Materials Science Forum Vols. 539-543

Abstract: Interstitials N, C and H in steels are compared in terms of their effect on the electronic structure and stacking fault energy, atomic distribution, phase transformations and precipitation, mobility of dislocations, mechanisms of deformation, strengthening and fracture. It is shown that the observed similarities and differences are essentially controlled by the decrease of the electron state density at the Fermi level of the iron due to carbon and its increase due to nitrogen and hydrogen. The increase in the concentration of vacancies with increasing content of interstitials and its possible role in mechanical properties are considered. The nature of nitrogen- and hydrogenincreased localized plasticity of austenitic steels is discussed and some consequences for their fracture are analyzed.

Abstract: The past years have triggered considerable scientific efforts towards the predictive analysis of the reliability of solder connections in micro-electronics. Undoubtedly, the replacement of the classical Sn-Pb solder alloy by a lead-free alternative constitutes the main motivation for this. This paper concentrates on the theoretical, computational and experimental multi-scale analysis of the microstructure evolution and degradation of the conventional solder material Sn-Pb and its most promising lead-free alternative, a Sn-Ag-Cu (SAC) alloy. Special attention is given to the thermal anisotropy of bulk SAC and the interfacial fatigue failure of SAC interconnects.

Abstract: National Research Council (NRC) as the premier research and development organization within the government of Canada has the mandate of providing vital scientific and technological services to research and industrial communities. The NRC Institute for Fuel Cell Innovation (IFCI) is leading NRC’s National Fuel Cell Program and is working closely with academic, government, and industrial organizations to support fuel cell cluster in Vancouver and across Canada and to fulfill the innovation needs of Canadian fuel cell companies. The key programs at IFCI include: Proton Exchange Membrane Fuel Cells (PEMFC), Solid Oxide Fuel Cells (SOFC), Hydrogen generation and infrastructure, and technology demonstration. NRC-IFCI’s impact on the fuel cell industry can be seen through the development and transfer of targeted and collaborative research projects addressing strategic and current technical gaps and providing infrastructure for research, development and demonstration. IFCI has been a catalyst in the coordination of industry’s responses to current commercialization barriers. This paper presents the latest research and development activities as well as demonstrations at NRC-IFCI.

Abstract: Although high-pressure torsion (HPT) is now a well-established technique for imposing severe plastic deformation on metallic materials, the technique is generally restricted to use with thin disk samples. This paper describes experiments which were conducted to evaluate the potential for making use of HPT with bulk samples in the form of small cylinders. The experiments were performed using an Al-Mg-Sc alloy and the samples were strained under a pressure of 1 GPa at room temperature. The results demonstrate excellent microstructural refinement in the vicinity of the central zone around the periphery of the cylinders.

Abstract: Zr702 and commercial purity Ti were grain refined through severe plastic deformation and their microstructure and texture characteristics were studied by optical microscopy, transmission electron microscopy, X-ray diffraction and electron back-scattered diffraction. The equal channel angular pressing was conducted at 350°C using a 90°/20° die. Up to 8 passes of pressing were performed via three different routes, A, C and BC, which resulted in reduction of the grain size down to 0.2-0.5.m. The two materials showed similar evolution characteristics of microstructure and texture. While the specimens pressed via route A showed lamellar grain shapes those via route C or BC exhibited equiaxed grains. A split basal texture was developed in the microstructure obtained by the route A pressing, which was similar to the cold rolling texture, whereas an asymmetric texture was produced in the specimens pressed via route BC. In the case of route C pressing, the texture characteristics were closer to those of the route BC case.

Abstract: This paper reviews our recent results of the formation, fundamental properties, workability and applications of late transition metal (LTM) base bulk glassy alloys (BGAs) developed since 1995. The BGAs were obtained in Fe-(Al,Ga)-(P,C,B,Si), Fe-(Cr,Mo)-(C,B), Fe-(Zr,Hf,Nb,Ta)-B, Fe-Ln-B(Ln=lanthanide metal), Fe-B-Si-Nb and Fe-Nd-Al for Fe-based alloys, Co-(Ta,Mo)-B and Co-B-Si-Nb for Co-based alloys, Ni-Nb-(Ti,Zr)-(Co,Ni) for Ni-based alloys, and Cu-Ti-(Zr,Hf), Cu-Al-(Zr,Hf), Cu-Ti-(Zr,Hf)-(Ni,Co) and Cu-Al-(Zr,Hf)-(Ag,Pd) for Cu-based alloys. These BGAs exhibit useful properties of high mechanical strength, large elastic elongation and high corrosion resistance. In addition, Fe- and Co-based glassy alloys have good soft magnetic properties which cannot be obtained for amorphous and crystalline type magnetic alloys. The Feand Ni-based BGAs have already been used in some application fields. These LTM base BGAs are promising as new metallic engineering materials.

