In-situ nuclear magnetic resonance investigation of strain, temperature, and strain-rate variations of deformation-induced vacancy concentration in aluminum

Abstract

Critical strain to serrated flow in solid solution alloys exhibiting dynamic strain aging (DSA) or Portevin-LeChatelier effect is due to the strain-induced vacancy production. Nuclear magnetic resonance (NMR) techniques can be used to monitor in situ the dynamical behavior of point and line defects in materials during deformation, and these techniques are nondestructive and noninvasive. The new CUT-sequence pulse method allowed an accurate evaluation of the strain-enhanced vacancy diffusion and, thus, the excess vacancy concentration during deformation as a function of strain, strain rate, and temperature. Due to skin effect problems in metals at high frequencies, thin foils of Al were used and experimental results correlated with models based on vacancy production through mechanical work (vs thermal jogs), while in situ annealing of excess vacancies is noted at high temperatures. These correlations made it feasible to obtain explicit dependencies of the strain-induced vacancy concentration on test variables such as the strain, strain rate, and temperature. These studies clearly reveal the power and utility of these NMR techniques in the determination of deformationinduced vacancies in situ in a noninvasive fashion.

Microstructural evolution and formation of nanocrystalline intermetallic compound during surface mechanical attrition treatment of cobalt

“…High strain rates are expected to expedite diffusion of migrating species [24,28,29]. Diffusion may also be enhanced by deformation-induced ''mechanical interdiffusion'' [45].…”

“…jog dragging and dipole annihilation [26,27]. As such, the mobile vacancy concentration will be proportional to both the imposed strain rate and the dislocation density [28,29].…”

Interface Formation in the US-Wedge/Wedge-Bond Process of AlSi1/CuNiAu Contacts

“…Now the question remains whether such a vacancy concentration can be reached during the bonding process. In situ nuclear magnetic resonance (NMR) measurements by Murty et al 23 on highly deformed aluminum foils have shown that strain rates on the order of 10 À4 s À1 can generate even higher excess vacancy concentrations at room temperature (X V ¼ 10 À5 ). Geißler, however, estimated the strain rates during the bonding process to be 10 s À1 to 30 s À1 .…”

Modelling of grain boundary impurity segregation during high temperature plastic deformation

“…where  is a structural parameter describing the distribution of dislocations which depends on temperature, and the value can be fitted as 0.75exp(0.0039T) at a temperature above 700°C [14];  is the shear modulus;  0 is the atomic volume; A and n are empirical constants; D v is the diffusion coefficient of vacancies; Q def is the apparent activation energy for deformation;  is a constant associated with the mechanical production of jogs; b is the Burgers vector; C j is the concentration of thermal jogs, given by exp(E j /kT) [12] where E j is the jog formation energy being calculated by 0.1b 3 [15];  is a parameter which describes the neutralization effect produced by the presence of vacancy emitting and vacancy absorbing jogs and may be calculated by 0.5-10C j if C j <0.05, and else  is equal to 0 [12].…”