Creep of Metals
Mercer University Physics
When a metal wire, such as nearly pure polycrystalline silver, is placed under load in an extensometer, the length increases irreversibly due to creep. The primary phase of the creep is characterized by a near exponential increase in length as a function of time. Once secondary creep has been established (constant rate of length increase), the trend toward a work-hardened state can be dramatically influenced by external impulsive blows to the table supporting the extensometer. Not only can these external impulsive stimuli cause quantal (as opposed to smooth) length increases, but they can also result in length decreases of similar magnitude. Thus there are mechanisms for stimulated creep recovery at the meso-scale; however, the atomic defect structures reponsible for this behavior are not yet understood.
When a wire increases its length, the surface to volume ratio must increase, since the number of atoms in the wire is constant. If there were no stable points of metastability (by "pinning" processes to hold the wire in the extended state) the wire would spontaneously return to its shorter configuration. Some preliminary measurements suggest that the quantal length changes brought about by stimulation impulses (permitting "jumps" over the metastable potential "hills") may result from a monlayer (or multiples thereof) of atoms being shuttled back and forth between the bulk and the surface. It is thought that this tendency is facilitated by the manner in which the wire is formed--pulling through successively smaller diameter dies.
Students of this project will be concerned with gathering sufficient "jump" data to make a statistically confident statement concerning the conjectured quantal length changes.