Physics and phenomenology of strain hardening: the FCC case

The mechanical properties of thin films on substrates are described and studied. It is shown that very large stresses may be present in the thin films that comprise integrated circuits and magnetic disks and that these stresses can cause deformation and fracture to occur. It is argued that the approaches that have proven useful in the study of bulk structural materials can be used to understand the mechanical behavior of thin film materials. Understanding the mechanical properties of thin films on substrates requires an understanding of the stresses in thin film structures as well as a knowledge of the mechanisms by which thin films deform. The fundamentals of these processes are reviewed. For a crystalline film on a nondeformable substrate, a key problem involves the movement of dislocations in the film. An analysis of this problem provides insight into both the formation of misfit dislocations in epitaxial thin films and the high strengths of thin metal films on substrates. It is demonstrated that the kinetics of dislocation motion at high temperatures are expecially important to the understanding of the formation of misfit dislocations in heteroepitaxial structures. The experimental study of mechanical properties of thin films requires the development and use of nontraditional mechanical testing techniques. Some of the techniques that have been developed recently are described. The measurement of substrate curvature by laser scanning is shown to be an effective way of measuring the biaxial stresses in thin films and studying the biaxial deformation properties at elevated temperatures. Submicron indentation testing techniques, which make use of the Nanoindenter, are also reviewed. The mechanical properties that can be studied using this instrument are described, including hardness, elastic modulus, and time-dependent deformation properties. Finally, a new testing technique involving the deflection of microbeam samples of thin film materials made by integrated circuit manufacturing methods is described. It is shown that both elastic and plastic properties of thin film materials can be measured using this technique.

Mechanical properties of thin films

The current understanding of the fundamentals of recrystallization is summarized. This includes understanding the as-deformed state. Several aspects of recrystallization are described: nucleation and growth, the development of misorientation during deformation, continuous, dynamic, and geometric dynamic recrystallization, particle effects, and texture. This article is authored by the leading experts in these areas. The subjects are discussed individually and recommendations for further study are listed in the final section.

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Current issues in recrystallization: a review

http://meyersgroup.ucsd.edu/papers/journals/meyers 213.pdf

The onset of twinning in metals: a constitutive description

This article is concerned with the mechanisms by which type II superconductors can carry currents. The equilibrium properties of the vortex lattice are described and the generalized driving force in gradients of temperature and field is derived using irreversible thermodynamics. This leads to expressions for thermal cross effects which can include pinning forces. The field distributions which occur in a range of situations are derived and a number of useful solutions of the critical state given. In particular, the distribution in a longitudinal field is obtained, and the conditions under which force-free configurations can break down by the cutting of vortices discussed. The effects of lattice rigidity on the summation of pinning forces is considered and it is shown that a summation based on statistical arguments uses the same approximations and leads to the same results as a dissipation argument. Theoretical expressions are derived for the vortex pinning interaction to a number of different metallurgical...

Flux vortices and transport currents in type II superconductors

A new mechanism of work hardening is proposed to explain the athermal hardening in Stage IV of f.c.c. and diamond cubic crystals. The mechanism is related to a cellular dislocation microstructure in which during Stage III, hardening by dislocation accumulation and recovery by various mechanisms occurs primarily in the cell walls. Hardening of the cells is through the build-up of long range misfit stresses that result when the primary dislocation flux cuts trough the geometrically required dislocation density of the cell walls that is associated with the lattice misorientations between cells. Experiments show that these misorientations increase monotonically with increasing strain. There is no recovery in the cells. At the end of Stage III, hardening in the cell walls saturates, but the hardening due to misfit stresses in the cells continues unabated, giving rise to the rate independent hardening of Stage IV. Eventually this hardening is also terminated in Stage V when the misfit stresses inside cells reach a critical level that triggers rate dependent stress relaxation in the cells by secondary glide processes. The new mechanism makes successful predictions for Stage IV processes, including: hardening rate, plastic resistance levels, the gradual increase in hardening rate with plastic resistance, the residual lattice strains on unloading that can be measured with X-ray peak distortions and broadening, and for the Baushinger effect.

A new mechanism of work hardening in the late stages of large strain plastic flow in F.C.C. and diamond cubic crystals

The brittle-to-ductile transition in doped silicon as a model substance

On the generation of new orientations during recrystallization: Recent results on the recrystallization of tensile-deformed fcc single crystals

The velocities vi of α, β, and screw dislocations are measured by double etching in GaAs crystals with different doping and at different temperatures. Also the maximum creep rates in compression are determined. A method of averaging the vi is proposed to obtain from the theory of multiplication-limited creep proposed by Haasen for Ge and Si. The dependence of vi on doping is attributed to an effect of the dislocation charge on kink formation energy.



In unterschiedlich dotierten GaAs-Kristallen werden die Geschwindigkeiten vi der α-, β- und Schraubenversetzungen bei verschiedenen Temperaturen nach der Doppelatzmethode gemessen. Ebenso werden die maximalen Kriechgeschwindigkeiten im Kompressionsversuch bestimmt. Um nach der von Haasen fur Ge und Si vorgeschlagenen Theorie des durch Versetzungsmultiplikation bestimmten Kriechens einen Zusammenhang zwischen und vi zu gewinnen, wird eine Methode zur Mittelung uber die Geschwindigkeiten der Versetzungstypen vorgestellt. Die Abhangigkeit der vi von der Dotierung wird dem Einflus einer Versetzungsladung auf die Kinkenbildungsenergie zugeschrieben.

Creep and dislocation velocities in gallium arsenide

The reversible contribution to the flow stress in α-iron single crystals was investigated by temperature and strain rate changes between −72°c and + 22°c and over several orders of magnitude of strain rate. The corresponding flow-stress changes were, compared with f.c.c. metals, quite large and increased with increasing deformation. The results were quantitatively evaluated using the assumption that the activation volume decreased with increasing stress. The initial distance between obstacles was found to be ∼5 × 10−6 cm and the activation energy 0.55 ev. Impurities are considered most likely to form the obstacles responsible for the thermally activated flow-stress contribution in the temperature range studied.

Peter Haasen

Papers

A new mechanism of work hardening in the late stages of large strain plastic flow in F.C.C. and diamond cubic crystals

The brittle-to-ductile transition in doped silicon as a model substance

On the generation of new orientations during recrystallization: Recent results on the recrystallization of tensile-deformed fcc single crystals

Creep and dislocation velocities in gallium arsenide

The influence of temperature and strain rate on the flow stress of α-iron single crystals