Effects of Cold Deformation Prior to Sensitization on Intergranular Stress Corrosion Cracking of Stainless Steel

The effects of deformation, prior to sensitization, on intergranular stress corrosion cracking (IGSCC) were studied on the AISI 304 (UNS S30400) stainless steel (SS). The degree of sensitization (DOS) was quantified by the double loop electrochemical potentiokinetic reactivation (DL-EPR) method. The susceptibility to IGSCC was investigated by the slow strain rate test (SSRT) carried out in polythionic acid (PTA) solutions. The results were complemented by scanning electron microscopy (SEM) fractographs. Deformation was found to accelerate sensitization, and a peak in sensitization vs. deformation was always observed. This peak was found to shift toward lower deformations with an increase in sensitization temperature. At 700°C, prior deformation is able to desensitize or heal the SS after 24 h. IGSCC was observed in AISI 304 SS after some treatments. No one-to-one correspondence was observed between IGSCC and DOS; this could be explained by the fact that the DOS measured by the DL-EPR indicates the depleted regions below ~15% Cr, whereas IGSCC depends on the availability of continuous grain boundary paths that are chromium-depleted, along with strain rate and environment (pH, temperature, etc.). Deformation prior to sensitization causes carbide formation and chromium depletion to occur near dislocations within the grain interiors, in addition to along grain boundaries. The DOS does not differentiate between these interior regions and the grain boundary regions, and shows Sensitization is a common phenomenon in stainless steels (SS) when they are exposed to temperatures ranging from about 400°C to 800°C.1-20 Classical sensitization results from the nucleation and growth of chromium carbide along grain boundaries (in solution-annealed SS and nickel alloys) with simultaneous depletion of chromium in adjacent grain boundary regions. The extent of chromium depletion in near grain boundary regions is limited by the equilibrium concentration of chromium at the carbide-matrix interface. The equilibrium chromium level depends on the temperature, the chromium activity coefficient, the carbon activity, and the equilibrium constant for carbide formation. Hall and Briant showed the equilibrium chromium concentrations to be 6.6, 8.4, and 10.8 wt% in AISI 316LN(1) (UNS S31603)(2) sensitized at 600, 650, and 700°C, respectively.21 Sensitization occurs in the temperature range where carbide is thermodynamically stable (500°C)

On the correlation between Nd:YAG laser-induced wettability characteristics modification and osteoblast cell bioactivity on a titanium alloy

The factors responsible for modifications to the wettability characteristics of a titanium (Ti6Al4V) alloy bio-metal following Nd:YAG laser treatment and the effects thereof on the response of osteoblast cells were considered in this work. It was found that interaction of the Nd:YAG laser beam with the Ti6Al4V alloy resulted in the wettability characteristics of the bio-metal improving. Such improvements in the wettability characteristics of the Ti6Al4V alloy were found to be due to: an increase in the surface roughness; and increase in the surface oxygen content and an increase in the polar component of the surface energy. From the cell response tests it was determined that the osteoblast cell adhesion and proliferation on the Nd:YAG laser treated Ti6Al4V alloy samples was considerably greater than on the untreated samples. By isolating the effects of surface roughness it was possible to confirm or refute the existence of a correlation between wettability characteristics and osteoblast cell bioactivity for the Nd:YAG laser treated Ti6Al4V alloy. The findings indicated that the aspects of wettability characteristics: surface oxygen content and polar component of the surface energy play an important role in promoting cell proliferation, particularly when surface roughness was simultaneously increased. Thus it was possible to conclude that the wettability characteristics of the Nd:YAG laser treated Ti6Al4V alloy were correlated to osteoblast cell bioactivity

High power diode laser modification of the wettability characteristics of an Al2O3/SiO2 based oxide compound for improved enamelling

High power diode laser (HPDL) surface melting of a thin layer of an amalgamated Al2O3/SiO2 oxide compound (AOC) resulted in significant changes in the wettability characteristics of the material. This behaviour was identified as being primarily due to: (i) the polar component of the AOC surface energy increasing after laser melting from 2.0 to 16.2 mJm-2, (ii) the surface roughness of the AOC decreasing from an Ra value of 25.9 to 6.3 μm after laser melting and (iii) the relative surface oxygen content of the AOC increasing by 36% after laser melting. HPDL melting was consequently identified as affecting a decrease in the enamel contact angle from 1180 prior to laser melting to 330 after laser melting; thus allowing the vitreous enamel to wet the AOC surface. The effective melt depth for such modifications was measured as being from 50 to 125 μm. The morphological, microstructural and wetting characteristics of the AOC were determined using optical microscopy, scanning electron microscopy, energy disperse X-ray analysis, X-ray diffraction techniques and wetting experiments by the sessile drop technique. The work has shown that laser radiation can be used to alter the wetting characteristics of the AOC only when surface melting occurs

