Forming 1imit predictions that incorporate crystal plasticity rnodels still cannot adequately predict the defonnation perlonnance of polycrystalline matenals. The reason for the limitation in prrdictive power is that the constitutive equations used to connect to the atomic scale assume an affine deformation which do not have a physicd basis. but give general trends. This study was undertaken to better eiucidate the micropiastic process and how it manifests itself phenomenologically. ui this endeavour, the strain rate sensitivity of the flow stress was identified as one parameter that greatly affects the fomiing limit. Hence, an attempt was made to properly define and measure the strain rate sensitivity according to the dictates of thennodynamics.

The thermodynamics of systems can delineate the evolution of the state of a materid if the state variables can be characterized and measured. Inevitably, these variables must be determined at constant structure. Using the theory of thermally activated flow, where the movement of dislocations past obstacles is the rate controlling step, the mechanical testing techniques have ken designed to statistically assess the dynarnic evolution of the microstructure by controlling the temperature, T, and strain rate, ̇ε, and measuring the stress,

, mean slip distance, λ, and mean slip velocity, ̇λ, to define σ = f(λ,̇λ,T). The apparent activation volume, which characterizes the obstacle resistance of strain centres, is determined at constant structure by applying the strain rate change technique. Strain rate sensitivity data are compared to the Comell-Stokes relation, and the Haasen plot is used to separate the diffennt contributions to the flow stress. Using these precise measurements at intempted segments of saain, the evolution of a microstructure during plastic flow can be monitored. By this examination of different rate controlling obstacles, the microstructural parameters which correlate to formability were assessed.

Detailed expenmental evidence is given for different aluminum alloys containing mainly fast or slow diffking solute species, transition precipitates, dispersed particles, and/or dislocation debris. These systems of Al-Fe, Al-Cr, Al-Cu, Al-Mg, and Al-Mg-Si, all displayed unique dislocation-defect interactions which could be elucidated by the current theory of thermally activated fiow.