Materials Science Forum Vols. 638-642

Abstract: The intermetallic compound Nb3Al is widely investigated because of its high temperature strength, superior superconductivity and relatively small density. As Nb3Al has an extremely high melting point and lack of deformability, it is impossible to prepare it by using the conventional metallurgy. In this study, a Nb-Al intermetallic compound was prepared by multi-layered roll-bonding of elemental Nb and Al foils. The process consisted of the accumulative roll-bonding (ARB) for making a laminated Nb/Al sheet and the subsequent heat treatment promoting a solid-phase reaction in the laminated Nb/Al sheet. Accumulated foils were roll-bonded at 573 K. The rolling reduction at 1 pass was ~50%, and the final rolling reduction at 4 passes was ~94%. A pulsed electric current sintering (PECS) process was used for the subsequent heat treatment. The microstructures produced at each processing stage were characterized by X-ray diffraction (XRD), optical microscopy and scanning electron microscopy (SEM) equipped with energy-dispersive X-ray spectroscopy (EDS). A homogeneous intermetallic compound of Nb3Al could be obtained after the subsequent heat treatment for 1.8 ks at 873 K and for 0.9 ks at 1673 K.

Abstract: In view of the development of improved TiAl cast alloys the potential of the  transformation and its dependence on the addition of several alloying elements has been investigated. It was found that microstructural refinement in  solidifying alloys can be attributed to the alloying effect on the kinetics of the  transformation. This also holds for grain refinement through Borides which apparently serve as nucleation sites for the  phase in the solid-state transformation.

Abstract: A Laves phase Ir2Y is found to exhibit a temperature dependent solubility, which yields Ir3Y precipitation within the Ir2Y matrix of an arc-melted Ir-30at.%Y alloy. Microstructure observations have confirmed that the precipitation of Ir3Y has already started in the as-cast state. Upon annealing of an as-cast Ir-30at.%Y, the volume fraction of Ir3Y precipitates increases and finally reaches about 50%. The orientation relationship between Ir3Y and Ir2Y has been identified as: (0001)Ir3Y//(111)Ir2Y and [21 _ 1 _ 0]Ir3Y//[1 _ 10]Ir2Y. The orientation relationship is rationalized by the atom matching at the habit plane between the two phases.

Abstract: The microstructures and tensile properties of a fully lamellar Ti-48Al-2Cr-2Nb, and two tungsten-modified versions, Ti-48Al-2Cr-2Nb-0.5W and Ti-48Al-2Cr-2Nb-1.0W (atomic percent) are investigated. Gas atomized powders are consolidated by hot isostatic pressing followed by solution treatment and aging. The microstructures are characterized by optical, scanning electron, and transmission electron microscopy and mechanical properties are characterized by room temperature tensile testing. The solution heat treatment, combined with controlled cooling, generates relatively fine, fully lamellar grains. Tungsten reduces the propensity for martensitic gamma formation during cooling, and slows down lamellar coarsening as well as the formation of equiaxed gamma phase during aging. The aging treatment stabilizes the microstructure and, in the tungsten-modified alloys, causes beta phase precipitation at lamellar interfaces and grain boundaries. Both aging and tungsten additions increase the alloy strength and reduce ductility. The fracture morphologies of the alloys are similar and exhibit mixed-mode fracture consisting of inter- and intra-granular cracking, as well as inter-lamellar cracking.

Abstract: FeAs-based single crystals of Ba1-xKxFe2As2 and Sr1-xKxFe2As2 were grown in a Mo capsule at the temperatures from 1200oC down to 850oC by a FeAs-self-flux method. Most of their sizes were approximately 0.2-0.5 mm. A larger single crystal of Sr Fe2As2 about 7mm in diameter was obtained by the same procedure, as appeared in a round-shape. Most of the time, potassium-doped crystals tends to be smaller than undoped ones. An example of Tc for a Ba1-xKxFe2As2 single crystal measured with a SQUID magnetometer, which indicates Tc at 38 K. Tc s are sometimes lower dependent on the potassium composition. The substitution reaction between SrFe2As2 and potassium was examined. An undoped crystal absorbed potassium at around 600oC and showed superconductivity by the reaction.