Wettability characteristics of an Al2O3/SiO2-based ceramic modified with CO2, Nd:YAG, excimer and high-power diode lasers

Interaction of CO2, Nd:YAG, excimer and high power diode laser (HPDL) radiation with the surface of an Al2O3/SiO2 based ceramic was found to effect significant changes in the wettability characteristics of the material. It was observed that interaction with CO2, Nd:YAG and HPDL radiation reduced the enamel contact angle from 1180 to 310, 340 and 330 respectively. In contrast, interaction with excimer laser radiation resulted an increase in the contact angle to 1210. Such changes were identified as being due to: (i) the melting and partial vitrification of the Al2O3/SiO2 based ceramic surface as a result of interaction with CO2, Nd:YAG HPDL radiation. (ii) the surface roughness of the Al2O3/SiO2 based ceramic increasing after interaction with excimer laser radiation. (iii) the surface oxygen content of the Al2O3/SiO2 based ceramic increasing after interaction with CO2, Nd:YAG and HPDL radiation. The work has shown that the wettability characteristics of the Al2O3/SiO2 based ceramic could be controlled and/or modified with laser surface treatment. In particular, whether the laser radiation had the propensity to cause surface melting. However, a wavelength dependance of the change of the wetting properties could not be deduced from the findings of this work

Minerological aspects of lead sintering

A brief overview on lead sinter microstructure is presented. Characteristic micro-structural features of a good and bad sinter are highlighted and these are used in a case study involving use of a low grade and complex concentrate of lead (-40% Pb) in the sintering operation. The plant sinter produced exhibited low strength and its nticrostructural examination revealed non-uniform distribution of porosity along with unsintered galena and low melting lead silicate phase. Part replacement of limestone by lime helped in producing sinter with good physical properties and desirable microstructure. The sinter with modified feed chemistry had more uniform distribution of porosity and presence of primarily a Pb-Fe silicate phase characterised by a (Pb+Fe):Si mole ratio of 3:1. Ca-Pb-Zn-Fe-Al-silicate phase identified as hardysonite and a spine! phase of the type (Fe,Zn)O.(Fe,Al),OJ. Lead nietal/oxide/sulphide occurred in the sinter only rarely. The likely implications of lime addition to the sinter charge mix are discussed Key Words: Lead. Complex and low grade concentrate. Sintering. Process Mineralog

The influence of a high power diode laser (HPDL) generated glaze on the wetting characteristics and the subsequent HPDL enamelling of ordinary Portland cement

High power diode laser (HPDL) surface glazing of the ordinary Portland cement (OPC) surface of concrete was found to effect significant changes in the wettability characteristics of the OPC. This behaviour was identified as being primarily due to: (i) the polar component of the OPC surface energy increasing after HPDL glazing from 3.46 to 15.56 mJm-2, (ii) the surface roughness of the OPC decreasing from an Ra value of 21.91 to 2.88 m after HPDL glazing and (iii) the relative surface O2 content of the OPC increasing by 4.5at% after HPDL glazing. HPDL glazing was consequently identified as occasioning a decrease in the enamel contact angle from an initial value of 1090 to 310, thus allowing the vitreous enamel to wet the OPC surface

Identification of the principal elements governing the wettability characteristics of ordinary Portland cement following high power diode laser surface treatment

The elements governing modifications to the wettability characteristics of ordinary Portland cement (OPC) following high power diode laser (HPDL) surface treatment have been identified. Changes in the contact angle, , and hence the wettability characteristics of the OPC after HPDL treatment were attributed to: reductions in the surface roughness of the OPC; the increase in the surface O2 content of the ceramic and the increase in the polar component of the surface energy, . What is more, the degree of influence exerted by each element has been qualitatively ascertained and was found to differ markedly. Surface energy, by way of microstructural changes, was found to be by far the most predominant element governing the wetting characteristics of the OPC. To a much lesser extent, surface O2 content, by way of process gas, was also seen to influence to a changes in the wettability characteristics of the OPC, whilst surface roughness was found to play a minor role in inducing changes in the wettability characteristics

