Materials Science Forum Vols. 561-565

Abstract: This paper reports the heat-resistance of plasma electrolytic oxidation (PEO) coatings formed on AZ91D alloy in phosphate electrolytes (P-film) and silicate electrolytes (Si-film). The results showed that the P-film was mainly composed of MgAl2O4, MgO and the Si-film was composed of Mg2SiO4, MgO. Thermoanalysis results verified that all these phases contained in the two type coatings had excellent thermal stability below 800oC. Meanwhile, the total area of heat erosion formed on the surface of samples tended to be increased with the exposure time prolonged in high temperature (410 oC and air atmosphere).

Abstract: Structure and configuration of boundary dislocations on the low angle tilt grain boundaries in alumina were considered based on the ideas that the boundary is composed of regularly arrayed edge dislocations and that the dislocations could dissociate into partial dislocations as well as glide dislocations in bulk. Moreover, the structure of the dissociated boundary dislocations were evaluated by the calculations based on an elastic theory. The calculations indicated that the largeness of the stacking fault region between partial dislocations formed by the dissociation will decrease with increasing tilt angles. It can be said that the idea and calculations used here will be powerful in considering the dislocation structure of low angle tilt grain boundaries that are not or are difficult to be identified.

Abstract: Polyimide surfaces after macroscopic wear at 80 to 260°C are studied by atomic force microscopy to give additional insight in the tribophysical and -chemical processes during sliding. Three sliding regimes are distinguished with hydrolysis resulting in rough surfaces, imidisation resulting in orientation of polymer molecules and melting resulting in short-range arrangements.

Abstract: In order to modify grain boundary character distribution (GBCD) and to improve intergranualr corrosion (IGC) resistance of 304 stainless steel, laser surface remelting experiments were conducted on 304 stainless steel using a 2kW CW Nd: YAG laser, and the effects of laser processing parameters on GBCD and corrosion resistance were investigated in detail under the optimal annealing condition (1220K 28h). The experimental results showed that combination of laser surface remelting and the following annealing treatment could change the GBCD remarkably and improve the IGC resistance of 304 stainless steel. However, there are no obvious effects of laser processing parameters on the final depth of the processed zone, although the depth of the molten pool increases with the increase of the laser output power or the decrease of the scanning velocity, and the subsequent GBCD and corrosion resistance.

Abstract: Surface layer hardness and concentration profiles of austenitic stainless steels after plasma carburizing and /or nitriding at 673 K were investigated. Carbon and nitrogen concentration were measured by glow discharge optical emission spectrometry (GDOES) and carbides or nitrides were detected by x-ray diffraction analysis (XRD) and TEM. The state of carbon at the treated surface was investigated by Raman spectroscopy. Separation of carburized layer and nitrided layer was observed in a simultaneous carburizing and nitriding plasma treatment.

Abstract: Interfacial microstructure of TiN-TiB2 composite, which was synthesized by hot shock compaction combined explosively shock condolidation and self-propagating high-temperature synthesis, was investigated by transmission electron microscopy (TEM). In the TiN-TiB2 composite included 60mol% TiN, an experimentally measured average grain size of the both TiN and TiB2 was approximately 500nm, and it decreased rather than those of the raw powders. By the conventional TEM observations, we clarified that there was a specific orientation relationship between cubic TiN and hexagonal TiB2. The high resolution electron microscopy (HREM) observations revealed that the TiN/TiB2 interphase boudnaries were atomically flat. We also observed grain boundaries of the composite and found that no secondary phases such as amorphous phase and precipitates were observed at the grain boundaries in the composite.

Abstract: Water cavitation peening (WCP) with aeration is a recent promising method in the surface enhancement technique, which can induce compressive residual stress in the near surface of mechanical components by the bubble collapse on the surface of components in the similar way as conventional shot peening. In this paper, the effect of WCP on fatigue crack growth behavior was investigated in single-edge-notched flat tensile specimens of S45C steel. The notched specimens were treated by WCP, and the compressive residual stress distributions in the near surface layer were measured by X-ray diffraction method. The tension-tension (R = Smin/Smax = 0.1, f = 10 Hz) fatigue tests were conducted. A Shimadzu servo-hydraulic fatigue test machine with in-situ observation by JSM-5410LV scanning microscope was used for all testing. Compared with those without WCP treatment, WCP can induce the residual compressive stress in the near surface layer, and delay the fatigue crack initiation, and decrease the rate of fatigue crack growth.

Abstract: Deformation Induced Ferrite Transformation (DIFT), i.e. transformation occurs during deformation applied in the temperatures above Ar3, has received wider attention since it has been proved to be a very effective way to produce ultrafine grained ferrite in low carbon steels. Although numerous works have been done on this topic in the past decade, the systematic works on DIFT in microalloyed steel, especially on the role of microalloying elements are still lacking compared with those in plain carbon steel. In this paper, the common features of DIFT will reviewed firstly, then an attempt will be made to elucidate the role of microalloying elements (niobium and vanadium) in DIFT, and the application of DIFT technology in microalloyed steels will be presented finally.