Newtonian viscous creep in metals

All materials exhibit Newtonian viscous creep behavior at low stresses and high temperatures. We review here such creep behaviors in metals comprising of pure metals and alloys. The underlying creep mechanism(s) depends mainly on the grain size and test temperature while other factors such as the initial dislocation density might also be a factor. Coble creep due to diffusion of point defects through grain boundaries is known to be the dominant creep mechanism in metals with very small grain sizes and relatively low temperatures while Nabarro-Herring creep becomes important for intermediate grain sizes and/or high temperatures. Large grain size and bulk single crystalline metals exhibit Harper-Dorn creep due to dislocation motion rather than point defect diffusion-dominated mechanisms albeit the underlying mechanism is still unclear. Microstructural studies of the specimens deformed in the Harper-Dorn regime have provided some insights. Recent studies suggest that microstructural characterization of deformed specimens is necessary for accurate determination of the rate controlling mechanism. The aim of this paper is two fold namely, to first review the viscous creep mechanisms and to present recent results on Ti3Al2.5V alloy emphasizing the importance of post creep microstructural characterization in establishing the rate controlling mechanism(s).