The coincidence Doppler broadening ͑CDB͒ technique is widely used to measure one-dimensional momentum distributions of annihilation photons, with the aim of obtaining information on the chemical environment of open-volume defects. However, the quantitative analysis of CDB spectra needs to include also purely geometrical effects. A demonstration is given here, on the basis of CDB spectra measured in quenched and in deformed pure aluminum. The comparison of the experimental results with ab initio computations shows that the observed differences come from the difference in free volume seen by positrons trapped in quenched-in vacancies or in vacancylike defects associated to dislocations. The computation reproduces accurately all details of CDB spectra, including the peak near the Fermi break, which is due to the zero-point motion of the confined positron.
This is an initial report of a multi-technique study on the effect of Mg alloying on solute-vacancy interactions during the early stages of ageing of dilute 2xxx Al-Cu-Mg alloys so as to better understand the early rapid hardening (RH) that occurs in certain compositions of these alloys and the more general phenomena of secondary hardening (SH) at ambient temperatures. Therefore, RH at 150 °C and SH at room temperature from the as-quenched condition and after 60 sec ageing at 150 °C were studied in Al-1.1Cu and Al-1.1Cu-0.5Mg (at. %) variously by positron annihilation lifetime spectroscopy (PALS), coincidence Doppler broadening (CDB) spectroscopy and atom probe tomography (APT) and monitored by Vickers hardness measurements. The present results indicate that Cu-Cu, Mg-Mg and Cu-Mg clusters are formed in the ternary alloy already in the asquenched state and that they persist during ageing at 150 °C. The fraction of the solutes Cu and Mg that were associated with vacancies after ageing was increased 10-fold and double, respectively and the strength of the Cu clustering is enhanced greatly after 60 sec at 150 °C.
Low background measurements of 1D momentum spectra of the positron annihilation radiation by the coincidence Doppler broadening (CDB) method are currently used to obtain the quantitative chemical analysis of the environment of vacancy-like defects in supersaturated solid solution of metals. Fractional concentrations are obtained by fitting the CDB spectra with linear combination of reference spectra. Examples regarding Al-based alloys are presented to show the quality of the fitting and the consistency of the CDB analysis with positron lifetime measurements. The appropriate choice of the references is considered a key factor for reducing systematic errors. Nevertheless the method still awaits a theoretical validation. Preliminary first-principles calculations regarding Cu-decorated vacancies in aluminium are presented.1 Introduction Most high-strength light alloys owe their excellent mechanical properties to the presence of a fine and uniform distribution of nanometric precipitates, which hinders the motion of dislocations under an applied stress. The possibility to achieve the ideal microstructure of an alloy by controlling the precipitation phenomena is based on a good knowledge of the physical mechanisms behind these phenomena. An important aspect is the role of the vacancies retained after quenching from high temperature, which affect the kinetics of precipitation by helping the transport of solute atoms as well as the stability of the precipitates by relieving the local stresses due to different atomic sizes. Among the experimental techniques that are currently applied in Physical Metallurgy, the one that gives direct information on vacancies is positron annihilation spectroscopy. Lifetime spectroscopy and conventional Doppler broadening measurements have been applied to the study of precipitation in Al-based alloys since about 30 years (see reviews in [1, 2]); however the high potential of the coincidence Doppler broadening technique [3] for the identification of the chemical composition at the positron annihilation site has begun to be exploited much more recently [4,5]. A step toward a quantitative chemical analysis of vacancy-solute complexes in alloys was made by Somoza et al. [6], who showed that accurate fitting of the CDB spectra measured for Al-Cu-Mg alloys can be obtained by a linear combination of CDB spectra for annealed Al together with spectra for cold-worked Al, Mg and Cu. According to these Authors, the coefficients of the Cu and Mg terms in the linear combination are an approximation to the fractional concentrations of these elements in the nearest neighbour sites of the vacancy. In the present work the linear combination approach is presented in a revised version, with a more reproducible definition of the reference spectra. This version has been now adopted as a standard for current work on Al-and Mg-based
Diffusion parameters of Cr diffusion along the ␣/ interphase boundaries of a Zr-2.5 wt pct Nb alloy are presented. The conventional radiotracer technique combined with serial sectioning of the samples was applied. In the Arrhenius plot, it is possible to consider only one straight line (with Q ϭ 133 kJ/mol for 615 Ͻ T Ͻ 953 K) or two zones (with Q ϭ 230 kJ/mol for 773 Ͻ T Ͻ 953 K and Q ϭ 77 kJ/mol for 615 Ͻ T Ͻ 773 K). An analysis is made of these results together with previous data concerning diffusion along short circuits paths in ␣-Zr (grain boundaries) and Zr-2.5 wt pct Nb (interphase boundaries): Zr and Nb as the alloy component elements and Ni, Fe, and Co as other relevant impurities. Different mechanisms are proposed: a vacancy mechanism for Zr and Nb and an interstitial-like mechanism for the impurities, for both kind of boundaries. The influence on diffusion and the estimated values of the impurities segregation in the ␣ phase are discussed in the work.
The role of vacancy-solute association in the formation of coherent precipitates at room temperature (RT) and at 70 °C was studied by means of coincidence Doppler broadening (CDB) spectroscopy. The formation of clusters and GP zones was monitored by differential scanning calorimetry (DSC). Two conditions were explored at low temperature: a) one-step aging after solution treatment and quenching and b) secondary aging (after solution treatment and quenching + 5 days at RT + 8 min at 150 o C). The assessment of the average fractional concentration of solute in contact with the vacancies was obtained by analyzing the CDB data. The results show considerable aggregation of Cu to vacancies at the very early stage of decomposition of the supersaturated solid solution; the contribution of Zn and Mg increases progressively with the formation of coherent precipitates. The preliminary exposure at 150 °C does not alter this sequence, although a reduction in the dissolution enthalpy observed during DSC scans shows retarded formation of coherent precipitates. 1 Introduction The Al-based alloy 7012 has been the subject of several investigations using positron lifetime spectroscopy (PALS) [1][2][3][4][5] with the purpose to correlate the strong hardening response to aging of this alloy with the sequence of precipitation. The value of PALS in the study of precipitation kinetics comes from the sensitivity of the positron lifetime to changes in the distribution of vacancy-like defects associated to different precipitates (solute clusters, GP zones and η' phase in the present study). The present investigation includes PALS measurements at the very early stage of aging at room temperature (RT) on a time scale never explored before. However, the main attention is shifted on the interaction of vacancies with the different atomic species, with the aim to provide basic elements of knowledge regarding the mechanisms by which the vacancies affect the transport of the solute and the stability of the precipitates. To this end, the chemical environment of the vacancies has been characterized by low-noise measurements of the momentum distribution of the positron annihilation radiation by the coincidence Doppler broadening (CDB) method [6]. Differential scanning calorimetry (DSC) has been adopted to identify the species of precipitates formed during the progression of a multi-step heat treatment including secondary aging at moderate temperature [7].
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