Positron affinities for elemental metals
ABSTRACT The relevant quantity in the comparison of the absolute positron energy levels in different materials is the sum of the internal electron and positron chemical potentials, i.e. the sum of the Fermi level and the bottom of the lowest positron band relative to a common, well-defined reference energy. This sum is defined as the positron affinity. The positron affinity reflects the preference of the positron for different components in heterostructures made of different materials and the preference between the host matrix and precipitates in alloys. Moreover, the affinity is closely related to the positron work function and positronium formation potential which are important parameters in the slow-positron-beam experiments. The authors have determined the positron affinity for the alkaline and alkaline-earth metals, 3d-, 4d-, and 5d-transition metal series, and for some metals on the right in the Periodic Table. The diamond structure semiconductors are also considered. The determination is based on the self-consistent electron structure calculations and the subsequent calculation of the positron band structure within the local-density approximation. The trends are studied and interpreted along the different columns and rows of the Periodic Table. The results are also compared with available experiments.
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ABSTRACT: The influence of the addition of C to the Fe-Mn-Si-Cr-Ni base material is investigated at room temperature. Steel samples were deformed during a tensile experiment up to a strain of 17%. Light optical microscopy (OM) and x-ray diffraction (XRD) gave information about the different micro-structural phases that exist in the deformed and the undeformed alloys. The evolution of the defect structure is followed by positron annihilation techniques such as Doppler broadening of annihilation radiation spectroscopy (DBAR) and the positron annihilation lifetime spectroscopy (PALS). During deformation a martensitic ε-phase is induced. The size of the martensite plates increases with increasing deformation.Journal of Materials Engineering and Performance 08/2009; 18(5):575-581. DOI:10.1007/s11665-009-9474-y · 0.98 Impact Factor
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ABSTRACT: Positron lifetime tau and momentum distribution (curve shape parameter S) measurements are performed to study the decomposition in an Al-5.9 at% Ag alloy during isochronal and isothermal heat treatment of water-quenched samples. The positron studies are supplemented by small-angle X-ray scattering experiments. It is proved that positrons become trapped by G.P. zones being free of any vacancy-type or dislocation-type defect and by semicoherent gamma'-precipitates. Positron trapping rates show the coarsening of G.P. zones simultaneously with the formation and growth of gamma' during isothermal aging above 170-degrees-C. From the temperature dependence of tau and S the transformation of eta-zones into epsilon-zones is concluded. The Ag content of zones changes from about 60 at% at 120-degrees-C (eta-zones) to 35 at% at 190-degrees-C (epsilon-zones) which is in good agreement with SAXS and AP-FIM experiments. Evidence of a reversible change of the G.P. zone composition when alternating the temperature between 190 and 120-degrees-C is found in both positron and SAXS experiments.physica status solidi (a) 12/1993; 140(2):311 - 328. DOI:10.1002/pssa.2211400203 · 1.21 Impact Factor
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ABSTRACT: This paper reports a comparative investigation of the effect of quenching on the Cu–Al–Be and Cu–Zn–Al shape memory alloys by the use of several experimental techniques. In a first stage, the order–disorder transitions in these alloys have been characterized by means of modulated calorimetry. Results have proved that the A2⇋DO3 transition in Cu–Al–Be is first order with a latent heat of 1160 J/mol; the B2⇋L21 transition in Cu–Zn–Al is second order, and a peak in the specific-heat vs temperature curve has been observed. Secondly, the post-quench behaviour of these alloys, when subjected to some of the typical heat treatments used to stabilize the β phase, has also been studied by means of neutron diffraction, positron annihilation and highly sensitive calorimetry. A different post-quench time evolution of the martensitic transition temperatures has been found for the two alloys. For Cu–Al–Be, this evolution has been shown to be correlated with positron annihilation data, while, for Cu–Zn–Al, a correlation with neutron diffraction data has been established. These results show that the measured shifts in the transition temperatures induced by a quench are mostly due to an excess of vacancies in the case of Cu–Al–Be, and to an incomplete degree of L21 atomic order in Cu–Zn–Al.Acta Materialia 01/1998; 46(3):1045-1053. DOI:10.1016/S1359-6454(97)00301-7 · 4.47 Impact Factor