Quantitative Non-Destructive Detection of Residual Stresses of the 2 nd and 3 rd Order by Using Micro Magnetic Methods
Micro residual stresses (MRS) of the 2 and 3 order play an important role in the lifetime analysis of thermally-cycled materials. The coherent residual stresses (MRS of 3 order) appear when the lattice parameter of the second phase particles (coherently embedded in the matrix) and the lattice parameter of the matrix are different from each other. Such differences in case of the Fe-Cu alloys between the lattice parameters of coherent Cu precipitates and the lattice parameters of the αFe matrix lead to distortion of the lattice around the Cu particles causing coherent tensile MRS of 3 order. Thermally induced MRS of the 2 order appear when individual material phases exhibit different thermal expansion coefficients. Such differences in temperature-related expansion coefficients of copper precipitates and the α-Fe matrix promote the development of thermally induced compressive MRS of the 2 order. The main objective of the presented research project is to develop an integral, economical and fast micro-magnetic non-destructive method for quantitative characterisation of MRS of the 2 and 3 order in Fe-Cu and Fe-Cu-Ni alloys. For this purpose Fe-Cu and Fe-Cu-Ni alloy samples with different Cu and Ni contents were prepared.
Positron confinement in ultrafine embedded particles: Quantum-dot-like state in an Fe-Cu alloy
A quantum-dot-like positron state is demonstrated in the ``defect-free'' particles using coincidence Doppler broadening of positron annihilation radiation. The wave functions of all positrons are entirely confined spatially in all three directions within the embedded nanosize and subnanosize Cu particles in a dilute Fe-Cu alloy. With use of this probe, the ultrafine particles are revealed to have nearly the same electron momentum distribution as bulk Cu, to be free from Fe atoms and defects, three dimensional, and to have no open-volume defects at the interfaces which can trap the positron. These successes indicate that this positron state promises to be a powerful tool for the studies of mesoscopic systems in metals and semiconductors.
Thermal ageing of an Fe‒Cu alloy: Microstructural evolution and precipitation hardening
Abstract The microstructural evolution of an Fe-Cu alloy during ageing has been investigated by small-angle X-ray scattering and transmission electron microscopy (TEM). Its mechanical properties have been characterized by Vickers microhardness measurements and in situ straining TEM. Cu precipitates evolve during annealing from a small coherent bcc to a larger incoherent type. In both cases, in situ straining shows that long screw dislocations move between localised cusps owing to dislocation-precipitate interactions. In the case of incoherent precipitates, a particular three-dimensional bypassing mechanism is observed, which does not form any loop around the precipitate. On the basis of these observations, we propose a model of hardening combining the viscous motion of screws and the dislocation-precipitate interaction, which yields a monotonic decrease in stress with annealing time in both cases of particle shearing and bypassing. As a consequence, the hardness peak shown by microhardness experiments is ...