temperature range from 20 °C to 80 °C and the water content changing from 20 vol% to 35 vol%. For the investigation of BC composition, morphological transition of vesicle-to-fiber, for high PS length, is observed as compared with bead-to-capsule for low PS length, as the temperature changes from 20 °C to 80 °C. Our research complements the protocols to control over the morphologies and the phase diagram describing P4VP-b-PS-b-P4VP micellar nanostructures in aqueous solution.

Dependence of size and morphology on shear flow for PS-based amphiphilic block copolymer micelles in aqueous solution

This contribution focuses on the impact of shear flow on size and nanostructure of PS-based amphiphilic block copolymer (BC) micelles by varying the stirring rate and copolymer composition. The results show that the vesicles formed from diblock copolymer (di-BC) of PS-b-PAA remain with vesicular morphology, although the average size decreases, with the increase of stirring rate. However, the multi-compartment micelles (MCMs) formed from tri-block copolymer (tri-BC) of PS-b-P2VP-b-PEO are quite intricate, in which the copolymer first self-assembles into spheres, then to clusters, to large compound micelles (LCMs), and finally back to spheres, as stirring rate increases from 100 r/min to 2200 r/min. Formation mechanism studies manifest that vesicles form simultaneously as water is added to the di-BC solution, termed as directassembly, and remain with vesicular structure in the flowing process. While for the PS-b-P2VP-b-PEO copolymer, spherical micelles at initial stage can further assemble into clusters and LCMs, termed as second-assembly, due to the speeding-up-aggregation of the favorable stirring. As a result, an invert V-relationship between tri-BC micelle dimension and stirring rate is observed in contrast to the non-linear decreasing curve of di-BC vesicles. It is by investigating these various amphiphilic BCs that the understanding of shear dependence of size and morphology of micelles is improved from self-assembly to second-assembly process.

SCC initiation for 4340 in distilled water

SCC initiation was studied for 4340 in distilled water at room temperature. SCC initiation was consistent with the following model which couples SCC initiation with cracking of a surface protective oxide by means of a dynamic interaction between oxide formation, the applied stress, oxide cracking, pitting and the initiation of SCC. An aspect of the dynamic interaction is cracks forming in a protective surface oxide due to the applied stress, exposing to the water bare metal at the oxide crack tip, and oxidation of the bare metal causing crack healing. Oxide crack healing would be competing with the initiation of intergranular SCC if an oxide crack meets the metal surface at a grain boundary. If the intergranular SCC penetration is sufficiently fast along the metal grain boundary, then the crack yaws open preventing healing of the oxide crack. If intergranular SCC penetration is not sufficiently fast, then the oxidation process could produce sufficient oxide to fill both the stress corrosion crack and the oxide crack; in this case there would be initiation of SCC but only limited propagation of SCC. Stress induced cracks in very thin oxide can induce pits which initiate SCC, and under some conditions such stress induced cracks in a thin oxide can directly initiate SCC.