Defect and Diffusion Forum Vols. 273-276

Abstract: Powder metallurgy is a new method for mass production of precision components with appropriate mechanical properties, but in this kind of materials (PM parts) with special microstructures (pores act as local stress risers), fracture due to fatigue is expected as an important destructive factor. Various microstructures in powder metallurgy steels, depending on alloying methods, have different response against cyclic loading. diffusion bonding is an effective method to obtain high fatigue performance in PM steels. The main characteristic of this materials consists of well-organized phases distribution due to incomplete diffusion of alloying elements. In this study fatigue behavior of diffusion-bonded, distaloy AE, steel with two carbon contents under different periodic loading are investigated. The effect of carbon content and various loading mode upon fatigue performance is analyzed. Metallugraphy and fractography examination on fatigue loaded samples revealed the positive effect of microstructure heterogeneity on fatigue crack behavior and this concept is a reason for increasing of diffusion-bonded powders application to manufacturing of components that are subjected to cyclic stresses.

Abstract: The electrochemical behavior of perovskite type LaMnO3 (LMO) oxides with different mean particle size was studied by voltammetry with the use of a carbon paste electroactive electrode. Three stages of electrochemical reduction were recognized. The first two of them are related to the release of oxygen from the crystal lattice in the range of two side nonstoichiometry of LaMnO3±δ whereas the final stage is conditioned by the decomposition of LaMnO3-δ into new phases. The nature of these phases and their formation mechanisms are different for nano- and microparticles. The utmost size effect appears on cathodic curves recorded from the stationary potential. The effect is not only due to the size factor but also due to the difference in electrochemical properties of nano- and microparticles. While the decomposition of LaMnO3 microparticles proceeds into La2O3 and MnO oxides, the nanoparticles decompose through the intermediate stage of Mn3O4 formation in accordance with the transformation sequence principle.

Abstract: A single crystal superalloy including 6 wt.% Re, CMSX10 was studied by directional solidification at various thermal gradients, 13 ~ 23oC/mm and solidification rates, 1~100 μm/s. A high thermal gradient could be obtained by applying the liquid metal, such as Ga-In, as a cooling media in directional solidification apparatus, and also by adjusting the cold chamber in the Bridgman system. As increasing the solidification rate, the planar interface changed to cellular and dendritic interfaces. The higher thermal gradient contributed to reducing the dendrite arm spacing effectively, which results in reducing the size of eutectic, as well as higher solidification rate. The length of the mushy zone decreased with increasing the thermal gradient and increased with increasing the solidification rate.

Abstract: Zr62Cu17Ni13Al8 in the supercooled liquid state is expected to be micro-formable at a relatively low stress. We used X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and quantitative high-resolution TEM (HRTEM) to investigate the microstructures of Zr62Cu17Ni13Al8 amorphous alloy after compression test. The alloy exhibited the homogeneous amorphous microstructure with some crystalline phases dispersed in the matrix. According to the XRD results, under the certain strain rate in the supercooled liquid state, the alloy showed higher crystallization at the higher heat treatment temperature. However, at the same heat treatment temperature, the alloy deformed under low strain rate showed higher crystallization. The β crystalline phase particles with spherical shape were detected by SEM and TEM. The sample with higher strain rate and temperature showed longer shear bands. Nano-voids formed by the coalescence of excess free volume in shear bands were investigated by quantitative HRTEM. Compared with the undeformed area, in the shear band, nanovoids were identified in the deformed area through quantitative HRTEM simulation.

Abstract: The thermal diffusivities of some industrially important alloys have been measured as a part of the EU funded Intermetallic Materials Processing in Relation to Earth and Space Solidification (IMPRESS) project which is coordinated by the European Space Agency (ESA). The thermal diffusivities of the alloys were measured by the Laser flash method with a carefully designed gas cleaning system to remove traces of oxygen from the argon atmosphere. In the present work, the thermal diffusivity of TiAlNb (Ti46.1Al45.9Nb8 at %) and AlNi alloy (Al-Ni31.5 at %) alloys have been measured independently at Royal Institute of Technology, Sweden (KTH) and National Physical Laboratory, UK (NPL). The results from both laboratories were consistent, and have been compared with predictions of phase transformation temperatures calculated using Thermo Calc and MTDATA software. Generally the variation of thermal diffusivity appears to be related to the phase transformation. However, one anomaly observed in the present work on TiAlNb was a maximum thermal diffusivity value at about 1100K. No corresponding peak was found for the density, ρ, the specific heat capacity, Cp, or the electrical resistivity, 1/σ, which were also measured as part of the project. In view of the fact that the thermal diffusivity could be related to electrical conductivity by the Wiedemann-Franz law describing electronic contribution to heat conduction, the present results indicate a non-electron contribution. This aspect is being currently investigated further. The recommended thermal diffusivity value of TiAlNb and AlNi alloys were obtained as follows. TiAlNb alloy: α = 3.75+ 5.16 ·10-3T+1.89·10-6 T2 – 2.69·10-9 T3 [10-6 m2 s-1] (293 K < T < 1573 K) AlNi alloy: α = 4.77+ 5.41·10-2T – 7.14·10-5T2 + 2.88·10-8T3 [10-6 m2 / s] (373K

375