G. Boom

Solution Hardening in Al-Zn Alloys. Mean Jump Distance and Activation Length of Moving Dislocations

Abstract Pulsed nuclear magnetic resonance proved to be a complementary new technique for the study of moving dislocations in AlZn alloys. The NMR technique, in combination with strain-rate change experiments and transmission electron microscopy have been applied to study dislocation dynamics in AlZn alloys (1–2 at.% Zn). Spin-lattice relaxation measurements clearly indicate that fluctuations in the quadrupolar field caused by moving dislocations in AlZn are different compared to those in ultra-pure Al. From the motion induced part of the spin-lattice relaxation rate the mean jump distance of mobile dislocations has been measured as a function of strain. Based on the NMR data and data obtained from strain-rate change experiments it could be concluded that moving dislocations advance over a number of solute atoms (order of 10) as described by Mott-Nabarros model and interact with forest dislocations as predicted by Friedels model. The strain rate change experiments confirm the linear additivity of flow stresses and the additivity of inverse activation length.

Solution hardening in aluminium-magnesium alloys: A nuclear magnetic resonance and transmission electron microscopic study

Pulsed nuclear magnetic resonance techniques as well as transmission electron microscopy have been applied to study dislocation motion in aluminium magnesium alloys (0.2–1.6 at.% Mg). The spin lattice relaxation rate in the rotating frame of 27A1 has been been measured at 77 K as a function of strain at constant plastic strain rate ϵ. For finite strain rates, the movement of dislocations induces an additional relaxation rate arising from time fluctuations in the nuclear quadrupole interactions. From the motion-induced part of the relaxation rate the mean free path of mobile dislocations can be calculated. The NMR experiments are combined with transmission electron microscopic investigations to reveal the static structure of defects in the samples. The NMR measurements clearly indicate that fluctuations in the quadrupolar field caused by moving dislocations in AlMg are different compared to those in ultra pure Al. From the NMR data it could be concluded that moving dislocations advance over a number of solute atoms (order of 7) as described by Mott-Nabarros model. On the other hand, Mott-Nabarros model does not predict the effective solute spacing as a function of the concentration of solute atoms in accordance with NMR experiments.

DISLOCATION DYNAMICS IN AL-MG-ZN ALLOYS - A NUCLEAR MAGNETIC-RESONANCE AND TRANSMISSION ELECTRON-MICROSCOPIC STUDY

Pulsed nuclear magnetic resonance (NMR) proved to be a complementary new technique for the study of moving dislocations in Al-Mg-Zn alloys. The NMR technique, in combination with transmission electron microscopy (TEM), has been applied to study dislocation motion in Al-0.6 at. % Mg-1 at. % Zn and Al-1.2 at. % Mg-2.5 at. % Zn. Spin-lattice relaxation measurements clearly indicate that fluctuations in the nuclear quadrupolar interactions caused by moving dislocations in Al-Mg-Zn are different compared to those in ultra pure Al. From the motion induced part of the spin-lattice relaxation rate the mean jump distance of mobile dislocations has been determined as a function of strain. From the NMR data it is concluded that moving dislocations advance over a number of solute atoms in these alloys as described by Mott-Nabarros model. At large strains there exists a striking difference between the mean jump distances in Al-0.6 at. % Mg-1 at. % Zn and in Al-1.2 at. % Mg-2.5 at. % Zn. The latter is about five times smaller than the former one. This is consistent with TEM observations that show dislocation cell formation only in Al-0.6 at. % Mg-1 at. % Zn and the macroscopic stress-strain dependences of these alloys.

Effects of Cl+ and F+ implantation of oxidation-induced stacking faults in silicon

Three implantation effects were investigated in floating‐zone‐grown silicon: (a) the effect of Cl+ implantation resulting in the shrinkage of oxidation‐induced stacking faults; (b) the effect of F+ implantation giving rise to defaulting of the 1/3[111] Frank dislocations into 1/2[110] perfect dislocations due to the interaction with 1/6[112] Shockley partials; (c) the effect of a combined F+ and Cl+ implantation of dislocation motion. Notwithstanding the limited magnification of double‐crystal x‐ray topography it proves to be a valuable technique for determination of the effects of implantation, as removal of F; the oxidation layer is unnecessary for observation of the oxidation‐induced stacking faults. Moreover, the role of the oxide layer in the Si‐SiO2 interface can be followed more appropriately.

Quasi-crystals Studied with Convergent Beam Electron Diffraction

Chill-cast Al6CuLi3 was studied together with roller-quenched Al4Mn and melt-spun Al7Mn2. With convergent beam electron diffraction it was established that Al6CuLi3 and Al4Mn exhibited a pentagon as the dominant Kikuchi band pattern while Al7Mn2 showed a decagon. The quasi-crystalline m3¯5¯ and the crystalline Im3 structures seem to occur simultaneously.

Microstructure of Plastically Deformed Ni-base Superalloys

ABSTRACT The deformation mechanism of PM Astroloy in LFC experiments has been investigated. At low temperature (300 °C) and high strain rate (10-2s-1) planar character of slip is demonstrated by superdislocation shearing precipitates. At high temperature (730 °C) deformation is more homogeneous. In both MA 6000 and PM Astroloy deformation induced stacking faults are the dominant mechanism at high temperature. Initially stacking faults are limited to precipitates but ultimately extended faults transform into deformation twins. After 2% plastic deformation at 760 °C in MA 6000 cross-slip of a/3 partials occurs.