Modification of the wettability characteristics of polymethyl methacrylate (PMMA) by means of CO2, Nd:YAG, excimer and high power diode laser radiation

The surface of the bio-material polymethyl methacrylate (PMMA) was treated with CO2, Nd:YAG, excimer and high power diode laser (HPDL) radiation. The laser radiation was found to effect varying degrees of change to the wettability characteristics of the material depending upon the laser used. It was observed that interaction with CO2, Nd:YAG and HPDL effected very little change to wettability characteristics of the PMMA. In contrast, interaction of the PMMA with excimer laser radiation resulted an increase in a marked improvement in the wettability characteristics. After excimer laser treatment the surface O2 content was found to have increased and the material was seen to be more polar in nature. The work has shown that the wettability characteristics of the PMMA could be controlled and/or modified with laser surface treatment. However, a wavelength dependence of the change of the wetting properties could not be deduced from the findings of this work

Sociobiological Control of Plasmid copy number

Background:
All known mechanisms and genes responsible for the regulation of plasmid replication lie with the plasmid rather than the chromosome. It is possible therefore that there can be copy-up mutants. Copy-up mutants will have within host selective advantage. This would eventually result into instability of bacteria-plasmid association. In spite of this possibility low copy number plasmids appear to exist stably in host populations. We examined this paradox using a computer simulation model.

Model:
Our multilevel selection model assumes a wild type with tightly regulated replication to ensure low copy number. A mutant with slightly relaxed replication regulation can act as a “cheater” or “selfish” plasmid and can enjoy a greater within-host-fitness. However the host of a cheater plasmid has to pay a greater cost. As a result, in host level competition, host cell with low copy number plasmid has a greater fitness. Furthermore, another mutant that has lost the genes required for conjugation was introduced in the model. The non-conjugal mutant was assumed to undergo conjugal transfer in the presence of another conjugal plasmid in the host cell.

Results:
The simulatons showed that if the cost of carrying a plasmid was low, the copy-up mutant could drive the wild type to extinction or very low frequencies. Consequently, another mutant with a higher copy number could invade the first invader. This process could result into an increasing copy number. However above a certain copy number within-host selection was overcompensated by host level selection leading to a rock-paper-scissor (RPS) like situation. The RPS situation allowed the coexistence of high and low copy number plasmids. The non-conjugal “hypercheaters” could further arrest the copy numbers to a substantially lower level.

Conclusions:
These sociobiological interactions might explain the stability of copy numbers better than molecular mechanisms of replication regulation alone

Mechanical Responses and Stress Fluctuations of a Supercooled Liquid in a Sheared Non-Equilibrium State

A steady shear flow can drive supercooled liquids into a non-equilibrium state. Using molecular dynamics simulations under steady shear flow superimposed with oscillatory shear strain for a probe, non-equilibrium mechanical responses are studied for a model supercooled liquid composed of binary soft spheres. We found that even in the strongly sheared situation, the supercooled liquid exhibits surprisingly isotropic responses to oscillating shear strains applied in three different components of the strain tensor. Based on this isotropic feature, we successfully constructed a simple two-mode Maxwell model that can capture the key features of the storage and loss moduli, even for highly non-equilibrium state. Furthermore, we examined the correlation functions of the shear stress fluctuations, which also exhibit isotropic relaxation behaviors in the sheared non-equilibrium situation. In contrast to the isotropic features, the supercooled liquid additionally demonstrates anisotropies in both its responses and its correlations to the shear stress fluctuations. Using the constitutive equation (a two-mode Maxwell model), we demonstrated that the anisotropic responses are caused by the coupling between the oscillating strain and the driving shear flow. We measured the magnitude of this violation in terms of the effective temperature. It was demonstrated that the effective temperature is notably different between different components, which indicates that a simple scalar mapping, such as the concept of an effective temperature, oversimplifies the true nature of supercooled liquids under shear flow. An understanding of the mechanism of isotropies and anisotropies in the responses and fluctuations will lead to a better appreciation of these violations of the FDT, as well as certain consequent modifications to the concept of an effective temperature.Comment: 15pages, 17figure