Materials Science Forum Vols. 638-642

Abstract: The microstructural characteristics and room temperature tensile ductility in some cast TiAl alloys were analyzed. It was found that grain refinement, either through Q&A treatment in boron-free alloys or through low dose of boron addition in beta solidifying alloys with lamellar microstructures, is not sufficient for good ductility. For cast alloys with convoluted microstructures and fine lamellar microstructures the orientation distribution of gamma grains/lamellar colonies plays an important role in controlling ductility. In those fine-grained alloys a highly random orientation distribution of gamma grains/lamellar colonies is essential to improvement in ductility.

Abstract: The values of all the three independent single-crystal elastic constants and polycrystalline elastic constants of Co3(Al,W) experimentally determined by resonance ultrasound spectroscopy at liquid helium temperature are 15~25% larger than those of Ni3(Al,Ta) but are considerably smaller than those previously calculated. Because of the large value of E111/E100 and cij of Co3(Al,W), two-phase microstructures with cuboidal L12 precipitates well aligned parallel to <100> and well faceted parallel to {100} are expected to form very easily in Co-base alloys, as confirmed indeed by experiment. Values of yield stress obtained for [001]-oriented L12/fcc two-phase single crystals moderately decrease with the increase in temperature up to 800°C and then decrease rapidly with temperature above 800°C without any anomaly in yield stress.

Abstract: NiAl possesses the superior oxidation resistant and Ni3Al shows the good high temperature strength. Various castings with NiAl and Ni3Al were used in order to investigate the influence of Ni content on the properties. The heats were made by using vacuum induction furnace and its properties were evaluated. This article described the density, hardness, oxidation resistance, corrosion resistance and compression strength of the castings in comparison of other structural materials.

Abstract: A new material is available that displays the high temperature mechanical properties of nickel alloys at half the density –the intermetallic Titaniumaluminides. These extraordinary properties give it great potential for lightweight construction in aeroengines. The high Young’s modulus compared to common Titanium alloys offers advantages even in light weight applications, which do not need high heat resistance, e. g. connection rods. However, its characteristic rather like ceramics than metals presents a challenge in terms of forming. A forging technology has been developed with respect to this behaviour. Using isothermal forging at high temperatures with extremely low strain rates allows a material related forming. High pressure compressor blades and low pressure turbine blades have been forged as well as con rod for sports car engines. For economical improvement several blades may be forged at the same time.

Abstract: Nd2Fe14B magnets could be sintered using terbium fluoride coated Nd2Fe14B powders. Fluorine atoms were segregated at the triple junctions and grain boundary after sintering and aging process. NdOF with fcc structure was grown at the triple junction. Terbium atoms were also distributed near grain boundaries by interdiffusion between the fluoride and Nd2Fe14B. No fluorine atoms were detected in Nd2Fe14B phase. The coercivity for 0.5wt% terbium fluoride coated magnet showed 1.23 MA/m which was 1.2 times higher than that for non-coated magnet with 1.04MA/m. The distribution of terbium and fluorine atoms increases the coercivity without reduction of remanence.

Abstract: The major problem restricting universal employment of intermetallic phase base alloy is their low plasticity which leads to hampering their development as construction materials. The following work concentrates on the analysis of microstructure and plasticity of ordered Fe3Al (D03) and FeAl (B2) during hot plastic deformation process.. The microstructure analyses applying optical electron microscopy and EBSD method have revealed the structure reconstruction processes occurring in Fe3Al and FeAl alloys. It has been shown that different mechanisms of the structural changes ensue from the thermal plastic strain in the investigated alloys, which influences their technological plasticity.

Abstract: Microstructure and mechanical properties of a new cast air-hardenable intermetallic Ti-46Al-8Ta (at.%) alloy are studied. Primary solidification phase, solidification path and high temperature phase equilibria are determined using quenching during directional solidification (QDS) experiments combined with microstructural analysis. Vickers hardness, tensile, compression and creep properties are presented. The effect of short-term high temperature exposure on room temperature (RT) ductility of uncoated specimens is evaluated and discussed. Creep properties such as minimum creep rate, time to 1 % creep deformation, fracture time and creep fracture mechanisms are presented and compared to other TiAl base alloys developed for industrial applications. Long-term high temperature microstructural stability and the effect of ageing on Vickers hardness and RT tensile properties are presented. Based on the achieved experimental results, the potential of this new alloy to fulfill industrial specifications is discussed.

Abstract: Surfaces of titanium aluminides were treated by fluorine either physically using Plasma Immersion Ion Implantation (PI³) or chemically with a F-based polymer. By controlling the fluorination parameters, both treatments improve the oxidation resistance even in the presence of sulfur dioxide (0.1 vol%). No sulfur was detected in the oxide scale although thermodynamic calculations predict the formation of sulfides. The inward diffusion of oxygen and nitrogen in the alloy was found to be reduced in the presence of SO2.

Abstract: The relationship between the microstructure and mechanical properties on the ternary ZrCoNi alloys has been investigated. Zr50Co50-xNix alloys undergo martensitic transformation from the B2 to B33 structure by the substitution of Ni for Co. The tensile ductility at room temperature increases remarkably without the loss of yield strength by substituting Ni. Zr50Co39Ni11 alloy has the extremely high plastic elongation of 21%. There are many {021}B33 deformation twins in the B2 parent phase just near the tensile fractured area. These twins are the strain induced martensitic phase due to the tensile deformation. Consequently, the remarkable improvement of the ductility of ZrCoNi alloys is due to the transformation induced plasticity.

Abstract: Using the halogen effect TiAl-alloys can be protected against high-temperature oxidation. Two different fluorination methods were applied to turbine blades. The mass increase due to oxidation can be drastically reduced compared to untreated specimen. A new vacuum chamber for ion beam analysis was developed to analyze the real parts. Using PIGE-technique the F-content as a function of depth before and after oxidation was detected. Thickness and composition of the oxide scale were measured by RBS. Both ion beam methods were non destructive and thus enabled for the first time quality assurance of the halogen treatment on real components.