Abstract: Experiments were carried out in which the dependence of the fractional softening on temperature, time and strain rate was determined in a 304H stainless steel. Three prestrain ranges were identified pertaining to three different post-deformation softening behaviors: 1) prestraining to below the DRX critical strain: strongly strain dependent softening by SRX alone with softening kinetics controlled by growth rate of the nuclei; 2) prestraining to above the DRX critical strain: SRX + MDRX softening with weaker strain dependence of the kinetics but still controlled by grain growth; 3) at a prestrain of ε* and beyond: nucleation-controlled MDRX softening with the full inhibition of SRX. The transition prestrain ε* can exceed the peak strain if the DRX grain refinement ratio g = D0/DDRX > 4. The transition to MDRX-dominated softening can be attributed to a constant value of the normalized strain hardening rate independent of the preloading temperature and strain rate. The softening data from the compression tests show that at ε*, the time for half softening t50 exhibits a minimum. These data differ somewhat from observations obtained in the torsion testing of solid bars, in which no strain dependence of t50 was detected at ε* and beyond. Whether or not the strain dependence of t50 vanishes in the MDRX range is sensitive to the test method employed to study the post-deformation softening.

Abstract: Magnesium applications for structural components in the automotive industry are constantly rising. This is based on the recent development of new alloys, new fabrication processes, and the ambition of car manufacturers to reduce the vehicles weight and CO2 emissions according to the EU and US policy [1, 2]. A rising quantity of magnesium per vehicle leads to a rising quantity of scrap which needs to be recycled according to the European Directive on end-of life vehicles [3]. So far post consumer scrap has not been used for structural parts. But since the metal is still expensive compared to aluminium or steel, and remelting saves more than 90 % of the energy for primary production, magnesium recycling will significantly contribute to cost savings. In comparison to steel or aluminium a recycling cycle for magnesium has not yet been established. Concerning post consumer scrap it is likely that many vehicles will end up in the shredder fraction or at least will be mixed up instead of being dismantled and separated according to their alloy. Thus it is reasonable to define secondary alloys which allow the use of post consumer scrap for structural applications. Creep resistant alloys have the potential of a broad application concerning the weight of the components and therefore a secondary alloy would be reasonable. The aim of this work is to examine a row of AM50-based alloys, modified with additions of Sr, Ca, and Si due to the importance of these elements to increase creep resistance and their usage in modern magnesium alloys. The corrosion properties as well as the mechanical properties and microstructures are investigated in the as-cast and annealed condition. Salt spray tests (using 5 % NaCl) and electrochemical corrosion methods are applied to investigate the corrosion properties which are then compared to the unmodified AM50. Tensile and compression tests at temperatures ranging from 20 °C to 200 °C are applied to investigate the mechanical properties.

Abstract: Nitrogen evolution research has been widely carried out in Japan after the Forth International Conference on High Nitrogen Steels held in 1995 in Kyoto (HNS1995). The first research forum on nitrogen evolution research has been started in The Japan Society of Corrosion Engineering in 1996, followed by a series of research forums in the Iron and Steel Institute of Japan (ISIJ). In the meantime, a pressurized electro-slag remelting furnace was newly developed in National Institute for Material Science in 1998. A new research group regarding the availability of nitrogen on the improvement in steel properties has been started in ISIJ in 2004. Current research activities of the group cover the fabrication of HNS such as mechanical alloying, pressurized ESR and nitrogen absorption, evaluation technologies of HNS such as microstructure and precipitation behavior of nitride, mechanical properties, corrosion properties and physical properties, joining/welding technologies. As one of the applications of HNS, Ni-free high nitrogen steels with high strength, anti-corrosion, non-magnetic properties are very attractive in the fields of bio/medical ones as anti-Ni allergy material. Recent research activities including some topics mentioned above will be discussed.

Abstract: Development of wrought Mg alloys, particularly in sheet form, is essential to support the growing interest for lightweight components in the automotive industry. However, development of Mg alloy sheets has been quite slow due to the complexity of sheet production originated from limited deformability of Mg. In this respect, twin-roll strip casting, a one-step processing of flat rolled products, can be an alternative for the production of Mg alloy sheets. In this study, AZ31 and experimental ZM series alloys are twin-roll strip cast into 2 mm thick sheets. The microstructure of the as-cast AZ31 alloy sheet consists of columnar zones near the roll side and equiaxed zones in the mid-thickness region. On the other hand, as-cast ZM series alloy sheets show equiaxed dendritic structure through the thickness of sheet. These alloys were subjected to various thermo-mechanical treatments and their tensile properties were evaluated. Twin-roll strip cast AZ31 alloy in H24 condition has equivalent yield and tensile strengths with similar ductility compared to commercial ingot cast AZ31-H24 alloy, indicating that twin-roll strip casting is a viable process for the fabrication of Mg alloy sheets. The experimental ZM series alloys have a large volume fraction of fine dispersoid particles in the microstructure, resulting from the beneficial effect of twin-roll strip casting on microstructural refinement. It has been shown that the experimental ZM series alloys have superior tensile properties compared to commercial ingot cast AZ31-H24 alloy, suggesting the possibility of the development of new wrought Mg alloy sheets by twin-roll strip casting.