Medium Mn steels

Medium Mn steels containing 4-12 wt\% Mn are a relatively new class of steels which has gained significant research attention over the past decade. These steels are typically duplex (austenite$+$ferrite) and can be processed in various ways to produce different microstructure variants. Additional Twinning Induced Plasticity (TWIP) and/or Transformation Induced Plasticity (TRIP) effects can also be enabled in the austenite phase. Medium Mn steels therefore have great flexibility in design and can be processed to exhibit a large range of tensile properties. This thesis describes the alloy development journey to develop novel TWIP-assisted medium Mn steels with high strength and high ductility. The aim was to produce industrially scalable medium Mn steels compared to high Mn TWIP steels (16-30 wt\% Mn) which faced many problems during industrial trials due to the high Mn content. Various tools such as Thermo-Calc were used to guide an iterative alloy design process that involved vacuum arc melting, hot rolling, tensile testing and electron microscopy. Findings from each alloy would then be used to guide the next iteration. As a result, a series of novel medium Mn steels with decreasing Mn contents and different strain hardening mechanisms have been developed. It is hoped that the relationships between composition, processing and tensile properties established in this work will continue to guide future medium Mn steel development. In Chapter 3, a novel medium Mn steel with 8 wt\% Mn was produced that displayed both TWIP and TRIP effects. Cold rolling was investigated as a means to improve strength. It was observed that cold rolling introduced a larger density of twins and therefore twin intersections into the microstructure compared to uniaxial tension. It was postulated that the higher density of twin intersections which can act as $\alpha'$-martensite nucleation sites led to an enhanced TRIP response in the cold rolled samples, improving strain hardenability while retaining a signifcant amount of ductility ($\sim30\%$). Chapter 4 describes a scaling up study on the same steel. It was found that after a standard slab reheating cycle of 1250 \degree C for 2 h, some $\delta$-ferrite remained untransformed but the matrix was mostly compositionally homogeneous, even in Mn. When thermomechanically processed, the $\delta$-ferrite was observed to form stringers, resulting in a slight loss in yield and tensile strength but the strain hardening behaviour was preserved. Finally, the relative effects of TWIP and TRIP were investigated in chapter 5. A new medium Mn steel with 5 wt\% Mn was developed and processed differently to produce two microstructure types (lamellar and mixed equiaxed$+$lamellar). Electron microscopy on interrupted tensile specimens showed different twinning kinetics but a similar $\alpha'$-martensite nucleation and growth mechanism between the two microstructure variants. However, constitutive modelling of their strain hardening rate curves showed that the contribution to strength from twinning was insignificant compared to the TRIP effect, questioning the relevance of the TWIP effect in medium Mn steels.Open Acces

Mining Term Association Rules for Global Query Expansion: A Case Study with Topic 202 from TREC4

The sudden growth of the World Wide Web and its unprecedented popularity as a de facto global digital library exemplified both the strengths and weaknesses of the Information Retrieval techniques used by popular search engines. Most queries are short and incomplete attempts to describe or characterize the possible documents relevant to the query. It seems then natural to try and expand the queries with additional terms, which are semantically and/or statistically associated with the original query terms. In this paper we are looking at the mining of associations between terms for the exploration of the terminology of a corpus as well as for the automatic expansion of queries. The technique we use for the discovery of the associations is association rules mining [Agrawal 96]. The technique we propose is more flexible than previous techniques based on term co-occurrence since it takes into account not only the co-occurrence frequency but also the confidence and direction of the association rules. Our preliminary experiment results show we can get benefit from this novel technique

Molecular characterization of carbapenem-resistant acinetobacter baumannii isolated from the intensive care unit in a tertiary teaching hospital in Malaysia

The emergence of carbapenem-resistant Acinetobacter baumannii (CRAB) has now become a global sentinel event. CRAB infections often instigate severe clinical complications and are potentially fatal, especially for debilitated patients. The present study aimed to conduct molecular characterization on CRAB isolated from patients in the intensive care unit from 2015 to 2016 and determine the risk factors associated with patients’ mortality. One hundred CRAB isolates were retrospectively selected and included in this study. Antimicrobial susceptibility testing showed that all isolates remained susceptible to colistin, even though 62% of them conferred resistance to all other classes of antibiotics tested. OXA carbapenemase gene was found to be the predominant carbapenemase gene, with 99% of the isolates coharbouring bla(OXA-23)-like and bla(OXA-51)-like carbapenemase genes. All isolates were carrying intact CarO genes, with the presence of various degree of nucleotide insertion, deletion and substitution. Overall, PFGE subtyped the isolates into 13 distinct pulsotypes, with the presence of 2 predominant pulsotypes. Univariate analysis implied that age, infection/colonization by CRAB, ethnicity, comorbidity and CRAB specimen source were significantly associated with in-hospital mortality. Multivariate analysis identified a higher risk of mortality for patients who are of Chinese ethnicity with diabetes as an underlying disease. As CRAB infection could lead to high rate of mortality, comprehensive infection control measures are needed to minimize the spread of this pathogen

Application of Hanging Drop Technique for Kidney Tissue Culture

Background/Aims: The hanging drop technique is a well-established method used in culture of animal tissues. However, this method has not been used in adult kidney tissue culture yet. This study was to explore the feasibility of using this technique for culturing adult kidney cortex to study the time course of RNA viability in the tubules and vasculature, as well as the tissue structural integrity. Methods: In each Petri dish with the plate covered with sterile buffer, a section of mouse renal cortex was cultured within a drop of DMEM culture medium on the inner surface of the lip facing downward. The tissue were then harvested at each specific time points for Real-time PCR analysis and histological studies. Results: The results showed that the mRNA level of most Na+ related transporters and cotransporters were stably maintained within 6 hours in culture, and that the mRNA level of most receptors found in the vasculature and glomeruli were stably maintained for up to 9 days in culture. Paraffin sections of the cultured renal cortex indicated that the tubules began to lose tubular integrity after 6 hours, but the glomeruli and vasculatures were still recognizable up to 9 days in culture. Conclusions: We concluded that adult kidney tissue culture by hanging drop method can be used to study gene expressions in vasculature and glomeruli