Abstract: In this work, significant modification of the microstructure of a rolled Ti-45Al-5Nb (at.%) alloy has been achieved by a two-step heat treatment. A combination of high cooling rates for the first step of the treatments with a subsequent annealing at high temperatures, results in refined microstructures where substantial discontinuous coarsening seems to play an important role for the grain refinement. In order to fully understand the transformation mechanisms responsible for such a refinement, Electron Back Scattered Diffraction (EBSD) technique has been extensively used. In particular, a study of orientation relationships between the phases which are present in the transformed microstructures, was performed by this technique. Besides discontinuous coarsening process, the obtained results also suggest that a modified discontinuous coarsening mechanism is operative, in order to justify the formation of novel microstructures, whose development could not be explained by the classical discontinuous coarsening.

Abstract: The fatigue behaviour of a powder-metallurgy (PM) + heat-treated Ti49Al47Cr2Nb2 alloy is investigated using detailed SEM characterization. Based on the results of comparative static and cyclic loadings at RT, the fracture origins for the different test specimens is investigated. Conventional fractographic analyses revealed that internal structural defects inherent to powder metallurgy such as compaction defects, porosities and non-metallic inclusions can promote crack initiation. However, the fracture surface of test specimens is markedly affected by the microstructure, which is indicative of the microstructure dependence on crack initiation and propagation. In sub-transus heat treatment conditions, the detrimental effect of structural defects is illustrated by clear crack initiation sites onto the fracture surfaces. In super-transus conditions, crack propagation from defects can be blunted due to crack deflection, branching through lamellar interfaces, thus leading to lower defect sensitivity. Correlation of the results of these microfractographic examinations with the stress-strain curves corresponding to the various specimens allows identifying the role of such structural defects on the static and cyclic deformation behaviours. Finally, implications of such dependencies will be assessed relative to the requirements for aerospace gas turbine applications.

Abstract: High silicon steel with Si content of 6.5wt.% is perspective to be applied in magnetic devices at high frequencies. It is very hard to produce Fe-6.5wt%Si alloy thin sheet by conventional hot-cold rolling method because of its embrittlement at room temperature resulted from the formation of intermetallic phases like B2 and D03. In this paper, embrittlement mechanism, rolling processes, and magnetic properties for the cold rolled Fe-6.5wt.% Si alloy sheets are discussed. Our study indicates that thermal mechanical treatments can improve the ductility of the Fe-6.5wt.%Si alloy. The Fe-6.5wt.%Si alloy sheets of 0.05 mm thickness can be successfully obtained after hot-cold rolling and heat treatments. Lamination pieces can be punched directly from the sheets. Tensile strength as high as 1048MPa and elongation over 3% were measured at room temperature.

Abstract: In this paper, high-quality single crystals of (Mo0.85Nb0.15)Si2, around 8 mm in diameter and 90 mm in length, have been grown by optical heating floating zone method. Effects of chemical composition and growth rate on forming C40 structured single crystal were studied. Aligned C40/C11b lamellar structured can be formed in the as-grown crystals after post annealing at temperatures between 1473 and 1873 K. Chemical composition as well as annealing temperature are found to be two important factors to form C11b lamellae in the C40 matrix. Fully C40/C11b lamellar structure was formed after annealing at 1873 K in the present work. The aligned C40 and C11b lamellae follow a crystallographic orientation relationship of (0001)C40//(110)C11b. Dislocations were observed in some coarse C11b lamellae but never in C40 lamellae of the duplex structure. This is probably due to accumulation of misfit strain during formation of C40/C11b lamellae.

Abstract: The grain sizes of two kinds of orthorhombic alloys, namely (O+B2) Ti-22Al-11Nb-2Mo -1Fe and (O+2) Ti-27.5Al-13Nb have been successfully reduced by the addition of trace boron (B) (less than 0.12 wt.%). For example, the grain size in the B2 solution-treated condition was reduced from 1 mm to 80 m by the addition of 0.05% B for both alloys. The tensile elongation of Ti-22Al-11Nb-2Mo-1Fe at room temperature and 650C was increased from 0.3% to 4.3%, and from 8.2% to 30.3%, respectively, by the addition of 0.10% B. Ti-27.5Al-13Nb also showed an improved room temperature ductility by the minor B addition.